Where to get a choke for a class-A single-ended MOSFET power stage?
Morris Slutsky
http://img524.imageshack.us/img524/4038/simplefet.jpg
Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.
What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like. No need
for expensive output transformer. A cheap FET like a IRF510, compare
to a 300B, yeah. No need for heater supply! And the choke . . . the
choke doesn't need to be NEARLY as large. A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range. Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys. Should be easy to find one I think.
But I'm not sure where. Any ideas where to get a 50-100 mH choke for
this? Needs to take an amp or three of current without saturating.
That's the hard part there I think. Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?
If I could find a choke this would be a nice cheap thing to build
though. Easy enough.
Stephen Cowell
"Morris Slutsky" <morris....@gmail.com> wrote
...
> If I could find a choke this would be a nice cheap thing to build
> though. Easy enough.
Roll your own, Morris... it is not space technology.
Obtain any copy of the ARRL Amateur Radio Handbuch
and start windin'. Big stick of red-coded ferrite would
probably get you there in a hurry... the choke can be
*way* over and not hurt anything, right? Hell, use a PT
primary.
__
Steve
.
Phil Allison
"Morris Slutsky"
> Thinking of a circuit like this. Simple and elegant-looking:
>
> http://img524.imageshack.us/img524/4038/simplefet.jpg
>
> this? Needs to take an amp or three of current without saturating.
** The nearest is a Hammond choke:
http://datasheet.octopart.com/195P5-Hammond-datasheet-7863.pdf
A 195p5 is closest to your spec.
30mH is a minimum and say 1 ohm resistance max.
Absolutely FORGET using ferrite for this application.
..... Phil
jh
Hi Morris,
have you got a defunct switch mode computer power supply laying around?
The coils in there might fit your bill. Air gap, MOL "serious" iron.
regards
Jochen
legg
Up until today, I would have said Hammond, but their catalog seems to
have been chopped. You're looking at 2.5lbs of part, measuring 4 x 2.5
x 2.25 inches, for the low end of the inductance range quoted, at 2A.
Double the weight for the upper end - 4 x 4 x 3 inches.
RL
JJTj
>>Thinking of a circuit like this. Simple and elegant-looking:
>>
>>http://img524.imageshack.us/img524/4038/simplefet.jpg
>>
If it hasn't been mentioned, or you already know, I hope
you visit and d/l all the free data available at:
Mr. Pass in the KING of class-a design, offers enough
free info to fill a book, and answers ?s in the forum.
When reading, you will find that a choke there is big,
heavy, and wasteful. Just use a constant current design
(cheaper to build too). I've built his ZEN designs,
which look alot like your pix using CCS(s). There are
even sources for PCBs! I promise you you'll LOVE the site.
JJTj
GregS
Did I miss something, but I didn't see what the amplifier was for.
Thats funny Pass came out with that SE about the same time I drew up
mine.
Greg
JJTj
I have a tee shirt from the old Georgia Champ wrestling folks
that says "EVIL MEAN AND NASTY". A gift from Ole.
>Did I miss something, but I didn't see what the amplifier was for.
>Thats funny Pass came out with that SE about the same time I drew up
>mine.
>
>Greg
Well, back when the humbucking PU was made, about 6 people said they
'invented' it (not saying that you claim the same) at the same time.
Mr. Pass has too awesome of a history in amp design for anyone to
doubt his actions. He also knows it's simple designs. Then again,
he also YEARS ago stated that chokes are too much a bother to deal
with, something you were searching for. I remember buying the white
paper "Power to the People" about hi voltage power supply design, a
great read (LONG LONG out of print) and the guy stated he 'designed'
a quad gated MOSFET preamp. He did. I've built a few, they work
great. And the same basic design was also in a book dealing with
the same Mosfet called : Mosfet Circuits Handbook, by Mr. Turner.
Years and worlds apart. Both 'designed' by talented Icons in the field.
It's like when Mesa/Boogie 'designed' the 1/2 power switch, by lifting
the cathode off 2 tubes..and the 'mid' switch by lifting the tone circuit
ground (sometimes bypassed by a cap). Sheit, I'd been doing that too,
and know folks in the UK who also did. No big deal. But no big 'design',
although M/B made it most available. Carvin and Acoustic 'stole' it
from M/B, sometimes as DIRECT copies..like the Acoustic. Even the
schematic was almost a carbon copy..right down to the mistakes.
At least 'Earth' was smart. The Xeroxed the PCBs that Peavey made, even
down to the etching 'PV' on the copper when they copied/stole PV's designs.
