Discussing audio amplifier design -- BJT, discrete
Jon Kirwan
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.
I imagine this will be broken up into three sections; input
transconductance, transimpedance VAS, and output driver. But
other arrangements (such as combining the VAS and output
driver using a signal splitting BJT) would work for me, in
learning.
I said "BJT" and "discrete" but I'm also open to the idea of
using BJT pairs, such as the BCV61 and BCV62. In the case of
current mirrors, that may make sense. But not high-priced,
elite and/or hard to get, or obsolete. And no FETs. This is
to be about learning to design with BJTs.
SMT vs through-hole isn't an issue for learning about a
design, I suppose. If I need to build up some section and
test it with a signal, I'll probably want to do it quickly
and without having to buy services every step of the way. So
I may 'dead bug' SMT parts to get there. (The basic idea
here is to learn, not to make something tiny.)
Although I have some other applications, right now I'd like
the target use to be as a computer speaker system (not unlike
those dirt cheap, sadly almost all of them 10% THD, systems
sold today into this market. Except that I'd like to work
through the design on my own, from start to end.
Given what I understand right now from a very short search on
the topic, the input should be taken as a maximum of 1.0Vrms
and the input's load should appear to be something like 10k
ohms. If someone knows different from that, I'll accept the
criticism and change that spec.
I'd like to consider a tone control and a volume control to
be included.
Output is to be into a small 8 ohm speaker. With that
maximum 1.0Vrms at the input and the volume control set to
maximum the wattage into 8 ohms should be around 10 watts.
Since human hearing won't tell much difference between 8
watts and 12 watts, this is a bit of a sloppy spec and I'm
open to anything in the area of 5-20 watts... though I'm
really wanting to keep the rail voltages down to something
modest and the BJTs not having to tolerate hugish Vce.
Now that I say this, an odd idea comes to mind because the
CFL light bulbs include two TO-220 BJTs that can handle quite
a high Vce on them. I could cannibalize those. But to be
honest, I'm still not needing high watt outputs. So there's
no reason to think about scavenging such parts.
I would like to design it to work into 4 ohms as a margin
bound and not as a design goal, but even 5.6 ohms would be
acceptable.
I'm not looking for this to be done quickly, either. If it
takes months of only occasional back-and-forth, I'm fine with
that. Also, I expect to do my work and don't expect someone
else to hand-hold me from complete ignorante to complete
enlightenment. :) I just need someone to slap my face when
I say something terribly stupid and/or point in a truely
useful direction when I need it. Or else someone who is
wanting to explore this with me and willing to work for it.
Is anyone here willing to consider a sincere discussion?
Jon
Jon Kirwan
<jo...@infinitefactors.org> wrote:
>... ignorante ...
... ignorance ...
Jon
David Eather
Yes, I've done a few amps... Truthfully it would be nice to do something
useful and it dovetails well with something else I am doing.
Jon Kirwan
Oh, my gosh! This is great to hear!!
Could you tell me what you know about the outputs of a PC
sound chip/board? As I gather the details right now, the
idea of 1Vrms max into a load of 10k ohm may be right. Do
you know any of the details?
Thanks,
Jon
David Eather
Don't get too excited, my brain has been badly scrambled. I often miss
things on a first pass - don't be afraid to check - I won't take offense.
John Larkin
<jo...@infinitefactors.org> wrote:
Back when transistors were young, and transistor manuals (GE, RCA)
were published, there were tons of such circuits around. They all
pretty much converged to a few forms, and haven't changed much since.
I could post some circuits from the old manuals, it that wouldn't
spoil what you want to do.
10 watts into computer speakers sounds like a lot. Most AM radios
didn't make one watt. You might experiment first to see how much power
you really need.
John
Jon Kirwan
I remember reading in popular electronics about some audio
amps that I couldn't even come close to following at the
time. The series of them with the name 'tiger' in them.
I'm not so much interested in _circuits_, per se, as I am in
learning about topologies, various ideas built upon them, and
then the specific details of designing towards a specific
implementation. For example, I enjoyed learning about
bootstrapping as a general idea _and_ as it applies to a
couple of specific areas. Having both theory _and_ specific
practice helps firm up the ideas better.
>I could post some circuits from the old manuals, it that wouldn't
>spoil what you want to do.
It may serve as a point of discussion. Would you be willing
to discuss their details and the broader theories as applied?
>10 watts into computer speakers sounds like a lot. Most AM radios
>didn't make one watt. You might experiment first to see how much power
>you really need.
Oh, I figure one watt is enough, too. As a practical matter
and as a consumer using a device and not as a designer trying
to learn something. That's what... 3Vrms? Into 8 ohms? A
voltage gain of 3, given 1Vrms input? I'm wanting to learn
some things, not place one BJT (okay, not really, but it
almost seems like that) down as an emitter follower and then
calling it good. ;)
Up front, I thought I'd like to deal with perhaps something
on the order of about 10Vrms into 8 ohms. I figured that is
enough 'bad' that I'd have to cope with some interesting
corners along the way; but not enough 'bad' that I'd have to
deal with too much all at once.
For example, at around 10 watts or so, it's enough that I may
need to seriously consider avoiding class-A operation of the
output stage and move to class-B, instead. But it is low
enough that there is some room to discuss each, as well as
class-AB biasing, too. More power and I'm almost certain I'm
pushed into class-B. Less power and.. well, who cares that
much? At one watt or so, just class-A and be done with it? I
won't learn the reasoning behind trade-offs that way.
There's more. I just figured at about 10 watts I'm likely to
learn some things but not be forced to learn so much that I'm
overwhelmed.
I'm open to specific advice about all this, of course.
Jon
Jon Kirwan
<eat...@tpg.com.au> wrote:
>Jon Kirwan wrote:
>>><snip of offer to discuss amplifier design>
>>
>> Oh, my gosh! This is great to hear!!
>
>Don't get too excited, my brain has been badly scrambled. I often miss
>things on a first pass - don't be afraid to check - I won't take offense.
><snip of trailer unresponded to>
hehe. Okay. It's just a topic I'd like to play with and I'm
glad there might be someone else out there to talk it over
with. What can I say? Except _thanks_!
Jon
David Eather
I like 10 watts as a starting size - at this size you have to start
doing things the way the big amps do, but it is not so big as to be
outrageously expensive, for example you still use a relatively small
power supply, heatsinks, and inexpensive transistors, and at the end you
can use it with your PC and really blow those 320 watt PFPO (peak
fantasy power output)speakers away.
I like your hesitation on class A. You want an amp with some power
output and class A is very inefficient, never more than 25% and often
way less. This would add greatly to the cost - a 40 watt power supply,
heatsinks capable of getting rid of the same as heat while keeping the
transistor junction temperature low, and beefier transistors. You also
get to put up with a shorter service life from all that heat. The "big
thing" with class A is there is no crossover distortion, which can lead
to better overall distortion figures, but the cost is huge - a kit for 2
x 20 watt class A sells for $600.
My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
and this is, I think, I great reason to go down this design path (what
you learn is applicable in the most number of situations). I should hunt
down a schematics of what I'm seeing in the distance (which can/will
change as decisions are made) - some of the justifications will have to
wait
The first step is to think about the output. The basic equations are
(1).....Vout = sqrt(2*P*R)
With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
actually +/- 12.7 volts with a split power supply.
(2).....Imax = sqrt(2*P/R)
This comes out to 1.6 amps. You should probably also consider the case
when R speaker = 4 ohms when initially selecting a transistor for the
output 2.2 amps - remember this is max output current. The power supply
voltage will have to be somewhat higher than Vout to take into account
circuit drive requirements, ripple on the power supply and transformer
regulation etc.
Are you OK with connecting mains to a transformer? or would you rather
use an AC plug pack (10 watts is about the biggest amp a plugpack can be
used for)? The "cost" for using an AC plug pack is you will need larger
filter capacitors.
I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.
Phil Allison
"David Eather" <
** Learn to trimv- wanker!!
>
> I like your hesitation on class A. You want an amp with some power output
> and class A is very inefficient, never more than 25% and often way less.
** Class A amplifiers are up to 50 % efficient.
>This would add greatly to the cost - a 40 watt power supply,
** 20 watts is all that is needed.
> heatsinks capable of getting rid of the same as heat while keeping the
> transistor junction temperature low,
** Junction temps can settle at 125 C with no problems.
> and beefier transistors.
** Nonsense.
> You also get to put up with a shorter service life from all that heat.
** Bull.
> The "big thing" with class A is there is no crossover distortion, which
> can lead to better overall distortion figures, but the cost is huge - a
> kit for 2 x 20 watt class A sells for $600.
** Irrelevant what some unspecified kit sells for.
A 20 watt class A power stage only needs bigger heatsinks compared to usual
low bias, class AB operation.
> My particular bias for an amp this size is to go class AB with a split
> power supply. The majority of quality audio amps follow this topology
** Bollocks.
Only some very high powered hi-fi and pro-audio amps use additional DC
rails.
> The power supply voltage will have to be somewhat higher than Vout to take
> into account circuit drive requirements, ripple on the power supply and
> transformer regulation etc.
** There is no variation in the DC current draw from a PSU with class A
amplifiers - in fact, this is the very definition of class A operation of
an audio amp. So the DC rails will remain steady from no drive to full
output.
Also, the heatsink will cool considerably when the amp is operated at full
sine wave power.
Why is SOOO much bollocks posted about something so very simple ??
.... Phil
Jon Kirwan
You appear to confirm my instincts.
>and at the end you
>can use it with your PC and really blow those 320 watt PFPO (peak
>fantasy power output)speakers away.
Well, mostly I'm just trying to learn... not impress others
about the results. :)
>I like your hesitation on class A. You want an amp with some power
>output and class A is very inefficient, never more than 25% and often
>way less. This would add greatly to the cost - a 40 watt power supply,
>heatsinks capable of getting rid of the same as heat while keeping the
>transistor junction temperature low, and beefier transistors. You also
>get to put up with a shorter service life from all that heat. The "big
>thing" with class A is there is no crossover distortion, which can lead
>to better overall distortion figures, but the cost is huge - a kit for 2
>x 20 watt class A sells for $600.
Egads. My instincts said class-A would add a lot to weight
and cost, but no idea a mere 20W kit could sell for $300!
>My particular bias for an amp this size is to go class AB with a split
>power supply. The majority of quality audio amps follow this topology
>and this is, I think, I great reason to go down this design path (what
>you learn is applicable in the most number of situations). I should hunt
>down a schematics of what I'm seeing in the distance (which can/will
>change as decisions are made) - some of the justifications will have to
>wait
I'm fine with taking things as they come.
As far as the class, I guessed that at 10 watts class-A would
be too power-hungry and probably not worth its weight but
that class-AB might be okay.
I have to warn you, though, that I'm not focused upon some
20ppm THD. I'd like to learn, not design something whose
distortion (or noise, for that matter) is around a bit on a
16-bit DAC or less. I figure winding up close to class-B
operation in the end. But I'd like to take the walk along
the way, so to speak.
>The first step is to think about the output. The basic equations are
>
>(1).....Vout = sqrt(2*P*R)
>
>With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
>actually +/- 12.7 volts with a split power supply.
If you don't mind, I'd like to discuss this more closely. Not
just have it tossed out. So, P=V*I; or P=Vrms^2/R with AC.
Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
equation. Which suggests the +/-12.7V swing. Which further
suggests, taking Vce drops and any small amounts emitter
resistor drops into account, something along the lines of +/-
14-15V rails?
Or should the rails be cut a lot closer to the edge here to
improve efficiency. What bothers me is saturation as Vce on
the final output BJTs goes well below 1V each and beta goes
away, as well, rapidly soaking up remaining drive compliance.
>(2).....Imax = sqrt(2*P/R)
>
>This comes out to 1.6 amps. You should probably also consider the case
>when R speaker = 4 ohms when initially selecting a transistor for the
>output 2.2 amps - remember this is max output current. The power supply
>voltage will have to be somewhat higher than Vout to take into account
>circuit drive requirements, ripple on the power supply and transformer
>regulation etc.
Okay. I missed reading this when writing the above. Rather
than correct myself, I'll leave my thinking in place.
So yes, the rails will need to be a bit higher. Agreed. On
this subject, I'm curious about the need to _isolate_, just a
little, the rails used by the input stage vs the output stage
rails. I'm thinking an RC (or LC for another pole?) for
isolation. But I honestly don't know if that's helpful, or
not.
>Are you OK with connecting mains to a transformer? or would you rather
>use an AC plug pack (10 watts is about the biggest amp a plugpack can be
>used for)? The "cost" for using an AC plug pack is you will need larger
>filter capacitors.
I'd much prefer to __avoid__ using someone else's "pack" for
the supply. All discrete parts should be on the table, so to
speak, in plain view. And I don't imagine _any_ conceptual
difficulties for this portion of the design. I'm reasonably
familiar with transformers, rectifiers, ripple calculations,
and how to consider peak charging currents vs averge load
currents as they relate to the phase angles available for
charging the caps. So on this part, I may need less help
than elsewhere. In other words, I'm somewhat comfortable
here.
>I should also ask if you have a multi meter, oscilloscope (not necessary
>but useful)and how is your soldering? But it would be wise to keep this
>whole thing as a paper exercise before you commit to anything.
I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on. For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area. I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well. I also have a room set aside
for this kind of stuff, when I get time to play.
Jon
David Eather
> "David Eather" <
>
> ** Learn to trimv- wanker!!
>
>> I like your hesitation on class A. You want an amp with some power output
>> and class A is very inefficient, never more than 25% and often way less.
>
> ** Class A amplifiers are up to 50 % efficient.
Yes, if you use a matching transformer, not really an option here
>
>> This would add greatly to the cost - a 40 watt power supply,
>
> ** 20 watts is all that is needed.
IBID
>
>
>> heatsinks capable of getting rid of the same as heat while keeping the
>> transistor junction temperature low,
>
> ** Junction temps can settle at 125 C with no problems.
Most transistors are spec'ed at 125 C for 1000 or 2000 hours. Most
people would like there devices to run a bit longer than this. At least
one semiconductor manufacturer believes that for each lowering of the
junction temperature by 10 C doubles the life of transistor
>
>> and beefier transistors.
>
> ** Nonsense.
needed for lower Th(jc) and Th(ch)
>
>> You also get to put up with a shorter service life from all that heat.
>
> ** Bull.
I think just about everyone will disagree with you on that point. heat
dries out electrolytics and heat stress cycling is a major failure mode
of semiconductors
>
>> The "big thing" with class A is there is no crossover distortion, which
>> can lead to better overall distortion figures, but the cost is huge - a
>> kit for 2 x 20 watt class A sells for $600.
>
> ** Irrelevant what some unspecified kit sells for.
No, it is an example of how costly class A can become. I don't think
anyone can realistically find a way to make a 2 x 20 watt class AB amp
kit sell for anything like that price (short of gold plated everything
and mil spec components). The kit price I quoted was the magazine
"Silicon Chip" May-September 2007 design from Altronics
http://www.altronics.com.au/index.asp?area=item&id=K5125
>
> A 20 watt class A power stage only needs bigger heatsinks compared to usual
> low bias, class AB operation.
That was my part of my point. Thanks for making it so exactly.
>
>
>> My particular bias for an amp this size is to go class AB with a split
>> power supply. The majority of quality audio amps follow this topology
>
> ** Bollocks.
Well, somewhat recently the trend has gone to mono block IC amps so I'll
take that on the chin
>
> Only some very high powered hi-fi and pro-audio amps use additional DC
> rails.
>
So what. Even you say "some". Class G is not yet very common.
>
>> The power supply voltage will have to be somewhat higher than Vout to take
>> into account circuit drive requirements, ripple on the power supply and
>> transformer regulation etc.
>
>
> ** There is no variation in the DC current draw from a PSU with class A
> amplifiers - in fact, this is the very definition of class A operation of
> an audio amp. So the DC rails will remain steady from no drive to full
> output.
True! But you may still need extra voltage for the circuit and you also
need extra voltage for the power supply ripple (unless you use a
*regulated* power supply) - which neither you or I specified.
>
> Also, the heatsink will cool considerably when the amp is operated at full
> sine wave power.
Not relevant at the moment.
>
> Why is SOOO much bollocks posted about something so very simple ??
>
>
>
> ..... Phil
>
>
Phil,
Your welcome to take the lead and go through an amplifier design. Just
say the word.
Bob Masta
<jo...@infinitefactors.org> wrote:
>I'd like to take a crack at thinking through a design of an
>audio amplifier made up of discrete BJTs and other discrete
>parts as an educational process.
>
<snip>
When I was first getting interested in power amp
design (back in the '70s) I started collecting
schematics for all the power amps I could get my
hands on, to compare them. I noticed that the
schematics for simple bipolar op-amp ICs were
remarkably similar to those for big discrete power
amps. If you have an old National Linear Databook
(or don't mind a lot of rooting around on the Web
for individual datasheets), you might take a look.
You can build a pretty decent amp with only a
handful of transistors. The same basic circuit
can be used for a wide range of output powers,
just by changing the power supply voltages and the
output device ratings.
Best regards,
Bob Masta
DAQARTA v5.00
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Phil Allison
"David Eather = a LIAR and a Wanker "
>> ** Learn to trim - wanker!!
>>
>>> I like your hesitation on class A. You want an amp with some power
>>> output and class A is very inefficient, never more than 25% and often
>>> way less.
>>
>> ** Class A amplifiers are up to 50 % efficient.
>
> Yes, if you use a matching transformer,
** 100%, absolute BULLSHIT !!!!!!!
YOU KNOW NOTHING MORON !!!!!!!
>>> This would add greatly to the cost - a 40 watt power supply,
>>
>> ** 20 watts is all that is needed.
>
> IBID
** Fuck you - you damn imbecile.
>>> heatsinks capable of getting rid of the same as heat while keeping the
>>> transistor junction temperature low,
>>
>> ** Junction temps can settle at 125 C with no problems.
>
> Most transistors are spec'ed at 125 C for 1000 or 2000 hours.
** More, 100% absolute BULLSHIT !!!!!!!
This RATBAG just keeps piling on the LIES !!!!!
> people would like there devices to run a bit longer than this. At least
> one semiconductor manufacturer believes that for each lowering of the
> junction temperature by 10 C doubles the life of transistor
** MORE 100% absolute BULLSHIT !!!!!!!
This RATBAG just keeps piling on the damn LIES !!
>>> You also get to put up with a shorter service life from all that heat.
>>
>> ** Bull.
>
> I think just about everyone will disagree with you on that point.
** What some fuckwit like you choses to "think" is of zero consequence.
> heat dries out electrolytics
** ROTFLMAO !!!
Shame BJTs are not electrolytic.
You FUCKING MORON !!!!!!!!
> and heat stress cycling is a major failure mode of semiconductors
** No heat cycling occurs with class A.
Huge amounts occur with all class B and AB designs.
You FUCKING KNOW NOTHING MORON !!!!!!!!
>>> The "big thing" with class A is there is no crossover distortion, which
>>> can lead to better overall distortion figures, but the cost is huge - a
>>> kit for 2 x 20 watt class A sells for $600.
>>
>> ** Irrelevant what some unspecified kit sells for.
> No, it is an example of how costly class A can become.
** Absolute fucking CRAP !!!!!!!!
YOU are an example of how STUPID people can become.
An extreme example of an damn LIAR too.
>> A 20 watt class A power stage ONLY needs bigger heatsinks compared to
>> usual low bias, class AB operation.
>
** No big expense there.
>>> My particular bias for an amp this size is to go class AB with a split
>>> power supply. The majority of quality audio amps follow this topology
>>
>> ** Bollocks.
>
> Well, somewhat recently the trend has gone to mono block IC amps so I'll
> take that on the chin
** Go drop dead - you pig ignorant LYING WANKER !
>> Only some very high powered hi-fi and pro-audio amps use additional DC
>> rails.
>>
>
> So what.
** Proves you WRONG - you stinking LIAR !!!!!!
>>> The power supply voltage will have to be somewhat higher than Vout to
>>> take into account circuit drive requirements, ripple on the power supply
>>> and transformer regulation etc.
>>
>>
>> ** There is no variation in the DC current draw from a PSU with class A
>> amplifiers - in fact, this is the very definition of class A operation
>> of an audio amp. So the DC rails will remain steady from no drive to
>> full output.
>
> True!
** Proves you WRONG AGAIN - you stinking LIAR !!!!!!
>> Also, the heatsink will cool considerably when the amp is operated at
full
>> sine wave power.
>
> Not relevant at the moment.
** Course it is - you DAMN LIAR !!!!!!!!!
GO DROP FUCKING DEAD
- YOU BULLSHITTING IMBECILE !!!!!!!!!
.... Phil
Phil Allison
"Jon Kirwan is a FUCKWIT TROLL "
DO NOT FEED THE TROLL !!!!!!!!!!!!!!!
DO NOT FEED THE TROLL !!!!!!!!!!!!!!!
TROLLS are DESTROYERS of all NEWSGROUPS.
-------------------------------------------------------------
... Phil
amdx
"Phil Allison" <phi...@tpg.com.au> wrote in message
news:7sau8i...@mid.individual.net...
Well Jon,
When you said,
"I just need someone to slap my face......"
Phil was the first one I thought of. :-)
Mike
pimpom
> On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin
> <jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>
>> On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
>> <jo...@infinitefactors.org> wrote:
>>
>>> I'd like to take a crack at thinking through a design of an
>>> audio amplifier made up of discrete BJTs and other discrete
>>> parts as an educational process.
>>>
>>
>> Back when transistors were young, and transistor manuals (GE,
>> RCA)
>> were published, there were tons of such circuits around. They
>> all
>> pretty much converged to a few forms, and haven't changed much
>> since.
>
> I remember reading in popular electronics about some audio
> amps that I couldn't even come close to following at the
> time. The series of them with the name 'tiger' in them.
>
That was probably an adaptation of RCA's 70-watt power amp,
published in their transistor manual of the mid-60s. A true
70-watt continuous output was pretty hefty then. Still is, in
fact, for many applications. The design was state-of-the-art,
using their 4000 series transistors which were specifically made
for audio. THD was <0.25% at 70W (pretty good for the time).
IIRC, Popular Electronics published it virtually unchanged (I
don't think they credited RCA with the design). I vaguely recall
their calling it 'tiger' something and the article title included
the word "indestructible". The latter term was because it
included current clamping for the output transistors as
short-circuit protection, a thermal fuse and a normal fuse. PE
claimed that they abused it with short-circuits and reactive
loads, and the worst they got was a blown fuse.
I remember that design fondly, if not perfectly, because I spent
a lot of time analysing it and others in the RCA manual.
Jon Kirwan
>On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>>I'd like to take a crack at thinking through a design of an
>>audio amplifier made up of discrete BJTs and other discrete
>>parts as an educational process.
>>
><snip>
>
>When I was first getting interested in power amp
>design (back in the '70s) I started collecting
>schematics for all the power amps I could get my
>hands on, to compare them. I noticed that the
>schematics for simple bipolar op-amp ICs were
>remarkably similar to those for big discrete power
>amps. If you have an old National Linear Databook
>(or don't mind a lot of rooting around on the Web
>for individual datasheets), you might take a look.
>
>You can build a pretty decent amp with only a
>handful of transistors. The same basic circuit
>can be used for a wide range of output powers,
>just by changing the power supply voltages and the
>output device ratings.
>
>Best regards,
Thanks, Bob. Audio amplifiers, especially ones delivering
_some_ power, seem to offer such an excellent way to learn.
The basic idea, at a behavioral level, is fairly simple. An
implementation requires some knowledge and thought in the
end. So the destination is arrived at by taking a great path
to walk, with such wonderful vistas to see, I think. Much of
interest is along the way of getting there.
I may have an old National databook on linear parts
somewhere. I keep a lot, but I also have several thousand
books in my library which covers all of the walls in one of
the rooms. I'm at a point now where to get room for more
books, others must be boxed and stored or simply destroyed
and pulped. So it's a _maybe_.
One of the nice things (to me) about this kind of a path,
too, is that what I learn can be used for lots of things. An
audio amplifier is, in effect, not that much different from
an op amp. There is the usual basic idea of open loop gain
and closed loop gain with negative feedback, phase margins,
problems to solve over a frequency range spanning many
decades, and so on.
A completely separate project I'd like to play with, which
this learning will help prepare me for, is designing a pin
driver. I'd like to sink or source a programmable current
spanning decades from perhaps 100nA to perhaps 100uA while
reading the voltage at the node, as well as being able to
program a low impedance voltages spanning from -15V to +15V
there and read the current, or read a voltage at the same
node while presenting a fairly high impedence to it. I
imagine what I learn here will aid me there. And I'd like to
do this at some speed, as well. I may then start with a BJT
tester, for example, making up only three of these to start
and tying them into a micro for playing. Expanding that for
other purposes, later. It would be fun.
Jon
Michael A. Terrell
That was the RCA 404xx series of their house numbered transistors. I
think they used:
1 40406
1 40407
1 40408
1 40409
1 40410
2 40411
and a couple 1N series metal cased diodes for temperature sensing.
It's been 40 years since I built that PE Tiger amp and the preamp
that went with it.
> IIRC, Popular Electronics published it virtually unchanged (I
> don't think they credited RCA with the design). I vaguely recall
> their calling it 'tiger' something and the article title included
> the word "indestructible". The latter term was because it
> included current clamping for the output transistors as
> short-circuit protection, a thermal fuse and a normal fuse. PE
> claimed that they abused it with short-circuits and reactive
> loads, and the worst they got was a blown fuse.
>
> I remember that design fondly, if not perfectly, because I spent
> a lot of time analysing it and others in the RCA manual.
--
Greed is the root of all eBay.
Jon Kirwan
Thanks for that bit. I remember looking at the first article
and wondering about trying my hand at building it. There was
no way I was prepared to understand it, though. The largest
problem I faced at the time, besides my own limitations in
education, was funds. I couldn't afford to even buy the
boards they offered, let alone the parts. So it was a non-
starter for me. I got my parts by scavenging TV sets and
radios others threw away. One of my larger hauls was when a
tornado knocked down a bowling alley and I called up the
owner and received permission to walk through the mess and
extract parts. I _never_ paid for anything. (Dad had died
when I was 7 and I literally had to work the fields picking
vegetables to earn enough to survive.)
Now, I might go back. But to be honest, I'd much prefer
being able to ask questions as they arise and work on
refining as I go. I learn more from a "movie in progress"
than studying a "snapshot," I guess.
Jon
David Eather
Well it would cost something less at 10 watts.
>
>> My particular bias for an amp this size is to go class AB with a split
>> power supply. The majority of quality audio amps follow this topology
>> and this is, I think, I great reason to go down this design path (what
>> you learn is applicable in the most number of situations). I should hunt
>> down a schematics of what I'm seeing in the distance (which can/will
>> change as decisions are made) - some of the justifications will have to
>> wait
>
> I'm fine with taking things as they come.
>
> As far as the class, I guessed that at 10 watts class-A would
> be too power-hungry and probably not worth its weight but
> that class-AB might be okay.
>
> I have to warn you, though, that I'm not focused upon some
> 20ppm THD. I'd like to learn, not design something whose
> distortion (or noise, for that matter) is around a bit on a
> 16-bit DAC or less. I figure winding up close to class-B
> operation in the end. But I'd like to take the walk along
> the way, so to speak.
>
10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
a few detours to see what would help or harm that.
Mostly not needed, if you use a long tailed pair for the input / error
amplifier, but you might prefer some other arrangement so keep it in
mind if your circuit "motorboats"
>
>> Are you OK with connecting mains to a transformer? or would you rather
>> use an AC plug pack (10 watts is about the biggest amp a plugpack can be
>> used for)? The "cost" for using an AC plug pack is you will need larger
>> filter capacitors.
>
> I'd much prefer to __avoid__ using someone else's "pack" for
> the supply. All discrete parts should be on the table, so to
> speak, in plain view. And I don't imagine _any_ conceptual
> difficulties for this portion of the design. I'm reasonably
> familiar with transformers, rectifiers, ripple calculations,
> and how to consider peak charging currents vs averge load
> currents as they relate to the phase angles available for
> charging the caps. So on this part, I may need less help
> than elsewhere. In other words, I'm somewhat comfortable
> here.
Ah, then there are questions of what voltage and VA for a transformer.
So there are questions of usage (music, PA, PA with an emergency alert
siren tied in etc) and rectifier arrangement and capacitor size /
voltage to get your required voltage output at full load.
>
>> I should also ask if you have a multi meter, oscilloscope (not necessary
>> but useful)and how is your soldering? But it would be wise to keep this
>> whole thing as a paper exercise before you commit to anything.
>
> I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
> handheld, two oscilloscopes (TEK 2245 with voltmeter option
> and an HP 54645D), three triple-output power supplies with
> two of them GPIB drivable, the usual not-too-expensive signal
> generator, and a fair bunch of other stuff on the shelves.
> Lots of probes, clips, and so on. For soldering, I'm limited
> to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
> somewhat wider spade tips in the 1.5mm area. I have tubs and
> jars of various types of fluxes, as well, and wire wrap tools
> and wire wrap wire, as well. I also have a room set aside
> for this kind of stuff, when I get time to play.
OK. Next serious project, I'm coming around to your place! Your gear is
better than mine. I had to ask, rather than just assume just in case my
assumptions got you building something you didn't want to, and got you
splattered all over the place from the mains, or suggesting you choose
the miller cap by watching the phase shift of the feedback circuit - I
don't read a lot of the posts so I didn't know what you could do.
