Need experts for vexing hum problem
to...@fullnet.com
the ones I trust the most.
There is a guy who has built a tube-based preamp, from a commercial
DIY kit. The preamp causes significant hum at the main amplifier's
output, even when the preamp is NOT powered, and also when unpowered
and unplugged. (It also causes hum when powered.) Other signal
sources do not cause hum when connected to the amplifier. Other than
the hum, the preamp sounds good.
Some great minds at diyaudio.com have, so far, been unable to find a
solution. The discussion-thread is here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=116374
(Please ignore my embarrassing contributions to that thread.)
Here is a link to thumbnails of the preamp's schematics, pcb-layout
diagram, and photos:
http://home.comcast.net/~garyworld/site/?/photos/
[Please note that the filaments' (DC) supply's smoothing capacitance
is much larger than what is indicated on the power supply schematic.]
The grounding scheme is quite ugly, and IMO should at least eventually
be completely converted to a proper star-grounding scheme.
But my current thinking is that if we can solve the hum problem for
when the preamp is NOT powered, then we will probably have mostly
solved the problem for when it IS powered, probably with minimal
modifications.
Thanks, guys.
- Tom Gootee
MooseFET
<to...@fullnet.com> wrote in message
news:f48586f1-db70-4a87...@e25g2000prg.googlegroups.com...
Check your rectifier, use Inductance+Cap as your filter, and Do not overload
your final Tube.
The waving at the top of your DC supply is where you need to clean up.
Stick an AC volt-meter on DC line to check your progress. This tip brought
to you from the Retarded Community.
to...@fullnet.com
> <t...@fullnet.com> wrote in message
Thanks for the tips, MooseFET.
But none of that would explain why it also causes hum when it is not
even powered on.
MooseFET
<to...@fullnet.com> wrote in message
news:707e00cc-2cff-4920...@q78g2000hsh.googlegroups.com...
Then your suspected target is not the cause of the hummmmmmmmmm..........
Fred
Sounds like a "ground loop" to me. You must allow only one path
between your system and earth. Isolate all the round grounding pins on
power cords (green wire).
Not paying attention to this allows a.c. current to flow in your
signal return connections, inducing him, whether the "offending
unit's" power is on or off.
Fred
JeffM
>>This tip brought to you from the Retarded Community
>>
Unless Ken had a brain aneurysm AND switched providers,
this is a hijacked identity.
I suspect this is the same 13 year old
who hijacked Mike Terrell's identity previously.
Jim Thompson
wrote:
And mine.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
to...@fullnet.com
In the preamp, all circuit and signal i/o grounds now connect to the
chassis through only one point, and there is a 'safety disconnect'
network between that point and the chassis, consisting of 10R, 0.1uF,
and anti-parallel diodes, all in parallel. Only the earth ground
connects directly to the chassis. The chassis is steel, by the way.
The preamp causes hum even when unplugged.
But, when plugged in, the earth ground should stay connected to the
chassis, and to the circuit etc (only) through the disconnect network,
for safety (and maybe even legal) reasons, right?
Spehro Pefhany
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Fri, 29 Feb 2008 11:06:20 -0800 (PST), JeffM <jef...@email.com>
>wrote:
>
>>MooseFET <kensmith@ rahul.net> (IDENTITY FRAUD) wrote:
>>>>This tip brought to you from the Retarded Community
>>>>
>>tom@ fullnet.com wrote:
>>>Thanks for the tips, MooseFET.
>>> - Tom Gootee
>>
>>Unless Ken had a brain aneurysm AND switched providers,
>>this is a hijacked identity.
>>
>>I suspect this is the same 13 year old
>>who hijacked Mike Terrell's identity previously.
>
>And mine.
>
> ...Jim Thompson
Hijacking something that old is more along the lines of salvage. ;-)
Happy BD!
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Jim Thompson
<spef...@interlogDOTyou.knowwhat> wrote:
>On Fri, 29 Feb 2008 12:14:24 -0700, Jim Thompson
><To-Email-Use-Th...@My-Web-Site.com> wrote:
>
>>On Fri, 29 Feb 2008 11:06:20 -0800 (PST), JeffM <jef...@email.com>
>>wrote:
>>
>>>MooseFET <kensmith@ rahul.net> (IDENTITY FRAUD) wrote:
>>>>>This tip brought to you from the Retarded Community
>>>>>
>>>tom@ fullnet.com wrote:
>>>>Thanks for the tips, MooseFET.
>>>> - Tom Gootee
>>>
>>>Unless Ken had a brain aneurysm AND switched providers,
>>>this is a hijacked identity.
>>>
>>>I suspect this is the same 13 year old
>>>who hijacked Mike Terrell's identity previously.
>>
>>And mine.
>>
>> ...Jim Thompson
>
>
>Hijacking something that old is more along the lines of salvage. ;-)
>
>Happy BD!
>
>Best regards,
>Spehro Pefhany
Thanks! (I think ;-)
to...@fullnet.com
OK. But he has at least attempted to contribute to the on-topic
portions of this thread, which is more than can be said for all of the
off-topic posts about his alleged identity hijacking.