Sometimes the same idea hits many people at the same time. But sometimes,
people like the 'Earth/Plush' people just downright steal from the designers.
Gawd, this is a fun biz. So much history, so many tales untold.
JJTj
PREPARE FOR 'FRITZMANIA 2010'
Aug 16, 2010 Hampton Beach NH.
Live bands! Pro Wrestling!
Dancing girls! BBQ and Clambake!
Tickets Available Soon.
Keep informed at:
Morris Slutsky
Wasteful is relative. Electrically, a choke-loaded single-ended amp
is twice as efficient as a current-source-loaded single ended amp
(because you have dissipation in the CCS that you don't with a choke),
which is turn is twice as efficient as a resistor-loaded single ended
amp.
Yes the Zen designs are cool. But if I wanted a badass single ended
FET, a choke would be the most electrically efficient way. Cost
efficient? Weight efficient? Maybe not.
It is depressing that, despite the relatively low inductance needed in
a solid-state single ended circuit, the currents are higher and I need
just as much iron. Huh.
Morris Slutsky
Huh. Hammond quit making chokes?
I have the laminations from a burned power transformer originally
belonging to a little Gibson guitar amp. Provided something like 40
VA I'd guess. Wonder if that's enough iron to make a choke, or better
yet even two. It would have to be air-gapped, right? Or not?
Phil_S
"Morris Slutsky" <morris....@gmail.com> wrote in message
news:9360f4f1-c838-41fc...@o26g2000vbd.googlegroups.com...
> Thinking of a circuit like this. Simple and elegant-looking:
<snip for brevity>
> If I could find a choke this would be a nice cheap thing to build
> though. Easy enough.
I have no idea if it's the right thing but I've got a modest size choke I
can't use. It says on the case:
Filter Reactor
678-118-00 C.S.T.C 6516A
15 Henries at 10V 60CPS .020ADC (20mA)
DCR 515 +/-10% (meters 548 ohms) at 25 degrees C.
PWV 1070
Chicago Standard Transformer Corp
Square grey military style case, two terminal lugs on the bottom. This one
is real old school.
The case is about 2" x 2" x 2 5/8"h.
Mounting holes are on wings, 2 3/8" across and 1 1/2" along the wings.
Email me off list if you want it. Cheaper than cheap. Never been soldered
as far as I can tell.
Phil
Morris Slutsky
> "Morris Slutsky" <morris.slut...@gmail.com> wrote in message
Thanks for the offer, but I don't think that would do for this. The
low saturation current and extremely high dc resistance would make
this unworkable for the applications. I do appreciate it, though!
Lord Valve
Wind your own.
Use 16G magnet wire. For 50-100 mH, it won't be all that large.
If you have an LCR meter and a variable speed drill, you'll be
done in less than 15 minutes.
LV
Morris Slutsky
Hi LV,
Do you have any core suggestions? Besides my previously mentioned
pieces of an old Gibson power transformer, which might do, hopefully I
could squeeze a pair of chokes out of them. I thought about doing it
air-core but basically it'd be like 500 feet of wire to do it that way
and DC resistance would become a problem with any reasonable gauge for
that.
Les Cargill
Wouldn't it have to be iron? It's audio band...
> Besides my previously mentioned
> pieces of an old Gibson power transformer, which might do, hopefully I
> could squeeze a pair of chokes out of them. I thought about doing it
> air-core but basically it'd be like 500 feet of wire to do it that way
> and DC resistance would become a problem with any reasonable gauge for
> that.
--
Les Cargill
Jim Thompson
Years ago I bought from these people. Good prices, good service and
good advice...
http://www.mcdavisco.com/
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
legg
Burnt, it might be easier to dissassemble than otherwise, but a 40W
isolation transformer is physically too small (by about 25%). If it
was for a 40W PA, then it would be oversized by about the same amount
and allow for the scrapping of material that was distorted in the
effort. A lot of work.....but a choke has fewer things that can go
wrong than other types of structures, and the non-overlapping
laminations allow for fairly easy iterations.
Depends on what you want it to end up looking like. End covers and a
good paint job can cover a multitude of sins.
I'm surprised that a topology requiring a capacitively coupled output
stage is given serious consideration - there must be a thousand
reasons why not. If all the parts fell into your lap, it might make
sense. Thump at turn-on/off might be avoided here by loading the
speaker to the same rail as the choke, but then it would need to be a
non-polarized part. Like I said, there's no end of headaches being
embraced here.