>
> Jon
Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier
The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.
Phil Allison
"David Eather is a Fuckwit LIAR"
> Have a look at
> http://en.wikipedia.org/wiki/Electronic_amplifier
> The bits on class A might be interesting as it says 25% efficiency and 50%
> obtainable with inductive output coupling (i.e. with a transformer)
> which is what I said, not what blow hard Phil said.
** ROTFLMAO !!!
So this Eather WANKER gets his WRONG info from bloody Wiki !!!!
Here is the actual quote:
" Class A amplifiers are the usual means of implementing small-signal
amplifiers. They are not very efficient; a theoretical maximum of 50% is
obtainable with inductive output coupling and only 25% with capacitive
coupling."
So, the para is clearly about "small signal" class A stages
- ie RC coupled pre-amp stages !!!!!!!
Not class A ** POWER AMPS ** !!!!!
Wot a fucking FUCKWIT !!
BTW
Inductive coupling also refers to the use of chokes as the collector or
plate loads for ( single ended ) class A operation.
Way over this Eather Google Monkey's pointy head.
.... Phil
David Eather
> "David Eather = a LIAR and a Wanker "
>
>
>>> ** Learn to trim - wanker!!
>>>
>>>> I like your hesitation on class A. You want an amp with some power
>>>> output and class A is very inefficient, never more than 25% and often
>>>> way less.
>>> ** Class A amplifiers are up to 50 % efficient.
>> Yes, if you use a matching transformer,
>
> ** 100%, absolute BULLSHIT !!!!!!!
>
> YOU KNOW NOTHING MORON !!!!!!!
http://en.wikipedia.org/wiki/Electronic_amplifier
agrees with me and not with you. As does every textbook - they agree
with me about the efficiency of class A. If you are up to it prove it -
work an example where you get better than 25% - or show a text book
making your claim 50% (without using a transformer, which you said
wasn't needed)
>
>
>>>> This would add greatly to the cost - a 40 watt power supply,
>>> ** 20 watts is all that is needed.
>> IBID
>
> ** Fuck you - you damn imbecile.
Sorry it is you
>
>
>
>>>> heatsinks capable of getting rid of the same as heat while keeping the
>>>> transistor junction temperature low,
>>> ** Junction temps can settle at 125 C with no problems.
>> Most transistors are spec'ed at 125 C for 1000 or 2000 hours.
>
>
> ** More, 100% absolute BULLSHIT !!!!!!!
>
> This RATBAG just keeps piling on the LIES !!!!!
>
How long do you think they are rated for Phil?
>
>> people would like there devices to run a bit longer than this. At least
>> one semiconductor manufacturer believes that for each lowering of the
>> junction temperature by 10 C doubles the life of transistor
>
>
> ** MORE 100% absolute BULLSHIT !!!!!!!
>
> This RATBAG just keeps piling on the damn LIES !!
RCA Power Transistors Application Note AN-6688 "A Practical Approach to
Audio Amplifier Design" page 5
Would you like me to post you a copy?
>
>
>>>> You also get to put up with a shorter service life from all that heat.
>>> ** Bull.
>> I think just about everyone will disagree with you on that point.
>
>
> ** What some fuckwit like you choses to "think" is of zero consequence.
>
>
>> heat dries out electrolytics
>
> ** ROTFLMAO !!!
>
> Shame BJTs are not electrolytic.
>
> You FUCKING MORON !!!!!!!!
>
>
>> and heat stress cycling is a major failure mode of semiconductors
>
>
> ** No heat cycling occurs with class A.
>
> Huge amounts occur with all class B and AB designs.
>
> You FUCKING KNOW NOTHING MORON !!!!!!!!
If no heat cycling occurs then no signal output occurs either.
>
>
>
>>>> The "big thing" with class A is there is no crossover distortion, which
>>>> can lead to better overall distortion figures, but the cost is huge - a
>>>> kit for 2 x 20 watt class A sells for $600.
>>> ** Irrelevant what some unspecified kit sells for.
>
>> No, it is an example of how costly class A can become.
>
> ** Absolute fucking CRAP !!!!!!!!
>
> YOU are an example of how STUPID people can become.
>
> An extreme example of an damn LIAR too.
Please Phil, show us all how to build a cheap 2 x 20 watt class A amp
>
>
>>> A 20 watt class A power stage ONLY needs bigger heatsinks compared to
>>> usual low bias, class AB operation.
>
> ** No big expense there.
>
>
>
>>>> My particular bias for an amp this size is to go class AB with a split
>>>> power supply. The majority of quality audio amps follow this topology
>>> ** Bollocks.
>> Well, somewhat recently the trend has gone to mono block IC amps so I'll
>> take that on the chin
>
> ** Go drop dead - you pig ignorant LYING WANKER !
>
>
>>> Only some very high powered hi-fi and pro-audio amps use additional DC
>>> rails.
>>>
>> So what.
>
>
> ** Proves you WRONG - you stinking LIAR !!!!!!
No, it shows you can't construct a logical argument.
>
>
>
>>>> The power supply voltage will have to be somewhat higher than Vout to
>>>> take into account circuit drive requirements, ripple on the power supply
>>>> and transformer regulation etc.
>>>
>>> ** There is no variation in the DC current draw from a PSU with class A
>>> amplifiers - in fact, this is the very definition of class A operation
>>> of an audio amp. So the DC rails will remain steady from no drive to
>>> full output.
>> True!
>
> ** Proves you WRONG AGAIN - you stinking LIAR !!!!!!
I agreed with you and you said that proves you wrong and makes you a
stinking liar. Correct!, you're wrong and you are a stinking liar
>
>
> >> Also, the heatsink will cool considerably when the amp is operated at
> full
>>> sine wave power.
I thought there was no heat stress cycling, so why would the heatsink be
"cooler" wouldn't it be at a constant temperature?.
>> Not relevant at the moment.
>
> ** Course it is - you DAMN LIAR !!!!!!!!!
How is the heat sink (currently) relevant when there is no spec for the
power supply, no spec for transistor de-rating, no spec for heat
dissipation?
Please tell us all what is the correct size of heatsink and how did you
arrive at that magical number?
>
> GO DROP FUCKING DEAD
>
> - YOU BULLSHITTING IMBECILE !!!!!!!!!
>
>
>
>
> ..... Phil
>
>
>
>
>
Phil,
Is it hard to walk around with so much malice and vitriol? What does
your wife think of that? How do you treat her and others close to you?
You are simply a stain on humanity. When you die, as we all one day
must, those few who remember you will remember you as a negative
influence and the few people you would have influenced will be better
off without you.
That is a sad monument to your life. The best you are is an internet
troll and a stain on life.
David Eather
>
>
>
>
>
So how are class A POWER AMPS more efficient? A little math please..
Oh, and you sniped or forgot to mention that the textbooks agree with me
too..
Phil Allison
>>
>>> Have a look at
>>> http://en.wikipedia.org/wiki/Electronic_amplifier
>>> The bits on class A might be interesting as it says 25% efficiency and
>>> 50% obtainable with inductive output coupling (i.e. with a transformer)
>>> which is what I said, not what blow hard Phil said.
>>
>>
>> ** ROTFLMAO !!!
>>
>> So this Eather WANKER gets his WRONG info from bloody Wiki !!!!
>>
>> Here is the actual quote:
>>
>> " Class A amplifiers are the usual means of implementing small-signal
>> amplifiers. They are not very efficient; a theoretical maximum of 50% is
>> obtainable with inductive output coupling and only 25% with capacitive
>> coupling."
>>
>> So, the para is clearly about "small signal" class A stages
>>
>> - ie RC coupled pre-amp stages !!!!!!!
>>
>> Not class A ** POWER AMPS ** !!!!!
>>
>> Wot a fucking FUCKWIT !!
>>
>> BTW
>>
>> Inductive coupling also refers to the use of chokes as the collector or
>> plate loads for ( single ended ) class A operation.
>>
>> Way over this Eather Google Monkey's pointy head.
>>
>
>
** Cos they operate in push pull - you IMBECILE !!
Look it up - you pig ignorant, arrogant, shit for brains LIAR !!
Forget stupid bloody Wiki cos it is full of missing info and mistakes.
.... Phil
Phil Allison
** ABSOLUTELY IGNORE this
KNOW NOTHING IDIOT !!!!!!!!!!
He has ZERO tech knowledge.
He has ZERO actual experience with any audio electronics at all.
He is another pathetic GOOGLE MONKEY !!!
ALL his advice is WRONG !!!
.... Phil
Jon Kirwan
<eat...@tpg.com.au> wrote:
>Jon Kirwan wrote:
>> On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
>> <eat...@tpg.com.au> wrote:
>>> <snip>
>>
>>> My particular bias for an amp this size is to go class AB with a split
>>> power supply. The majority of quality audio amps follow this topology
>>> and this is, I think, I great reason to go down this design path (what
>>> you learn is applicable in the most number of situations). I should hunt
>>> down a schematics of what I'm seeing in the distance (which can/will
>>> change as decisions are made) - some of the justifications will have to
>>> wait
>>
>> I'm fine with taking things as they come.
>>
>> As far as the class, I guessed that at 10 watts class-A would
>> be too power-hungry and probably not worth its weight but
>> that class-AB might be okay.
>>
>> I have to warn you, though, that I'm not focused upon some
>> 20ppm THD. I'd like to learn, not design something whose
>> distortion (or noise, for that matter) is around a bit on a
>> 16-bit DAC or less. I figure winding up close to class-B
>> operation in the end. But I'd like to take the walk along
>> the way, so to speak.
>
>10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
>a few detours to see what would help or harm that.
Hehe. I'm thinking of some numbers I saw in the area of
.002% THD. I hate percentages and immediately convert them.
In this case, it is 20e-6 or 20 ppm. Which is darned close
to a bit on a 16-bit dac. That's why I wrote that way. I
just don't like using % figures. They annoy me just a tiny
bit.
Regarding .1% to .05%, I'm _very_ good with that. Of course,
I'm going to have to learn about how to estimate it from
theory as well as measure it both via simulation before
construction and from actual testing afterwards. More stuff
I might _think_ I have a feel for, but I'm sure I will
discover I don't as I get more into it.
But speaking from ignorance, I'm good shooting for the range
you mentioned. It was about what I had in mind, in fact,
figuring I could always learn as I go.
Okay. I've _zero_ experience for audio. It just crossed my
mind from other cases. I isolate the analog supply from the
digital -- sometimes with as many as four caps and three
inductor beads. There, it _does_ help.
>>> Are you OK with connecting mains to a transformer? or would you rather
>>> use an AC plug pack (10 watts is about the biggest amp a plugpack can be
>>> used for)? The "cost" for using an AC plug pack is you will need larger
>>> filter capacitors.
>>
>> I'd much prefer to __avoid__ using someone else's "pack" for
>> the supply. All discrete parts should be on the table, so to
>> speak, in plain view. And I don't imagine _any_ conceptual
>> difficulties for this portion of the design. I'm reasonably
>> familiar with transformers, rectifiers, ripple calculations,
>> and how to consider peak charging currents vs averge load
>> currents as they relate to the phase angles available for
>> charging the caps. So on this part, I may need less help
>> than elsewhere. In other words, I'm somewhat comfortable
>> here.
>
>Ah, then there are questions of what voltage and VA for a transformer.
>So there are questions of usage (music, PA, PA with an emergency alert
>siren tied in etc) and rectifier arrangement and capacitor size /
>voltage to get your required voltage output at full load.
I figure on working out the design of the amplifier and then
going back, once that is determined and hashed out, with the
actual required figures for the power supply and design that
part as the near-end of the process. Earlier on, I'd expect
to have some rough idea of how "bad" it needs to be -- if the
initial guesses don't raise alarms, then I wouldn't dig into
the power supply design until later on. The amplifier, it
seems to me, dictates the parameters. So that comes later,
doesn't it?
>>> I should also ask if you have a multi meter, oscilloscope (not necessary
>>> but useful)and how is your soldering? But it would be wise to keep this
>>> whole thing as a paper exercise before you commit to anything.
>>
>> I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
>> handheld, two oscilloscopes (TEK 2245 with voltmeter option
>> and an HP 54645D), three triple-output power supplies with
>> two of them GPIB drivable, the usual not-too-expensive signal
>> generator, and a fair bunch of other stuff on the shelves.
>> Lots of probes, clips, and so on. For soldering, I'm limited
>> to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
>> somewhat wider spade tips in the 1.5mm area. I have tubs and
>> jars of various types of fluxes, as well, and wire wrap tools
>> and wire wrap wire, as well. I also have a room set aside
>> for this kind of stuff, when I get time to play.
>
>OK. Next serious project, I'm coming around to your place!
You come to the west coast of the US and I'll have a room for
you!
>Your gear is
>better than mine. I had to ask, rather than just assume just in case my
>assumptions got you building something you didn't want to, and got you
>splattered all over the place from the mains, or suggesting you choose
>the miller cap by watching the phase shift of the feedback circuit - I
>don't read a lot of the posts so I didn't know what you could do.
To be honest, I can do a few things but I'm really not very
practiced. My oscilloscope knowledge is lacking in some
areas -- which becomes all too painfully obvious to me when I
watch a pro using my equipment. And I'm still learning to
solder better. It's one of a few hobbies.
>> Jon
>
>Have a look at
>http://en.wikipedia.org/wiki/Electronic_amplifier
Done.
>The bits on class A might be interesting as it says 25% efficiency and
>50% obtainable with inductive output coupling (i.e. with a transformer)
>which is what I said, not what blow hard Phil said.
What I first see there is the amplifier sketch at the top of
the page (I don't really care too much about arguing about
efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair. First thing I remember about is that R4
shouldn't be there and better still both R3 and R4 should be
replaced with a current mirror. R5 should be a replaced with
a BJT, as well. I assume the input impedance of that example
is basically the parallel resistance of R1 and R2, but if we
use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point. There's no
miller cap on Q3, I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)
The feedback ... well, I need to think about that a little
more. There's no degen resistors in the emitters of Q4 and
Q5.
Um.. okay, I need to sit down and think. Mind is spinning,
but I've not set a finger to paper yet and there is lots to
think about in that one. I could be way, way off base.
Jon
Jon Kirwan
>The amplifier, it
>seems to me, dictates the parameters. So that comes later,
>doesn't it?
By "that" I mean "the power supply." In case it isn't clear.
Jon
pimpom
Either the circuit was designed by someone with a limited
experience or it was deliberately presented this way for clarity
as an illustration of the basic topology.
In a practical design using an unregulated power supply, R1
should be split into two with a capacitor from the split point to
ground. This is to decouple the input stage for stability as well
as for hum filtering. R6 should also be split and the split point
bootstrapped with a capacitor to the mid point of the output
stage.
Talking about RCA's 70W amp got me nostalgic about those days.
Here's a 1W amp using _germanium_ transistors:
http://img716.imageshack.us/img716/2583/1wamp.png
This is one of my early solid-state designs based, of course, on
topologies I'd learned by studying others' designs. It's no hi-fi
by any stretch of imagination, but I actually constructed a few
of these in the early 70s for myself and for friends. One of them
fed the input from an early Sony Walkman to drive an 8-inch
Philips dual-cone "Hi-Q" speaker and gushed over how good it
sounded!
Jon Kirwan
Well, having looked a little more at the web site, I see them
talking about everything from opamps to servo amps so maybe
they just did the basics. But they missed the signal split
technique aka the old tube days, then.
>In a practical design using an unregulated power supply, R1
>should be split into two with a capacitor from the split point to
>ground. This is to decouple the input stage for stability as well
>as for hum filtering.
Now _that_ makes a lot of sense. I missed it.
>R6 should also be split and the split point
>bootstrapped with a capacitor to the mid point of the output
>stage.
That one I really need to think about. This is what I wanted
to happen here. Throwing out things (I'm assuming correct
things, of course) that force me to consider and think.
Thanks.
>Talking about RCA's 70W amp got me nostalgic about those days.
>Here's a 1W amp using _germanium_ transistors:
>http://img716.imageshack.us/img716/2583/1wamp.png
>This is one of my early solid-state designs based, of course, on
>topologies I'd learned by studying others' designs. It's no hi-fi
>by any stretch of imagination, but I actually constructed a few
>of these in the early 70s for myself and for friends. One of them
>fed the input from an early Sony Walkman to drive an 8-inch
>Philips dual-cone "Hi-Q" speaker and gushed over how good it
>sounded!
Okay. So lets talk about some aspects. It'll expose my
terrible ignorance, but what the heck.
Input loading. I think I can ignore the R2 feedback as it is
10k. At least, for now. C1 will present about Z=800 at
20Hz, Z=160 at 100Hz, and Z goes down from there. R1 is 1k,
obviously in series with C1. Then there is R3=1k in parallel
with Q1's impedance, which maybe I can approximate as R4
times beta, or call it 50*33 or about 1500 ohms? So about
600 ohms counting that and R3 in parallel, that itself in
series with 1k and whatever C1 presents? So call it around
2k ohms loading, or so? (Which adds to the idea that the R2
feedback can be mostly ignored as a load.) Would that be an
okay, off-the-hip guess? Or how would you go about it?
D1 is, I guess, silicon and given that you said _germanium_,
I'll take that to suggest that the Vbe on those are about
half that of a silicon BJT. Which is why only one DR25 was
needed there.
DC bias point of Q1... hmm. Well, assuming no signal, SPK1
is roughly a dead short, so R5 is tied one side to a rail.
The other side moves Q2's base and Q2's emitter follows. As
Q2's emitter rises with it, R2 and R3 act to split that as
1/11th to the Q1 base. Q1's emitter follows up for a ways,
allowing DC current via R4 which must go through R5, dropping
Q2's base and thus Q2's emitter, lowering Q1's base voltage
in opposition. So there will be a middle point found.
Assuming Q1's Vbe should be something on the order of 300mV
(random guess), and I(R4) roughly equals I(R5), let's
establish where Q1's base will wind up. Call it Vb. The
value at Q2's emitter (which is also the other side of R2
from the Q1 base) will be 11 times higher because R2 and R3
split things that way. And Q2's base will be 300mV (same
random guess, again) higher than that. The difference
between there and the 9V battery voltage sets the current in
R5 and, by implication, in R4 as well. Of course, Q1's
emitter is 300mV away from that Vb value we are fussing over.
The equation looks like:
I(R5) = (9V - Vb*11 - 300mV) / 560
I(R4) = (Vb - 300mV) / 33
I(R4) = I(R5)
So,
(9V - Vb*11 - 300mV) / 560 = (Vb - 300mV) / 33
33/560 * (9V - Vb*11 - 300mV) = (Vb - 300mV)
Vb = 33/560 * (9V - Vb*11 - 300mV) + 300mV
Vb = 33/560*9V - 33/560*Vb*11 - 33/560*300mV + 300mV
Vb + 33/560*Vb*11 = 33/560*9V - 33/560*300mV + 300mV
Vb * (1 + 33/560*11) = (33/560*9V - 33/560*300mV + 300mV)
Vb = (33/560*9V - 33/560*300mV + 300mV) / (1 + 33/560*11)
or,
Vb = 493mV
and thus the current routing through R5, D1, Q1, and R4 is
about 193mV/33 or 5.85mA. That's not the total quiescent
current because D1 uses that 5.85mA to develop a voltage
across it that is probably on the order of 700mV. With that
between the Q2 and Q3 bases, both Q2 and Q3 are passing
collector currents, rail to rail. Hard to know how much
without data sheets, I suppose. But something. Their shared
emitter node would be on the order of 11*490mV or about 5.4V.
That neglected the base current for Q1 flowing via R2. As
I'm now guessing almost 6mA as Ic, and since we are talking
germanium here, I will pick a beta of about 60 and figure
about 100uA base current, then. That's about another 1V
across R2, less than that a little because that lowers Vb a
bit which lowers the 5.85mA figure a bit, which probably then
gets things very darned close to the midpoint of 4.5V one
might wish there.
Not too bad given I have no idea about the BJTs and am using
a lot of random guesses as I go.
R2 is not only a DC divider but also NFB, I think. Can you
talk a little about how you figure on calculating both the
NFB you want _and_ the DC biasing of this thing, both of
which affect R2's value, I think? And although I've _seen_
miller feedbacks in the small nF range, could you talk a
little about how that was set at 2.2nF? Also, I think I
_almost_ get the idea of hooking one side of R5 to SPK1
instead of to the (-) side of 9V... but not quite sure. Can
you talk about that choice, as well?
Have at me. I probably got a lot wrong in the above, but
that's my thinking exposed like a soft worm to be crushed. If
I learn in the process, crush away!
Jon
pimpom
> On Thu, 28 Jan 2010 14:00:18 +0530, "pimpom"
> <pim...@invalid.invalid> wrote:
>
>
>> Talking about RCA's 70W amp got me nostalgic about those days.
>> Here's a 1W amp using _germanium_ transistors:
>> http://img716.imageshack.us/img716/2583/1wamp.png
>> This is one of my early solid-state designs based, of course,
>> on
>> topologies I'd learned by studying others' designs. It's no
>> hi-fi
>> by any stretch of imagination, but I actually constructed a
>> few
>> of these in the early 70s for myself and for friends. One of
>> them
>> fed the input from an early Sony Walkman to drive an 8-inch
>> Philips dual-cone "Hi-Q" speaker and gushed over how good it
>> sounded!
>
> Okay. So lets talk about some aspects. It'll expose my
> terrible ignorance, but what the heck.
I'm no expert myself, but I'm willing to help where I can.
> Input loading. I think I can ignore the R2 feedback as it is
> 10k. At least, for now.
R2 provides both dc and ac feedback. DC for bias stabilisation
and setting the emitters of the output transistors to about half
of Vcc (more about that later). For ac, it may be easier to think
of it as current feedback. Q2 needs about +/-50uA peak of base
current at full drive. At signal frequencies, R2 (plus the much
smaller input impedance of Q1) is effectively in parallel with
the output. The output swings by about 4V peak at max power,
which has 400uA of negative feedback current going back through
R2. The input current requirement goes up by a factor of 9. IOW,
a negative feedback of 19db. This is substantially better than
nothing and should significantly reduce distortion and improve
frequency response.
> C1 will present about Z=800 at
> 20Hz, Z=160 at 100Hz, and Z goes down from there. R1 is 1k,
> obviously in series with C1.
The already low input impedance of Q1 is further reduced by the
negative feedback, so R1 represents practically the whole input
impedance of the amp. The -3db cutoff frequency is 1/2*pi*C1*R1
which is about 16Hz.
> Then there is R3=1k in parallel
> with Q1's impedance, which maybe I can approximate as R4
> times beta, or call it 50*33 or about 1500 ohms?
No. R4 is bypassed by C3 and has little effect on input impedance
except at very low frequencies.
> So about
> 600 ohms counting that and R3 in parallel, that itself in
> series with 1k and whatever C1 presents? So call it around
> 2k ohms loading, or so? (Which adds to the idea that the R2
> feedback can be mostly ignored as a load.) Would that be an
> okay, off-the-hip guess? Or how would you go about it?
>
It's mostly the internal dynamic emitter resistance that
determines Q1's input impedance. That resistance is 26/Ie at 20
deg C. Q2 is biased at about 7.7mA emitter current, giving about
3.4 ohms. Multiply that by hfe, add the ohmic base resistance and
you get Q1's basic input Z. I don't have my old data book handy,
but I think the AC126 had a typical hfe of about 150 and rbb of
maybe 100 ohms. This gives an input Z of about 600 ohms.
> D1 is, I guess, silicon and given that you said _germanium_,
> I'll take that to suggest that the Vbe on those are about
> half that of a silicon BJT. Which is why only one DR25 was
> needed there.
>
The output transistors need only about 0.1V each of Vbe to bias
them at a few mAs of Ic. D1 is germanium, but at the dc current
level flowing through it, two of them in series will have too
much voltage drop (I measured several samples). Ge transistors
have a more rounded knee than their Si counterparts in the Vbe
vs. Ic curve. So I felt that a single diode would present less
chance of thermal runaway for the output Trs and still cause a
reasonably low crossover distortion.
Oops. Have to go out for a while. Will take up the rest later.
pimpom
> On Thu, 28 Jan 2010 14:00:18 +0530, "pimpom"
> <pim...@invalid.invalid> wrote:
>
>
>> R6 should also be split and the split point
>> bootstrapped with a capacitor to the mid point of the output
>> stage.
>
> That one I really need to think about. This is what I wanted
> to happen here. Throwing out things (I'm assuming correct
> things, of course) that force me to consider and think.
> Thanks.
>
conduct on the negative half-cycle of the signal, the base drive
current has to come via R6. At the same time, Q5's current is
pulling its emitter - and therefore the base - down towards
ground, decreasing the voltage drop across R6. This decreases the
base drive current available just when it's needed.
Now look at it modified with a boostrap:
http://img715.imageshack.us/img715/4259/boostrap.png
For simplicity, let R6 = R7. At steady-state, C will be charged
to about a quarter of Vcc. When Q5 pulls its emitter (and
therefore the positive electrode of C) towards ground, the
voltage across C cannot change instantaneously and will push its
negative terminal down too. Beyond a certain level of drive, the
junction of C, R6 and R7 will even go down past oV and become
negative with respect to ground. This maintains the voltage
across R6 at an approximately constant level.
Oops again. Guests this time. Will be back when I can.
pimpom
>
>
> D1 is, I guess, silicon and given that you said _germanium_,
> I'll take that to suggest that the Vbe on those are about
> half that of a silicon BJT. Which is why only one DR25 was
> needed there.
>
Your reasoning is correct. However, the output transistors Q2 and
Q3 need only about 100mV each of Vbe for Class AB bias. IIRC,
beta of Q1 is about 150 and Vbe at that level of current is about
0.12V.
I don't know about others, but with low voltage circuits, I
usually try to fix the quiescent voltage at the output mid-point
at slightly more than half of Vcc. This is because Q1's Ve plus
its Vcesat reduces the available downward swing of Q3's base.
For this design, I tentatively chose a target of 4.6V at Q3's
emitter. Add Q2's Vbe and that leaves 4.3V for R5 plus the
speaker's dc resistance. The speaker's resistance has only a
minor effect but, just for the heck of it, let's take it as 6
ohms. So Q1's Ic = 4.3/566 = 7.6mA.
Q1's dc beta = 150, so Ib is about 50uA, and Ie = 7.65mA = I(R4).
7.65*33 places Q1's emitter at 252.45mV above ground. That plus
Vbe of 0.12V gives Vb = 372.45mV.
I(R3) = 372.45uA
I(R2) = I(R3) + Ib = 422.45uA
I(R2)*R2 = 4.2245V
V(R2) + Vb = (4.2245 + 0.37245)V = 4.59695V.
It just so happens that, in this case, common resistor values
produce almost exactly the desired quiescent bias level. If they
didn't, a slight departure from the target voltages would be
acceptable. In any case, tolerances on resistor values and
transistor characteristics could throw off actual values a bit.
> R2 is not only a DC divider but also NFB, I think. Can you
> talk a little about how you figure on calculating both the
> NFB you want _and_ the DC biasing of this thing, both of
> which affect R2's value, I think?
For such a simple design without a high level of audio quality as
the target, I wasn't too particular about the amount of signal
feedback as long as it's a reasonable amount. I chose a
compromise value for R3 first - low enough for bias stability so
that the current through it would be several times Ib, but not
too low to avoid excessive shunting of the signal input current.
Then I let the value of R2 be what it needs to be for correct
bias.
Then I calculate the amount of NFB as outlined in one of my
earlier replies and accept it if it's within reason. If I really
wanted more NFB, I'd parallel R2 with another resistor, but with
a capacitor in series to avoid upsetting the dc levels. BTW, that
can be used to provide some bass boost by choosing the proper
values of cap and resistor.
> And although I've _seen_
> miller feedbacks in the small nF range, could you talk a
> little about how that was set at 2.2nF?
That's a guesstimated value, partly empirical and partly based on
observation of other people's designs. No PCs and simulation
software 40 years ago. For such a simple circuit, I didn't bother
with complex calculations for loops and phase shifts that
wouldn't be precise anyway due to wide tolerances in component
characteristics.
The reason for the relatively high capacitance is that this was a
low-Z low-gain circuit. But I might have made a mistake in
showing it now as 2.2nF. I might have used something like 1nF.
> Also, I think I
> _almost_ get the idea of hooking one side of R5 to SPK1
> instead of to the (-) side of 9V... but not quite sure. Can
> you talk about that choice, as well?
>
This is a variation of the bootstrap circuit I described in my
other post. R5 and the speaker serve the same functions as R6 and
R7 respectively in the other circuit.
Jon Kirwan
This much I can see, immediately. And thanks for dealing
with more, later on.
>For ac, it may be easier to think of it as current feedback.
Okay.
>Q2 needs about +/-50uA peak of base
>current at full drive. At signal frequencies, R2 (plus the much
>smaller input impedance of Q1) is effectively in parallel with
>the output.
R2 is connected from the output to an input, which
effectively doesn't move much after arriving at it's DC bias
point. As you later point out, the _AC_ input impedance is
lowish (near 600 ohms), so the 10k is pretty close to one of
the rails at AC, anyway. Is that a different way of saying
what you just said? Or would you modify it?
>The output swings by about 4V peak at max power,
>which has 400uA of negative feedback current going back through
>R2. The input current requirement goes up by a factor of 9. IOW,
>a negative feedback of 19db. This is substantially better than
>nothing and should significantly reduce distortion and improve
>frequency response.