To MooseFET:
Dude! Since you do know something about electronics, why not use your
own unique identity?
If you're as young as they say you are, then your level of electronics
knowledge is quite impressive.
And you can think whatever you want. But I gotta say I don't think ID
highjacking is too cool.
- Tom
legg
Sounds (reads) like you've tackled a lot of issues, so far. There's
always a problem with iterations, that makes it possible to
accidentally pass over or defeat solutions.
First thong to fix is hum when off. Ignore all other issues.
With hum present, disconnect all other equipment. Leave disconnected
until powered-off hum is solved. This includes your 'computer scope'.
These connections will just complicate things. Use your ears to
measure the hum. When you can't hear objectionable levels, the problem
is on the road to being solved.
trannybox - the orange wire should follow the brown wire from the AC
line socket to the switch. Twisting uses the least hardware. You are
switching the neutral wire in this photo. This could explain a 60Hz
hum when the preamp is off.
You said that you reversed wires going to the switch. This wording is
subject to misinterpretation. The live terminal of the IEC inlet
(brown wire in your trannybox photo) should go through the fuse, to
the switch. From the switched terminal the returning connection should
run to the transformer primary.I am surprised that both transformer
primary wires are colour-coded brown. The orange wire on the polarized
neutral of the IEC inlet should follow the brown wire to the switch
and return unbroken to the transformer primary. Twisting it around the
live wire uses less hardware to accomplish this routing.
This is actually how the wiring is laid out in the 'preamp1' photo,
using printed wiring, but without the polarized inlet. The 2-pin 120V
hardware can also be polarized - the wider blade is supposed to be
neutral. Old Dynaco hardware may predate this convention.
There is no ground wire connection. Assuming plastic hardware, this
would still have connected to the transformer case, followed by a lead
to the chassis of the target load, to allow for leakage current return
to safety earth. If not present, connect the transformer case to the
chassis, anyways, even from a remote plastic box.
Check the line cord phasing of the power amplifier, it should match
that of the preamp, even if line and neutral are not properly wired
into your wall socket.
The power supply schematic does not use the same transformer you are
using. The original has a colour-coded primary, a grounded
core/screen. If you have an isolation transformer, use this
temporarily for the preamp, power amp and combined set to see if
there's a difference one way or another.
Ignore other issues until powered-off hum is defeated.
RL
to...@fullnet.com
But the amplifier hums when the preamp is its signal source, and
doesn't hum with any other signal source. And it hums with the preamp
as it source whether or not the preamp is powered, unpowered, or
unplugged.
As I said, it's 'vexing'.
Whoever solves this one will be given genius/hero credit, at
diyaudio.com .
- T
to...@fullnet.com
First things first: I wouldn't want to fix 'thongs'. I'd want to
unfix them. And I always ignore all other issues when the thong is
off and there's a hum. :-)) [Sorry. Couldn't stop myself.]
THANKS, RL! After first quick read, it sounds like you have
uncovered some significant-sounding issues that have NOT yet been
mentioned in the diyaudio.com thread! I will paste your post into
the discussion over there, if you don't object.
And I certainly agree that solving the hum problem for when the preamp
is unpowered is probably the key.
Michael A. Terrell
|------||-----|
| DO NOT |
| FEED THE |
| aioe.org |
| TROLL! |
|------||-----|
||
||
||
/|\\|/||||//|||/\???\\//\\\\/|?\/
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
JeffM
>>>I suspect this is the same 13 year old
>>>who hijacked Mike Terrell's identity previously.
>>
>>And mine.
>>
Spehro Pefhany wrote:
>Hijacking something that old is more along the lines of salvage. ;-)
>
Have a happy day, JT.
Jim Thompson
wrote:
>JeffM wrote:
Thanks! Feels just like any other day... happy, happy ;-)
Rich Grise
> But none of that would explain why it also causes hum when it is not
> even powered on.
Your preamp has a loose ground on its output, and is acting like an
antenna.
Good Luck!
Rich
to...@fullnet.com
It's your birthday?! "To the wine!", then, Jim.
I hope that your birthday is as happy as possible.
- Tom Gootee
Jim Thompson
>On Feb 29, 2:14 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@My-
>Web-Site.com> wrote:
>> On Fri, 29 Feb 2008 11:06:20 -0800 (PST), JeffM <jef...@email.com>
>> wrote:
>>
>> >MooseFET <kensmith@ rahul.net> (IDENTITY FRAUD) wrote:
>> >>>This tip brought to you from the Retarded Community
>>
>> >tom@ fullnet.com wrote:
>> >>Thanks for the tips, MooseFET.
>> >> - Tom Gootee
>>
>> >Unless Ken had a brain aneurysm AND switched providers,
>> >this is a hijacked identity.
>>
>> >I suspect this is the same 13 year old
>> >who hijacked Mike Terrell's identity previously.
>>
>> And mine.
>>
>> ...Jim Thompson
[snip]
>
>It's your birthday?! "To the wine!", then, Jim.
>
>I hope that your birthday is as happy as possible.
>
>- Tom Gootee
Thanks! I won't disappoint you ;-)
to...@fullnet.com
Thanks, Rich!