RL
Morris Slutsky
Thank you for your advice!
I've SPICEd it and I think I could get away with as little as 10 mH.
I think that possibly I could just squeeze out 2 chokes from the iron
that I have. Air-gapping would, I think, be trial and error here.
Might be as little as a single sheet of paper, perhaps. Not sure.
The difference between high-voltage triode and low-voltage MOSFET
operation here seems to just be the number of turns of wire. The coil
holds the same amp-turns either way, that's all the iron cares about.
In fact the only difference here between para-feed and transformer
operation is whether I'd want to wind a secondary or not.
What I like about parafeed with an output capacitor is that I get good
PSRR by letting the choke filter the power rail. Also, I don't have
to fear frying somebody's audiophile speakers if they try this thing
out someday. I'm not really sure why everyone in the audiophile world
hates output capacitors so much - bypass them with film if you worry
about the highs, right? - nobody NEEDS to be ABLE to put DC out the
speakers. And lots of people love transformer coupled Williamson and
McIntosh style amplifiers which can't go down to DC either.
But if one was mad, absolutely mad for minimalism, they could simply
connect the speaker ACROSS the choke. The advantage of the FET here,
as opposed to the triode, is that the DC resistance of the choke can
be low enough due to the low number of turns that not much current
would flow through the speaker. If as LV suggests I use 16 gauge
wire, I could likely keep the DCR below 0.1 ohm meaning that an 8 ohm
speaker would pull only about a milliamp of current. Yes that's well
enough to make a nasty pop at turnon but probably won't damage it.
It'd be madness, I'm sure, absolutely insanity in the cause of
minimalism. PSRR would suck rocks, though.
Morris Slutsky
It's already disassembled. I took it apart out of curiosity as much
as anything, especially as the owner of the amplifier for some reason
didn't actually want his blown old parts back.
Anyway I just took the stack of E's, wrapped 64 turns of ordinary 22
gauge wire (with plastic insulation) around it, wired it up to a
doorbell transformer with a power resistor in series and tried to
measure the voltage drop to guess the reactance at 60 Hz.
Unfortunately, I don't own a LC meter. As it turns out, the
inductance varies wildly with how tightly the lams are squeezed
together, although definitely a millihenry or 10 is not impossible
here. Holding the I pieces in position was kind of tricky as the
whole assembly was vibrating. Probably these measurements don't
normally involve half an amp of current? I imagine that trying to do
a precision 'air gap' here is going to be a nightmare.
legg
<morris....@gmail.com> wrote:
>On Feb 6, 12:02�am, legg <l...@nospam.magma.ca> wrote:
<snip>
>> Burnt, it might be easier to dissassemble than otherwise, but a 40W
>> isolation transformer is physically too small (by about 25%). If it
>> was for a 40W PA, then it would be oversized by about the same amount
>> and allow for the scrapping of material that was distorted in the
>> effort. A lot of work.....but a choke has fewer things that can go
>> wrong than other types of structures, and the non-overlapping
>> laminations allow for fairly easy iterations.
>
>It's already disassembled. I took it apart out of curiosity as much
>as anything, especially as the owner of the amplifier for some reason
>didn't actually want his blown old parts back.
>
>Anyway I just took the stack of E's, wrapped 64 turns of ordinary 22
>gauge wire (with plastic insulation) around it, wired it up to a
>doorbell transformer with a power resistor in series and tried to
>measure the voltage drop to guess the reactance at 60 Hz.
>Unfortunately, I don't own a LC meter. As it turns out, the
>inductance varies wildly with how tightly the lams are squeezed
>together, although definitely a millihenry or 10 is not impossible
>here. Holding the I pieces in position was kind of tricky as the
>whole assembly was vibrating. Probably these measurements don't
>normally involve half an amp of current? I imagine that trying to do
>a precision 'air gap' here is going to be a nightmare.
>
Laminations are laminated for a reason - don't let them short out at
the edges, where the insulation stops. Insulate the clamping bolts.
The L value shouldn't change appreciably with lamination pressure, as
the x-sectional area of the magnetic media doesn't change, but there
will be increased fringing in the gap for looser stacks. Your I-piece
and E-Lams should be tapped flat to get good gap control, regardless
of the spacer medium used.
RL
Paul E. Schoen
"Morris Slutsky" <morris....@gmail.com> wrote in message
news:9360f4f1-c838-41fc...@o26g2000vbd.googlegroups.com...