Okay. This goes past me a little (as if maybe the earlier
point didn't.) I'd like to try and get a handle on it.
Let's start with the 4V peak swing at max power.
Since you are discussing AC and converting it 400uA current
via the 10k, I would normally take this to mean 4Vrms AC.
Which in Vp-p terms would be 2*SQRT(2) larger, or 11.3V which
I know is impossible without accounting for the BJTs, given
the 9V supply. So this forces me to think in terms of
something else. But what? Did you mean 4Vpeak, which would
be 8Vp-p? If so, that would be about 2.8Vrms. In that case,
wouldn't a better "understanding" come from then saying that
the negative feedback is closer to 280uA?
The next point is on your use of "goes up by a factor of 9."
Can you elaborate more on this topic? Where the 9 comes
from? For volts, not power, I think I can gather the point
that 20*log(9) = 19.085), so I'm not talking about that
conventional formula. I'm asking about the 9, itself, and
also your thinking along the lines of concluding that it
significantly reduces distortion. How does one decide how
much is enough?
> > C1 will present about Z=800 at
>> 20Hz, Z=160 at 100Hz, and Z goes down from there. R1 is 1k,
>> obviously in series with C1.
>
>The already low input impedance of Q1 is further reduced by the
>negative feedback, so R1 represents practically the whole input
>impedance of the amp. The -3db cutoff frequency is 1/2*pi*C1*R1
>which is about 16Hz.
I think I follow. Leading towards a comment you make soon
below (about C3), C3's very low impedance even after beta
multiplication almost bypasses R3 completely so the impedance
is as you say, mostly depending upon C1 and R1. I think.
>> Then there is R3=1k in parallel
>> with Q1's impedance, which maybe I can approximate as R4
>> times beta, or call it 50*33 or about 1500 ohms?
>
>No. R4 is bypassed by C3 and has little effect on input impedance
>except at very low frequencies.
Thanks for the knock in the head there. I had been ignoring
C3 for DC bais-point thinking and forgot to put it back in
when talking about AC loading. Your point is made.
>> So about
>> 600 ohms counting that and R3 in parallel, that itself in
>> series with 1k and whatever C1 presents? So call it around
>> 2k ohms loading, or so? (Which adds to the idea that the R2
>> feedback can be mostly ignored as a load.) Would that be an
>> okay, off-the-hip guess? Or how would you go about it?
>
>It's mostly the internal dynamic emitter resistance that
>determines Q1's input impedance. That resistance is 26/Ie at 20
>deg C. Q2 is biased at about 7.7mA emitter current, giving about
>3.4 ohms. Multiply that by hfe, add the ohmic base resistance and
>you get Q1's basic input Z. I don't have my old data book handy,
>but I think the AC126 had a typical hfe of about 150 and rbb of
>maybe 100 ohms. This gives an input Z of about 600 ohms.
Okay. I get your point about small-case 're' based upon kT/q
and Ie. I'm mostly following here.
>> D1 is, I guess, silicon and given that you said _germanium_,
>> I'll take that to suggest that the Vbe on those are about
>> half that of a silicon BJT. Which is why only one DR25 was
>> needed there.
>
>The output transistors need only about 0.1V each of Vbe to bias
>them at a few mAs of Ic. D1 is germanium, but at the dc current
>level flowing through it, two of them in series will have too
>much voltage drop (I measured several samples). Ge transistors
>have a more rounded knee than their Si counterparts in the Vbe
>vs. Ic curve. So I felt that a single diode would present less
>chance of thermal runaway for the output Trs and still cause a
>reasonably low crossover distortion.
Thanks.
>Oops. Have to go out for a while. Will take up the rest later.
Well, I think I'm roughly following so far. Please kick me
where I'm still off-track, if you feel you can afford the
moment to do it. I appreciate it very much.
Jon
Jon Kirwan
I'm appreciating this very much. Printed the two and am
looking at both. I think I'm following, but need to sit down
and do a little paper calcs to make sure I burn it in a
little more. If I think of something useful, I'll post a
question or two. For now, I feel like I'm following you.
Jon
sparky
>
> - Show quoted text -
Phils panties are all in a twist ! Again !
Jon Kirwan
Thanks for the adjustments. At least, I didn't wander too
far afield, for once. It's nice to hear I'm not totally out
of my depth.
>I don't know about others, but with low voltage circuits, I
>usually try to fix the quiescent voltage at the output mid-point
>at slightly more than half of Vcc. This is because Q1's Ve plus
>its Vcesat reduces the available downward swing of Q3's base.
Okay, that I follow.
>For this design, I tentatively chose a target of 4.6V at Q3's
>emitter. Add Q2's Vbe and that leaves 4.3V for R5 plus the
>speaker's dc resistance. The speaker's resistance has only a
>minor effect but, just for the heck of it, let's take it as 6
>ohms. So Q1's Ic = 4.3/566 = 7.6mA.
I follow. Ve(Q2)=4.6V, Vbe(Q2)=0.1V, so Vb(Q2)=4.7V.
Assuming a solid 9V power source (not what a 9V battery is,
if that were used, but...), that leaves 9V-4.7V = 4.3V across
R5 and SPK1. Which sets a current to the Vb node of Q2 that
needs to be disposed of via D1 and then Q1. 7.6mA it is.
>Q1's dc beta = 150,
That high? I had anticipated germaniums were lower. Okay.
>so Ib is about 50uA,
Followed.
>and Ie = 7.65mA = I(R4).
Yes. 7.6mA+0.05mA = 7.65mA that must be dumped into R4.
There will be some Q1 base current (another 50uA?) added to
Ie(Q1) that is ignored here. No problem. That would only
mean 7.7mA instead of 7.65mA for your calcs below.
>7.65*33 places Q1's emitter at 252.45mV above ground.
or 7.7mA*33 = 254.1mV, if you add Q1's base drive?
>That plus
>Vbe of 0.12V gives Vb = 372.45mV.
Okay.
>I(R3) = 372.45uA
>I(R2) = I(R3) + Ib = 422.45uA
>I(R2)*R2 = 4.2245V
>V(R2) + Vb = (4.2245 + 0.37245)V = 4.59695V.
>
>It just so happens that, in this case, common resistor values
>produce almost exactly the desired quiescent bias level. If they
>didn't, a slight departure from the target voltages would be
>acceptable. In any case, tolerances on resistor values and
>transistor characteristics could throw off actual values a bit.
I'm with you. Thanks for the care, here.
>> R2 is not only a DC divider but also NFB, I think. Can you
>> talk a little about how you figure on calculating both the
>> NFB you want _and_ the DC biasing of this thing, both of
>> which affect R2's value, I think?
>
>For such a simple design without a high level of audio quality as
>the target, I wasn't too particular about the amount of signal
>feedback as long as it's a reasonable amount. I chose a
>compromise value for R3 first - low enough for bias stability so
>that the current through it would be several times Ib, but not
>too low to avoid excessive shunting of the signal input current.
>Then I let the value of R2 be what it needs to be for correct
>bias.
Okay. So I remember in an earlier post you talking about
400uA being a factor of 9 higher. Guessing that Q1's base
current is around the same area of 50uA, this would be a
factor of 8... not the 9 you mentioned before. But at least
I'm starting to see where that number 9 came from?
>Then I calculate the amount of NFB as outlined in one of my
>earlier replies and accept it if it's within reason. If I really
>wanted more NFB, I'd parallel R2 with another resistor, but with
>a capacitor in series to avoid upsetting the dc levels.
Got it. That would provide additional AC feedback but keep
the DC biasing. So you don't have to screw around balancing
R2 against two different considerations, you just "fix it"
with a patch like that. Makes total sense.
>BTW, that
>can be used to provide some bass boost by choosing the proper
>values of cap and resistor.
Hmm... Lower frequencies would have less NFB, higher
frequencies more. Okay. There is also other areas where
higher frequencies are going to see less gain in this
design... Now I'm starting to wonder about phase shift not
exactly 180 degrees in the NFB over frequency. But I need to
sit down and think more.
>> And although I've _seen_
>> miller feedbacks in the small nF range, could you talk a
>> little about how that was set at 2.2nF?
>
>That's a guesstimated value, partly empirical and partly based on
>observation of other people's designs. No PCs and simulation
>software 40 years ago. For such a simple circuit, I didn't bother
>with complex calculations for loops and phase shifts that
>wouldn't be precise anyway due to wide tolerances in component
>characteristics.
>
>The reason for the relatively high capacitance is that this was a
>low-Z low-gain circuit. But I might have made a mistake in
>showing it now as 2.2nF. I might have used something like 1nF.
Okay. Nuff said. I'll leave that for later thinking.
>> Also, I think I
>> _almost_ get the idea of hooking one side of R5 to SPK1
>> instead of to the (-) side of 9V... but not quite sure. Can
>> you talk about that choice, as well?
>
>This is a variation of the bootstrap circuit I described in my
>other post. R5 and the speaker serve the same functions as R6 and
>R7 respectively in the other circuit.
Hmm. I generally get the thrust. I need to think more
closely about the value of it. But I'll take it as something
to explore more.
Jon
Paul E. Schoen
"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
news:vcm2m5lhelb2t4ims...@4ax.com...
>
> Have at me. I probably got a lot wrong in the above, but
> that's my thinking exposed like a soft worm to be crushed. If
> I learn in the process, crush away!
Have you used LTSpice (AKA SwitcherCAD)? Some time ago I tried some ideas
for linear amplifiers, and I came up with a design that is DC coupled and
uses just three MOSFETs. You can trim the quiescent current to trade
efficiency for crossover distortion. I originally made it with capacitor
coupling and a single supply, but I redid it with a dual supply and DC
coupling.
As a practical matter, the amplifier may be a bit unstable and prone to
overheating and even self-destruction if the bias current is set high
enough to eliminate crossover distortion. If you can tolerate a few percent
(tens of thousands of PPM, if you must), then it should be fairly stable.
This design is basically an output stage only, and has no voltage
amplification. That can be easily achieved with a simple class A input
stage.
When the output is driven just about to the rails, it puts out 5.5 watts
into 8 ohms, with input power of 8.9 watts, or 62% efficiency. I'm using
+/- 12 VDC rails and 7.7 VRMS input. There are two IRL3915 NMOS output
transistors (STD30NF06 also work), and one IRF7205 PMOS. I have not
actually built this circuit, and there are probably a number of problems
with making it work using actual components, but I think it's worth a try.
Of course, this is a MOSFET design and not BJT, but a similar circuit could
be built using BJTs if that is a requirement.
The ASCII file follows.
Paul
---------------------------------------------------------------
Version 4
SHEET 1 1304 744
WIRE 736 -176 192 -176
WIRE 976 -176 736 -176
WIRE 736 -144 736 -176
WIRE 976 -96 976 -176
WIRE 736 -48 736 -64
WIRE 736 -48 592 -48
WIRE 736 -16 736 -48
WIRE 928 -16 736 -16
WIRE 192 16 192 -176
WIRE 736 80 736 64
WIRE 736 208 400 208
WIRE 976 224 976 0
WIRE 1216 224 976 224
WIRE 1248 224 1216 224
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WIRE 192 256 144 256
WIRE 592 256 592 -48
WIRE 400 288 400 208
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WIRE 976 304 976 224
WIRE 976 304 848 304
WIRE 736 352 736 336
WIRE 192 400 192 256
WIRE 400 432 400 368
WIRE 848 432 848 304
WIRE 736 448 736 432
WIRE 800 448 736 448
WIRE 976 448 976 304
WIRE 1248 448 1248 368
WIRE 592 512 592 336
WIRE 736 512 736 448
WIRE 736 512 592 512
WIRE 928 528 848 528
WIRE 736 544 736 512
WIRE 848 560 848 528
WIRE 192 672 192 480
WIRE 736 672 736 624
WIRE 736 672 192 672
WIRE 848 672 848 640
WIRE 848 672 736 672
WIRE 976 672 976 544
WIRE 976 672 848 672
FLAG 144 256 0
FLAG 400 208 Vin
FLAG 1216 224 Vout
FLAG 400 432 0
FLAG 1248 448 0
SYMBOL voltage 192 0 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 12
SYMBOL voltage 400 272 R0
WINDOW 3 -127 203 Left 0
WINDOW 123 24 44 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 10 200 1u 0 0 5000)
SYMATTR Value2 AC 1
SYMATTR InstName V1
SYMBOL nmos 928 -96 R0
SYMATTR InstName M1
SYMATTR Value IRL3915
SYMBOL nmos 928 448 R0
SYMATTR InstName M2
SYMATTR Value STD30NF06L
SYMBOL pmos 800 528 M180
SYMATTR InstName M3
SYMATTR Value IRF7205
SYMBOL res 832 544 R0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL res 1232 272 R0
SYMATTR InstName R8
SYMATTR Value 8
SYMBOL res 720 -160 R0
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL res 720 528 R0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL diode 720 80 R0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 720 144 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 720 208 R0
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 720 272 R0
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL voltage 192 384 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL res 576 240 R0
SYMATTR InstName R3
SYMATTR Value 470k
SYMBOL res 720 -32 R0
SYMATTR InstName R4
SYMATTR Value 220
SYMBOL res 720 336 R0
SYMATTR InstName R5
SYMATTR Value 220
TEXT 264 704 Left 0 !.tran .5
TEXT 416 704 Left 0 !;ac oct 5 20 20000
Jon Kirwan
<pa...@peschoen.com> wrote:
>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>news:vcm2m5lhelb2t4ims...@4ax.com...
>>
>> Have at me. I probably got a lot wrong in the above, but
>> that's my thinking exposed like a soft worm to be crushed. If
>> I learn in the process, crush away!
>
>Have you used LTSpice (AKA SwitcherCAD)?
Oh, yes. However, my interest is in _learning_. I use
LTspice to explore my own ignorance, at times. And I use it
to test what I think I've learned. But I don't "hack" with
it. Much. ;)
>Some time ago I tried some ideas
>for linear amplifiers, and I came up with a design that is DC coupled and
>uses just three MOSFETs. You can trim the quiescent current to trade
>efficiency for crossover distortion. I originally made it with capacitor
>coupling and a single supply, but I redid it with a dual supply and DC
>coupling.
No FETs. I think I stated that in the beginning. There are
a variety of reasons why. But suffice it that I don't want
to go there... for now.
>As a practical matter, the amplifier may be a bit unstable and prone to
>overheating and even self-destruction if the bias current is set high
>enough to eliminate crossover distortion. If you can tolerate a few percent
>(tens of thousands of PPM, if you must), then it should be fairly stable.
>
>This design is basically an output stage only, and has no voltage
>amplification. That can be easily achieved with a simple class A input
>stage.
>
>When the output is driven just about to the rails, it puts out 5.5 watts
>into 8 ohms, with input power of 8.9 watts, or 62% efficiency. I'm using
>+/- 12 VDC rails and 7.7 VRMS input. There are two IRL3915 NMOS output
>transistors (STD30NF06 also work), and one IRF7205 PMOS. I have not
>actually built this circuit, and there are probably a number of problems
>with making it work using actual components, but I think it's worth a try.
>Of course, this is a MOSFET design and not BJT, but a similar circuit could
>be built using BJTs if that is a requirement.
Yeah. That's a requirement. I've still some learning ahead
of me. But I'll take a look at the schematic and tuck it
away, at least. Thanks.
Jon
>The ASCII file follows.
>
>Paul
><snip of LTspice .ASC file>
Paul E. Schoen
OK. So I made a similar amplifier output stage using two 2N3055s, and a
2N3904 and a 2N3906. With 8.4 VRMS input the output is 7.3 VRMS into 8 ohms
for 6.66 watts. Input power is 9.97 watts, efficiency is 67%. Some very
slight crossover distortion. 6 mA drive current (about 1.2k input
impedance). Looks good 20 Hz to 20 kHz. I added an output inductor which
affects output at higher frequencies. LTSpice ASCII follows.
Paul
-----------------------------------------------------------
Version 4
SHEET 1 1304 744
WIRE 736 -176 192 -176
WIRE 864 -176 736 -176
WIRE 976 -176 864 -176
WIRE 736 -144 736 -176
WIRE 864 -96 864 -176
WIRE 976 -96 976 -176
WIRE 736 -48 736 -64
WIRE 800 -48 736 -48
WIRE 912 -48 896 -48
WIRE 736 0 736 -48
WIRE 736 0 592 0
WIRE 896 0 896 -48
WIRE 896 0 864 0
WIRE 192 16 192 -176
WIRE 736 32 736 0
WIRE 736 128 736 96
WIRE 736 208 736 192
WIRE 736 208 400 208
WIRE 976 224 976 0
WIRE 1056 224 976 224
WIRE 1216 224 1136 224
WIRE 1248 224 1216 224
WIRE 592 240 592 0
WIRE 192 256 192 96
WIRE 192 256 144 256
WIRE 736 272 736 208
WIRE 400 288 400 208
WIRE 1248 288 1248 224
WIRE 976 304 976 224
WIRE 976 304 848 304
WIRE 592 352 592 320
WIRE 736 352 736 336
WIRE 736 352 592 352
WIRE 192 400 192 256
WIRE 848 400 848 304
WIRE 400 432 400 368
WIRE 736 448 736 352
WIRE 784 448 736 448
WIRE 976 448 976 304
WIRE 1248 448 1248 368
WIRE 912 496 848 496
WIRE 736 544 736 448
WIRE 848 560 848 496
WIRE 192 672 192 480
WIRE 736 672 736 624
WIRE 736 672 192 672
WIRE 848 672 848 640
WIRE 848 672 736 672
WIRE 976 672 976 544
WIRE 976 672 848 672
FLAG 144 256 0
FLAG 400 208 Vin
FLAG 1216 224 Vout
FLAG 400 432 0
FLAG 1248 448 0
SYMBOL voltage 192 0 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 12
SYMBOL voltage 400 272 R0
WINDOW 3 -127 203 Left 0
WINDOW 123 24 44 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 12 200 1u 0 0 5000)
SYMATTR Value2 AC 1
SYMATTR InstName V1
SYMBOL res 832 544 R0
SYMATTR InstName R4
SYMATTR Value 470
SYMBOL res 1232 272 R0
SYMATTR InstName R1
SYMATTR Value 8
SYMBOL res 720 -160 R0
SYMATTR InstName R2
SYMATTR Value 2.7k
SYMBOL res 720 528 R0
SYMATTR InstName R3
SYMATTR Value 2.7k
SYMBOL diode 720 128 R0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 720 272 R0
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL voltage 192 384 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL res 576 224 R0
SYMATTR InstName R5
SYMATTR Value 470
SYMBOL npn 912 -96 R0
SYMATTR InstName Q1
SYMATTR Value 2N3055
SYMBOL npn 912 448 R0
SYMATTR InstName Q2
SYMATTR Value 2N3055
SYMBOL pnp 784 496 M180
SYMATTR InstName Q3
SYMATTR Value 2N3906
SYMBOL npn 800 -96 R0
SYMATTR InstName Q4
SYMATTR Value 2N3904
SYMBOL ind 1040 240 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 5 56 VBottom 0
SYMATTR InstName L1
SYMATTR Value 100�
SYMBOL diode 720 32 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
Jon Kirwan
<pa...@peschoen.com> wrote:
><snip>
>OK. So I made a similar amplifier output stage using two 2N3055s, and a
>2N3904 and a 2N3906. With 8.4 VRMS input the output is 7.3 VRMS into 8 ohms
>for 6.66 watts. Input power is 9.97 watts, efficiency is 67%. Some very
>slight crossover distortion. 6 mA drive current (about 1.2k input
>impedance). Looks good 20 Hz to 20 kHz. I added an output inductor which
>affects output at higher frequencies. LTSpice ASCII follows.
><snip>
Got it and saved it. Sziklai pair on one side, Darlington on
the other. 3 NPN, 1 PNP. Now I'm thinking about tubes (they
don't come in complementary form) and using a signal splitter
and NPNs "all the way down!" ;)
Jon
pimpom
> On Thu, 28 Jan 2010 19:19:06 +0530, "pimpom"
> <pim...@invalid.invalid> wrote:
>
>
>> Q2 needs about +/-50uA peak of base
>> current at full drive. At signal frequencies, R2 (plus the
>> much
>> smaller input impedance of Q1) is effectively in parallel with
>> the output.
>
> R2 is connected from the output to an input, which
> effectively doesn't move much after arriving at it's DC bias
> point. As you later point out, the _AC_ input impedance is
> lowish (near 600 ohms), so the 10k is pretty close to one of
> the rails at AC, anyway. Is that a different way of saying
> what you just said? Or would you modify it?
>
>> The output swings by about 4V peak at max power,
>> which has 400uA of negative feedback current going back
>> through
>> R2. The input current requirement goes up by a factor of 9.
>> IOW,
>> a negative feedback of 19db. This is substantially better than
>> nothing and should significantly reduce distortion and improve
>> frequency response.
>
> Okay. This goes past me a little (as if maybe the earlier
> point didn't.) I'd like to try and get a handle on it.
>
> Let's start with the 4V peak swing at max power.
>
> Since you are discussing AC and converting it 400uA current
> via the 10k, I would normally take this to mean 4Vrms AC.
> Which in Vp-p terms would be 2*SQRT(2) larger, or 11.3V which
> I know is impossible without accounting for the BJTs, given
> the 9V supply. So this forces me to think in terms of
> something else. But what? Did you mean 4Vpeak, which would
> be 8Vp-p? If so, that would be about 2.8Vrms.
Yes. It's 4Vp, 8Vp-p and 2.8Vrms. I wanted to give you a mental
picture of how much the output voltage can swing. Each output Q
has about 4.4V of Vce available, and about 4V before hard
saturation is reached (these are all round figure values). That's
4V peak for a sinusoidal wave form.
> In that case,
> wouldn't a better "understanding" come from then saying that
> the negative feedback is closer to 280uA?
>
Yes, it's 280uA rms. But I was talking in terms of the maximum
amplitude of instantaneous change, which is why I used the terms
"swing" and "peak".
> The next point is on your use of "goes up by a factor of 9."
> Can you elaborate more on this topic? Where the 9 comes
> from? For volts, not power, I think I can gather the point
> that 20*log(9) = 19.085), so I'm not talking about that
> conventional formula. I'm asking about the 9, itself, and
Without feedback, the input transistor Q1 needs 50uA of AC input
signal to drive the output Qs to full power output (still talking
in terms of peak to avoid confusion). With NFB, we need an
additional 400uA to overcome the current fed back from the
output. That's a total of 450uA peak, which is 9 times the
original 50uA.
Actually, I made an error when I cited the 50uA figure. Q1 is
biased at Ic = 7.6mA, Ib = 50uA. But only 5mA peak is needed from
Q1's collector to drive the output transistors. Divide that by
Q1's hfe of 150 and you get 33uA (peak) of AC signal current
needed into the base of Q1. The corrected total needed from the
signal source is now 433uA. The gain reduction factor due to NFB
is now 13 instead of 9. That's 22db (feedback is usually given in
db).
> also your thinking along the lines of concluding that it
> significantly reduces distortion.
The basic principle of NFB is that it reduces THD and extends
frequency response by a factor equal to the feedback ratio. So,
in our example, if you have 10% THD without feedback, it will
drop to 0.77% with the feedback factor of 13. But there are
caveats. E.g., phase shifts can cause undesireable effects,
especially with large amounts of feedback. I'm afraid a detailed
treatment of such things is really outside the scope of this
discussion - unless someone else is willing to take it up.
> How does one decide how much is enough?
For one thing, how much distortion one is willing to put up with.
Another factor is input sensitivity, or IOW, how much gain is
needed. E.g., to drive the 1W amp to full output, we need 433uA
peak (306uA rms) from the signal source into 1k. That's 306mV
rms, plus some millivolts at the b-e junction. Say about 0.32V
rms total input voltage into about 1k input impedance.
To present the basic concepts, I've made several approximations.
E.g., I neglected the shunting effect of R2. Besides, the input
resistance of Q1 is constant at 600 ohms only for very small
signal amplitudes relative to the quiescent dc levels. This
dynamic input resistance changes significantly with large signal
swings and adds distortion while also complicating precise
calculations.
George Herold
> "Jon Kirwan is a FUCKWIT TROLL "
>
> DO NOT FEED THE TROLL !!!!!!!!!!!!!!!
>
> DO NOT FEED THE TROLL !!!!!!!!!!!!!!!
>
> TROLLS are DESTROYERS of all NEWSGROUPS.
> -------------------------------------------------------------
>
> ... Phil
Dang Phil, take it easy. This seems like the first good thread that
there's been on this news group for a while.
George H.
George Herold
> On Wed, 27 Jan 2010 13:23:28 GMT, Bob Masta wrote:
> >On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
> ><j...@infinitefactors.org> wrote:
>
> >>I'd like to take a crack at thinking through a design of an
> >>audio amplifier made up of discrete BJTs and other discrete
> >>parts as an educational process.
>
>
> >When I was first getting interested in power amp
> >design (back in the '70s) I started collecting
> >schematics for all the power amps I could get my
> >hands on, to compare them. I noticed that the
> >schematics for simple bipolar op-amp ICs were
> >remarkably similar to those for big discrete power
> >amps. If you have an old National Linear Databook
> >(or don't mind a lot of rooting around on the Web
> >for individual datasheets), you might take a look.
>
> >You can build a pretty decent amp with only a
> >handful of transistors. The same basic circuit
> >can be used for a wide range of output powers,
> >just by changing the power supply voltages and the
> >output device ratings.
>
> >Best regards,
>
> Thanks, Bob. Audio amplifiers, especially ones delivering
> _some_ power, seem to offer such an excellent way to learn.
> The basic idea, at a behavioral level, is fairly simple. An
> implementation requires some knowledge and thought in the
> end. So the destination is arrived at by taking a great path
> to walk, with such wonderful vistas to see, I think. Much of
> interest is along the way of getting there.
>
> I may have an old National databook on linear parts
> somewhere. I keep a lot, but I also have several thousand
> books in my library which covers all of the walls in one of
> the rooms. I'm at a point now where to get room for more
> books, others must be boxed and stored or simply destroyed
> and pulped. So it's a _maybe_.
>
> One of the nice things (to me) about this kind of a path,
> too, is that what I learn can be used for lots of things. An
> audio amplifier is, in effect, not that much different from
> an op amp. There is the usual basic idea of open loop gain
> and closed loop gain with negative feedback, phase margins,
> problems to solve over a frequency range spanning many
> decades, and so on.
>
> A completely separate project I'd like to play with, which
> this learning will help prepare me for, is designing a pin
> driver. I'd like to sink or source a programmable current
> spanning decades from perhaps 100nA to perhaps 100uA while
> reading the voltage at the node, as well as being able to
> program a low impedance voltages spanning from -15V to +15V
> there and read the current, or read a voltage at the same
> node while presenting a fairly high impedence to it. I
> imagine what I learn here will aid me there. And I'd like to
> do this at some speed, as well. I may then start with a BJT
> tester, for example, making up only three of these to start
> and tying them into a micro for playing. Expanding that for
> other purposes, later. It would be fun.
>
> Jon- Hide quoted text -
>
> - Show quoted text -
Hi Jon, I'm enjoying your posts. What's a pin driver? I made a nice
switchable current source (10nA to 1mA) from a voltage reference,
opamp and switchable resistors. (circuit cribbed from AoE.)
George H.
George Herold
>
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
"I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)"
First Jon I know less about amplifier design than you do... That said,
I would be careful about replacing the diodes in the push-pull stage.
Way back in college I had a Sony stero amp that I had to fix. It came
with a nice circuit diagram. I seem to recall that the bias diodes
in the push pull stage were thermally attached to the same heat sink
that held the output transistors. As the output transistors warm up
their Vbe drop decreases. You want the bias diodes to track this
change. Or else the whole thing could 'run-away' on you. ...
degenerative emmiter resistors (as you suggest) will help some.
George H.
pimpom
I like the biasing scheme mentioned by Jon and use it for all my
designs except the early ones using germanium transistors, though
I don't know the name either. The biasing transistor can be
mounted on the output transistors' heatsink for temperature
tracking.
I like it because it's versatile and a single transistor can be
used to bias several transistors with their b-e junctions in
series as long as they are mounted on a common heatsink.
http://img691.imageshack.us/img691/2075/bias.png
My personal preference is to place the bias adjustment pot R3 in
this position rather than with R1. It ensures that any accidental
loss of contact by the pot's wiper arm will reduce the total bias
whereas placing it with R1 will have the opposite effect and
could cause excessive quiescent current in the output
transistors, possibly getting them to overheat.
Jon Kirwan
<gghe...@gmail.com> wrote:
>Hi Jon, I'm enjoying your posts.
Thanks. I feel like I'm way behind some curves, but it's fun
taking a moment to think about things and it is fantastic
that anyone else is willing to help talk about things with
me. That is priceless. So the real thanks go to those who
are sharing their knowledge and experience here.
>What's a pin driver?
Hmm. I think I first heard the idea when talking about
testing ICs, to be honest. But imagine instead a micro with
software to test some discrete part (could be an IC, too,
that that's more complex.) For example, to automatically
derive some modeling parameters for a BJT.
Take a look at this datasheet, for an example of the features
one might support:
http://www.analog.com/static/imported-files/Data_Sheets/AD53040.pdf
>I made a nice
>switchable current source (10nA to 1mA) from a voltage reference,
>opamp and switchable resistors. (circuit cribbed from AoE.)