I think it was suggested that he check that. But you know how it
goes. I'll suggest it again.
This one has really 'been through the mill', already. The preamp's
owner/kit-builder had been on the verge of despairing and giving up.
The diyaudio.com discussion got rolling again, but a solution STILL
hasn't been found. That's why I decided to ask for help, here.
Here is a quote from the preamp owner's most-recent post (last night),
at diyaudio.com:
--------- quote:
measured at preamp out, preamp on:
http://home.comcast.net/~garyworld/preampon.jpg
the little numbers indicate a 2.5V 120 Hz wave, a bit modulated
(perhaps by small 60 Hz)
measured at speaker out of amp with both on:
http://home.comcast.net/~garyworld/bothon.jpg
this is a 25-30V 120 Hz wave with a more pronounced modulation by
something else.
Once I corrected the probe ground problem Tom noticed, the amp out
read flat when on and off, and the preamp measured flat when off but
plugged in.
For what it's worth, with the changes I made today the preamp hum
continues.
-------- end quote
Thanks again, Rich.
Regards,
Tom Gootee
Michael A. Terrell
>
> Thanks! I won't disappoint you ;-)
So, you've passed 'Old fart', and 'Geezer'. What's the next level?
;-)
Jim Thompson
<mike.t...@earthlink.net> wrote:
>Jim Thompson wrote:
>>
>> Thanks! I won't disappoint you ;-)
>
>
> So, you've passed 'Old fart', and 'Geezer'. What's the next level?
>;-)
Curmudgeon ?:-)
MooseFET
> MooseFET <kensmith@ rahul.net> (IDENTITY FRAUD) wrote:
>
> >>This tip brought to you from the Retarded Community
>
> tom@ fullnet.com wrote:
> >Thanks for the tips, MooseFET.
> > - Tom Gootee
>
> Unless Ken had a brain aneurysm AND switched providers,
> this is a hijacked identity.
I am fairly certain I haven't switched providers.
Michael A. Terrell
>
> On Fri, 29 Feb 2008 19:09:30 -0500, "Michael A. Terrell"
> <mike.t...@earthlink.net> wrote:
>
> >Jim Thompson wrote:
> >>
> >> Thanks! I won't disappoint you ;-)
> >
> >
> > So, you've passed 'Old fart', and 'Geezer'. What's the next level?
> >;-)
>
> Curmudgeon ?:-)
I guess that will work for the next decade or two. :)
Michael A. Terrell
You're still on Comcast, and in California.
legg
>On Feb 29, 6:28 pm, Rich Grise <r...@example.net> wrote:
>> On Fri, 29 Feb 2008 10:10:49 -0800, tomg wrote:
>> > But none of that would explain why it also causes hum when it is not
>> > even powered on.
>>
>> Your preamp has a loose ground on its output, and is acting like an
>> antenna.
>>
>> Good Luck!
>> Rich
>
>Thanks, Rich!
>
>I think it was suggested that he check that. But you know how it
>goes. I'll suggest it again.
>
>This one has really 'been through the mill', already. The preamp's
>owner/kit-builder had been on the verge of despairing and giving up.
>The diyaudio.com discussion got rolling again, but a solution STILL
>hasn't been found. That's why I decided to ask for help, here.
>
>Here is a quote from the preamp owner's most-recent post (last night),
>at diyaudio.com:
>
So this isn't your problem?
I doubt there will be any joy for news server users working on it
third-hand. It's bad enough when the OP is posting directly.
RL
MooseFET
<to...@fullnet.com> wrote in message
news:1bfe071e-fcc8-4ecb...@n77g2000hse.googlegroups.com...
- T
Ah, I've seen this problem, your input MIC/SIGNAL ground maybe opened. I
used to trace stereo and I've found a broken wire inside a MIC cable.
MooseFET
"MooseFET" <kens...@rahul.net> wrote in message
news:fqantg$s6t$1...@aioe.org...
> cable.
beware of missing words on my posts. My typing is bad, it can't keep up
with my thinking speed.
>
to...@fullnet.com
Hi RL,
Well, it's not actually MY preamp. But a few of us have made it our
problem. There's a collaborative effort underway to help a previously-
despairing preamp builder whose hum problem has persisted for far too
long, and has defied many valiant efforts to defeat it.
I copied your earlier long post about it to the thread at
diyaudio.com, where it helped spark a flurry of new activity and
discussion, and also helped to attract new participants. It has helpd
to build both momentum and morale.
I don't know how gratifying all of this might be, for anyone here who
contributes ideas and suggestions. But it HAS given new hope to Gary,
the preamp owner/builder, and to those who have been doggedly pursuing
The Answer, with him.
There have been many bright people involved in this effort, and still
'no joy'. And I must admit, that even when participating in the
original thread, trying to help diagnose and solve this type of
problem remotely, via a discussion thread, is often a frustrating,
slow process. That is one reason I finally decided to post a request
for help, here, in SED. We needed some fresh perspectives and ideas.
And I figured that '200 heads would be better than 20'. And I am
quite familiar with SED, and have the greatest respect for the minds,
here.
The link to the original and ongoing diyaudio.com thread is in post
#1, of this thread, by the way.