Just got back on-line after being without power for a couple days due to
that nasty snowstorm that left over two feet of snow sticking to trees and
power lines...
Signal transformer makes a choke (CH-1) that is 100 mH at 1 amp, 1.5 ohms.
It weighs 2.3 lb and is about 3" cube.
I tried an LTSpice of the circuit and I could not come up with a
combination of resistors that resulted in any better than about 10%
efficiency. I don't see any advantage to this design except that the output
voltage exceeds the rail voltage when the MOSFET is off. Following is my
LTSpice ASCII file. I'm just curious if there is a real reason to use this
design.
Paul
--------------------------------------------------------------------
Version 4
SHEET 1 880 680
WIRE 192 -80 -80 -80
WIRE 192 -64 192 -80
WIRE 192 16 64 16
WIRE 192 32 192 16
WIRE 288 32 192 32
WIRE 384 32 352 32
WIRE 192 48 192 32
WIRE -80 80 -80 -80
WIRE 64 128 64 96
WIRE 144 128 64 128
WIRE 384 128 384 32
WIRE 64 224 64 208
WIRE 64 224 -16 224
WIRE 64 240 64 224
WIRE -80 320 -80 160
WIRE -16 320 -16 304
WIRE -16 320 -80 320
WIRE 64 320 -16 320
WIRE 192 320 192 144
WIRE 192 320 64 320
WIRE 304 320 192 320
WIRE 384 320 384 208
WIRE 384 320 304 320
WIRE 304 336 304 320
FLAG 304 336 0
SYMBOL nmos 144 48 R0
SYMATTR InstName M1
SYMATTR Value IRF510
SYMBOL res 48 0 R0
SYMATTR InstName R1
SYMATTR Value 400
SYMBOL res 48 112 R0
SYMATTR InstName R2
SYMATTR Value 100
SYMBOL res 48 224 R0
SYMATTR InstName R3
SYMATTR Value 10k
SYMBOL ind 176 -80 R0
SYMATTR InstName L1
SYMATTR Value 100m
SYMATTR SpiceLine Rser=.1
SYMBOL cap 352 16 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1000�
SYMBOL res 368 112 R0
SYMATTR InstName R4
SYMATTR Value 8
SYMBOL voltage -80 64 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 24
SYMBOL voltage -16 208 R0
WINDOW 3 -14 133 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(0 2 200 0 0 0 100)
TEXT -18 378 Left 0 !.tran .5 startup
Morris Slutsky
> "Morris Slutsky" <morris.slut...@gmail.com> wrote in message
> SYMBOL res 368 112 R0
> SYMATTR InstName R4
> SYMATTR Value 8
> SYMBOL voltage -80 64 R0
> WINDOW 123 0 0 Left 0
> WINDOW 39 0 0 Left 0
> SYMATTR InstName V1
> SYMATTR Value 24
> SYMBOL voltage -16 208 R0
> WINDOW 3 -14 133 Left 0
> WINDOW 123 0 0 Left 0
> WINDOW 39 0 0 Left 0
> SYMATTR InstName V2
> SYMATTR Value SINE(0 2 200 0 0 0 100)
> TEXT -18 378 Left 0 !.tran .5 startup
Hi Paul,
Yeah, I appreciate it. I really do have to think this through more.
Obviously there's an impedance mismatch here - 8 ohms versus 24. I
do, regrettably, only have 24 or 48 volts available with the junkbin
power transformer that I have, and it probably isn't beefy enough to
run 3 amps of idle current at 24 volts even if I could make a choke
that would take it. If I do 1 Amp of idle current a 12 volt supply
would be a better match. It's almost feeling like I should just put a
second winding on my 'choke' and make it a transformer output like a
tube designer would.
morris
Morris Slutsky
Hi Paul,
As in my previous post, yes, the efficiency is low. I guess I've got
a bad combination of supply voltage and idle current - doesn't really
match an 8 ohm speaker very well. A shame that I'm trying to use a
supply I already have.
But a choke load is much more efficient than a resistor load. About 4
times so. And twice as efficient as an active constant current
source. Only the choke load can actually give back the idle current
as necessary.
JJTj
>But a choke load is much more efficient than a resistor load.
Yes, but more hum and magnetic interference.
>And twice as efficient as an active constant current source.