I'd require at least one that can either sink _or_ source to
the pin. And that would be only one of the pin driver's
required features. I think the datasheet mentioned above
provides some more. But that part is expensive and not
readily available to us hobbyist types and doesn't teach me
anything about various trade-offs I might want to make or how
to design it at all, besides.
Jon
George Herold
>
> My personal preference is to place the bias adjustment pot R3 in
> this position rather than with R1. It ensures that any accidental
> loss of contact by the pot's wiper arm will reduce the total bias
> whereas placing it with R1 will have the opposite effect and
> could cause excessive quiescent current in the output
>
> - Show quoted text -
Ahh Thanks Pimpom, I've never done any temperature tracking type
calculations.
George H.
David Eather
> On Thu, 28 Jan 2010 11:17:02 +1000, David Eather
>
>
>
Sorry Jon,
I'm stuck on google groups for a little while - I can't believe people
actually use it full time or that google could make an interface this
bad. (I suspect it is very fine for simple threads) anyway...
Sorry.
>
> Regarding .1% to .05%, I'm _very_ good with that. Of course,
> I'm going to have to learn about how to estimate it from
> theory as well as measure it both via simulation before
> construction and from actual testing afterwards. More stuff
> I might _think_ I have a feel for, but I'm sure I will
> discover I don't as I get more into it.
A little experience will get you into the right ballpark when
estimating what you could expect for distortion. It is basically the
same "rules" as you would see with op-amps - the more linear it is to
start with the better. Higher bandwidth stages generally mean you can
use more negative feedback to eliminate distortion - but the lower the
final gain the more instability is likely to become a problem. And bad
circuit layout can increase distortion (and even more so hum and
noise) easily by a factor of 10.
As for how low you need distortion to be one rule of thumb (I forget
the reference) is to be clearly audible the message must be 20db above
the background noise and to be inaudible distortion has to be 20db
below the background noise - which pretty much sets "low" distortion
for PA and similar uses at 1% or 10000 ppm. For HiFi the "message" has
a high dynamic range and you (allegedly) want a distortion figure at
least 20db below that. So a 60 db signal range 0.0001% (or 100PPM).
The you start getting into all kinds of trouble with power output /
dynamic range of the amp etc and you relies that it is all a
compromise anyway. You do the best you can within the restrictions of
the job description.
Yes and No. All the published circuits are made by people who want to
sell transistors, not audio systems, power supplies or transformers.
As a result the power supply is often assumed to be regulated, which
is not true in this case, or the power supply is treated in a very
perfunctory manner that is not at all compatible with good design.
In this case you have the voltage you need for the 10 watts, plus
voltage drop for the driver circuitry and output stage , plus ripple
voltage, plus whatever is required for transformer regulation and
mains regulation. When you add it all up you might find that a chosen
transistor/component is actually not at all suitable for the job. Back
to the drawing board. Change this change that recheck everything again
etc.
If you do the power supply first you have the figures needed for your
worst case already. It saves time and makes a better result (no
tendency to comprimise to save all the calculations already done).
I wasn't going to prompt, but it is close to the sort of thing, I
think, you should be aiming for . As someone has already noted (I
would attribute you if I wasn't on GG, I'm sorry) it has been drawn up
for a single supply, rather than a more common (for this size /
configuration) split supply.
(I don't really care too much about arguing about
> efficiencies right now -- I'm more concerned about learning.)
> The input stage shown is a voltage-in, current-out bog
> standard diff-pair. First thing I remember about is that R4
> shouldn't be there
Correct. Theory says it does nothing. I practice the theory but have
the occasional heretical belief about that.
and better still both R3 and R4 should be
> replaced with a current mirror.
This would provide more differential gain.
R5 should be a replaced with
> a BJT, as well.
In the right configuration it would reduce the common mode signal gain
of things like mains hum and supply ripple (you mentioned power supply
isolation before).
Also, from another (what do you call it branch? thread?) you were
discussing boot-strapping R6. This is not done so much as amplifiers
get bigger but a BJT configured in the same way as the replacement for
R5 is very common. I'm leaving the details to you - perhaps there is a
way to reduce component count without affecting performance. (I am
hoping this is what you wanted "nutting it out for yourself")
I assume the input impedance of that example
> is basically the parallel resistance of R1 and R2, but if we
Yes.
> use split supplies I'd imagine replacing the two of them with
> a single resistor to the center-ground point.
Yes, but you should probably think of a whole passive network to
filter out low and high frequency - (think what happens if you amp is
operated near a source of RF)
There's no
> miller cap on Q3,
Depending on transistors layout etc it might not be needed, but more
often it is the size that is the question.
I'd probably replace the two diodes with
> one of those BJT and a few resistor constructions I can't
> remember the name of (which allows me to adjust the drop.)
Vbe multiplier...
> The feedback ... well, I need to think about that a little
> more. There's no degen resistors in the emitters of Q4 and
> Q5.
>
Why would/should you use them?
> Um.. okay, I need to sit down and think. Mind is spinning,
> but I've not set a finger to paper yet and there is lots to
> think about in that one. I could be way, way off base.
Not at all.
>
> Jon
Is there a way you could post a schematic of where your thinking is
and what you would like to discuss - there is no need for a complete
circuit.
Jon Kirwan
<eat...@tpg.com.au> wrote:
>On Jan 28, 12:51�pm, Jon Kirwan <j...@infinitefactors.org> wrote:
>> On Thu, 28 Jan 2010 11:17:02 +1000, David Eather
>
>Sorry Jon,
>
>I'm stuck on google groups for a little while - I can't believe people
>actually use it full time or that google could make an interface this
>bad. (I suspect it is very fine for simple threads) anyway...
Cripes. Google didn't even show the thread when I'd looked,
a day ago or so. And it had been around for at least 24
hours by then. Used to be the case that google groups would
show the posts within an hour or so. Doesn't seem to be
true, anymore. If not, there is no possibility of having a
discussion very quickly via google. It would greatly
lengthen out the interactions. Maybe that's on purpose, now,
to cause people to find some other solution?
Don't be. I was just explaining myself, not complaining
about your usage.
Understood.
A concern I care not the least about. My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices. One part. That's
it. Why? Because I've got thousands of them. ;)
Literally. Something like 22,000 of the bastards. I give
them away like popcorn to students at schools. Got them
_very cheaply_. So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.
What to go there? :)
>not audio systems, power supplies or transformers.
Got it.
>As a result the power supply is often assumed to be regulated, which
>is not true in this case, or the power supply is treated in a very
>perfunctory manner that is not at all compatible with good design.
>
>In this case you have the voltage you need for the 10 watts, plus
>voltage drop for the driver circuitry and output stage , plus ripple
>voltage, plus whatever is required for transformer regulation and
>mains regulation. When you add it all up you might find that a chosen
>transistor/component is actually not at all suitable for the job. Back
>to the drawing board. Change this change that recheck everything again
>etc.
In this case, though, there is nothing particularly
remarkable about the rails. Taken across the entire span,
even, doesn't exceed the maximum Vce of a great many BJTs. So
no real worry there. But I see some of where problems may
arise. Luckily, at this level I can side-step worrying about
that part and get back to learning about amplifier design,
yes?
>If you do the power supply first you have the figures needed for your
>worst case already. It saves time and makes a better result (no
>tendency to comprimise to save all the calculations already done).
Well, does this mean we should hack out the power supply
first? I'm perfectly fine with that and can get back to you
with a suggested circuit and parts list if you want to start
there. We could settle that part before going anywhere else
and I'd be happy with that approach, too, because to be
honest I don't imagine it to put a horrible delay into
getting back to amplifier design. So I'm good either way.
I had assumed we'd be using a split supply.
I had assumed a speaker would be hooked up via a cap to the
output, so DC currents into a speaker coil would be removed
from any concern. But I was also holding in the back of my
mind the idea of tweaking out DC bias via the speaker and
removing the coupling cap as an experiment to try. And if
so, I'd pretty much want the ground as a "third rail."
(Playing just a bit upon the Chicago parlance about the once
dangerous rail in their transit system.)
>(I don't really care too much about arguing about
>> efficiencies right now -- I'm more concerned about learning.)
>> The input stage shown is a voltage-in, current-out bog
>> standard diff-pair. �First thing I remember about is that R4
>> shouldn't be there
>
>Correct. Theory says it does nothing. I practice the theory but have
>the occasional heretical belief about that.
Actually, I think I've read that theory says it is _better_
to be removed. The reason seemed pretty basic, as it's
easier to get close to a balanced current split; and this, I
gather, lowers 2nd harmonic distortions produced in the pair
-- notable more on the high frequency end I suppose because
gain used for linearizing feedback up there is diminishing
and can't compensate it.
In other words, it's not neutral. It's considered to be
better if I gathered the details. Then even better, the
current mirror enforces the whole deal and you've got about
the best to be had.
Of course, mostly just being a reader means I have no idea
which end is up. So I might have all this wrong.
> and better still both R3 and R4 should be
>> replaced with a current mirror. �
>
>This would provide more differential gain.
_and_ improve distortion because the currents are forced to
be balanced in the pair, yes?
>>R5 should be a replaced with
>> a BJT, as well. �
>
>In the right configuration it would reduce the common mode signal gain
>of things like mains hum and supply ripple (you mentioned power supply
>isolation before).
Yes, that's how I thought about it.
>Also, from another (what do you call it branch? thread?) you were
>discussing boot-strapping R6. This is not done so much as amplifiers
>get bigger but a BJT configured in the same way as the replacement for
>R5 is very common. I'm leaving the details to you - perhaps there is a
>way to reduce component count without affecting performance. (I am
>hoping this is what you wanted "nutting it out for yourself")
Yes! I don't want things handed on a platter. But I also
don't want to have to rediscover all of the ideas by making
all of the mistakes, either. This is the kind of "pointer"
towards something that I like a lot. It gives me a place to
think about something, but leaves me some reason to have to
do so and that helps me own it better.
One general truth about learning is that you don't present
someone with a problem so out of their depth that they have
no chance at it. Doing that means they fail, they feel like
a failure, and it causes a student to just want to go away.
They lose motivation, usually, in cases like that. On the
other hand, providing no difficulty at all merely means
repetition of what they already know and they grow bored from
that, too. Finding the sweet spot where a student is faced
with interesting problems that are not already known, but
perhaps within reach of grasping at with some effort, is the
key. Then it can be fun, educational, and motivate.
That's what you just did for me.
>I assume the input impedance of that example
>> is basically the parallel resistance of R1 and R2, but if we
>Yes.
Okay.
>> use split supplies I'd imagine replacing the two of them with
>> a single resistor to the center-ground point. �
>Yes, but you should probably think of a whole passive network to
>filter out low and high frequency - (think what happens if you amp is
>operated near a source of RF)
Well, every trace picks up like little antennae. All kinds
of trace voltages appearing here and there. Not good.
So. Can you make an audio amplifier that can withstand a
microwave oven environment and deliver good performance while
irradiated with 1kW banging around in there? ;)
>> There's no
>> miller cap on Q3,
>
>Depending on transistors layout etc it might not be needed, but more
>often it is the size that is the question.
I was thinking it helped locally linearize the VAS section
and that such would be "good" most anywhere. But I am just
taking things without having worked through them on my own.
So...
>> I'd probably replace the two diodes with
>> one of those BJT and a few resistor constructions I can't
>> remember the name of (which allows me to adjust the drop.)
>
>Vbe multiplier...
Okay. Thanks.
>> The feedback ... well, I need to think about that a little
>> more. �There's no degen resistors in the emitters of Q4 and
>> Q5.
>
>Why would/should you use them?
I'm still thinking about that. In general, I was thinking
about them because of the "little re" that is kT/q based in
each BJT, and varies on Ie. Since Ie is varying around, I
was thinking about something fixed there to overwhelm it and
"make it knowable" for the design, I suppose. Maybe that's
all wet, given your query. I'll toss the idea off the side,
for now.
>> Um.. okay, I need to sit down and think. �Mind is spinning,
>> but I've not set a finger to paper yet and there is lots to
>> think about in that one. �I could be way, way off base.
>
>Not at all.
Thanks for that. I'm just glad to be able to talk to someone
about any of this, at all. So please accept my thanks for
the moments you are offering.
>Is there a way you could post a schematic of where your thinking is
>and what you would like to discuss - there is no need for a complete
>circuit.
Yes. I can use ASCII here, for example. But before I go off
into the wild blue with this, do you want to focus on the
power supply first? Or just jump in on the amplifier?
Jon
Jon Kirwan
>A concern I care not the least about. My _real_ preference,
>were I to impose it on the design, would be to use ONLY
>PN2222A BJTs for all the active devices. One part. That's
>it. Why? Because I've got thousands of them. ;)
>
>Literally. Something like 22,000 of the bastards. I give
>them away like popcorn to students at schools. Got them
>_very cheaply_. So if I were pushing something, I'd be
>pushing a 10W PN2222A design, use signal splitting approach
>probably (because it's the only way I think think of, right
>now), and distribute the dissipation across lots and lots of
>the things.
>
>What to go there? :)
I meant "Want to go there?"
One part of the brain says "type out word for concept X on
the keyboard," and that gets passed down to a low-level
manager function which maps concept to English word, gets the
wrong hash bucket ID for a "nearby word" and the passes on
motor instructions for mirror neurons driving the hand which
then types the wrong word. Meanwhile, eye reads "want" text,
this gets translated into a concept which is promptly ignored
because the concept already resides in local cache storage
and doesn't need replacement. So the brain checks that what
I wrote is what I intended, glancing quickly over it and gets
the cached version and matches everything up nicely and moves
on.
hehe. This is either a hardware problem or a software
problem, depending upon point of view... ;)
Jon
Jon Kirwan
<jo...@infinitefactors.org> wrote:
>eye reads "want" text
hehe. Or "What" text....
Nesting all the way down...
Jon
Jon Kirwan
Yeah. That's what I was thinking about. Just some thought
here to add. As you carefully point out, tying it to the
heat sink of the output BJTs to hold them near each others'
temperatures seems important. Vbe varies a great deal over
temperature.
The voltage across this structure is:
Vbe * (1 + R1/(R2+R3))
However, it's also true that the output BJTs are also
experiencing similar (but _not_ the exact same as they aren't
necessarily even from the same manufacturer or family)
changes in Vbe. So it's actually a kind of "good thing" to
have the voltage held between the output BJT bases vary as
the output BJTs temperatures vary.
Question is, is a random selection of a BJT for this purpose
okay? Or does it need to be carefully considered, taken
together with the output BJT characteristics? It seems to me
that some care is needed here, even assuming good temperature
coupling occurs.
Also, I think I've seen some examples where there is a
collector resistor added to this structure, with Q2's base
kept tied directly to Q1's collector lead. What is the
reasoning here? (I believe in the cases I saw, there was a
current source [not a resistor] feeding at the top. I
started to work the equations to show the relationship, but
then realized that there is also base current drive to the
upper side of the output transistors involved and then
decided to just ask, instead of wandering all over the place
right now.)
>My personal preference is to place the bias adjustment pot R3 in
>this position rather than with R1. It ensures that any accidental
>loss of contact by the pot's wiper arm will reduce the total bias
>whereas placing it with R1 will have the opposite effect and
>could cause excessive quiescent current in the output
>transistors, possibly getting them to overheat.
Makes sense.
Thanks,
Jon
Phil Allison
"George Herold"
Dang Phil, take it easy. This seems like the first good thread that
there's been on this news group for a while.
** As usual - you are 100% fucking WRONG.
It is easily one of the VERY WORST bullshit fests here ever.
Piss off and die ASAP -
you know nothing TROLLING TWAT.
.... Phil
Paul E. Schoen
>
> A concern I care not the least about. My _real_ preference,
> were I to impose it on the design, would be to use ONLY
> PN2222A BJTs for all the active devices. One part. That's
> it. Why? Because I've got thousands of them. ;)
>
> Literally. Something like 22,000 of the bastards. I give
> them away like popcorn to students at schools. Got them
> _very cheaply_. So if I were pushing something, I'd be
> pushing a 10W PN2222A design, use signal splitting approach
> probably (because it's the only way I think think of, right
> now), and distribute the dissipation across lots and lots of
> the things.
I design a lot of things that way as well. I have (hundreds of) thousands
of parts that I got about 20 years ago, and I really like to use them
wherever possible. You can check my website where I have some of these
parts listed as surplus sales and I'd really like to get rid of them where
they might be used rather than hauling them to the dump. Take a look and if
you can use anything I'll see if it's worthwhile to send them to you for
little more than the cost of shipping (probably USPS flat rate). My website
is www.pstech-inc.com, and just look for the link to surplus parts. If that
doesn't work, try http://www.smart.net/~pstech/surplus.htm and
http://www.smart.net/~pstech/PARTS.txt and
http://www.smart.net/~pstech/PARTS.xls.
I have a lot of MPSA06 NPN transistors, so I use them wherever possible. I
also have a few thousand MJE170 PNP Power transistors (40V, 3A, 12W). And
lots of 2N6312 in TO-66 metal cans (PNP 40V 5A 75W) and about 600
Thermalloy 6060 heat sinks that can be used for them, as well as other case
types. If you like SCRs I have about 600 of 2N6504 which is 35A at 50 V.
If you need a transformer for a power supply I have a couple hundred Signal
241-6-16 which can be used to make a raw +/- 8-10 VDC supply and with a few
more capacitors and diodes makes a nice +/- 20 VDC supply at about 1 amp.
And I have an armload of capacitors such as 500 uF 50V, 1500 uF 50V, and
even some 4500 uF 50V in big blue metal cans. And a few handsful of 1N4003
and 1N4004 rectifiers.
If you could come to my place near Baltimore, MD I could give you a
"shopping spree" where you could fill a few bags and boxes with all sorts
of goodies. Lately I am realizing that almost any new design I do will be
with SMT components and newer parts, and there are only a few one-off
projects that I might make using these older components. Some of them have
been stores so long in a damp, unheated building that the leads are
difficult to solder, and some resistors have actually soaked up enough
moisture to change value. (That is what a friend told me, and he also said
they were restored to normal by baking them for a while).
I've sent "care packages" to others in the past. I don't expect to make any
money selling/giving away these parts but I just want to be compensated for
shipping cost. I don't know if I have some of these parts and if I do I
might not even be able to find them, but I think I can supply enough parts
for you to build a good amplifier and other projects.
Paul
Paul E. Schoen
As a quick precaution against thermal runaway, a thermistor from base to
emitter, and thermally tied to the case, should shut off base drive if
things get too hot. Or just put a thermal switch on the heat sinks and use
it to shut off the supply to the whole shebang.
Paul
Jon Kirwan
<pa...@peschoen.com> wrote:
>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>news:l6n6m5pd53835g9q7...@4ax.com...
>>
>> A concern I care not the least about. My _real_ preference,
>> were I to impose it on the design, would be to use ONLY
>> PN2222A BJTs for all the active devices. One part. That's
>> it. Why? Because I've got thousands of them. ;)
>>
>> Literally. Something like 22,000 of the bastards. I give
>> them away like popcorn to students at schools. Got them
>> _very cheaply_. So if I were pushing something, I'd be
>> pushing a 10W PN2222A design, use signal splitting approach
>> probably (because it's the only way I think think of, right
>> now), and distribute the dissipation across lots and lots of
>> the things.
>
><snip>
>
>If you could come to my place near Baltimore, MD I could give you a
>"shopping spree" where you could fill a few bags and boxes with all sorts
>of goodies. Lately I am realizing that almost any new design I do will be
>with SMT components and newer parts, and there are only a few one-off
>projects that I might make using these older components. Some of them have
>been stores so long in a damp, unheated building that the leads are
>difficult to solder, and some resistors have actually soaked up enough
>moisture to change value. (That is what a friend told me, and he also said
>they were restored to normal by baking them for a while).
>
>I've sent "care packages" to others in the past. I don't expect to make any
>money selling/giving away these parts but I just want to be compensated for
>shipping cost. I don't know if I have some of these parts and if I do I
>might not even be able to find them, but I think I can supply enough parts
>for you to build a good amplifier and other projects.
>
>Paul
Paul, I'll write under separate cover, directly. There are
some thoughts I'd like to explore more, if that's okay. I
can also provide a 501(c)3 for tax purposes, as well as cash
compensation. That may also help a little. But we can talk
about that off-line.
Jon
John Larkin
<gghe...@gmail.com> wrote:
>
>"I'd probably replace the two diodes with
>one of those BJT and a few resistor constructions I can't
>remember the name of (which allows me to adjust the drop.)"
"Vbe multiplier."
The classic output stage biasing scheme uses small emitter resistors
and biases the output transistors to idle current using a couple of
junction drops between the bases, or a Vbe multiplier with a pot. Both
are good ways to have a poorly defined idle current and maybe fry
transistors. Two alternates are:
1. Use zero bias. Connect the complementary output transistors
base-to-base, emitter-to-emitter. Add a resistor from their bases to
their emitters, namely the output. At low levels, the driver stage
drives the load through this resistor. At high levels, the output
transistors turn on and take over.
2. Do the clasic diode or Vbe multiplier bias, but use big emitter
resistors. Parallel the emitter resistors with diodes.
In both cses, the thing will be absolutely free frfom thermal runaway
issues and won't need adjustments. Bothe need negative feeback to kill
crossover distortion.
Or...
3. Use mosfets
John
Phil Allison
"John Larkin is lying IDIOT
> The classic output stage biasing scheme uses small emitter resistors
> and biases the output transistors to idle current using a couple of
> junction drops between the bases, or a Vbe multiplier with a pot. Both
> are good ways to have a poorly defined idle current and maybe fry
> transistors.
** Done correctly, either way produces a stable bias situation in the output
stage.
Larkin has no idea how it is done - cos Larkin is bullshitting asshole.
> 1. Use zero bias. Connect the complementary output transistors
> base-to-base, emitter-to-emitter. Add a resistor from their bases to
> their emitters, namely the output. At low levels, the driver stage
> drives the load through this resistor. At high levels, the output
> transistors turn on and take over.
** Guarantees serious x-over distortion.
Zero bias can be done, but never so crudely as that.
> 2. Do the clasic diode or Vbe multiplier bias, but use big emitter
> resistors. Parallel the emitter resistors with diodes.
** No need to ever use emitter resistors of more than 1 ohm.
With the usual 0.33 to 0.47 ohm resistors, parallel diodes have barely any
effect.
Very few power amp designs have ever used them - SAE brand amps from the
late 1970s being one exception.
> In both cses, the thing will be absolutely free frfom thermal runaway
> issues and won't need adjustments.
** Vbe multipliers always need adjustment to suit the actual devices in use.
> Both need negative feeback to kill crossover distortion.
** One thing that NFB is notoriously very poor at doing.
.... Phil
John Larkin
wrote:
Discrete-transistor audio design, being such an ancient practice,
tends to refer to history and authority rather than design from
engineering fundamentals.
If I were designing an audio amp nowadays (which I certainly aren't)
I'd use mosfets with an opamp gate driver per fet. That turns the fets
into almost-perfect, temperature-independent, absolutely identical
gain elements. That's what I do in my MRI gradient amps, whose noise
and distortion are measured in PPMs.
ftp://jjlarkin.lmi.net/Amp.jpg
Why keep repeating a 50-year-old topology when you could have a little
fun?
John
Paul E. Schoen
"John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> wrote in message
news:4ch8m516qf4n46fqm...@4ax.com...
>
> Discrete-transistor audio design, being such an ancient practice,
> tends to refer to history and authority rather than design from
> engineering fundamentals.
>
> If I were designing an audio amp nowadays (which I certainly aren't)
> I'd use mosfets with an opamp gate driver per fet. That turns the fets
> into almost-perfect, temperature-independent, absolutely identical
> gain elements. That's what I do in my MRI gradient amps, whose noise
> and distortion are measured in PPMs.
>
> ftp://jjlarkin.lmi.net/Amp.jpg
>
> Why keep repeating a 50-year-old topology when you could have a little
> fun?
Nice photo. But how about a schematic to show how you implemented the
design? I made a simulation of an amplifier design where I used a MOSFET
output stage similar to my other post, and I closed the loop with a single
op-amp and appropriate negative feedback.
I think the OP is looking for a basic learning experience using the
simplest components. It may be argued that a MOSFET is simpler than a BJT,
but experience with both is a good idea. If the object were just to make an
audio amp, there are single package designs and kits that will do the job
nicely.
I prefer making the circuit using LTSpice, but it can be a thrill to build
something with real components. There are usually some gotchas that cause
unwanted behavior not indicated in the simulation. But I need a real reason
to build something, other than practice with soldering and handling
components, so I rarely commit these designs to copper and silicon.
Paul
Jon Kirwan
>On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold
><gghe...@gmail.com> wrote:
>>
>>"I'd probably replace the two diodes with
>>one of those BJT and a few resistor constructions I can't
>>remember the name of (which allows me to adjust the drop.)"
>
>"Vbe multiplier."
Got it. Since that time, I've found "rubber diode" as
another term mentioned on wiki. ;)
>The classic output stage biasing scheme uses small emitter resistors
>and biases the output transistors to idle current using a couple of
>junction drops between the bases, or a Vbe multiplier with a pot. Both
>are good ways to have a poorly defined idle current and maybe fry
>transistors.
I've already expressed my concern about that.
>Two alternates are:
>
>1. Use zero bias. Connect the complementary output transistors
>base-to-base, emitter-to-emitter. Add a resistor from their bases to
>their emitters, namely the output. At low levels, the driver stage
>drives the load through this resistor. At high levels, the output
>transistors turn on and take over.
>
>2. Do the clasic diode or Vbe multiplier bias, but use big emitter
>resistors. Parallel the emitter resistors with diodes.
>
>In both cses, the thing will be absolutely free frfom thermal runaway
>issues and won't need adjustments. Bothe need negative feeback to kill
>crossover distortion.
I need to get some basics down before I return to these. I'm
not there, yet. But I _do_ see an issue with the Vbe
multiplier if it isn't crafted carefully for the situation.
>Or...
>
>3. Use mosfets
No FETs.
Jon
Jon Kirwan
<pim...@invalid.invalid> wrote:
>Jon Kirwan wrote:
>> On Thu, 28 Jan 2010 19:19:06 +0530, "pimpom"
>> <pim...@invalid.invalid> wrote:
>>
>>
>>> Q2 needs about +/-50uA peak of base
>>> current at full drive. At signal frequencies, R2 (plus the
>>> much
>>> smaller input impedance of Q1) is effectively in parallel with
>>> the output.
>>
>> R2 is connected from the output to an input, which
>> effectively doesn't move much after arriving at it's DC bias
>> point. As you later point out, the _AC_ input impedance is
>> lowish (near 600 ohms), so the 10k is pretty close to one of
>> the rails at AC, anyway. Is that a different way of saying
>> what you just said? Or would you modify it?
>>
>That's another way of putting it, yes.
Okay. Thanks.
>>> The output swings by about 4V peak at max power,
>>> which has 400uA of negative feedback current going back
>>> through
>>> R2. The input current requirement goes up by a factor of 9.
>>> IOW,
>>> a negative feedback of 19db. This is substantially better than
>>> nothing and should significantly reduce distortion and improve
>>> frequency response.
>>
>> Okay. This goes past me a little (as if maybe the earlier
>> point didn't.) I'd like to try and get a handle on it.
>>
>> Let's start with the 4V peak swing at max power.
>>
>> Since you are discussing AC and converting it 400uA current
>> via the 10k, I would normally take this to mean 4Vrms AC.
>> Which in Vp-p terms would be 2*SQRT(2) larger, or 11.3V which
>> I know is impossible without accounting for the BJTs, given
>> the 9V supply. So this forces me to think in terms of
>> something else. But what? Did you mean 4Vpeak, which would
>> be 8Vp-p? If so, that would be about 2.8Vrms.
>
>Yes. It's 4Vp, 8Vp-p and 2.8Vrms. I wanted to give you a mental
>picture of how much the output voltage can swing. Each output Q
>has about 4.4V of Vce available, and about 4V before hard
>saturation is reached (these are all round figure values). That's
>4V peak for a sinusoidal wave form.
Got it.
>> In that case,
>> wouldn't a better "understanding" come from then saying that
>> the negative feedback is closer to 280uA?
>
>Yes, it's 280uA rms. But I was talking in terms of the maximum
>amplitude of instantaneous change, which is why I used the terms
>"swing" and "peak".
Understood.
>> The next point is on your use of "goes up by a factor of 9."
>> Can you elaborate more on this topic? Where the 9 comes
>> from? For volts, not power, I think I can gather the point
>> that 20*log(9) = 19.085), so I'm not talking about that
>> conventional formula. I'm asking about the 9, itself, and
>
>Without feedback, the input transistor Q1 needs 50uA of AC input
>signal to drive the output Qs to full power output (still talking
>in terms of peak to avoid confusion). With NFB, we need an
>additional 400uA to overcome the current fed back from the
>output. That's a total of 450uA peak, which is 9 times the
>original 50uA.
Okay. Let me put it in my own words. Since you chose to
stick a 10k in for the NFB, and since it supplies a peak of
400uA into the input node and since Q1 itself requires its
own 50uA (corrected below), then the signal source itself
must "comply" by supplying 450uA peak into the input node.
And that is where you got your 9. If that is it, I've got
it.