Doh! (Slaps forehead.) I hope that everyone here will consider
joining diyaudio.com, not only to directly participate in this effort,
but, mainly, because there is a whole lot of cutting-edge electronic
and scientific work being discussed there. (I kid you not in the
slightest. There are discussions covering in-depth component and
device physics, new and old digital stuff, PCB effects, extremely-
advanced transistor and MOSFET modeling topics, ALL kinds of circuits,
details of component selection, RF and electromagnetic field theory,
magnetics, waveguides, phased arrays, SMPS, every known type of
voltage and current regulation, materials science, optics,
woodworking, metalworking, and on and on and on (oh, and amplifier and
speaker designs of every type, too); plus... music!) Not only that,
but there are a lot of world-renowned designers and scientists who are
regulars, there, as well as a large number of other very-technically-
sophisticated people, from all over the world. And many of the people
there are just simply great to communicate with. (It is also very
well-moderated. So there are virtually no out-of-control jerks
allowed, which, after seeing what has happened to SED, is very, very
nice; quite civilized, by comparison.) As a bonus, there is also a
vitually unlimited supply of highly-motivated newbies who would be
forever grateful for even the smallest bit of mentoring (And as they
say, "You can't take it with you.".).
to...@fullnet.com
>
> - Show quoted text -
Thanks for the further suggestion, MooseFET!
I will pass it along. (Actually, I already posted this thread's URL
in the diyaudio.com thread. So your suggestion will be seen whether I
mention it there or not.)
- Tom
to...@fullnet.com
>
> - Show quoted text -
For those people who are using actual newsgroup-traffic feeds, who
might not be able to easily view previous posts in this thread, the
entire thread can be viewed and navigated at
http://groups.google.com/group/sci.electronics.design/browse_thread/thread/d4037b16bf44591d/28e4c3665610bf53
. The URL for the original diyaudio.com thread is in Post #1, as is a
link to the relevant schematics, layout diagrams, and photos.
- Tom
Robert Baer
Does "not powered" mean "turned off but not unplugged"?
If so, the hum comes from its transformer - capacitance from primary
to secondary...
legg
Yes, it really sounds like a lousy patch cord with an open ground on
one end. - the extreme case of 'ground loop'.
RL
legg
I copied the thread, but it still sounded like there were computers
and such still attached. Keep it simple.
RL
legg
Looking at the schematic, it seems that the output impedance of the
preamplifier is extremely high.
I mean 330K? Get real. Although the reciever may have an input that's
equally high, the idea is always to get the source impedance lower
than the receiver, if noise is a concern.
I think these tube could drive 33K and still develop a 3V signal with
only a 2% change in cathode current. (coupling cap 0.12 for 40Hz)
Hell, why isn't it 3K3? ......1uF /240V coupling still gives -3dB at
50Hz
RL
to...@fullnet.com
>
> - Show quoted text -
Thanks for taking the time to respond, Robert!
Either or both, unfortunately. Unplugging it from the AC Mains does
not change the unpowered hum behavior at all.
Regards,
Tom Gootee
to...@fullnet.com
> On Fri, 29 Feb 2008 23:28:49 GMT, Rich Grise <r...@example.net> wrote:
> >On Fri, 29 Feb 2008 10:10:49 -0800, tomg wrote:
>
> >> But none of that would explain why it also causes hum when it is not
> >> even powered on.
>
> >Your preamp has a loose ground on its output, and is acting like an
> >antenna.
>
> Yes, it really sounds like a lousy patch cord with an open ground on
> one end. - the extreme case of 'ground loop'.
>
> RL
Since this has been mentioned at least twice, I will definitely re-
emphasize checking it, in the diyaudio.com thread. Thanks!
to...@fullnet.com
>
> - Show quoted text -
I'm afraid you've completely lost me, there!
to...@fullnet.com
Hi RL,
This is a very interesting point, that had not yet been raised. Thank
you VERY much for the insight.
The fix-finding effort had mostly been concentrated toward star-
grounding problems, loop-area/coupling issues, and possible faulty
components, with (some but) relatively little thought given to whether
or not the actual circuit toplogy and component values were
reasonable. Since this was a commercial 'kit' (i.e. build-it-
yourself), which had been purchased by the builder, everyone tended to
assume, especially at first, that the fault could not be because of
such basic original design considerations. However, after seeing the
abysmally-inept grounding scheme, we should have become more
suspicious of the rest of the design, too, and probably sooner, but
mostly hadn't yet. Even with the grounding scheme issues, when I
first mentioned modifications that it appeared to need, I was reminded
that since it was a commercial design, it 'must be' OK, and any actual
design changes should not be needed. Whether or not my approach was
correct, in that case, that illustrates the mind-set that we all have
had to try to evolve beyond, apparently too gradually, as all of the
usual suspected problems have been found to not be the cause, or at
least as far as we can tell from remote locations. (And I don't know
whether or not it reinforces or negates the possibility that the
original design is at fault, but, the unit was also already sent back
to the manufacturer of the kit, who apparently could not fix the hum
problem.)