I'd bet you don't know how to design CCS(s) that
interact with the circuit. Didn't Mr. Pass teach
you anything? Maybe it was over your head. Was for
me till I built a few. Once you see what happens
with SE designs, and how current is used, you get the
picture. I'm not talking single device CCS devices,
which are little better then resistors, I'm talking
controlled transistor/Mosfet devices that read the amp
and adjust to the need. A choke just sits there doing
the best it can. Fighting what the PS feeds it.
>Only the choke load can actually give back the idle current
>as necessary.
See what I said above. A choke is a one trick pony, often
done well, but a interactive CCS reads the circuit and does
what is needed to keep the current flowing full tilt.
Yeah, more parts. But better operation.
JJTj
Paul E. Schoen
"JJTj" <up yers.con> wrote in message
news:5u43n55kd1gd8k7k1...@4ax.com...
I was able to make an LTSpice simulation that gives about 37% maximum
efficiency with a 6 VDC power supply, 1.94 VAC input, and 3.13 VAC output
into an 8 ohm load, at 200 Hz. It appears to work from 20 Hz to 20 kHz. But
the biasing is very critical, and probably will be unstable with power
supply, temperature and device variations. I used an STD30NF06L MOSFET
which has a low turn-on voltage. R3 in the schematic is not needed, but the
V2 signal source should be low impedance.
I have some 100 mH chokes rated at 10 amps that I found at a Hamfest in a
pile of freebies. But they are probably 30 lb each.
I agree that this is not really a very practical design, and much better
alternatives exist.
Paul
--------------------------------------------------------------------------
SYMATTR Value STD30NF06L
SYMBOL res 48 0 R0
SYMATTR InstName R1
SYMATTR Value 6800
SYMBOL res 48 112 R0
SYMATTR InstName R2
SYMATTR Value 3900
SYMBOL res 48 224 R0
SYMATTR InstName R3
SYMATTR Value 10k
SYMBOL ind 176 -80 R0
SYMATTR InstName L1
SYMATTR Value 100m
SYMATTR SpiceLine Rser=1.5
SYMBOL cap 352 16 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 2200�
SYMBOL res 368 112 R0
SYMATTR InstName R4
SYMATTR Value 8
SYMBOL voltage -80 64 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 6
SYMBOL voltage -16 208 R0
WINDOW 3 -14 133 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 2.8 200 0 0 0 10000)
SYMATTR InstName V2
TEXT -16 376 Left 0 !.tran .5 startup
Morris Slutsky
> Yeah, more parts. But better operation.
>
> JJTj
Probably more parts would be better in every measurable way! You
don't build a single-ended class A amplifier for any objective reason
anyway. It's something you do for subjective, not objective reasons.
Like, say, building a guitar amplifier that uses 2 EL34 tubes with 300
V of plate supply and a 5K load. Not ideal in any design sense. I do
like the way it sounds, though.
> >But a choke load is much more efficient than a resistor load.
>
> Yes, but more hum and magnetic interference.
I understand that in general all coils will pick up interference. But
seriously, a choke that runs into an 8 ohm speaker?
> >And twice as efficient as an active constant current source.
>
> I'd bet you don't know how to design CCS(s) that
> interact with the circuit. Didn't Mr. Pass teach
> you anything? Maybe it was over your head.
Maybe it was over my head, maybe it wasn't, you decide I guess. Sure
I read his website already. And I guess I see his Aleph-type current
sources as not really single ended amplifiers. In fact, I know
someone with a Pass power amp and it sounds very very good. I could
almost see switching from a choke design to an Aleph type thing. But
that's really, really push-pull from my point of view. Nothing wrong
with that, I could have more than 1 transistor per channel - it'd be
so hilariously cool to do it with 1, though, you can understand that.
Efficiency is something that single ended designs often suck at. Push-
pull stages - ideal class B is 78.5% efficient, ideal class A push-
pull is 50% efficient, class AB lies in between. Ideal single ended
class A with an ideal choke or transformer is also 50% efficient.
With an ideal CCS load, 25%, and with a resistor load 12.5%.
Pass has a few designs up on his DIY site that lie between these
situations. He has, for example, resistor-loaded single ended amps
(with horribly low efficiency). He also suggests use of incandescent
lamps as loads, which have a strong temperature coefficient and could
perhaps even be seen as something BETWEEN a resistor and a CCS load.
He has CCS loaded single ended class A, the "Zen" amplifier.
And the Aleph is also sort of an in between thing, between CCS load
and push-pull class A.
> I'm not talking single device CCS devices,
> which are little better then resistors, I'm talking
> controlled transistor/Mosfet devices that read the amp
> and adjust to the need.