>Actually, I made an error when I cited the 50uA figure. Q1 is
>biased at Ic = 7.6mA, Ib = 50uA. But only 5mA peak is needed from
>Q1's collector to drive the output transistors. Divide that by
>Q1's hfe of 150 and you get 33uA (peak) of AC signal current
>needed into the base of Q1. The corrected total needed from the
>signal source is now 433uA. The gain reduction factor due to NFB
>is now 13 instead of 9. That's 22db (feedback is usually given in
>db).
Okay. I can compute the numbers. I think the problem I'm
having with this, as a mental concept, is that the feedback
is _from_ the complementary BJT outputs at their emitters
_backwards_ into the node at the base of Q1. Not outwards
from that node _towards_ the emitters.
Let me think a little about this from an AC point of view,
not DC. The output is supposedly running with about 4Vpeak
into an AC divider made up of what you've earlier described
as about 600 ohms to ground via Q1 and about 1k for R1. So
given that, we are talking about 1k from input to base node,
600 ohms from base node to ground, and 10k from 4V peak
output into base node. The 4V peak has the opposite sign as
the input because of the relationship of Q1's collector to
its base voltage. I'm just spouting things here without a
lot of understanding, so bear with me.
The change in Ic of Q1 for a change in the base voltage
(computing a transconductance of Q1) is 1 divided by re,
which you computed as 3.4 ohms earlier. So gm=294mSeimens,
assuming the emitter (small signal wise) follows the base
exactly. A 1mV input change at the base yields 1mV*294mS or
294uA change in the collector, ignoring the 100 ohm rbb you
earlier mentioned for now. Multiplied by the collector load
of about 566 ohms (your figure) produces 166mV change at the
collector. A voltage gain (base node to output) of -166.
This 166mV change is fed back via the 10k into an existing
divider composed of the 1k and the beta-multiplied 3.4 ohms
to ground (if I'm following you.) So with the rbb of 100,
about 600 ohms as you mentioned (still AC-minded.)
So back to the 1k from input, 600 to ground, 10k in from that
-166mV change in response to a +1mV change at the base node.
The 1k/600 divider means the input had to vary by about
2.66mV to achieve that node change. That means our voltage
gain wasn't really -166, but more like 62.5 -- audio input to
Q1 collector and then to output drive.
Am I going around the barn about right, so far? Here's what
worries me now. The postulated thevenin base node change of
1mV is through a thevenin of about 375 ohms (the 1k and 600
ohm splitter.) The 10k feeds into this from the other side
with -166mV there. If I imagine 1mV on one side via 375 ohms
and -166mV on the other side via 10k ohms, what is the node
itself at? Well, (1mV*10k+-166mv*375)/(10k+375) is -5mV or
so. This is a lot, isn't it? And it is more than enough to
oppose the postulated thevinin change at the input node of
+1mV.
So I follow the calculation of 22db. The problem I'm having
is with what kind of signal will result at Q1's collector.
Okay. Granted. I am sure my reasoning fails on some points
you will make clearer. I'm just trying to see this in a
variety of ways rather than just let you tell me stuff
without running through different thinking to see if I get to
the same place. So what did I do wrong here? I can't argue
with success and I know I'm not doing this right. But there
it is.
>> also your thinking along the lines of concluding that it
>> significantly reduces distortion.
>
>The basic principle of NFB is that it reduces THD and extends
>frequency response by a factor equal to the feedback ratio. So,
>in our example, if you have 10% THD without feedback, it will
>drop to 0.77% with the feedback factor of 13. But there are
>caveats. E.g., phase shifts can cause undesireable effects,
>especially with large amounts of feedback. I'm afraid a detailed
>treatment of such things is really outside the scope of this
>discussion - unless someone else is willing to take it up.
I can do phase shift calcs given simple cases and if I take
into account all the necessary parts (which, being ignorant
about all this, I'm unlikely to do without more thought
experiments to clarify my thinking.) So when I get to that
point where I can actually walk myself along better, I'll be
able to handle that (I hope.)
>> How does one decide how much is enough?
>
>For one thing, how much distortion one is willing to put up with.
>Another factor is input sensitivity, or IOW, how much gain is
>needed. E.g., to drive the 1W amp to full output, we need 433uA
>peak (306uA rms) from the signal source into 1k. That's 306mV
>rms, plus some millivolts at the b-e junction. Say about 0.32V
>rms total input voltage into about 1k input impedance.
>
>To present the basic concepts, I've made several approximations.
>E.g., I neglected the shunting effect of R2. Besides, the input
>resistance of Q1 is constant at 600 ohms only for very small
>signal amplitudes relative to the quiescent dc levels. This
>dynamic input resistance changes significantly with large signal
>swings and adds distortion while also complicating precise
>calculations.
Okay. I'm going to leave things with the above "issue" laid
out for you. It's bugging me right now.
I am refusing, by the way, to attempt any simulation. I
don't want to be "told" something by a simulator or handed
things on some platter. I want to try and work through my
thinking, find the flaws, slap myself for them, get back and
try again, until I'm good from a paper-and-brain point of
view. _Then_ I'll go and check it out to see where the chips
fall in the simulator. Might bring up a good question at
that point. But until I can get the gross aspects down, that
won't really matter.
Thanks so much for your efforts so far. It's been helpful to
me, at least.
Jon
Jon Kirwan
So there's a need to refresh our minds about this. You
talked about FET designs, but before one can understand
whether or not they compare well to BJT designs it seems to
me that one needs to understand what can be done with BJTs
first. Just _stating_ (or making a premise based on what you
say is more history and authority that actual _best_ design
practices) that they would be better isn't enough, I suspect.
>If I were designing an audio amp nowadays (which I certainly aren't)
>I'd use mosfets with an opamp gate driver per fet. That turns the fets
>into almost-perfect, temperature-independent, absolutely identical
>gain elements. That's what I do in my MRI gradient amps, whose noise
>and distortion are measured in PPMs.
Well, if I were opening the door to ICs there'd be no real
learning going on. An opamp doesn't teach one that much.
They are pretty close to ideal and what's learned by that?
Now designing one... that would be another case. But using
ICs with a FET tacked on the end teachs about as much about
deeper levels of analog design (getting closer to the physics
around us) as does using a Visual BASIC drop-down box teaches
about the Windows messaging layer. It's almost all hidden
from view in either case.
That doesn't mean a Visual BASIC drop-down isn't useful or
that people should make programs using them as a shortcut.
They should, and do. But if you want to know how to make
some new widget of your own... you may find yourself with
only one oar in the water... spinning in circles.
>ftp://jjlarkin.lmi.net/Amp.jpg
>
>Why keep repeating a 50-year-old topology when you could have a little
>fun?
The fun for me is in digging closer into the physics. Just
as the programming fun comes from seeing how a c compiler
implements the resulting code on the machine instruction
level or how a coroutine may be implemented using a thunk.
Put another way, one can move from understanding one's own
backyard in two directions. (1) Towards seeing how plants
participate in the meadow or a woods and how those
participate within an Earth/air/ice/water/sun system (in
other words, reaching towards larger and larger abtraction
levels.) (2) Or else, delve deeper towards seeing how organs
function within the organism, how cells function within that,
how proteins work, how peptide chains are brought together
into those, how atoms work in making peptide chains, and so
on. In other words, there are two telescoping directions to
head.
With electronics, this can be towards higher abstraction
levels using ICs or towards smaller, more concrete levels
towards depletion regions and eventually QM events at the
particle interaction level.
Which is more interesting depends on your goals. Right now,
I want to focus on the BJT design level of abstraction. This
has nothing to do with making an amplifier. It has to do
with using an amplifier as an excuse to learn but also as a
well defined outcome that can then be measured and observed
using well-understood measurement criteria (and the ability
to experience the result as a basic, visceral thing to the
ear, too.)
Jon
John Larkin
<jo...@infinitefactors.org> wrote:
>On Fri, 29 Jan 2010 20:01:53 -0800, John Larkin wrote:
>
>>On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold
>><gghe...@gmail.com> wrote:
>>>
>>>"I'd probably replace the two diodes with
>>>one of those BJT and a few resistor constructions I can't
>>>remember the name of (which allows me to adjust the drop.)"
>>
>>"Vbe multiplier."
>
>Got it. Since that time, I've found "rubber diode" as
>another term mentioned on wiki. ;)
>
>>The classic output stage biasing scheme uses small emitter resistors
>>and biases the output transistors to idle current using a couple of
>>junction drops between the bases, or a Vbe multiplier with a pot. Both
>>are good ways to have a poorly defined idle current and maybe fry
>>transistors.
>
>I've already expressed my concern about that.
The basic tradeoff is to use big emitter resistors to prevent thermal
runaway, but that wastes power at large signal swings. Another
tradeoff is to use a small Vbe multiplier voltage (ie, small quiescent
DC drops across the emitter resistors) to reduce idle power and
heatsink temp at the cost of more crossover distortion.
Or change the rules. Semiconductors are cheap, heatsinks are
expensive.
Take a crack at calculating the thermal runaway situation of a typical
class AB output stage. It's interesting.
John
Phil Allison
> tends to refer to history and authority rather than design from
> engineering fundamentals.
** How the fuck would you know ?
Clearly you don't and just make things up to suit your wacky prejudices.
> If I were designing an audio amp nowadays (which I certainly aren't)
** So shut the fuck up.
You clueless fucking wanker.
... Phil
John Larkin
wrote:
>"John Larkin is lying IDIOT
I showed everybody an amp I designed, 17KW peak power out, a few PPM
noise and absolute analog accuracy.
Hey, Mr Audio, show us a power amp that you designed.
John
Phil Allison
** FFS that monstrosity is NOT any kind of audio amp.
YOU have no experience with any aspect if the subject.
YOU are nothing but a LYING PIECE OF SHIT.
FUCK OFF
.... Phil
John Larkin
wrote:
>"John Larkin is lying IDIOT
Nothing to show, we see.
How about something you were allowed to repair?
John
George Herold
> On Fri, 29 Jan 2010 09:19:31 -0800 (PST), George Herold
>
Wow, that's some chip.
Thanks,
George H.
George Herold
<jjSNIPlar...@highTHISlandtechnology.com> wrote:
> On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold
>
Cool thanks John, I tend to only use transistors when I need more
poop on the output and always have an opamp in the loop.
George H.
George Herold
>
> - Show quoted text -
Hee Hee, OK I'm not real proud of it, but we've sold several hundred
of them so at least it's paid for my ~week(?) of design and testing
time.
http://www.teachspin.com/instruments/audio/index.shtml
Hey, at least it's called an audio amplifier. It's powered by a 15V /
1A switching supply. And uses two power opamps. One sets the ground
and the other does the work.
(Well both have to work when there is significant ground current.)
George H.
John Larkin
Things like this, laser drivers and gradient amplifiers and such, are
more interesting than actual audio, whose performance criteria are
fuzzy things like absurd power specs, "soundstaging", "microdynamics",
and similar fuzzy blather. They sell for a lot more, too.
John
John Larkin
Exactly. Opamps make gain and precision cheap, so every power
transistor deserves one.
John
John Larkin
But $25 at 100 pieces, and rather slow.
HFA1130 is interesting as a pin driver, run open-loop and slamming
into its voltage-set limits.
John
David Eather
> On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather
>
> <eat...@tpg.com.au> wrote:
> >On Jan 28, 12:51 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
> >> On Thu, 28 Jan 2010 11:17:02 +1000, David Eather
>
> >Sorry Jon,
>
> >I'm stuck on google groups for a little while - I can't believe people
> >actually use it full time or that google could make an interface this
> >bad. (I suspect it is very fine for simple threads) anyway...
>
> Cripes. Google didn't even show the thread when I'd looked,
> a day ago or so. And it had been around for at least 24
> hours by then. Used to be the case that google groups would
> show the posts within an hour or so. Doesn't seem to be
> true, anymore. If not, there is no possibility of having a
> discussion very quickly via google. It would greatly
> lengthen out the interactions. Maybe that's on purpose, now,
> to cause people to find some other solution?
>
The "Google Delay" is my delay. In a couple of days it will be back to
normal
> >> <eat...@tpg.com.au> wrote:
> >> >Jon Kirwan wrote:
> >> >> On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
> >> >> <eat...@tpg.com.au> wrote:
> >> >>> <snip>
>
> >> >>> My particular bias for an amp this size is to go class AB with a split
> >> >>> power supply. The majority of quality audio amps follow this topology
> >> >>> and this is, I think, I great reason to go down this design path (what
> >> >>> you learn is applicable in the most number of situations). I should hunt
> >> >>> down a schematics of what I'm seeing in the distance (which can/will
> >> >>> change as decisions are made) - some of the justifications will have to
> >> >>> wait
>
> >> >> I'm fine with taking things as they come.
>
> >> >> As far as the class, I guessed that at 10 watts class-A would
> >> >> be too power-hungry and probably not worth its weight but
> >> >> that class-AB might be okay.
>
> >> >> I have to warn you, though, that I'm not focused upon some
> >> >> 20ppm THD. I'd like to learn, not design something whose
> >> >> distortion (or noise, for that matter) is around a bit on a
> >> >> 16-bit DAC or less. I figure winding up close to class-B
> >> >> operation in the end. But I'd like to take the walk along
> >> >> the way, so to speak.
>
> >> >10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
> >> >a few detours to see what would help or harm that.
>
> >> Hehe. I'm thinking of some numbers I saw in the area of
> >> .002% THD. I hate percentages and immediately convert them.
> >> In this case, it is 20e-6 or 20 ppm. Which is darned close
> >> to a bit on a 16-bit dac. That's why I wrote that way. I
> >> just don't like using % figures. They annoy me just a tiny
> >> bit.
>
> >Sorry.
>
> Don't be. I was just explaining myself, not complaining
> about your usage.
>
> >> Regarding .1% to .05%, I'm _very_ good with that. Of course,
> >> I'm going to have to learn about how to estimate it from
> >> theory as well as measure it both via simulation before
> >> construction and from actual testing afterwards. More stuff
> >> I might _think_ I have a feel for, but I'm sure I will
> >> discover I don't as I get more into it.
>
> >A little experience will get you into the right ballpark when
> >estimating what you could expect for distortion. It is basically the
> >same "rules" as you would see with op-amps - the more linear it is to
> >start with the better. Higher bandwidth stages generally mean you can
> >use more negative feedback to eliminate distortion - but the lower the
> >final gain the more instability is likely to become a problem. And bad
> >circuit layout can increase distortion (and even more so hum and
> >noise) easily by a factor of 10.
>
> >As for how low you need distortion to be one rule of thumb (I forget
> >the reference) is to be clearly audible the message must be 20db above
> >the background noise and to be inaudible distortion has to be 20db
> >below the background noise - which pretty much sets "low" distortion
> >for PA and similar uses at 1% or 10000 ppm. For HiFi the "message" has
> >a high dynamic range and you (allegedly) want a distortion figure at
> >least 20db below that. So a 60 db signal range 0.0001% (or 100PPM).
> >The you start getting into all kinds of trouble with power output /
> >dynamic range of the amp etc and you relies that it is all a
> >compromise anyway. You do the best you can within the restrictions of
> >the job description.
>
> Understood.
>
> >> But speaking from ignorance, I'm good shooting for the range
> >> you mentioned. It was about what I had in mind, in fact,
> >> figuring I could always learn as I go.
>
> >> >>> The first step is to think about the output. The basic equations are
>
> >> >>> (1).....Vout = sqrt(2*P*R)
>
> >> >>> With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
> >> >>> actually +/- 12.7 volts with a split power supply.
>
> >> >> If you don't mind, I'd like to discuss this more closely. Not
> >> >> just have it tossed out. So, P=V*I; or P=Vrms^2/R with AC.
> >> >> Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
> >> >> equation. Which suggests the +/-12.7V swing. Which further
> >> >> suggests, taking Vce drops and any small amounts emitter
> >> >> resistor drops into account, something along the lines of +/-
> >> >> 14-15V rails?
>
> >> >> Or should the rails be cut a lot closer to the edge here to
> >> >> improve efficiency. What bothers me is saturation as Vce on
> >> >> the final output BJTs goes well below 1V each and beta goes
> >> >> away, as well, rapidly soaking up remaining drive compliance.
>
> >> >>> (2).....Imax = sqrt(2*P/R)
>
> >> >>> This comes out to 1.6 amps. You should probably also consider the case
> >> >>> when R speaker = 4 ohms when initially selecting a transistor for the
> >> >>> output 2.2 amps - remember this is max output current. The power supply
> >> >>> voltage will have to be somewhat higher than Vout to take into account
> >> >>> circuit drive requirements, ripple on the power supply and transformer
> >> >>> regulation etc.
>
> >> >> Okay. I missed reading this when writing the above. Rather
> >> >> than correct myself, I'll leave my thinking in place.
>
> >> >> So yes, the rails will need to be a bit higher. Agreed. On
> >> >> this subject, I'm curious about the need to _isolate_, just a
> >> >> little, the rails used by the input stage vs the output stage
> >> >> rails. I'm thinking an RC (or LC for another pole?) for
> >> >> isolation. But I honestly don't know if that's helpful, or
> >> >> not.
>
> >> >Mostly not needed, if you use a long tailed pair for the input / error
> >> >amplifier, but you might prefer some other arrangement so keep it in
> >> >mind if your circuit "motorboats"
>
> >> Okay. I've _zero_ experience for audio. It just crossed my
> >> mind from other cases. I isolate the analog supply from the
> >> digital -- sometimes with as many as four caps and three
> >> inductor beads. There, it _does_ help.
>
> >> >>> Are you OK with connecting mains to a transformer? or would you rather
> >> >>> use an AC plug pack (10 watts is about the biggest amp a plugpack can be
> >> >>> used for)? The "cost" for using an AC plug pack is you will need larger
> >> >>> filter capacitors.
>
> >> >> I'd much prefer to __avoid__ using someone else's "pack" for
> >> >> the supply. All discrete parts should be on the table, so to
> >> >> speak, in plain view. And I don't imagine _any_ conceptual
> >> >> difficulties for this portion of the design. I'm reasonably
> >> >> familiar with transformers, rectifiers, ripple calculations,
> >> >> and how to consider peak charging currents vs averge load
> >> >> currents as they relate to the phase angles available for
> >> >> charging the caps. So on this part, I may need less help
> >> >> than elsewhere. In other words, I'm somewhat comfortable
> >> >> here.
>
> >> >Ah, then there are questions of what voltage and VA for a transformer.
> >> >So there are questions of usage (music, PA, PA with an emergency alert
> >> >siren tied in etc) and rectifier arrangement and capacitor size /
> >> >voltage to get your required voltage output at full load.
>
> >> I figure on working out the design of the amplifier and then
> >> going back, once that is determined and hashed out, with the
> >> actual required figures for the power supply and design that
> >> part as the near-end of the process. Earlier on, I'd expect
> >> to have some rough idea of how "bad" it needs to be -- if the
> >> initial guesses don't raise alarms, then I wouldn't dig into
> >> the power supply design until later on. The amplifier, it
> >> seems to me, dictates the parameters. So that comes later,
> >> doesn't it?
>
> >Yes and No. All the published circuits are made by people who want to
> >sell transistors,
>
> A concern I care not the least about. My _real_ preference,
> were I to impose it on the design, would be to use ONLY
> PN2222A BJTs for all the active devices. One part. That's
> it. Why? Because I've got thousands of them. ;)
>
> Literally. Something like 22,000 of the bastards. I give
> them away like popcorn to students at schools. Got them
> _very cheaply_. So if I were pushing something, I'd be
> pushing a 10W PN2222A design, use signal splitting approach
> probably (because it's the only way I think think of, right
> now), and distribute the dissipation across lots and lots of
> the things.
>
> What to go there? :)
Signal Splitting? Can you sketch out what your thinking?
The wikipedia type circuit can use a few n2222 - I count a max of 5.
Even if you could use only n2222 it would not be a good idea - making
the circuit stable would be more difficult. On the good side the n2222
is a good choice for Q1,Q2 and as active replacements for R5,R6 and
one other (optional) we haven't met yet. What makes it a good
transistor is the large current gain / bandwidth product and the flat
DC current gain over a wide range of viable bias currents. Both
contribute to low distortion.
http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF (page 3
graph)
compared to say 2n3904
http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF
where the flat portion of the DC gain curve is over a very limited
range.
>
> >not audio systems, power supplies or transformers.
>
> Got it.
>
> >As a result the power supply is often assumed to be regulated, which
> >is not true in this case, or the power supply is treated in a very
> >perfunctory manner that is not at all compatible with good design.
>
> >In this case you have the voltage you need for the 10 watts, plus
> >voltage drop for the driver circuitry and output stage , plus ripple
> >voltage, plus whatever is required for transformer regulation and
> >mains regulation. When you add it all up you might find that a chosen
> >transistor/component is actually not at all suitable for the job. Back
> >to the drawing board. Change this change that recheck everything again
> >etc.
>
> In this case, though, there is nothing particularly
> remarkable about the rails. Taken across the entire span,
> even, doesn't exceed the maximum Vce of a great many BJTs. So
> no real worry there. But I see some of where problems may
> arise. Luckily, at this level I can side-step worrying about
> that part and get back to learning about amplifier design,
> yes?
I come up with a figure of 50 volts rail to rail no load voltage -
after picking out a common transformer with 15% regulation.
>
> >If you do the power supply first you have the figures needed for your
> >worst case already. It saves time and makes a better result (no
> >tendency to comprimise to save all the calculations already done).
>
> Well, does this mean we should hack out the power supply
> first? I'm perfectly fine with that and can get back to you
> with a suggested circuit and parts list if you want to start
> there. We could settle that part before going anywhere else
> and I'd be happy with that approach, too, because to be
> honest I don't imagine it to put a horrible delay into
> getting back to amplifier design. So I'm good either way.
I'm looking at some of your other posts and I don't think you need a
maths lesson from me. If you want to do a power supply great. Its a
small one so nothing much too it. If you don't want, I'm OK too.
>
> >> >>> I should also ask if you have a multi meter, oscilloscope (not necessary
> >> >>> but useful)and how is your soldering? But it would be wise to keep this
> >> >>> whole thing as a paper exercise before you commit to anything.
>
> >> >> I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
> >> >> handheld, two oscilloscopes (TEK 2245 with voltmeter option
> >> >> and an HP 54645D), three triple-output power supplies with
> >> >> two of them GPIB drivable, the usual not-too-expensive signal
> >> >> generator, and a fair bunch of other stuff on the shelves.
> >> >> Lots of probes, clips, and so on. For soldering, I'm limited
> >> >> to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
> >> >> somewhat wider spade tips in the 1.5mm area. I have tubs and
> >> >> jars of various types of fluxes, as well, and wire wrap tools
> >> >> and wire wrap wire, as well. I also have a room set aside
> >> >> for this kind of stuff, when I get time to play.
>
> >> >OK. Next serious project, I'm coming around to your place!
>
> >> You come to the west coast of the US and I'll have a room for
> >> you!
>
> >> >Your gear is
> >> >better than mine. I had to ask, rather than just assume just in case my
> >> >assumptions got you building something you didn't want to, and got you
> >> >splattered all over the place from the mains, or suggesting you choose
> >> >the miller cap by watching the phase shift of the feedback circuit - I
> >> >don't read a lot of the posts so I didn't know what you could do.
>
> >> To be honest, I can do a few things but I'm really not very
> >> practiced. My oscilloscope knowledge is lacking in some
> >> areas -- which becomes all too painfully obvious to me when I
> >> watch a pro using my equipment. And I'm still learning to
> >> solder better. It's one of a few hobbies.
>
> >> >> Jon
>
> >> >Have a look at
> >> >http://en.wikipedia.org/wiki/Electronic_amplifier
>
> >> Done.
>
> >> >The bits on class A might be interesting as it says 25% efficiency and
> >> >50% obtainable with inductive output coupling (i.e. with a transformer)
> >> >which is what I said, not what blow hard Phil said.
>
> >> What I first see there is the amplifier sketch at the top of
> >> the page
>
> >I wasn't going to prompt, but it is close to the sort of thing, I
> >think, you should be aiming for . As someone has already noted (I
> >would attribute you if I wasn't on GG, I'm sorry) it has been drawn up
> >for a single supply, rather than a more common (for this size /
> >configuration) split supply.
>
> I had assumed we'd be using a split supply.
I think that's very much the preferred way.
>
> I had assumed a speaker would be hooked up via a cap to the
> output, so DC currents into a speaker coil would be removed
> from any concern. But I was also holding in the back of my
> mind the idea of tweaking out DC bias via the speaker and
> removing the coupling cap as an experiment to try. And if
> so, I'd pretty much want the ground as a "third rail."
Exactly right! There are two common ways to reduce/remove any offset
from the output. Neither is shown on the wikipedia circuit. If you
have another split rail circuit it will certainly have one method -
both methods involved use the diff amp.
> (Playing just a bit upon the Chicago parlance about the once
> dangerous rail in their transit system.)
>
> >(I don't really care too much about arguing about
> >> efficiencies right now -- I'm more concerned about learning.)
> >> The input stage shown is a voltage-in, current-out bog
> >> standard diff-pair. First thing I remember about is that R4
> >> shouldn't be there
>
> >Correct. Theory says it does nothing. I practice the theory but have
> >the occasional heretical belief about that.
>
> Actually, I think I've read that theory says it is _better_
> to be removed. The reason seemed pretty basic, as it's
> easier to get close to a balanced current split; and this, I
> gather, lowers 2nd harmonic distortions produced in the pair
> -- notable more on the high frequency end I suppose because
> gain used for linearizing feedback up there is diminishing
> and can't compensate it.
>
> In other words, it's not neutral. It's considered to be
> better if I gathered the details. Then even better, the
> current mirror enforces the whole deal and you've got about
> the best to be had.
>
> Of course, mostly just being a reader means I have no idea
> which end is up. So I might have all this wrong.
No. Thats all correct. I'll show a different circuit latter
>
> > and better still both R3 and R4 should be
> >> replaced with a current mirror.
>
> >This would provide more differential gain.
>
> _and_ improve distortion because the currents are forced to
> be balanced in the pair, yes?
yes.
>
> >>R5 should be a replaced with
> >> a BJT, as well.
>
> >In the right configuration it would reduce the common mode signal gain
> >of things like mains hum and supply ripple (you mentioned power supply
> >isolation before).
>
> Yes, that's how I thought about it.
>
> >Also, from another (what do you call it branch? thread?) you were
> >discussing boot-strapping R6. This is not done so much as amplifiers
> >get bigger but a BJT configured in the same way as the replacement for
> >R5 is very common. I'm leaving the details to you - perhaps there is a
> >way to reduce component count without affecting performance. (I am
> >hoping this is what you wanted "nutting it out for yourself")
>
> Yes! I don't want things handed on a platter. But I also
> don't want to have to rediscover all of the ideas by making
> all of the mistakes, either. This is the kind of "pointer"
> towards something that I like a lot. It gives me a place to
> think about something, but leaves me some reason to have to
> do so and that helps me own it better.
>
> One general truth about learning is that you don't present
> someone with a problem so out of their depth that they have
> no chance at it. Doing that means they fail, they feel like
> a failure, and it causes a student to just want to go away.
> They lose motivation, usually, in cases like that. On the
> other hand, providing no difficulty at all merely means
> repetition of what they already know and they grow bored from
> that, too. Finding the sweet spot where a student is faced
> with interesting problems that are not already known, but
> perhaps within reach of grasping at with some effort, is the
> key. Then it can be fun, educational, and motivate.
>
> That's what you just did for me.
>
> >I assume the input impedance of that example
> >> is basically the parallel resistance of R1 and R2, but if we
> >Yes.
>
> Okay.
There is the parallel resistance of R5 x Beta Q1 as well, but this is
normally so high it won't affect the result. And if R5 is replaced
with an active device it can become essentially infinite.
>
> >> use split supplies I'd imagine replacing the two of them with
> >> a single resistor to the center-ground point.
> >Yes, but you should probably think of a whole passive network to
> >filter out low and high frequency - (think what happens if you amp is
> >operated near a source of RF)
>
> Well, every trace picks up like little antennae. All kinds
> of trace voltages appearing here and there. Not good.
>
> So. Can you make an audio amplifier that can withstand a
> microwave oven environment and deliver good performance while
> irradiated with 1kW banging around in there? ;)
If you can do that the military wants you to EMP harden all there
electronics. The input is a little different because some user always
want to stick a bloody gret big long wire onto it.
>
> >> There's no
> >> miller cap on Q3,
>
> >Depending on transistors layout etc it might not be needed, but more
> >often it is the size that is the question.
>
> I was thinking it helped locally linearize the VAS section
> and that such would be "good" most anywhere. But I am just
> taking things without having worked through them on my own.
> So...
It sets the bandwidth of the VAS stage so you can use negative
feedback without the whole thing turning into smoke. Do you know of
control theory / bode diagrams. There is a minuscule amount needed
for this app.
>
> >> I'd probably replace the two diodes with
> >> one of those BJT and a few resistor constructions I can't
> >> remember the name of (which allows me to adjust the drop.)
>
> >Vbe multiplier...
>
> Okay. Thanks.
>
> >> The feedback ... well, I need to think about that a little
> >> more. There's no degen resistors in the emitters of Q4 and
> >> Q5.
>
> >Why would/should you use them?
>
> I'm still thinking about that. In general, I was thinking
> about them because of the "little re" that is kT/q based in
> each BJT, and varies on Ie. Since Ie is varying around, I
> was thinking about something fixed there to overwhelm it and
> "make it knowable" for the design, I suppose. Maybe that's
> all wet, given your query. I'll toss the idea off the side,
> for now.