Your first point, I think it was, in your first post in this thread,
still both haunts and encourages me: Probably especially because all
of us except the unit's owner are trying to do all of this 'remotely',
I still often find myself assuming that there must be some simpler
problem that has been missed, or a proposed solution that would have
worked but didn't appear that it would, when tried, because of some
trivial mistake in interpretation or implementation, which couldn't
have been noticed without being there. Those types of lingering
doubts are probably not so good for remote contributors' morale.
Naturally, there are also problems with keeping the unit's owner on
track well-enough, so that he does provide needed answers,
measurements, etc, and try suggested modifications, which is
understandably difficult for him when there are too many suggestions
hitting him at once, for example. But it must be almost like hell,
sometimes, for HIM. :-) He has to try to discern what the concensus
is, and what the exact details are for suggested tests etc, after he
has tried to discern weightings for which contributors he should be
listening to, etc etc etc. However, the good news is that it appears
that the group is getting better at coordinating itself, and at
working together, better, to try to keep the unit's owner on one path
at a time, and following logical progressions, etc. But sometimes I
wish we could collect the entire 'best' progression of tests and
modifications, etc, and have him do it all over again.
Sorry to go on about all of that. It's still slow and unsure, doing
it remotely. And the peamp's owner has limited time to work on it,
usually. I'm still hoping it's something fairly trivial that simply
hasn't been mentioned, yet, although that appears to be getting less
and less likely.
Thanks again, very much, for the output impedance insight! I will
definitely pass it along to the group.
Highest regards,
Tom Gootee
JosephKK
> On Feb 29, 12:51 pm, "MooseFET" <kensm...@rahul.net> wrote:
>> <t...@fullnet.com> wrote in message
>>
>> news:f48586f1-db70-4a87...@e25g2000prg.googlegroups.com...
>>
>>
>>
>>
>>
>> your final Tube.
>>
>> The waving at the top of your DC supply is where you need to clean up.
>> Stick an AC volt-meter on DC line to check your progress. This tip brought
>> to you from the Retarded Community
>
> Thanks for the tips, MooseFET.
>
> even powered on.
>
My wild guess is a fully isolated power amp jack with no shield connection.
to...@fullnet.com
> On Fri, 29 Feb 2008 23:28:49 GMT, Rich Grise <r...@example.net> wrote:
> >On Fri, 29 Feb 2008 10:10:49 -0800, tomg wrote:
>
> >> But none of that would explain why it also causes hum when it is not
> >> even powered on.
>
> >Your preamp has a loose ground on its output, and is acting like an
> >antenna.
>
> Yes, it really sounds like a lousy patch cord with an open ground on
> one end. - the extreme case of 'ground loop'.
>
> RL
Hi RL,
Some good news!
I posted your message in the diyaudio.com thread. But, by the time I
got there to post it, I found the message below, from the preamp's
owner/builder
- Tom
------- QUOTE:
Great News...
It would seem the star grounding has worked; the 120 Hz hum is now
absent from the preamp out and amp out when preamp is off. There is
still 100mV 60Hz hum when they are on but I see this as an important
victory and feel very happy. It was previously theorized that this hum
was the sum of several issues possibly compounding each other.
Here is a pic of the current state. Once again, thanks for the
constant support and advice:
http://home.comcast.net/~garyworld/preampmarch1.jpg
Here is a close up of the starground:
http://home.comcast.net/~garyworld/starground.jpg
Please let me know if this can be improved to make it final.
Here is where you guys are going to get annoyed with me...I made the
grounding changes prescribed most recently by megajocke. I tested the
system without a successful outcome and proceeded to install the 4
(replacement) output caps and scrub all the pcb connections with
alcohol and toothbrush. Then I realized that the rca output cable was
still plugged into the old, now floating, output jack. So I can't
conclude which one of the three changes caused the improvement. I'm
assuming it's the ground scheme. Mark, I'm sorry about this. I will
gladly replace the old caps if you are interested in a conclusive
answer. Let me know.
Divide and conquer. Tom, I'm going to order resistors to step down the
scope probe signal to my computer. What is the next step?
You guys are the best. Talk to you on Monday.
gary
ps, I cleaned up the picture site: http://home.comcast.net/~garyworld/site/?/photos/
I will open up the tranny box and take pictures, I get the feeling
there are some improvements I can make with the fuse and switch
wiring.
------ END QUOTE
legg
The PC scope will be the largest source of 60Hz leakage current in an
ungrounded system. It's supply, being a major potential source of EMI,
has a built-in line filter that connects the chassis to the AC line
through typically 6800pf of safety capacitors. This is roughly ten
times what you'd find in audio or high quality low noise test
equipment.
With no ground wire on his wall socket, this current will travel
wherever it has to, to get back to ground or neutral.
RL
legg
Good news, it sounds like, anyways. Not quite as foolproof as a Dynaco
was, despite the simplicity of the new kit circuitry. Wonder what
happened to the guts of the PAT4 that's being canibalized. I hope he's
not using the same neon on-off switch.
I've heard no reference to what's connected to the preamp input or
output sockets, when powered-on testing is done.
What's the power amp input load look like?
What's the source - a shorting plug?
RL
Paul E. Schoen
<to...@fullnet.com> wrote in message
news:e9faffa5-c53a-4bee...@2g2000hsn.googlegroups.com...