What the Aleph current source does, as far as I can tell, is sense the
current into the load and adjust itself accordingly. It's responding
to the bottom FET that actually is providing the transconductance
which drives the load. The amount of current sensing/adjustment seems
to be variable - from pure CCS on one side, to fully compensating load
current swings on the other - at that point, what you've got is a
pretty clever way to do push-pull without an explicit phase inverter
stage. A lot like an SRPP or some push-pull pentode stages that don't
require a phase inverter - works so long as you stay in class A. And
so you can push it up to the same efficiency level as a choke loaded
stage if you want to.
I will consider such designs. I suppose chokes being expensive and
heavy and all. But there's something kind of elegant and brute force
about a single MOSFET just sitting there heating away.
Assuming I can keep a stable bias on the damned thing and get a decent
choke in the first place, which might be pretty hard actually.
Morris Slutsky
Not practical but fun perhaps. Probably I really need a fancier
circuit that separates DC bias from signal feedback. I'll look into
the CH-1 and it's relatives (the CH-2 might actually be better for
this).
Thanks for all your advice.
Les Cargill
> On Feb 9, 12:05 pm, JJTj <up yers.con> wrote:
>
>
> Maybe it was over my head, maybe it wasn't, you decide I guess. Sure
> I read his website already. And I guess I see his Aleph-type current
> sources as not really single ended amplifiers. In fact, I know
> someone with a Pass power amp and it sounds very very good. I could
> almost see switching from a choke design to an Aleph type thing. But
> that's really, really push-pull from my point of view.
Huh. No, I'd just say he's using the CCS as an "active anti-resistor."
It's just how the JFET is biased. I think JFETs switch current,
not voltage - like tubes do.
> Nothing wrong
> with that, I could have more than 1 transistor per channel - it'd be
> so hilariously cool to do it with 1, though, you can understand that.
>
> Efficiency is something that single ended designs often suck at. Push-
> pull stages - ideal class B is 78.5% efficient, ideal class A push-
> pull is 50% efficient, class AB lies in between. Ideal single ended
> class A with an ideal choke or transformer is also 50% efficient.
> With an ideal CCS load, 25%, and with a resistor load 12.5%.
>
> Pass has a few designs up on his DIY site that lie between these
> situations. He has, for example, resistor-loaded single ended amps
> (with horribly low efficiency). He also suggests use of incandescent
> lamps as loads, which have a strong temperature coefficient and could
> perhaps even be seen as something BETWEEN a resistor and a CCS load.
I saw that, and I am surprised they would not add horrible
nonlinear distortions. But then again, they've used filament
light bulbs as current limiters on PA speakers for years.
> He has CCS loaded single ended class A, the "Zen" amplifier.
>
That thing is awesome looking.
> And the Aleph is also sort of an in between thing, between CCS load
> and push-pull class A.
>
>> I'm not talking single device CCS devices,
>> which are little better then resistors, I'm talking
>> controlled transistor/Mosfet devices that read the amp
>> and adjust to the need.
>
> What the Aleph current source does, as far as I can tell, is sense the
> current into the load and adjust itself accordingly. It's responding
> to the bottom FET that actually is providing the transconductance
> which drives the load. The amount of current sensing/adjustment seems
> to be variable - from pure CCS on one side, to fully compensating load
> current swings on the other - at that point, what you've got is a
> pretty clever way to do push-pull without an explicit phase inverter
> stage.
Okay, then why does it look like it's just a bias element to me? Is
it actually "sensing", or it just literally adding a DC bias? A
CCS is like a voltage source with a Zener somewhere ( wherever
it makes the thing act as a CCS). It's just
essentially a regulator - but a current regulator. Voltage can vary;
current is constant.
> A lot like an SRPP or some push-pull pentode stages that don't
> require a phase inverter - works so long as you stay in class A. And
> so you can push it up to the same efficiency level as a choke loaded
> stage if you want to.
>
> I will consider such designs. I suppose chokes being expensive and
> heavy and all. But there's something kind of elegant and brute force
> about a single MOSFET just sitting there heating away.
>
Oh yeah. Cool as all get out. And I could be wrong, but it looks
like the thing swings *current* more than it swings voltage.
> Assuming I can keep a stable bias on the damned thing and get a decent
> choke in the first place, which might be pretty hard actually.
Heh. I don't understand how the choke helps much at all. Not in a power
path like that. I always think of chokes as filters, as big ferrite bead
arrangements.
--
Les Cargill