>
Try working through the thermal stabilization. Just make a stab at the
transistor junction temperatures - it will be pretty hot (unless you
can afford mega bucks for heatsinking)
> >> Um.. okay, I need to sit down and think. Mind is spinning,
> >> but I've not set a finger to paper yet and there is lots to
> >> think about in that one. I could be way, way off base.
>
> >Not at all.
>
> Thanks for that. I'm just glad to be able to talk to someone
> about any of this, at all. So please accept my thanks for
> the moments you are offering.
>
> >Is there a way you could post a schematic of where your thinking is
> >and what you would like to discuss - there is no need for a complete
> >circuit.
>
> Yes. I can use ASCII here, for example. But before I go off
> into the wild blue with this, do you want to focus on the
> power supply first? Or just jump in on the amplifier?
I don't mind. Earlier I put a stab at a no load worst case voltage,
you can use that if you want to. Until you get to output stage power
dissipation that is all you need.
>
> Jon
George Herold
Yup, I'm often making audio frequency stuff, but driving things other
than speakers. One difference is that we tend to care about DC.
"They sell for a lot more, too."
I was a little surprised when I scrolled down on my link and checked
the price. I think this started out selling for about $200. It's
probablly got about $100 worth of stuff in it so $300+ is a better
price.
George H.
Jon Kirwan
<eat...@tpg.com.au> wrote:
>On Jan 30, 9:06�am, Jon Kirwan <j...@infinitefactors.org> wrote:
>> On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather
><snip>
>
>> >Yes and No. All the published circuits are made by people who want to
>> >sell transistors,
>>
>> A concern I care not the least about. �My _real_ preference,
>> were I to impose it on the design, would be to use ONLY
>> PN2222A BJTs for all the active devices. �One part. �That's
>> it. �Why? �Because I've got thousands of them. �;)
>>
>> Literally. �Something like 22,000 of the bastards. �I give
>> them away like popcorn to students at schools. �Got them
>> _very cheaply_. �So if I were pushing something, I'd be
>> pushing a 10W PN2222A design, use signal splitting approach
>> probably (because it's the only way I think think of, right
>> now), and distribute the dissipation across lots and lots of
>> the things.
>>
>> What to go there? �:)
>
>Signal Splitting? Can you sketch out what your thinking?
Yeah, I think so. Something like this:
>: | |
>: \ |
>: / R2 |
>: \ |
>: / |
>: | |
>: | |/c Q2
>: +---------|
>: | |>e
>: | |
>: |/c Q3 |
>: -------| +-----
>: |>e |
>: | |
>: | |/c Q1
>: +---------|
>: | |>e
>: | |
>: \ |
>: / R1 |
>: \ |
>: / |
>: | |
The "signal splitter" here is Q3. It's also providing gain,
too, though. The emitter and collector move in opposite
directions and the signal "splits" at Q3. (The emitter
follows the base, the collector inverts the base.)
If I read with any understanding about these things, properly
biasing Q3 is a pain, the Q3 gain varies with the load itself
as well as its bias, and compensation issues are complicated
a bit.
>The wikipedia type circuit can use a few n2222 - I count a max of 5.
>Even if you could use only n2222 it would not be a good idea - making
>the circuit stable would be more difficult.
Yes, ignorant as I am still of the details, I think that's
very true. The splitter has significant signal voltage on
its input and I've read that pole-splitting methods for
improving stability are harder to apply here.
>On the good side the n2222
>is a good choice for Q1,Q2 and as active replacements for R5,R6 and
>one other (optional) we haven't met yet. What makes it a good
>transistor is the large current gain / bandwidth product and the flat
>DC current gain over a wide range of viable bias currents. Both
>contribute to low distortion.
>
>http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF (page 3
>graph)
>
>compared to say 2n3904
>http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF
>
>where the flat portion of the DC gain curve is over a very limited
>range.
Interesting point to consider. Something that had slipped by
me, so far.
>> >not audio systems, power supplies or transformers.
>>
>> Got it.
>>
>> >As a result the power supply is often assumed to be regulated, which
>> >is not true in this case, or the power supply is treated in a very
>> >perfunctory manner that is not at all compatible with good design.
>>
>> >In this case you have the voltage you need for the 10 watts, plus
>> >voltage drop for the driver circuitry and output stage , plus ripple
>> >voltage, plus whatever is required for transformer regulation and
>> >mains regulation. When you add it all up you might find that a chosen
>> >transistor/component is actually not at all suitable for the job. Back
>> >to the drawing board. Change this change that recheck everything again
>> >etc.
>>
>> In this case, though, there is nothing particularly
>> remarkable about the rails. �Taken across the entire span,
>> even, doesn't exceed the maximum Vce of a great many BJTs. So
>> no real worry there. �But I see some of where problems may
>> arise. �Luckily, at this level I can side-step worrying about
>> that part and get back to learning about amplifier design,
>> yes?
>
>I come up with a figure of 50 volts rail to rail no load voltage -
>after picking out a common transformer with 15% regulation.
Okay. This is going to force me to sit down with paper and
work through. I was stupidly imagining +/-18V max, or 36V
rail to rail. I haven't considered the details of the output
section yet, driving a load from rails that run up and down
on capacitors that charge and discharge at 1A-level currents
into the load, and perhaps I need to spend some more time
there before moving on.
There are so many ways to cut this. Start at the input and
that's one focus that may work okay. Start at the output
stage and that provides important power supply information,
though. So maybe I should start at that end?
>> >If you do the power supply first you have the figures needed for your
>> >worst case already. It saves time and makes a better result (no
>> >tendency to comprimise to save all the calculations already done).
>>
>> Well, does this mean we should hack out the power supply
>> first? �I'm perfectly fine with that and can get back to you
>> with a suggested circuit and parts list if you want to start
>> there. �We could settle that part before going anywhere else
>> and I'd be happy with that approach, too, because to be
>> honest I don't imagine it to put a horrible delay into
>> getting back to amplifier design. �So I'm good either way.
>
>I'm looking at some of your other posts and I don't think you need a
>maths lesson from me. If you want to do a power supply great. Its a
>small one so nothing much too it. If you don't want, I'm OK too.
I still haven't been down the path on my own, yet. So I
don't have strong opinions about this. It's like going to
Disneyland for the first time. Which land should I go to,
first? Later, after being there a few times, I may look at
the flow of people and decide that "Adventureland" is the
best first start. But first time out? Who knows? I'm open
to guidance. Everything is new.
I feel more comfortable assuming it, too.
>> I had assumed a speaker would be hooked up via a cap to the
>> output, so DC currents into a speaker coil would be removed
>> from any concern. �But I was also holding in the back of my
>> mind the idea of tweaking out DC bias via the speaker and
>> removing the coupling cap as an experiment to try. �And if
>> so, I'd pretty much want the ground as a "third rail."
>
>Exactly right! There are two common ways to reduce/remove any offset
>from the output. Neither is shown on the wikipedia circuit. If you
>have another split rail circuit it will certainly have one method -
>both methods involved use the diff amp.
Thanks.
>> (Playing just a bit upon the Chicago parlance about the once
>> dangerous rail in their transit system.)
>>
>> >(I don't really care too much about arguing about
>> >> efficiencies right now -- I'm more concerned about learning.)
>> >> The input stage shown is a voltage-in, current-out bog
>> >> standard diff-pair. �First thing I remember about is that R4
>> >> shouldn't be there
>>
>> >Correct. Theory says it does nothing. I practice the theory but have
>> >the occasional heretical belief about that.
>>
>> Actually, I think I've read that theory says it is _better_
>> to be removed. �The reason seemed pretty basic, as it's
>> easier to get close to a balanced current split; and this, I
>> gather, lowers 2nd harmonic distortions produced in the pair
>> -- notable more on the high frequency end I suppose because
>> gain used for linearizing feedback up there is diminishing
>> and can't compensate it.
>>
>> In other words, it's not neutral. �It's considered to be
>> better if I gathered the details. �Then even better, the
>> current mirror enforces the whole deal and you've got about
>> the best to be had.
>>
>> Of course, mostly just being a reader means I have no idea
>> which end is up. �So I might have all this wrong.
>
>No. Thats all correct. I'll show a different circuit latter
Okay. I'll enjoy the moment when it happens.
>> > and better still both R3 and R4 should be
>> >> replaced with a current mirror. �
>>
>> >This would provide more differential gain.
>>
>> _and_ improve distortion because the currents are forced to
>> be balanced in the pair, yes?
>
>yes.
Okay. So I am picking up details not too poorly, so far.
Okay. I've got that detail from other discussions, too. So
yes, understood. Also, I mentioned replacing R5, I think. In
replacing R5 with active parts, I'm thinking of two BJTs in a
usual form that seems to work pretty well over supply
variations.
>> >> use split supplies I'd imagine replacing the two of them with
>> >> a single resistor to the center-ground point. �
>> >Yes, but you should probably think of a whole passive network to
>> >filter out low and high frequency - (think what happens if you amp is
>> >operated near a source of RF)
>>
>> Well, every trace picks up like little antennae. �All kinds
>> of trace voltages appearing here and there. �Not good.
>>
>> So. �Can you make an audio amplifier that can withstand a
>> microwave oven environment and deliver good performance while
>> irradiated with 1kW banging around in there? �;)
>
>If you can do that the military wants you to EMP harden all there
>electronics. The input is a little different because some user always
>want to stick a bloody gret big long wire onto it.
:)
I actually _do_ work on low-mass, direct-contact temperature
measuring devices designed to work within a microwave
environment. (But no electronics or metals inside.)
But you brought up the microwave environment, so I hope you
don't mind the teasing about it.
>> >> There's no
>> >> miller cap on Q3,
>>
>> >Depending on transistors layout etc it might not be needed, but more
>> >often it is the size that is the question.
>>
>> I was thinking it helped locally linearize the VAS section
>> and that such would be "good" most anywhere. �But I am just
>> taking things without having worked through them on my own.
>> So...
>
>It sets the bandwidth of the VAS stage so you can use negative
>feedback without the whole thing turning into smoke. Do you know of
>control theory / bode diagrams. There is a minuscule amount needed
>for this app.
I am familiar with _some_ closed loop control theory,
sufficient to get me by with PID controls (using _and_
writing code for them.) Bode diagrams are something I have
not used, though I've seen them. My math is adequate, I
suspect. But I will have to read up on them, I suppose.
For Laplace analysis, I'm familiar with complex numbers,
poles and zeros, partial fraction extractions, and so on.
Just inexperienced in the "short cuts" that many use to get
(and think about) answers.
>> >> I'd probably replace the two diodes with
>> >> one of those BJT and a few resistor constructions I can't
>> >> remember the name of (which allows me to adjust the drop.)
>>
>> >Vbe multiplier...
>>
>> Okay. �Thanks.
>>
>> >> The feedback ... well, I need to think about that a little
>> >> more. �There's no degen resistors in the emitters of Q4 and
>> >> Q5.
>>
>> >Why would/should you use them?
>>
>> I'm still thinking about that. �In general, I was thinking
>> about them because of the "little re" that is kT/q based in
>> each BJT, and varies on Ie. �Since Ie is varying around, I
>> was thinking about something fixed there to overwhelm it and
>> "make it knowable" for the design, I suppose. �Maybe that's
>> all wet, given your query. I'll toss the idea off the side,
>> for now.
>
>Try working through the thermal stabilization. Just make a stab at the
>transistor junction temperatures - it will be pretty hot (unless you
>can afford mega bucks for heatsinking)
I need to understand the output configuration a little better
before I do that.
Including thinking more closely about swinging one end of an
output cap around so that 1Amp rms can pass through it at
20Hz. I = C dv/dt, but V=V0*sin(w*t), so I=C*w*V0*cos(w*t).
Assuming max current at the max slew rate for a sine at phase
angle zero, the w*t is some 2*PI*N thing, so cos(w*t) goes to
1. That makes I=w*C*V0. But w=2*pi*20, or about 126 or so.
So I=126*C*V0. So with I=1A, C=1/(126*V0). With V0=15V, I
get about 530uF for the output cap. That's an amp peak only
at the right phase, too. It'll be less elsewhere. To make
that an amp rms, the cap would need to be still bigger.
Peak current via the cap will take place right about the time
when the two BJTs's emitters are at their midpoint. One of
the BJTs will be supplying that. Not only that, but also
depending upon class mode of operation, supplying current to
the other one as well. How much is important to figuring out
the wattage.
I need to sit down with paper, I suspect. But if you want to
provide some suggested thinking process here, I'd also be
very open to that, as well. I'll take a shot at it either
way, but it helps to see your thinking, too. If you can
afford the moment for me.
>> >> Um.. okay, I need to sit down and think. �Mind is spinning,
>> >> but I've not set a finger to paper yet and there is lots to
>> >> think about in that one. �I could be way, way off base.
>>
>> >Not at all.
>>
>> Thanks for that. �I'm just glad to be able to talk to someone
>> about any of this, at all. �So please accept my thanks for
>> the moments you are offering.
>>
>> >Is there a way you could post a schematic of where your thinking is
>> >and what you would like to discuss - there is no need for a complete
>> >circuit.
>>
>> Yes. �I can use ASCII here, for example. �But before I go off
>> into the wild blue with this, do you want to focus on the
>> power supply first? �Or just jump in on the amplifier?
>
>I don't mind. Earlier I put a stab at a no load worst case voltage,
>you can use that if you want to. Until you get to output stage power
>dissipation that is all you need.
Maybe I'd like to focus on understanding different output
pair configurations, first. I frankly don't like the "haul
the output pair around with a collector on one side and a
resistor on the other with a rubber diode in between to keep
them biased up" approach. It's smacks of heavy-handedness
and I simply don't like the way it looks to me. Everything
tells me this works, but it is indelicate at the very least.
However, it is crucial that I understand it in detail before
deciding what I really think about it. For example, I might
want to replace the resistor with a current source. But
without apprehending the output stage more fully, its time
domain behavior over a single cycle for example, I'm not
comfortable with hacking it here and there, ignorantly.
Jon
Jon Kirwan
><snip>
>I like the biasing scheme mentioned by Jon and use it for all my
>designs except the early ones using germanium transistors, though
>I don't know the name either. The biasing transistor can be
>mounted on the output transistors' heatsink for temperature
>tracking.
><snip>
Okay. I'm giving this a little more thought -- as it applies
to temperature variation. The basic idea is that the two
bases of the two output BJTs (or output BJT structures) must
be separated a little bit in order to ensure both quadrants
are in forward conduction. With a "Vbe multiplier" in place
and with its own BJT tacked onto the same heat sink, the idea
is that the the Vbe multiplier's own voltage separation will
shrink as temperature rises, exactly in some proportion
needed to maintain the designed forward conduction
relationship of the output BJTs.
To be honest, this designed forward conduction mode may not
be critital. It might move a class-AB around a little within
its AB operation, for example, if the voltage tracking with
temperature weren't flawlessly applied. And that may be
harmless. I don't know. On the other hand, if tweaked for
class-A I can imagine that it might move the operation into
class-AB; if tweaked for lower-dissipation class-AB it might
move the operation into class-B; and if class-B were desired
it could move it into class-C with associated distortion.
There are several parts of the basic Shockley equation. One
is the always-in-mind part that includes a kT/q part in it
and relates that to Vbe. The other is the Is part and Eg is
the key there. So one thing that crosses my mind is in
selecting the BJT for the "rubber diode" thingy. Unless it's
Vbe (at 27C and designed constant current) and its Eg are the
same, even though it is a small signal device, doesn't that
mean that the variations over temperature will be two lines
that cross over only at one temperature point? In other
words, basically matches nowhere except at one temperature?
It seems crude.
I've seen this as a modification. In ASCII form:
>: A
>: |
>: ,---+---,
>: | |
>: | \
>: | / R3
>: \ \
>: / R2 /
>: \ |
>: / +--- C
>: | |
>: | |
>: | |/c Q1
>: +-----|
>: | |>e
>: \ |
>: / R1 |
>: \ |
>: / |
>: | |
>: '---+---'
>: |
>: B
We've already decided that R1 might be both a simple resistor
plus a variable pot to allow adjustment. The usual case I
see on the web does NOT include R3, though. However, I've
seen a few examples where R3 (small-valued) exists and one of
the two output BJTs' base is connected at C and not at A.
The above circuit is a somewhat different version of the Vbe
multiplier/rubber diode thing. The difference being R3,
which I'm still grappling with.
But does anyone know, before I go writing equations all over
the place, why R3 is added? Or is R3 just some book author's
wild ass guess?
This is all pressing me into studying the output structure
more, I guess. It basically looks simple when I wave my
hands over it, but I suspect the intimate details need to be
exposed to view. On to that part, I suppose.
Jon
pimpom
You lost me for a while with the Eg term. You mean the emitter
transconductance?
Perhaps a short diversion into my own background may be
appropriate here. Shortage of funds and scarcity of good books
even for those who could afford them in a technologically
primitive environment kept me from delving deeply into
semiconductor physics when I started teaching myself electronics
over 40 years ago. I had advanced Math in college, but lack of
practice has made me very rusty. You're probably much better at
that.
Over the years, I developed my own shortcuts and approximations
using mostly basic algebra, trigonometry and bits of calculus
here and there, blended with empirical formulas.
In any case, the Shockley equation seems to hold fairly well in
practice for the purpose of bias regulation within the
temperature range normally encountered. Temperature tracking with
simple circuits like diodes in series or a Vbe multiplier cannot
be more than approximate. Such a device can sense only the
heatsink temperature and,.except under long-term static
conditions, that temp will almost always be different from Tj of
the output devices. That Tj is what needs to be tracked and when
the output transistors are pumping out audio power, that
difference can be tens of degrees.
> I've seen this as a modification. In ASCII form:
>
>>> A
>>> |
>>> ,---+---,
>>> | |
>>> | \
>>> | / R3
>>> \ \
>>> / R2 /
>>> \ |
>>> / +--- C
>>> | |
>>> | |
>>> | |/c Q1
>>> +-----|
>>> | |>e
>>> \ |
>>> / R1 |
>>> \ |
>>> / |
>>> | |
>>> '---+---'
>>> |
>>> B
>
> We've already decided that R1 might be both a simple resistor
> plus a variable pot to allow adjustment. The usual case I
> see on the web does NOT include R3, though. However, I've
> seen a few examples where R3 (small-valued) exists and one of
> the two output BJTs' base is connected at C and not at A.
>
> The above circuit is a somewhat different version of the Vbe
> multiplier/rubber diode thing. The difference being R3,
> which I'm still grappling with.
>
I've seen R3 used in that position too, but never gave it much
thought until you brought it up. Offhand I still can't see a
reason for it either. Maybe for stability against a local
oscillation? Perhaps taking some time to think about it will
bring some revelation. Or someone else can save us the trouble
and enlighten us.
> But does anyone know, before I go writing equations all over
> the place, why R3 is added? Or is R3 just some book author's
> wild ass guess?
>
A possibility. But I wouldn't go out on a limb and call it that
:-)
Jon Kirwan
No. Eg is the effective energy gap, specified in electron
volts. Eg (and Tnom, which is the nominal temperature at
which the Is used in the Shockley equation is given at) are
used to account for and calibrate the variation of Is over
the BJT's temperature. In other words, Is is a function of
T, namely Is(T), and not a constant at all.
If you solve the Shockley equation for Vbe and then look at
the derivative (partial, since Is is momentarily taken as a
constant) of it with respect to temperature, you will see
that it varies in the _wrong_ direction... the sign is wrong:
Id(T) = Is(T) * ( e^( q*Vd / (k*T) ) - 1 )
which becomes:
Vd(T) = (k*T/q) * ln( 1 + Ic/Is(T) )
The derivative is then trivially:
d Vd(T) = (k/q) * ln( 1 + Ic/Is(T) ) dT
which is a positive trend, very nearly +2mV/K for modest
Ic... but __positive__.
Does that make sense? It just is wrong. BJTs don't _do_
that. The figure is more like -2mV/K. So why is the sign
wrong?
Because that isn't the whole picture. "Is" also varies with
temp. As in:
Is(T) = Is(Tn) * (T/Tn)^3 * e^( -(q*Eg/k) * (1/T-1/Tn) )
where "Tn" is the nominal calibration temperature point.
The new derivative is a bit large. To get it onto a silly
post page with some chance that it won't sprawl for lines and
lines, I have to set up these math phrases.
Assume:
X = T^3 * Isat * e^(q*Eg/(k*Tnom))
Y = Tnom^3 * Ic * e^(q*Eg/(k*T))
Then the derivative is (if you use fixed-spaced ASCII):
X+Y
k*Tn*T*((X+Y)*ln( -------- )-3*Y) - q*Eg*(X*T+Y*T+Y*Tn)
Isat*T^3
-------------------------------------------------------
q * Tnom * T * (X+Y)
What a mess, even then. Here again, Tn is the nominal
temperature (in Kelvin, of course) at which the device data
is taken and Eg is the effective energy gap in electron
volts for the semiconductor material. Of course, 'k' is the
usual Boltzmann's constant, q the usual electron charge
value, and T is the temperature of interest.
Eg often defaults to around 1.11eV in spice, I think. For an
Ic=10uA and a stock Isat of about 1E-15, the figure comes out
to about -2.07mV/K in the vicinity of 20 Celsius ambient.
Which is the more usual value.
The "Is" term is the y-axis intercept, which isn't actually
measured, by the way, but instead extrapolated from measured
values elsewhere.
All this is the reason I was asking about the voltage bias
mechanism (that rubber diode/Vbe multiplier thing) and
selecting its BJT vs those in the output stage. (Which, if
PNP _and_ NPN are used, probably themselves do not vary the
same as either other, even, so there is another problem there
as well.) It fries my brain thinking about selecting
"perfect" parts for this.
Another issue I'm starting to wonder about is sweeping out
charges in the BJTs at higher frequencies and providing
sufficient drive current to do it quickly enough. But one
thing at a time, I guess.
>Perhaps a short diversion into my own background may be
>appropriate here. Shortage of funds and scarcity of good books
>even for those who could afford them in a technologically
>primitive environment kept me from delving deeply into
>semiconductor physics when I started teaching myself electronics
>over 40 years ago. I had advanced Math in college, but lack of
>practice has made me very rusty. You're probably much better at
>that.
Your own experiences sound very much like mine, except that
you _did_ something with yourself in this area when I did
not. Something I very much respect in you and disrespect in
me. I grew up poor enough that I had to literally live in
homes without walls and work the fields as a laborer child
(before laws today now prevent that, sadly in some ways good
in others) in order to eat and survive. So I understand
"shortage of funds" in my very gut. Perhaps what differed a
little is that I also was living near Portland and there was
a library system I could access, riding my bicycle as a kid.
And I would sometimes even take a bus and use the university
library (particularly the 5th floor where the science
subjects were located.) I scored an 800 on the math section
of my SAT and was rewarded with entry into a university
scholars program at PSU. However, I had to work to pay for
the classes and books and in the end I simply couldn't handle
all of it on my own. Without a dad (he died when I was 7)
and no family to help out, I couldn't manage to do everything
and get by at school, too. So I dropped out well before the
1st year completed. Everything I know is self-taught. It's
a commitment.
I have been honored by being asked by Portland State
University to temporarily teach as an adjunct professor,
though. And I did that for a few years until they could find
their replacement professors. I enjoyed it and I think I did
well. When I visited the department, last year after some
dozen years of absence, I was greeted in the hallways by many
others who I sometimes barely remembered with sincere smiles
and talks of those days. So I must have made some kind of
impression there.
Maybe a difference is that I've made the study of mathematics
a centerpiece for me. Besides, it's central to the work I do
so I can't really ignore it. But since I love studying it, I
would do it, anyway. None of this means I'm properly trained
in it. However, even these days I get to spend time almost
every month or two with sit-down time with an active
physicist to get some additional education in Lie Groups or
catastrophe theory or reflection spaces and manifolds, and so
on. I find I really love both finite and infinite group
theory work.
>Over the years, I developed my own shortcuts and approximations
>using mostly basic algebra, trigonometry and bits of calculus
>here and there, blended with empirical formulas.
And here, most likely, is our fundamental difference. I
cannot remember things without understanding their deep
details. I lump this to my "autism." (I have two disabled
children on the spectrum, the youngest is almost exactly like
I was at his age.) When I took calculus at college, it was
all a blur trying to remember what was called what and how
they applied. However, if I _understood_ it deeply, could
picture it well, I could re-derive almost anything on the
spot when I needed it for a test. In other words, while most
of the other students appeared to simply take notes and keep
track of details (and shortcuts) many of which they'd
remember, I couldn't work like that. My memory was _zero_
for names of people, and similarly for names of math
formulas. I had to understand them viscerally and "see" them
well, in order to be able to remember the concept. However,
I still couldn't rememeber the specific "formula." Just the
concept -- the visualization, the image. That wouldn't
provide me with an answer to a problem, merely an approach
that "seemed right." So I would simply use that image to
guide me in re-deriving the formula from scratch, every time.
The upshot was that I took longer than most in completing my
tests, because I spent so much additional time quickly
running through the derivations of the rules I needed, but
where I answered the problems I got them right.
I've never been satisfied, as a result of my own limitations
here, to memorize shortcuts and approximations. It doesn't
give me "sight." They are useless to me, since I cannot use
them for any other derivations, since they are themselves
only blunted tools for specific purposes that cannot be used
to extrapolate anywhere else. Which then forces me to depend
upon a memory I don't have. What I need is to _see_ the
physics itself so that I can then derive those approximations
and shortcuts on the fly, deduced to the specific situation
I'm facing at the time.
>In any case, the Shockley equation seems to hold fairly well in
>practice for the purpose of bias regulation within the
>temperature range normally encountered.
No, it doesn't. Because the SIGN is wrong!! The Vbe doesn't
rise with rising temperature, it falls.
>Temperature tracking with
>simple circuits like diodes in series or a Vbe multiplier cannot
>be more than approximate.
That seems to be what I'm getting. One "lucky" circumstance
seems to be that the Vbe multiplier is supposed to produce
about two Vbes with k=2 in k*Vbe, just when there are two
Vbe's in the output structure. That way, if the Vbe in the
multiplier moves around with temperature, the multiplier
doubles it in just the right way to handle the actual two
Vbes in the output pair. If it had been needed to set k=3 or
k=1.5 or k= anything else, there'd have been a problem again
because they wouldn't vary together.
But this brings up the other problem I am talking about. If
Eg isn't the same figure, the slope over temperature for the
Vbe multiplier and the output BJTs won't be the same slope.
That means they can intersect at some temperature, but never
really be right anywhere else at all.
Worse than this is the fact that PNPs are used on one side
and NPNs on the other. They _cannot_ possibly vary their
Vbes in matched ways. It's got to be a nasty problem. And
it seems to argue, in my mind, for some modified version of a
quasicomplementary structure on the output. What argues
against it so much is that, again, the driving structure
before the quasi structure is driving two kinds of quadrants
and this means the cross-over area _must_ be nasty looking,
indeed.
Because of that, I searched around and found out that there
is a correction structure to fix the quasi crossover problem.
It appears to use something called a Baxandall diode, though
for now I haven't learned the details of how it does what it
does.
>Such a device can sense only the
>heatsink temperature
But which quadrant do you decide to attach it closer to?
>and,.except under long-term static
>conditions, that temp will almost always be different from Tj of
>the output devices. That Tj is what needs to be tracked and when
>the output transistors are pumping out audio power, that
>difference can be tens of degrees.
I can believe it!
It is often a small value, 10s of Ohms. It might just be
what you are talking about because I've often seen people
talk about needing a 33 ohm base resistor on emitter follower
BJTs to snub high frequency oscillations. So you might be on
the right track there. Hopefully, someone else does know and
will feel like saying.
>> But does anyone know, before I go writing equations all over
>> the place, why R3 is added? Or is R3 just some book author's
>> wild ass guess?
>
>A possibility. But I wouldn't go out on a limb and call it that
>:-)
Hehe. :)
Jon
Jon Kirwan
<jo...@infinitefactors.org> wrote:
>Assume:
> X = T^3 * Isat * e^(q*Eg/(k*Tnom))
> Y = Tnom^3 * Ic * e^(q*Eg/(k*T))
Sorry, should be consistent in terms with:
X = T^3 * Isat * e^(q*Eg/(k*Tn))
Y = Tn^3 * Ic * e^(q*Eg/(k*T))
Jon
Jon Kirwan
>Because of that, I searched around and found out that there
>is a correction structure to fix the quasi crossover problem.
>It appears to use something called a Baxandall diode, though
>for now I haven't learned the details of how it does what it
>does.
Here's some articles I found on the web by a single author on
audio amplifier design:
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205207238&printable=true
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205601405&printable=true
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205801115&printable=true
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205917273&printable=true
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=206103226&printable=true
The last one of the above links __mentions__ the Baxandall
diode.
In looking at those, there is this one also listed at the
bottom of the last article above. I haven't read this one
yet, but include it just the same:
http://www.planetanalog.com/article/printableArticle.jhtml?articleID=205202120&printable=true
I need to read all of these, I suppose.
Jon
Paul E. Schoen
"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
news:eg7hm51jl8ogq5k6f...@4ax.com...