Hi RL,
Some good news!
- Tom
------- QUOTE:
Great News...
http://home.comcast.net/~garyworld/preampmarch1.jpg
http://home.comcast.net/~garyworld/starground.jpg
gary
------ END QUOTE
I have a few observations and questions about the design and the layout. I
don't claim to be an audio expert, and I gave up using tubes about 40 years
ago, so there may be some element of "why" in the following:
1. The AC supply appears to be fed by a two-prong plug. There needs to be
an earth ground somewhere, and it should be at the same point for all
equipment enclosures. The shields for the signals should not be used for
this purpose. A separate ground wire should be used to connect the preamp
to the main amp.
2. The filaments of the tubes are fed by DC, which is a good idea to reduce
hum. But the filament supply ground seems to be connected to the same
ground as the signal reference, so the capacitor current will create some
120 Hz noise in the ground system. It would be better to have the entire
filament circuit isolated (and maybe tied to the chassis with a small RC
network so it doesn't float to a high potential)
3. I don't understand the purpose of V301A in the schematic2 for the
preamp. It seems like it is used as a constant current load resistance for
V300A, which actually provides the amplification. It is only dropping about
840 mW with 140 VDC and 6 mA. Why not a 22 kOhm 2 watt resistor?
4. What sort of output voltage is needed on R312? The +15 dB figure with no
reference indicates a power ratio of 34 times (6 mV/500 ohms), or about 2.5
mW. If it is in dBm, then it is 34 mW. The circuit can provide probably 50
VRMS at 3 mA, which is 150 mW into a 17 k ohm load.
5. What do you mean by ordering resistors to step down the scope probe
signal to the computer? You will be looking for signals in the microvolt
and millivolt range that are causing the hum, and stepping them down won't
help. And you can't just isolate the scope input with resistors. You really
need a differential input, with high impedance probes that can be placed at
nodes in the circuit to measure potentials across components to track down
the source of the noise. Otherwise, the capacitance of the scope ground
will induce noise into the circuit (if it is floating), or else it will
create a ground path. Either way, it won't help you.
6. It seems to me that this whole circuit could be made with an IC audio
amplifier and it could be powered by a 9 volt battery or maybe a wall wart
supply. But, then, I don't understand why anyone would design something
like this using tubes. Maybe there is some rationale behind using them for
high power output stages, where a "warmer" sound (IOW, pleasant distortion)
may be produced, but it seems that it is asking for trouble to use high
voltage components to amplify microvolts to millivolts or a few volts.
Good luck,
Paul
legg
wrote:
That is good news.
120Hz characteristic ripple is more likely in the powered-on circuit
that the powered-down version. The fact that the frequency didn't
change to predominantly 60Hz in the PC Scope waveforms in the
powered-down scope plots suggests that it originates in another piece
of operating equipment.- like the PC Scope itself.
RL
JosephKK
Yours may be poser, the real moose is more careful.
JosephKK
I hope you are ready for this. I just noticed that the shielded cables
have the shield connected at both ends. Ground loops. Select the ends
to cut and ground only from one end.
legg
wrote:
>to...@fullnet.com wrote:
>> ps, I cleaned up the picture site: http://home.comcast.net/~garyworld/site/?/photos/
>>
>> I will open up the tranny box and take pictures, I get the feeling
>> there are some improvements I can make with the fuse and switch
>> wiring.
>>
>> ------ END QUOTE
>
>I hope you are ready for this. I just noticed that the shielded cables
>have the shield connected at both ends. Ground loops. Select the ends
>to cut and ground only from one end.
I think that's something that only works on differential (two wire)
sheided loops. These are single ended signals, using the sheild as a
signal return path.
RL
Michael A. Terrell
Well, it works in TV and radio broadcast control rooms. if you
don't, you have an AC current flowing through the shield.
legg
<mike.t...@earthlink.net> wrote:
>legg wrote:
>>
>> On Sat, 01 Mar 2008 21:42:17 -0800, JosephKK <quiett...@yahoo.com>
>> wrote:
>>
>> >to...@fullnet.com wrote:
>>
>> >> ps, I cleaned up the picture site: http://home.comcast.net/~garyworld/site/?/photos/
>> >>
>> >> I will open up the tranny box and take pictures, I get the feeling
>> >> there are some improvements I can make with the fuse and switch
>> >> wiring.
>> >>
>> >> ------ END QUOTE
>> >
>> >I hope you are ready for this. I just noticed that the shielded cables
>> >have the shield connected at both ends. Ground loops. Select the ends
>> >to cut and ground only from one end.
>>
>> I think that's something that only works on differential (two wire)
>> sheided loops. These are single ended signals, using the sheild as a
>> signal return path.
>
>
> Well, it works in TV and radio broadcast control rooms. if you
>don't, you have an AC current flowing through the shield.
I'll have to try it on single ended audio myself sometime, but my
experience with duff patch cables suggests it's a misapplication.
I can see where it could work at HF.
RL
Paul E. Schoen
"Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
news:47CB0167...@earthlink.net...