This stuff is a bit too intense for my taste, but a Dogpile search of
Baxandall Diode turned up this manual for an amplifier that uses tubes and
transistors, and has a Baxandall diode in the output stage. Its stated
purpose was to "improve symmetry" and appears to add an additional diode
drop for the PNP-NPN pair to match the NPN-NPN darlington on the top side.
http://www.wimdehaan.nl/downloads/technicalmanualnishiki41.pdf
This article states that a Baxandall diode made little change in linearity:
http://www.embedded.com/design/206801065?printable=true
There is also a Baxandall Tone Control circuit which is discussed here:
http://digitalcommons.calpoly.edu/eesp/14/
For my own purposes, distortion of anything less than about 1% is probably
not worth paying for or striving to achieve. For audio, my ears are not all
that good and I would probably welcome distortion in the form of non-linear
frequency response to compensate for degraded sensitivity at the high end.
And there is also the argument that any sound that is naturally produced
will have some significant distortion that is actually part of the
listener's experience. Whatever the acoustics in a given auditorium may be,
they contribute to the waveshape as it is received by the listener's ears,
and it varies depending on where one is seated. Sometimes added distortion,
such as an echo, may enhance the enjoyment of the music, and coloration due
to an imperfect amplifier might just as easily be perceived as pleasant
rather than objectionable. It is in fact distortion that causes an
audiophool to prefer the "warm" sound of a tube amplifier over a laboratory
grade solid state amplifier.
I am more impressed with amplifiers that are extremely efficient, such as
PWM amps. And for some types of test equipment that I have designed, I had
to deal with maintaining phase shift to better than 1 degree into a range
of inductive, resistive, or capacitive loads, and with outputs of power
line frequencies of 45-450 Hz, for voltage sources up to 300 VAC, and
current sources up to 100 amperes, at 50 VA to 300 VA or higher. And they
had to be able to withstand overloads and short circuits.
Paul
Jon Kirwan
It's gravy to me. If only I had the good sense to be able to
tell if it is being comprehensively and accurately stated.
>but a Dogpile search of
>Baxandall Diode turned up this manual for an amplifier that uses tubes and
>transistors, and has a Baxandall diode in the output stage. Its stated
>purpose was to "improve symmetry" and appears to add an additional diode
>drop for the PNP-NPN pair to match the NPN-NPN darlington on the top side.
>
>http://www.wimdehaan.nl/downloads/technicalmanualnishiki41.pdf
I think that very same purpose is what I took it to mean,
too. In a quasi-complementary output stage, the gain curve
(less than 1 everywhere) shown over output would have to show
an interesting tweak, midrange. In class-B especially, from
one side it would look one way, from the other, somewhat
different. Simply because the two quadrants just aren't the
same structure. One uses two NPNs, the other an NPN and a
PNP. That weirdness in gain has to translate to distortion
of some kind. The fix, I'd read, is to use a diode on the
complementary NPN/PNP side (and a resistor in parallel, I
gather.) Supposedly, it flatten out the gain curve in just
the right amount to balance things pretty well. It's an
interesting point, if true, because the quasi-complementary
output stage is attractive in that it can use the exact same
NPN part number for both quadrants' output BJTs.
I need to post up some different output structures in the
wane hope someone will help me walk through an analysis of
them.
>This article states that a Baxandall diode made little change in linearity:
>http://www.embedded.com/design/206801065?printable=true
Thanks for the article. Same author!! If I read closely
enough what he is saying, he is saying that the Baxandall
diode adds little _in the case of class-A operation_. In the
case of class-B, I think he argues it is worth having!
He writes,
"The choice of class A output topology is now simple.
For best performance, use the CFP. Apart from greater
basic linearity, the effects of output device
temperature on Iq are servoed out by local feedback,
as in class B. For utmost economy, use the quasi
complementary with two NPN devices: these need only a
low Vce(max) for a typical class A amp, so here is an
opportunity to recoup some of the money spent on
heatsinking.
"The rules are different from class B; the simple quasi
configuration will give first class results with
moderate NFB, and adding a Baxandall diode to simulate
a complementary emitter follower stage makes little
difference to linearity."
I think I represented the meaning of these two paragraphs,
accurately. And if you look closely at Figure 4, you will
see that there are two curves -- both are for quasi-
complementary outputs. However, one of them is class-B --
the really nasty-looking one. That one cries out for fixing
and is exactly what I was just talking about, above!!! So
the Baxandall diode really seems to be useful in allowing one
to _select_ class-B operation without having to pay much for
it. Makes class-B lots more attractive with quasi-
complementary outputs.
Let me know if you think otherwise.
>There is also a Baxandall Tone Control circuit which is discussed here:
>http://digitalcommons.calpoly.edu/eesp/14/
I'd need to download that thing to read it, I guess. For
now, I merely suspect that Baxandall writes about ideas from
time to time and isn't known for one thing.
>For my own purposes, distortion of anything less than about 1% is probably
>not worth paying for or striving to achieve. For audio, my ears are not all
>that good and I would probably welcome distortion in the form of non-linear
>frequency response to compensate for degraded sensitivity at the high end.
I don't have a number in mind because I'm _very_ ignorant
about what I'd care about and what I wouldn't. I _do_ know
one thing.... I really _hate_ the 10% THD computer speaker
systems. That much I do know. There is a place in hell for
people who pawn those things off as amplifiers with a nickel.
>And there is also the argument that any sound that is naturally produced
>will have some significant distortion that is actually part of the
>listener's experience.
hehe. I'm imagining Mister Magoo right now and what he'd
consider "good." ;) With Magoo as the "listener" ....
Alfred E Neuman's "What, me worry?" comes to mind regarding
any amplifier system.
Slightly more seriously, best of all would be that we somehow
analyze each and every person's brain's responses to sound,
in real time if possible, from the conscious interpretation
back through to the cochlea and the transducers nearby, to
the environment around it, and use a DSP to process the
content first before driving a speaker system, at all.
I expect to be dead before that happens, though.
I'm good ignoring "listener's experience" and focusing on a
more objective measure of some kind, letting the chips fall.
>Whatever the acoustics in a given auditorium may be,
>they contribute to the waveshape as it is received by the listener's ears,
>and it varies depending on where one is seated. Sometimes added distortion,
>such as an echo, may enhance the enjoyment of the music, and coloration due
>to an imperfect amplifier might just as easily be perceived as pleasant
>rather than objectionable. It is in fact distortion that causes an
>audiophool to prefer the "warm" sound of a tube amplifier over a laboratory
>grade solid state amplifier.
Those arguments are "beyond my pay grade." I'll just retreat
to something I can actually compute.
>I am more impressed with amplifiers that are extremely efficient, such as
>PWM amps. And for some types of test equipment that I have designed, I had
>to deal with maintaining phase shift to better than 1 degree into a range
>of inductive, resistive, or capacitive loads, and with outputs of power
>line frequencies of 45-450 Hz, for voltage sources up to 300 VAC, and
>current sources up to 100 amperes, at 50 VA to 300 VA or higher. And they
>had to be able to withstand overloads and short circuits.
I like efficiency as one goal. Especially as I'm getting to
understand just how much power can be wasted without much
value. This 10W thing, if we are talking about class-A and
planning for 6db overhead (4X) as someone I read in one of
those articles saying about it, might mean a 40W capability
into 8 ohms, rails that are way out there and power waste
that starts to look like a toaster. So I'm beginning to get
my head turned 'round even at 10W!! Cripes, that spec is
rapidly becoming something I'm beginning to respect a lot
more and to realize that I might have landed on a number that
is better for teaching than I'd first imagined it to be.
Jon
David Eather
A 30 volt CT transformer with 15% regulation and 7% mains over-voltage,
less voltage drop for the diode bride would give rails of +/- 25.
A dual 12.6 volt transformer would give a minimum (worst case with
transformer at full load and mains 7% under voltage) of 16.something
volts meaning big filter caps if you were serious at getting 10 watts.
One of the reasons to go PSU first I think. (Also I live in a tiny
jerk-water town where no one knows what a custom made transformer is let
alone where you can get one wound)
(Seriously, I think you are doing amazing)
Two BJT's make a current sink with a nice sharp knee giving best CMRR.
I'll point out a small "optimization" (price/component reduction but
with a small degradation in performance - given the over abundance of
2n2222 in your area it probably doesn't count). You could use a single
BJT current sink with a voltage reference (2 diodes, low voltage zenner
or LED) tied to the negative rail. You could then use the same reference
voltage for a 1 BJT current sink in place of R6. Saving a couple of
components.
>
>>>>> use split supplies I'd imagine replacing the two of them with
>>>>> a single resistor to the center-ground point.
>>>> Yes, but you should probably think of a whole passive network to
>>>> filter out low and high frequency - (think what happens if you amp is
>>>> operated near a source of RF)
>>> Well, every trace picks up like little antennae. All kinds
>>> of trace voltages appearing here and there. Not good.
>>>
>>> So. Can you make an audio amplifier that can withstand a
>>> microwave oven environment and deliver good performance while
>>> irradiated with 1kW banging around in there? ;)
>> If you can do that the military wants you to EMP harden all there
>> electronics. The input is a little different because some user always
>> want to stick a bloody gret big long wire onto it.
>
> :)
>
> I actually _do_ work on low-mass, direct-contact temperature
> measuring devices designed to work within a microwave
> environment. (But no electronics or metals inside.)
>
> But you brought up the microwave environment, so I hope you
> don't mind the teasing about it.
>
No. :)
>>>>> There's no
>>>>> miller cap on Q3,
>>>> Depending on transistors layout etc it might not be needed, but more
>>>> often it is the size that is the question.
>>> I was thinking it helped locally linearize the VAS section
>>> and that such would be "good" most anywhere. But I am just
>>> taking things without having worked through them on my own.
>>> So...
>> It sets the bandwidth of the VAS stage so you can use negative
>> feedback without the whole thing turning into smoke. Do you know of
>> control theory / bode diagrams. There is a minuscule amount needed
>> for this app.
>
> I am familiar with _some_ closed loop control theory,
> sufficient to get me by with PID controls (using _and_
> writing code for them.) Bode diagrams are something I have
> not used, though I've seen them. My math is adequate, I
> suspect. But I will have to read up on them, I suppose.
>
> For Laplace analysis, I'm familiar with complex numbers,
> poles and zeros, partial fraction extractions, and so on.
> Just inexperienced in the "short cuts" that many use to get
> (and think about) answers.
Bode diagrams are really simple and (for this type of thing) will get
you where you want to go.
>
>>>>> I'd probably replace the two diodes with
>>>>> one of those BJT and a few resistor constructions I can't
>>>>> remember the name of (which allows me to adjust the drop.)
>>>> Vbe multiplier...
>>> Okay. Thanks.
>>>
>>>>> The feedback ... well, I need to think about that a little
>>>>> more. There's no degen resistors in the emitters of Q4 and
>>>>> Q5.
>>>> Why would/should you use them?
Jon,
Just in case the question misled you - I was asking only about things
that needed changing. So I asked a question about the degen Rs on Q4 and
Q5, even though you would/should use them and for a few good of reasons
of which I think thermal stabilization is the more important.
If I had a split power supply I would *always* get rid of the output
capacitor. It is not difficult to get the output DC to withing 50mv of
gnd. A weird thing I have noticed, and I think you would have noticed it
sooner, is that no one, even audio "golden ears" pay serious attention
to the output cap. They just stick a plain old electrolytic of no
particular type (some times it's a bipolar) in the output, make it
bigger than needed for the LF -3db corner and call it "good". It would
seem that some attention should be paid to "ripple" current at
frequencies like 20khz etc, so some low esr caps would seem mandatory.
That music has relatively less high frequency components is the only
reason I can think of that this very lax approach might work.
>
>>>>> Um.. okay, I need to sit down and think. Mind is spinning,
>>>>> but I've not set a finger to paper yet and there is lots to
>>>>> think about in that one. I could be way, way off base.
>>>> Not at all.
>>> Thanks for that. I'm just glad to be able to talk to someone
>>> about any of this, at all. So please accept my thanks for
>>> the moments you are offering.
>>>
>>>> Is there a way you could post a schematic of where your thinking is
>>>> and what you would like to discuss - there is no need for a complete
>>>> circuit.
>>> Yes. I can use ASCII here, for example. But before I go off
>>> into the wild blue with this, do you want to focus on the
>>> power supply first? Or just jump in on the amplifier?
>> I don't mind. Earlier I put a stab at a no load worst case voltage,
>> you can use that if you want to. Until you get to output stage power
>> dissipation that is all you need.
>
> Maybe I'd like to focus on understanding different output
> pair configurations, first. I frankly don't like the "haul
> the output pair around with a collector on one side and a
> resistor on the other with a rubber diode in between to keep
> them biased up" approach. It's smacks of heavy-handedness
> and I simply don't like the way it looks to me. Everything
> tells me this works, but it is indelicate at the very least.
Well it would be a Darlington or complementary pair (I can't remember
the name sz...? )for the output transistors but I have no objections
>
> However, it is crucial that I understand it in detail before
> deciding what I really think about it. For example, I might
> want to replace the resistor with a current source.
Sorry you lost me on which R
pimpom
>
> If I had a split power supply I would *always* get rid of the
> output
> capacitor. It is not difficult to get the output DC to withing
> 50mv of
> gnd. A weird thing I have noticed, and I think you would have
> noticed
> it sooner, is that no one, even audio "golden ears" pay serious
> attention to the output cap. They just stick a plain old
> electrolytic
> of no particular type (some times it's a bipolar) in the
> output, make
> it bigger than needed for the LF -3db corner and call it
> "good". It
> would seem that some attention should be paid to "ripple"
> current at
> frequencies like 20khz etc, so some low esr caps would seem
> mandatory.
> That music has relatively less high frequency components is the
> only
> reason I can think of that this very lax approach might work.
>
If I may inject a comment here: I strongly support the idea of
avoiding an output coupling capacitor. I always use a split-PS,
OCL configuration unless some other consideration makes it
necessary to use a single-ended PS.
The comment about DC offset at the output terminal reminds me of
an experience I had more than 20 years ago. I was asked to spruce
up the P.A system at our state legislators' main session hall.
One of the things I did was to replace the old tube power amp
with my own design. I built four 60-watt amps (3 in use, one
spare) using 2N3055 BJTs in quasi-complementary configuration (I
couldn't easily get true complementary pairs then). Since the
existing system distributed audio power to dozens of small
speakers, inside and outside the hall, over a standard 100-volt
line, I integrated a 4-ohm input, 100V output transformer in my
amps.
When I first tested the system, one output transistor each in two
of the amplifiers warmed up quickly even without any output - not
actually hot, but warmer than they should be. After a few moments
of puzzlement, I traced the culprit to slight DC offset at the
output terminal. It was only a small fraction of a volt and
wouldn't have mattered with direct coupling to a speaker. But the
DC resistance of the primary winding of the output transformer
was so low (a fraction of an ohm) that it forced one of the
output transistors to draw a substantil amount of DC current at
idle.
I further traced the cause of offset to poorly matched
transistors at the input differential stage. I didn't include
provision for manual balancing of the static DC level, so I tried
out a few transistors for the input stage until I got a pair that
matched closely enough to reduce the offset to within a millivolt
or so (there was no hope of obtaining a factory-matched pair).
I know this has no direct relevance to the discussion, but I was
partly reminiscing and partly thinking that it may not be a bad
idea to give a real-life example of how easy it is to overlook
something.
Paul E. Schoen
"pimpom" <pim...@invalid.invalid> wrote in message
news:hkcqg6$vda$1...@news.albasani.net...
Something else to consider is a bridge output connection. You just need two
output stages, invert the phase to one, and make sure the DC voltages on
each are balanced. Another bonus is that you can get nearly 24 volts P-P
with a 24 VDC single supply. You can get very close to the rails if the
driver stage uses a slightly higher power supply voltage, so you can
optimize efficiency but at the cost of distortion (clipping).
At one time I considered making an amplifier with a dynamically adjustable
power supply so that the rails would always be just a couple of volts above
the peak output signal. It would probably be workable for a high-power
signal generator where some clipping can be tolerated as the output is
increased, but for music or other complex signals that vary unpredictably
in amplitude, there would need to be a delay in the signal long enough to
allow the power supply to adjust to what will be needed. At low
frequencies, it might be possible even to have the power supply track the
waveshape and even higher efficiency could be obtained.
I have used serial analog delay lines which are basically a bucket brigade
of switched capacitors, clocked higher than the maximum frequency required.
I designed a phase-shifting circuit for power line frequency, using an IC
that was sold at Radio Shack at the time, an SAD1024
http://www.geofex.com/sad1024.htm. I think I clocked it at a rate which
produced a 90 degree phase shift at 60 Hz, which would be 1024/0.00416 =
246 KHz. But it was prone to distortion, and it was not long before the IC
was discontinued and replaced with a dual 512 stage device that was even
worse.
There are much better ways to accomplish such feats now, but in 1980 or so
there were not many alternatives. Now the way to do it might be to digitize
the signal and then use a circular buffer to achieve whatever delay is
needed. Probably 16 bit audio sampled at 44 kHz so you can get about a 1.5
second delay with a 16 bit x 64k word memory. But with all that, probably a
PWM amp would be the way to go.
Just draining the brain through my fingers and the keyboard into
cyberspace...
Paul
David Eather
The adjustable power supply approach has been dubbed type "H", but like
type "G", it is almost certainly dying now... (which is sad I think - it
does cut down lines of investigation for originality and inspiration)
In the search for optimal efficiency:
http://www.irf.com/product-info/datasheets/data/irs2092.pdf
(irf - the worlds most experimenter unfriendly company)
Paul E. Schoen
"David Eather" <eat...@tpg.com.au> wrote in message
news:_ZidnV6rPbIxhffW...@supernews.com...
>
> The adjustable power supply approach has been dubbed type "H", but like
> type "G", it is almost certainly dying now... (which is sad I think - it
> does cut down lines of investigation for originality and inspiration)
>
> In the search for optimal efficiency:
>
> http://www.irf.com/product-info/datasheets/data/irs2092.pdf
>
> (irf - the worlds most experimenter unfriendly company)
Here's what I found for Class H, which used a power supply boost under
certain high output conditions:
http://www.nxp.com/documents/data_sheet/TDA1562Q_ST_SD.pdf
But it has been discontinued, and . I didn't realize that it had already
been implemented and had a class letter. I came up with the idea in the
early 80s IIRC. And I also tried to design a switching amplifier a few
years later. Maybe they were novel ideas then and I should have pursued
them.
My idea for the switching amplifier planned to use, basically, two
programmable switching supplies driven by the input signal and its inverse.
But it ignored the necessity of supplying both positive and negative
current. The answer, of course, was an H-bridge configuration. And I think
sometimes that is known as Type H, as opposed to Class H.
Another engineer at the time thought that he could just pass a pulse-width
modulated high frequency signal through a ferrite transformer rated at
perhaps 200 VA and 40 kHz. It would be modulated by the 60 Hz nominal
signal that was to be provided as an output, and would use capacitors and
inductors to filter out the carrier. But the only way for this to work at
all would be to rectify the output, resulting in half of the waveform. And
that would also saturate the core. So it was doomed from the start, but he
actually had transformers made and PC boards built before he could be
convinced that it had a fatal error.
That IRF part sounds like a real beast. There's pretty much no need to
design a power amplifier from discrete parts if you can get that IC for
about $5 and some $2 MOSFETs to make a reliable, rugged, and efficient
amplifier. Their development kit is $200 but that's not bad for a 250W x 2
channel amp.
I guess that's what you mean about them being experimenter unfriendly. No
reason to design and breadboard and fiddle around with something if it's
already been done essentially to perfection..
Paul
David Eather
large budget.
Jon Kirwan
<eat...@tpg.com.au> wrote:
>A 30 volt CT transformer with 15% regulation and 7% mains over-voltage,
>less voltage drop for the diode bride would give rails of +/- 25.
I'm not entirely familiar with the formal terms, but I figure
the 15% regulation you mentioned above must mean that the
ripple voltage goes from 100% to 85%. In general, the
equation for the angle would be something like:
angle = arcsin( 1 - Rf*(1-Vd/Vpk) )
This is with Rf being the ripple factor (not in percent
terms, obviously) and Vd being the sum of the diode drops
(full wave would be something like 2V) and Vpk being the
sqrt(2)*Vrms. At least, that's what the equation works out
for me on paper. (I can develop out here, if needed.)
The peak diode current happens just as the first moment of
conduction (which is neatly defined by the ripple factor, if
I understand you) and would be something like:
Ipk = 2*pi*f*C*Vpk*cos( angle )
Since the cos(arcsin(x)) is just sqrt(1-x^2), the computation
looks like:
Ipk = 2*pi*f*C*Vpk*SQRT(1-(1-Rf*(1-Vd/Vpk))^2)
There's probably some other adjustments to nail it, but that
probably gets somewhat close.
For a full wave bridge (I know, I think you were talking
about a half wave, but let's go with this for a moment) with
Vpk=25.2*SQRT(2) and Vd=2V (for two diodes in conduction in
the bridge) and Rf=.15 and f=60 (US-centric) and let's say a
C=2200uF, that gets something like Ipk = 15.2A.
None of this takes into account the average load current or
peak load currents. It just assumes that the ripple factor
is somehow known to be correct. I started out assuming that
the average load current would be defined entirely by the
droop (which is, of course, based upon the ripple factor
assumption) and the time from the peak of a previous cycle to
the point at which the above angle occurs and conduction
again begins. But it is complicated slightly by the fact
that the transformer supplies the entire current draw (if it
can) for a short part of the cycle _after_ the peak, as its
slope is less than the droop slope of the cap. I played with
accounting for all that but then decided that in most
practical cases the droop is hopefully not too excessive and
if not, then the angle after the peak isn't that much.. maybe
5 to 8 degrees or so... So I decided to ignore it and just
rely upon the capacitor's droop only:
Iave = C*Vpk*Rf/((pi/2+angle)/(2*pi*f))
So in the case just mentioned, I get Iave = 1.7A. I take it
that Ipk can easily be a factor of 9 or 10 greater. Also, I
note that it would probably be helpful to have nifty charts
of some kind to help pick off details like these.
Can you expand a little on what you were talking about,
though? Was that a half wave suggestion? I'm not sure I can
make sense of the rails, if so. If not, then I'd still
appreciate some of the calculations so that I can sure I
follow all of it.
I'm guessing that if a rail is to have a minimum of 25V on it
at the bottom of the ripple, and you are talking about 15%
regulation, the peak is going to be 29.4V -- not counting the
diode drops. Add 1V for that and it's 30.4V. Another 7% on
top would be 32.7V for the peak. RMS would be that figure
divided by sqrt(2), wouldn't it? Or 23.1Vrms or so?
So would that suggest two of the cheap 25.2Vrms transformers
and plan on rails still slightly higher?
Oh, crap. The VA rating. That's another one to consider.
Later, I guess.
>A dual 12.6 volt transformer would give a minimum (worst case with
>transformer at full load and mains 7% under voltage) of 16.something
>volts meaning big filter caps if you were serious at getting 10 watts.
>One of the reasons to go PSU first I think. (Also I live in a tiny
>jerk-water town where no one knows what a custom made transformer is let
>alone where you can get one wound)
I suppose the old filament-type 12.6VAC transformers must be
common everywhere. I also see that Radio Shack (yes, I'm
holding my nose for a moment) still carries some "commodity"
type 25.2VAC CT 2.0A rated transformers for about US$10.
Their 12.6VAC CT 3.0A transformers are priced identically.
So what's considered to be generally available?
Jon
Jon Kirwan
output stage. A lot of talk seems to revolve around
"crossover distortion." Seems almost very first thing folks
talk about when discussing class of operation if not also at
other times.
Seems to me that in a three-rail power supply situation
without an output capacitor involved, the crossover takes
place near the midpoint (ground) voltage between the rails,
at a time when current into the speaker load is also near
zero. (I'm neglecting any thoughts about inductance in the
speaker and physical coupling into the air, for now.) In
other words, where power at the speaker is near zero. Is it
really that important to consider?
I was looking at that terrible large scale gain plot for the
quasicomplementary output stage on the web site recently
mentioned in the thread (the lower curve in Figure 4 on this
link):
http://www.embedded.com/design/206801065?printable=true
(It's not that terrible of a plot, as the variation is from
.96 to .98 with the "normal" middle at .97.)
What's experience say here? Is it really so terrible as to
worry too much about something that takes place near zero
voltage, anyway? I'm just questioning the concern, for now.
I have no understanding about it, at all. Just wondering.
Jon
Jon Kirwan
>But it is complicated slightly by the fact
>that the transformer supplies the entire current draw (if it
>can) for a short part of the cycle _after_ the peak, as its
>slope is less than the droop slope of the cap.
I overstated this. The capacitor does supply _some_ along
with the transformer windings during this short phase, as the
voltage on the cap is also declining with it.
Jon
Paul E. Schoen
> On Thu, 04 Feb 2010 05:46:33 +1000, David Eather
> <eat...@tpg.com.au> wrote:
>
>>A 30 volt CT transformer with 15% regulation and 7% mains over-voltage,
>>less voltage drop for the diode bride would give rails of +/- 25.
>
> I'm not entirely familiar with the formal terms, but I figure
> the 15% regulation you mentioned above must mean that the
> ripple voltage goes from 100% to 85%. In general, the
> equation for the angle would be something like:
Transformers are specified with a percentage regulation which means the
change in voltage from no load to full load conditions. A small,
inexpensive transformer might have 15% regulation so that the 30VCT unit
would have a 15% higher output voltage with no load, or 34.5 VRMS or 48.8
P-P. Mains voltage may vary +/- 7% or 120 VAC +/- 8, or 112 to 128 VAC. At
the high end of this range the tranny puts out about 52.2 V P-P. Assuming a
FWB rectifier and the CT as reference, with 0.7 V diode drop, you get 25.4
volts peak.
If you put a capacitor on the output, it eventually charges to the peak
voltage. This is the high limit that must be considered for design. It may
not be exact, and probably will be a bit lower, because a power transformer
is usually designed to operate in partial saturation, so the output will
not increase linearly above its design rating.
Under load, the output will drop, caused by the effects of primary and
secondary coil resistance as well as magnetic effects. These will cause
heating over a period of time, and the coil resistance will increase,
adding to the effect until a point of equilibrium is reached based on the
ambient conditions and removal of heat via conduction, convection, and
radiation.
Large power transformers, high quality audio transformers, and
instrumentation transformers are designed with perhaps 1% or 2% regulation,
which is usually accomplished by using more copper and iron, and also using
special cooling mechanisms such as oil flow and forced air.
> angle = arcsin( 1 - Rf*(1-Vd/Vpk) )
>
> This is with Rf being the ripple factor (not in percent
> terms, obviously) and Vd being the sum of the diode drops
> (full wave would be something like 2V) and Vpk being the
> sqrt(2)*Vrms. At least, that's what the equation works out
> for me on paper. (I can develop out here, if needed.)
>
> The peak diode current happens just as the first moment of
> conduction (which is neatly defined by the ripple factor, if
> I understand you) and would be something like:
>
> Ipk = 2*pi*f*C*Vpk*cos( angle )
>
> Since the cos(arcsin(x)) is just sqrt(1-x^2), the computation
> looks like:
>
> Ipk = 2*pi*f*C*Vpk*SQRT(1-(1-Rf*(1-Vd/Vpk))^2)
>
> There's probably some other adjustments to nail it, but that
> probably gets somewhat close.
Maybe it is useful to work out these equations to get a concept of what is
going on, but I prefer a more empirical method which may involve initial
rough estimates and prototyping and bench testing, as well as LTSpice
simulation. The simulator includes the equations that determine the
performance of the circuit, and may also include the effects of losses and
heating and temperature change. But usually I just use approximations and
best guesses of final operating conditions such as temperature, and use
parameters such as internal resistance based on these figures. Then it is
time to build the circuit and do real world bench testing.
[snip]
>
> Can you expand a little on what you were talking about,
> though? Was that a half wave suggestion? I'm not sure I can
> make sense of the rails, if so. If not, then I'd still
> appreciate some of the calculations so that I can sure I
> follow all of it.
>
> I'm guessing that if a rail is to have a minimum of 25V on it
> at the bottom of the ripple, and you are talking about 15%
> regulation, the peak is going to be 29.4V -- not counting the
> diode drops. Add 1V for that and it's 30.4V. Another 7% on
> top would be 32.7V for the peak. RMS would be that figure
> divided by sqrt(2), wouldn't it? Or 23.1Vrms or so?
>
> So would that suggest two of the cheap 25.2Vrms transformers
> and plan on rails still slightly higher?
>
> Oh, crap. The VA rating. That's another one to consider.
> Later, I guess.
I sense a lack of a real direction or intended purpose for this project. As
an academic exercise and learning experience, throwing all sorts of ideas
into the pot is worthwhile. But when it comes to the actual task of
building something useful, whether for production or a one-off hobby
project, it comes down to the three factors I offer. I can build it well, I
can build it quickly, and I can build it cheaply. Pick any TWO!
>>A dual 12.6 volt transformer would give a minimum (worst case with
>>transformer at full load and mains 7% under voltage) of 16.something
>>volts meaning big filter caps if you were serious at getting 10 watts.
>>One of the reasons to go PSU first I think. (Also I live in a tiny
>>jerk-water town where no one knows what a custom made transformer is let
>>alone where you can get one wound)
>
> I suppose the old filament-type 12.6VAC transformers must be
> common everywhere. I also see that Radio Shack (yes, I'm
> holding my nose for a moment) still carries some "commodity"
> type 25.2VAC CT 2.0A rated transformers for about US$10.
> Their 12.6VAC CT 3.0A transformers are priced identically.