> legg wrote:
>>
>> On Sat, 01 Mar 2008 21:42:17 -0800, JosephKK <quiett...@yahoo.com>
>> wrote:
>>
>> >to...@fullnet.com wrote:
>>
>> >> ps, I cleaned up the picture site:
>> >> http://home.comcast.net/~garyworld/site/?/photos/
>> >>
>> >> I will open up the tranny box and take pictures, I get the feeling
>> >> there are some improvements I can make with the fuse and switch
>> >> wiring.
>> >>
>> >> ------ END QUOTE
>> >
>> >I hope you are ready for this. I just noticed that the shielded cables
>> >have the shield connected at both ends. Ground loops. Select the ends
>> >to cut and ground only from one end.
>>
>> I think that's something that only works on differential (two wire)
>> sheided loops. These are single ended signals, using the sheild as a
>> signal return path.
>
>
> Well, it works in TV and radio broadcast control rooms. if you
> don't, you have an AC current flowing through the shield.
It is fine for the shield to act as the signal return path, and as such
needs to be connected to the signal ground at both the preamp and the amp.
But one problem I see is that there is no separate earth ground connection
to the chassis, and this should be done by means of a separate wire from
the preamp to the amp, with a single earth connection there. But it might
be OK if both units had 3-wire cords plugged into the same set of outlets.
Paul
Michael A. Terrell
>
> On Sun, 02 Mar 2008 14:35:03 -0500, "Michael A. Terrell"
> <mike.t...@earthlink.net> wrote:
> >
> >don't, you have an AC current flowing through the shield.
>
> I'll have to try it on single ended audio myself sometime, but my
> experience with duff patch cables suggests it's a misapplication.
>
> I can see where it could work at HF.
I was talking about audio equipment, nothing HF. The 60 HZ component
on a heavy braid on the video cable doesn't cause many problems. As far
as single ended, it depends on the power source for each piece of
equipment. OTOH, an AM radio transmitter too close to the control room
can cause fits, trying to filter the AM modulation out of the baseband
video. We had to add more 80 foot ground rods to lower the ground
resistance to an acceptable level.
Hum can be a pain in an older building, or where the equipment is
powered by different phases. If you want to run audio a long distance,
it should always be balanced, or converted to FO.
Michael A. Terrell
>
> It is fine for the shield to act as the signal return path, and as such
> needs to be connected to the signal ground at both the preamp and the amp.
> But one problem I see is that there is no separate earth ground connection
> to the chassis, and this should be done by means of a separate wire from
> the preamp to the amp, with a single earth connection there. But it might
> be OK if both units had 3-wire cords plugged into the same set of outlets.
Yes, good grounding is the first step in hum elimination. In the
areas i was talking about, all the equipment is bolted to grounded steel
racks.
memja...@yahoo.com
my name is Gary, I am the electronically impaired owner of the above
preamp. First let me thank Tom for going out of his way on my behalf.
We have been talking about this preamp on diyAudio.com. If it is not
mentioned above here is a link to some pictures and drawings:
http://home.comcast.net/~garyworld/site/?/photos/
It should become clear quickly that I am new to this but thanks to the
help I have been getting I am making progress.
The current issue is this: I have fixed the 120Hz ground related
problem. I have an FTT and oscilloscope application on my computer (I
notice has generated some concern) that tells me that I have a very
orderly looking 60Hz sine wave (100mV) measured at the amp's speaker
out. It is not subtle. I do not get this from either preamp or amp
when they are not connected to each other. The preamp has a 3-prong
outlet with earth ground connected to the chassis. The amp does not,
it has a 2 prong plug, it's tranny has a CT connected to the chassis.
One of the steps I took a while ago was to remove the preamp tranny
and place it in a box separated by a 20" braided umbillical. There is
a picture on the above page of it open, the black rectangle is the
back of a 3Amp fuse holder.
All help is greatly appreciated, please forgive tardy responses for
now I am juggling 3 forums.
thanks,
gary
legg
With 120Hz defeated, you'd go through the same series of iterations to
tackle the 60Hz problem, starting simply with a minimum of external
components or connections.
If you don't trust your ears, a battery-operated voltmeter permanently
across the speaker leads can give unambiguous AC output readings,
while individual items (such as your PC scope probe ground lead or
component line cords / line power ) are being alternately connected or
disconnected.
The input to the preamp should be shorted with a characteristic
impedance, when the source is disconnected.
You might occasionally venture to put the lid on the preamp to see if
there's any effect on a specific iteration.
RL
JosephKK
I strongly recommend converting the main amp to a three prong plug. It
is also a safety issue.
legg
wrote:
>
>I strongly recommend converting the main amp to a three prong plug. It
>is also a safety issue.
In an earlier post, the OP indicated that his house wiring is
two-wire.
RL
memja...@yahoo.com
> On Wed, 05 Mar 2008 03:30:25 GMT, JosephKK <quiettechb...@yahoo.com>
> wrote:
>
> In an earlier post, the OP indicated that his house wiring is
> two-wire.