>
> So what's considered to be generally available?
Certainly this depends on your location as well as your budget (time and/or
money) and criteria for the design. If you plan to go the cheapest monetary
route for a one-off project, look for locally available freebies in a
junkyard, flea markets, Hamfests, eBay, and www.freecycle.com. You also
must consider time and transportation or shipping expenses, which can be
high for items like transformers.
You must also balance what is readily available with what you actually need
for your project. If you have certain constraints and absolute design
criteria, you may be forced into a narrow range of what is acceptable. At
some point, you may need to modify a salvaged transformer or wind your own
(or have one custom made). There are many off-the-shelf transformers
available at reasonable cost, so it would be rare to need a custom design,
but sometimes it is the only option. You can do a lot with a MOT if you
don't mind spending the time messing with it.
And you can also get toroid transformer kits that have the primary already
wound, and you just add your own secondary. See www.toroid.com. They have
kits from 80VA ($52) to 1400VA ($110). I used four of the largest ones to
make a circuit breaker test set with an output of 2000 amperes at 2.8 volts
continuous, and the good regulation allowed it to provide pulses of over
12,000 amperes. If you find any equipment with toroid transformers, by all
means salvage them. You can also use Variacs and Powerstats and their
equivalents to make high power transformers. I have about a dozen damaged
units rated at 240 VAC at 8 amps, or 2 kVA, and I had plans to use them for
a 24 kVA test set, 4000 amps at 6 volts. Here are pictures of a 10 kVA test
set I designed for www.etiinc.com, using toroids:
http://www.smart.net/~pstech/PI2000-1-small.JPG
http://www.smart.net/~pstech/PI2000-2-small.JPG
http://www.smart.net/~pstech/PI2aux-5a.JPG
But I have digressed, and this thread has digressed from the discussion of
amplifiers to power supplies (which is related, of course), and line
powered transformers (which may not be the best choice). However, at some
point one must decide if this is to be an actual project or just an
academic discussion, and then proceed to get some parts and put something
together and plug it in. It can be done using as many "free" parts as
possible, or from the standpoint of what is the most cost-effective
overall, and in either case one must have a clear view of the end result.
Paul
Jon Kirwan
Actually, the only thing you could have been talking about is
a full wave bridge + a CT transformer to get to two rails and
ground. I knew that was the only way to get there, too, and
that's why I was staying on a bridge form of it. But I was
sadly not thinking clearly about your writing when I read it.
That's entirely my fault, of course.
I should have used 2V here, too. A bridge is the way to go
and that's two drops. No need to suddenly insert a half wave
thing here when massaging the numbers around. So it should
be something more like 31.4V, with 7% more being 33.8V with
margin added. And that is about 23.9Vrms, I think.
Luckily, it doesn't change the thought about 25.2Vrms xfrmrs.
(You might, of course.)
Jon
Jon Kirwan
<pa...@peschoen.com> wrote:
>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>news:upmjm55fpn755sdsh...@4ax.com...
>> On Thu, 04 Feb 2010 05:46:33 +1000, David Eather
>> <eat...@tpg.com.au> wrote:
>>
>>>A 30 volt CT transformer with 15% regulation and 7% mains over-voltage,
>>>less voltage drop for the diode bride would give rails of +/- 25.
>>
>> I'm not entirely familiar with the formal terms, but I figure
>> the 15% regulation you mentioned above must mean that the
>> ripple voltage goes from 100% to 85%. In general, the
>> equation for the angle would be something like:
>
>Transformers are specified with a percentage regulation which means the
>change in voltage from no load to full load conditions. A small,
>inexpensive transformer might have 15% regulation so that the 30VCT unit
>would have a 15% higher output voltage with no load, or 34.5 VRMS or 48.8
>P-P. Mains voltage may vary +/- 7% or 120 VAC +/- 8, or 112 to 128 VAC. At
>the high end of this range the tranny puts out about 52.2 V P-P. Assuming a
>FWB rectifier and the CT as reference, with 0.7 V diode drop, you get 25.4
>volts peak.
Hmm. Totally new thoughts. So that's what the term
"regulation" means. It's about the transformer design? And
here I was off and away on the capacitive-filtered ripple
side. Well, that's still useful to have gone back to,
anyway.
I'm not buying the 0.7V diode drop, yet. At peak currents
near 10 times larger than average load currents, I have to
imagine more than 0.7V drop with anything silicon and not
schottky. Do they use schottky's? (Leakage comes to mind.)
Okay. So the 25V was specifying the peak, not the bottom
side. And that is unloaded, basically. Which brings up the
question of what exactly does 15% regulation _actually_ mean.
What is the definition of "full load?" Since the peak diode
currents can be quite a lot more than the average load
current from my calculations, that seems to place quite a
burden on the transformer ratings.
So could you go further here? In other words, let's say I
know that the average load current will be 1.4A, but that the
peak diode current given the bridge/capacitor design will be
15A. The transformer is a 25.2Vrms CT unit. The DC rails
are at -15 and +15, with 2200uF caps on each side to ground,
and the ripple on them is about 3.8V peak to peak (+/-1.9V
around 15V.)
What's the VA rating here? And "regulation" number are you
looking for in the transformer and how does it relate back to
VA and other terms that might be used?
>If you put a capacitor on the output, it eventually charges to the peak
>voltage. This is the high limit that must be considered for design. It may
>not be exact, and probably will be a bit lower, because a power transformer
>is usually designed to operate in partial saturation, so the output will
>not increase linearly above its design rating.
Ah. Core saturation is __intended__ as part of the design? I
haven't done that one before. What guidance can you give on
that aspect?
>Under load, the output will drop, caused by the effects of primary and
>secondary coil resistance as well as magnetic effects. These will cause
>heating over a period of time, and the coil resistance will increase,
>adding to the effect until a point of equilibrium is reached based on the
>ambient conditions and removal of heat via conduction, convection, and
>radiation.
Now that, I understand and worry about.
>Large power transformers, high quality audio transformers, and
>instrumentation transformers are designed with perhaps 1% or 2% regulation,
>which is usually accomplished by using more copper and iron, and also using
>special cooling mechanisms such as oil flow and forced air.
Okay.
>> angle = arcsin( 1 - Rf*(1-Vd/Vpk) )
>>
>> This is with Rf being the ripple factor (not in percent
>> terms, obviously) and Vd being the sum of the diode drops
>> (full wave would be something like 2V) and Vpk being the
>> sqrt(2)*Vrms. At least, that's what the equation works out
>> for me on paper. (I can develop out here, if needed.)
>>
>> The peak diode current happens just as the first moment of
>> conduction (which is neatly defined by the ripple factor, if
>> I understand you) and would be something like:
>>
>> Ipk = 2*pi*f*C*Vpk*cos( angle )
>>
>> Since the cos(arcsin(x)) is just sqrt(1-x^2), the computation
>> looks like:
>>
>> Ipk = 2*pi*f*C*Vpk*SQRT(1-(1-Rf*(1-Vd/Vpk))^2)
>>
>> There's probably some other adjustments to nail it, but that
>> probably gets somewhat close.
>
>Maybe it is useful to work out these equations to get a concept of what is
>going on, but I prefer a more empirical method which may involve initial
>rough estimates and prototyping and bench testing, as well as LTSpice
>simulation.
I reverse this. I like understanding the _theory_ and don't
care at all about practice until _after_ I've mastered the
theoretical aspects that bear more on the problems. I _then_
use LTspice _after_ being able to work things on paper, just
to check and verify that I got it. The reason is, if I'm
missing something important it will then show up and that
will kick me to go back and find additional theory to cover
the gap in my paper knowledge. That's how I learn. It's the
only way I really feel that I understand something. (I think
I talked a little about that here.)
>The simulator includes the equations that determine the
>performance of the circuit, and may also include the effects of losses and
>heating and temperature change. But usually I just use approximations and
>best guesses of final operating conditions such as temperature, and use
>parameters such as internal resistance based on these figures. Then it is
>time to build the circuit and do real world bench testing.
I think there is always time to go build. And when I do
that, I will take measurements and make adjustments to get
where I want to be and I won't be sweating the theory so much
at that point. However, before I get there I like to make
sure I've mastered the relevant theories.
Let me put this in an entirely different context that may
shed some light on "how I think" and "why I think that way."
The reality of modern US surgery is that an anesthesiologist
uses well-worn practice with well-surveyed and well-studied
drugs and tools. They work. And as a general matter, they
work most of the time without the anesthesiologist having to
remember anything about chemistry or metabolites or liver
pathways or the kidney micropipette filtering system. They
don't care about memorizing any of that, or frankly, even
knowing much how it works.
I make it a matter of regular practice to check off the "yes"
box every time any of my family gets the surgery forms where
there always present the question, "Do you want to meet with
the anesthesiologist?" The very first question I ask is,
"What are you using and what are the liver pathways and
resulting metabolites for it?" I have yet to have a single
one of them be able to answer the question. Not once. I
have had one or two tell me that "Well, we studied all that
in school but I don't remember any of it." At least a frank
admission there.
So why do I care? I completely understand that in almost
every case on the operating table there will be no problems
and that the well-worn paths in anesthesia work on most
people most of the time. However, there is a reason. What
if something unusual takes place. A unique reaction, for
example. Something _outside_ the usual experience. What
then? They would have, let's say, minutes to make a
decision. No time to go to books.
I want someone there who knows the chemistry, knows what _is_
known about the pathways. How much of the primary pathway is
used in proportion to the other pathways? What are the
products in the other pathways? What are their effects
should they exceed some limit? How does that present or
manifest itself?
These are the kinds of things that might bring to bear an
answer -- something needed to mitigate a disaster in the
making when there is no time to hit a book but where if they
did understand the theories well and knew the pathways and
the effects of excessive amounts of the unusual pathway
metabolites they might know exactly what to do when it would
mean the difference of life and death.
There are MANY people who die in these circumstances that are
chocked up to "Oh, well. It happens rarely."
If I were an anesthesiologist, Paul, I'd know this stuff
cold. And I'd keep up on the current knowledge, too. And
more. Because that's the way I am.
Yes, they are practical people and they do a very
satisfactory professional job every day of their lives. But
quite frankly I don't think that's good enough.
Theory provides _all_ meaning. And it's the way I think
about things. It's how I function. Yes, others will be very
satisfied with "practical" results. I'm not. I need more.
>[snip]
>>
>> Can you expand a little on what you were talking about,
>> though? Was that a half wave suggestion? I'm not sure I can
>> make sense of the rails, if so. If not, then I'd still
>> appreciate some of the calculations so that I can sure I
>> follow all of it.
>>
>> I'm guessing that if a rail is to have a minimum of 25V on it
>> at the bottom of the ripple, and you are talking about 15%
>> regulation, the peak is going to be 29.4V -- not counting the
>> diode drops. Add 1V for that and it's 30.4V. Another 7% on
>> top would be 32.7V for the peak. RMS would be that figure
>> divided by sqrt(2), wouldn't it? Or 23.1Vrms or so?
>>
>> So would that suggest two of the cheap 25.2Vrms transformers
>> and plan on rails still slightly higher?
>>
>> Oh, crap. The VA rating. That's another one to consider.
>> Later, I guess.
>
>I sense a lack of a real direction or intended purpose for this project.
It's for education. I think I stated that at the outset. I
sure hope I did. However, I _do_ intend on producing a
practical result. Not because I need one. But because I
need to make sure that my mental models work, in real
practice. It's like some theorist saying that if you pass
electrons by a certain kind of magnetic field, they will
separate according to a certain observational spin. Great.
But until you build and test the idea, you really don't know.
So you build and test. I intend to build and test an
amplifier, not because I need one badly, but because I want
to see how all that theory works in practical building
circumstances. It may highlight yet something new that I
hadn't considered and will point me to still more theory to
gain a hold upon.
Where this knowledge will wind up "doing something" is
unknown at this stage. It might get used in ways two years
from now that I have no way to predict, today. Or ten years.
I hope to live long enough to see some utility, though it may
not be with amplifiers.
However, I had posted a different question a while back about
my autistic daughter's abuse of volume controls around the
house and ultimately I hope to use this knowledge in
designing a custom system for her that does include some
features probably few others will care about. So I foresee
something in the next year, to be honest.
But the main point is learning, right now. I need to grasp
this stuff from start to end to some _reasonable_ level.
>As
>an academic exercise and learning experience, throwing all sorts of ideas
>into the pot is worthwhile.
:) That's me.
>But when it comes to the actual task of
>building something useful, whether for production or a one-off hobby
>project, it comes down to the three factors I offer. I can build it well, I
>can build it quickly, and I can build it cheaply. Pick any TWO!
Hehe. I want to _learn_ to design to specified criteria,
have a comprehensive view of the theoretical concepts
involved, and that means I need to only pick the first one.
The 'quickly' is unimportant -- one to two years is good
enough. The 'cheaply' is equally unimportant. If it costs
me 10 times as much in terms of parts and time as it would
just buying something commercial, buying a commercial
solution will teach me exactly zero about what I need to
learn to design what my daughter needs. And there is NOTHING
on the market to get there, either. No one else has my
problem. Or few do.
This is a "give a person a fish and they eat for a day, teach
a person to fish and they eat for the rest of their lives"
thing. It won't just apply to the next solution for my
daughter. It will help me in other ways I poorly understand
right now.
>>>A dual 12.6 volt transformer would give a minimum (worst case with
>>>transformer at full load and mains 7% under voltage) of 16.something
>>>volts meaning big filter caps if you were serious at getting 10 watts.
>>>One of the reasons to go PSU first I think. (Also I live in a tiny
>>>jerk-water town where no one knows what a custom made transformer is let
>>>alone where you can get one wound)
>>
>> I suppose the old filament-type 12.6VAC transformers must be
>> common everywhere. I also see that Radio Shack (yes, I'm
>> holding my nose for a moment) still carries some "commodity"
>> type 25.2VAC CT 2.0A rated transformers for about US$10.
>> Their 12.6VAC CT 3.0A transformers are priced identically.
>>
>> So what's considered to be generally available?
>
>Certainly this depends on your location as well as your budget (time and/or
>money) and criteria for the design. If you plan to go the cheapest monetary
>route for a one-off project, look for locally available freebies in a
>junkyard, flea markets, Hamfests, eBay, and www.freecycle.com. You also
>must consider time and transportation or shipping expenses, which can be
>high for items like transformers.
I was about to write, earlier, that I already have a large
supply of scavenged transformers. I will have _no_ problem
finding a suitable one somewhere in the pile. The question
was brought up by David. So I asked, that's all.
>You must also balance what is readily available with what you actually need
>for your project. If you have certain constraints and absolute design
>criteria, you may be forced into a narrow range of what is acceptable. At
>some point, you may need to modify a salvaged transformer or wind your own
>(or have one custom made). There are many off-the-shelf transformers
>available at reasonable cost, so it would be rare to need a custom design,
>but sometimes it is the only option. You can do a lot with a MOT if you
>don't mind spending the time messing with it.
For now, I'm just planning to use what I can lay hands on...
when the time comes. However, I don't mind at all any
discussion about practical choices were someone to buy new
parts. That teaches me about others and their concerns and
helps me to help others, too.
In short, this topic is made even better when it isn't just
about me and my interests and my focus. I like it very much
when others chip in about other thoughts, other places and
times, and where ever that may take it. However, I am still
clear about what part of it makes the most difference for me
-- the learning part about everything from power supply
design, input stage design, class A, class B, class AB
considerations, output stages and drivers, VAS, splitters,
current mirrors, current sources, etc. It's all good to me.
That part of this discussion that went off on the direction
of ICs was also fine. I took note and figure on getting back
to thinking about that too, someday later on.
>And you can also get toroid transformer kits that have the primary already
>wound, and you just add your own secondary. See www.toroid.com. They have
>kits from 80VA ($52) to 1400VA ($110). I used four of the largest ones to
>make a circuit breaker test set with an output of 2000 amperes at 2.8 volts
>continuous, and the good regulation allowed it to provide pulses of over
>12,000 amperes. If you find any equipment with toroid transformers, by all
>means salvage them. You can also use Variacs and Powerstats and their
>equivalents to make high power transformers. I have about a dozen damaged
>units rated at 240 VAC at 8 amps, or 2 kVA, and I had plans to use them for
>a 24 kVA test set, 4000 amps at 6 volts. Here are pictures of a 10 kVA test
>set I designed for www.etiinc.com, using toroids:
>
>http://www.smart.net/~pstech/PI2000-1-small.JPG
>http://www.smart.net/~pstech/PI2000-2-small.JPG
>http://www.smart.net/~pstech/PI2aux-5a.JPG
>
>But I have digressed, and this thread has digressed from the discussion of
>amplifiers to power supplies (which is related, of course), and line
>powered transformers (which may not be the best choice). However, at some
>point one must decide if this is to be an actual project or just an
>academic discussion, and then proceed to get some parts and put something
>together and plug it in. It can be done using as many "free" parts as
>possible, or from the standpoint of what is the most cost-effective
>overall, and in either case one must have a clear view of the end result.
The digressions are great! I am NOT in a rush to build,
though. I'm wanting to engage the math and learn what can be
achieved by deducing from parsimonous theory. Then test a
few things on the bench, ask questions, learn some more. Etc.
So theory _and_ practical approaches are important. Not one,
or the other, but both!!
Pendulum motion is well understood. One might either have a
practical knowledge about it and some tables and just go with
that. Probably, lots of folks making pendulum clocks stop
there and go no further and are none the worse for that. It
is similarly very easy to develop the infinite series that
describes it (or use the sqrt(L/g) proportionality as a first
order approximation or for small starting angles) from the
simple differentials involved and to take an entirely
theoretical approach, as well.
But I'm interested in more than that. Theory by itself lacks
reality. Reality by itself lacks meaning sans theory. The
two go together like hand in glove, though. Building even
the most simple ones using a peg-in-hole method leads to the
discovery of still more interesting effects, if you know some
theory. For example, the rocking of the pin itself in the
larger hole has a measurable impact of perhaps as much as 2
or 3 percent. It's useful to know that and understand it.
Once that mechanism is itself understood, one can then dig
even deeper to find more subtle (and possibly useful) effects
to continue improvements. A practitioner lacking even the
basic theory might accidentally happen upon some idea, of
course. And a theoretician lacking practical reality to
interfere might accidentally imagine some realistic effect to
pursue, too. But it really takes a marriage of both to make
quick work of progress forward, I think.
Since theory is primary, I like to pursue that part of it
earlier and move to experience once I have the mental tools
required to make sense of the data that results. Without
theory, data is pure noise. Without the theory of a sphere,
even the gentle curvature at the horizon "seen" my a mountain
climber is just so much useless noise to them. But _with_
that theory, the data _means_ much.
Jon
Jon Kirwan
><snip>
>Since theory is primary, I like to pursue that part of it
>earlier and move to experience once I have the mental tools
>required to make sense of the data that results.
><snip>
Okay. On second thought... enough theory. I think it's time
for practice. I already have triple output power supplies,
but using them wouldn't be true to the actual amplifier
situation. And any testing of distortions needs to cope with
that reality.
So I'm moving forward on the power supply rails. I need to
scarf around and see what I have available. I'll post what I
find, the resulting design and thinking, photos perhaps, and
the results of testing with static loads. Once that is done,
I'd like some advice about the next step, though. But until
then, I'll just focus on getting that part put to bed. That
much I can do right now.
I've decided that your kick in the butt, Paul, was what I
needed. I have enough in mind to move out of the thinking
stage and into trying some different alternatives. I'll get
going.
Thanks,
Jon
Paul E. Schoen
>
> Hmm. Totally new thoughts. So that's what the term
> "regulation" means. It's about the transformer design? And
> here I was off and away on the capacitive-filtered ripple
> side. Well, that's still useful to have gone back to,
> anyway.
>
> I'm not buying the 0.7V diode drop, yet. At peak currents
> near 10 times larger than average load currents, I have to
> imagine more than 0.7V drop with anything silicon and not
> schottky. Do they use schottky's? (Leakage comes to mind.)
Silicon diodes are the norm except for high power, high efficiency, high
frequency, and low voltage. But they do have forward drops of 0.7 to 0.6
volts at normal operating temperatures, and when drawing minimal current,
as is the case at the waveform peak under no load conditions. Even with a
capacitor, the diode current drops to near zero at the voltage peak. A
different result is expected if there is inductance, of course.
There is a separate regulation spec for the DC output. It is typically much
worse than the regulation of the transformer, as the capacitors quickly
discharge between peaks and can be charged up only as quickly as the
transformer and diodes allow during the conduction cycle. So we use big
capacitors and linear regulators, or resort to a switching supply.
But if you are lucky enough to have three phase power, you can design a DC
supply with no capacitors and get something like 6% regulation (and
ripple). This is SOP for really high power DC, like 10kVA.
> Okay. So the 25V was specifying the peak, not the bottom
> side. And that is unloaded, basically. Which brings up the
> question of what exactly does 15% regulation _actually_ mean.
> What is the definition of "full load?" Since the peak diode
> currents can be quite a lot more than the average load
> current from my calculations, that seems to place quite a
> burden on the transformer ratings.
Transformers are rated at RMS current, which is pretty much all that
matters for heating effect, and it is mostly related to the resistance of
the copper and the allowable rise in temperature in the core. Efficiency
aside, what matters is the temperature the insulation can withstand before
deteriorating, and usually that is at least 130C, or 100C above ambient.
The smaller the tranny, the better it sheds heat (surface area/volume), so
regulation and efficiency of smaller ones tend to be poorer.
Full load is just the maximum RMS current at which the transformer is
rated. This may be further complicated by duty cycle ratings, which can be
continuous or intermittent. Generally intermittent duty is 50% duty cycle,
with ON times not greater than 30 minutes, at least for larger transformers
with more thermal mass. At 50% duty cycle the output rating is 1.4 times
the true continuous rating. And then the allowable duty cycle is the
inverse of the square of the overload. For the circuit breaker test sets I
design, we specify output up to 10x the continuous rating, at which the
duty cycle is only 1%. But the ON time is limited to about 100 mSec, which
is more than enough to trip a circuit breaker instantaneously, and then you
should wait 10 seconds before doing it again.
I designed a "Programmable Overload Device", or POD, which takes into
account the current and the time, as well as the actual temperature using a
thermistor, to enforce reasonable duty cycles. Fuses, circuit breakers, and
Motor Overloads do a similar function, but don't fully take into account
all the factors. The intelligence for this is buried in the PIC code, and
is rather involved and yet imperfect. If I could accurately model the
heating and cooling effects of current in a transformer, it would be ideal.
Now that's where theory can really help.
> So could you go further here? In other words, let's say I
> know that the average load current will be 1.4A, but that the
> peak diode current given the bridge/capacitor design will be
> 15A. The transformer is a 25.2Vrms CT unit. The DC rails
> are at -15 and +15, with 2200uF caps on each side to ground,
> and the ripple on them is about 3.8V peak to peak (+/-1.9V
> around 15V.)
>
> What's the VA rating here? And "regulation" number are you
> looking for in the transformer and how does it relate back to
> VA and other terms that might be used?
It's really easier (and perhaps even more appropriate) to use a tool such
as LTSpice for this purpose. You could look at all the variables over time,
quantized to steps small enough to minimize error, and finally arrive at a
steady state solution where you may be able to describe such complex
entities as RMS current with an equation, but all you will have done is
spend a lot of time doing what LTSpice does so well and so quickly. So I
cobbed together a simple power supply simulation, which in this case models
part of a power supply that I have been using on my Ortmasters, with a
Signal 241-6-16 transformer. The ASCII file is at the end of this post.
I'm using a voltage doubler circuit on each leg of the 16VCT transformer,
as I need to get at least 17 VDC for 15VDC linear regulators for the analog
portion of the circuit. I figure no more than 20 mA. So for simulation
puposes I use a 1k resistor as the load. The transformer is 32 VA, or 2A at
16V, and I estimate 15% regulation which is a 2.4 V drop at 16V or open
circuit 18.4 VRMS. I'm using a voltage source with 26 volts peak and 1.1
ohms internal resistance. The capacitors are 220uF, and MURS120 diodes. As
a result, I get 22.35 VDC outputs, and the transformer current is 104 mA
RMS, with peaks of about 360 mA.
Just for fun, I changed the output loads to 10 ohms, and I found that the
current is only 345 mA RMS, and the transformer current is 611 mA RMS, with
peaks of about 1 amp. The capacitively coupled design is inherently
current-limited, which can be a good thing.
>>If you put a capacitor on the output, it eventually charges to the peak
>>voltage. This is the high limit that must be considered for design. It
>>may
>>not be exact, and probably will be a bit lower, because a power
>>transformer
>>is usually designed to operate in partial saturation, so the output will
>>not increase linearly above its design rating.
>
> Ah. Core saturation is __intended__ as part of the design? I
> haven't done that one before. What guidance can you give on
> that aspect?
Maximum use of the iron occurs near the maximum flux density. It results in
increased current which actually occurs at 90 degrees to the applied
voltage, so the distortion is not in the form of a flattening of the
voltage waveform but rather like crossover distortion. But it does result
in a somewhat non-linear effect, as it interacts with the resistance of the
windings. See the following for more information:
http://openbookproject.net/electricCircuits/AC/AC_9.html
and more about regulation:
http://www.allaboutcircuits.com/vol_2/chpt_9/6.html
It is most pronounced in ferroresonant transformers:
http://www.ustpower.com/Support/Voltage_Regulator_Comparison/Ferroresonant_Transformer_CVT/Constant_Voltage_Transformer_Operation.aspx
>>Under load, the output will drop, caused by the effects of primary and
>>secondary coil resistance as well as magnetic effects. These will cause
>>heating over a period of time, and the coil resistance will increase,
>>adding to the effect until a point of equilibrium is reached based on the
>>ambient conditions and removal of heat via conduction, convection, and
>>radiation.
>
> Now that, I understand and worry about.
That's why most designs are made with a generous safety factor so you do
not need to worry about these effects. They can be predicted approximately
and that is good enough.
> Hehe. I want to _learn_ to design to specified criteria,
> have a comprehensive view of the theoretical concepts
> involved, and that means I need to only pick the first one.
> The 'quickly' is unimportant -- one to two years is good
> enough. The 'cheaply' is equally unimportant. If it costs
> me 10 times as much in terms of parts and time as it would
> just buying something commercial, buying a commercial
> solution will teach me exactly zero about what I need to
> learn to design what my daughter needs. And there is NOTHING
> on the market to get there, either. No one else has my
> problem. Or few do.
It might be worthwhile to discuss those details here to dig up some ideas.
> This is a "give a person a fish and they eat for a day, teach
> a person to fish and they eat for the rest of their lives"
> thing.
I've heard it said that, "teach a man to fish, and he'll spend all day in a
boat drinking beer!" :)
I think I had problems in the EE program at Johns Hopkins because it was
too theoretical for my mindset, and I had fundamental problems with
advanced calculus. I aced the lab courses and helped others because I had
already designed and built many circuits. But, looking back, I see where
having a stronger grasp of theory would have helped. I still design
circuits with a highly empirical approach, using rule of thumb and
experience to choose components. Now that SPICE is freely available I find
it fascinating to try different values and placements and configurations
"just to see what happens". And I learn by looking at the time domain
simulation plots and determining what may have caused certain glitches or
oscillations that I did not foresee.
My talents are more in the realm of imagination and thinking outside the
box. And sometimes it has gotten me into trouble. But I have also sometimes
been able to make a lot of progress in a short period of time. I think some
aspects of design are more of an art than a science, and I look for a sort
of elegance in the finished design of a circuit, even in the placement of
components on the schematic, and also in their placement on a PCB.
Paul
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pimpom
There's this saying "Practice without theory is blind and theory
without practice is lame". You've made it clear that you want to
thoroughly understand the hows and whys of amplifier design from
mathematical models. I have no quarrel with that approach and I
also use it myself within the limits of my own capability - *up
to a point*. But there comes a point at which striving for
absolute precision solely from theory results in diminishing
returns.
Take the case of the pendulum you brought up earlier. The basic
theory is well established, but to predict the behaviour of a
practical pendulum with 100% precision will require taking into
account the effects of so many factors that it may well be
impossible. E.g., the aerodynamics of the pendulum's shape
including minute irreguarities on its surface, the exact strength
and orientation of the earth's magnetic field at the location and
its effect on traces of magnetic materials in the alloy, friction
with suspended particles in the air in addition to the air
itself, friction at the point of suspension and elasticity of the
suspension, etc., etc. Even if all these influencing factors are
included in the equation, the physical values to be entered can
never be measured with 100% accuracy.
Take the case of the forward drop of the diode in the power
supply that you've been discussing with Paul. This what I did
before personal computers and simulation progs became widely
available: I drew a curve of the diode's V-I characteristics on
graph paper up to the expected peak current. Then I drew a
straight line, approximately following the dynamic curve, from
the peak point down to the voltage axis. I took that voltage as a
constant forward drop and the slope of the line as a constant
series resistor. I then added that resistance to other source
resistances like the transformer winding resistance and either
use it to calculate the rectified and filtered voltage or, more
often, to determine it from a graph such as that in RDH. It also
comes in useful for finding the peak and rms currents. I don't
know if anyone else uses that method or how well it agrees with
theory, but it agrees pretty well with practical measurements.
I don't do this every time I design a power supply. I just make a
mental estimate based partly on theory and partly on past
experience. In short, there's a point at which it makes more
sense to make informed assumptions and approximations even before
doing physical construction.