>
> RL
Allow me to clarify. Most of my apartment is 2-prong wiring, I suspect
knob and tube over 50 years old. Renovations long before my arrival
here required a grounded plug in the kitchen. I am using this outlet,
with nothing else plugged in, to power an isolation transformer off of
which I run my home stereo. I work on and test the preamp using a 2-
prong power source (desk/work bench location,) but check all my
changes by bringing the preamp back to the grounded outlet, (listening
room.) I have also brought the preamp, amp and a test speaker to a
brand new house and powered them up right off the pole, feet from the
earth ground stake with no improvement.
It was suggested on another forum that if the preamp manifests hum
immediately when switched on with all the tubes removed, the
transformer might be the problem. Any thoughts on this?
Thanks,
gary
legg
>It was suggested on another forum that if the preamp manifests hum
>immediately when switched on with all the tubes removed, the
>transformer might be the problem. Any thoughts on this?
>
Are you saying that there is no hum now, with the preamp power switch
blocking the live line? Is the preamp power switch in the remote box?
Is the swich a neon indicator?
How far can you actually get, hooking up the simplest of preamp
connections, before hum becomes audible in the outputs of the
powered-on power amp?
At this time:
Is it the same noise in both left and right channels?
Is it the same amplitude with the preamp lid on?
Is your PC scope connected?
RL
Paul E. Schoen
"legg" <le...@nospam.magma.ca> wrote in message
news:9j9ss39m1i22lj0lt...@4ax.com...
For safety and for optimal audio performance, I would recommend
retrofitting with 3-wire outlets, or at least get a good ground connection
to a water pipe or a ground rod for earthing the enclosures.
Paul
memja...@yahoo.com
> On Wed, 5 Mar 2008 17:05:39 -0800 (PST), memjaych...@yahoo.com wrote
>
> Are you saying that there is no hum now, with the preamp power switch
> blocking the live line? Is the preamp power switch in the remote box?
> Is the swich a neon indicator?
>
> How far can you actually get, hooking up the simplest of preamp
> connections, before hum becomes audible in the outputs of the
> powered-on power amp?
>
> At this time:
>
> Is it the same noise in both left and right channels?
> Is it the same amplitude with the preamp lid on?
> Is your PC scope connected?
>
> RL
There is no hum when the preamp is switched off (butt connected and
plugged in.) I replaced the original switch with the little neon
lamp. The switch is now in the remote box. There is a pic of it opened
up here:
http://home.comcast.net/~garyworld/site/?/photos/
The black rectangle is the back of a 3A fuse holder.
As far as connections, I hope I understand you here, I get the hum
with the inputs shorted or with an ipod connected (but not plugged in,
running on battery power.) I do not have my probes connected or any
other gear plugged in. The hum is always consistent in volume and
equal in both channels regardless of volume adjustment. (I have
removed the balance pot.) Putting the lid on does not effect the hum
at all. No scope or DMM connected. With the amp already on the hum is
at full volume as soon as I throw the preamp switch. And again, with
or without tubes and/or rectifier tube.
thanks for helping,
gary
memja...@yahoo.com
> "legg" <l...@nospam.magma.ca> wrote in message
>
> news:9j9ss39m1i22lj0lt...@4ax.com...
>
> > On Wed, 05 Mar 2008 03:30:25 GMT, JosephKK <quiettechb...@yahoo.com>
> > wrote:
>
> >>I strongly recommend converting the main amp to a three prong plug. It
> >>is also a safety issue.
>
> > In an earlier post, the OP indicated that his house wiring is
> > two-wire.
>
> > RL
>
> For safety and for optimal audio performance, I would recommend
> retrofitting with 3-wire outlets, or at least get a good ground connection
> to a water pipe or a ground rod for earthing the enclosures.
>
> Paul
So this is yet another recommendation to accommodate an earth ground
to my amp. I'll get on it presently. I have an Adcom GFA 5500, is
anywhere inside on the chassis suitable for the earth connection?
thanks,
gary
memja...@yahoo.com
rereading your last post I see you mean that I should install 3-prong
outlets in my apartment. Believe me, I would if I could. As it is I
have run an extension from the grounded outlet in the kitchen, which,
again, warrants converting my amp to have an earth ground.
gary
legg
wrote:
>
I appreciate that safety is of some concern. This is particularly true
of higher voltage circuits, with vacuum tubes in 'em and marketed by
scoundrels, grounded or not. On a test bench, a low impedance ground
can be more of a hazard than a safety feature ( to the technician or
his stray relatives) if not carefully identified and deployed.
The OP says, in this thread, that he's tapped a ground wire off the
stove for third wire uses where deemed suitable; that he's using an
isolation transformer for the stereo.
Let's fix the amplifier first. Rewiring the house probably ought to
wait for better weather.
RL
JosephKK
I must have missed that one. Just the same the preamp has a three
prong plug. Go figure.
JosephKK
Without absolutely accurate schematics it is difficult to do much
better than guess.
Just the same there are serious issues with the PA design.
legg
>There is no hum when the preamp is switched off (butt connected and
>plugged in.) I replaced the original switch with the little neon
>lamp. The switch is now in the remote box. There is a pic of it opened
>up here:
>
>http://home.comcast.net/~garyworld/site/?/photos/
>
>The black rectangle is the back of a 3A fuse holder.
>
>As far as connections, I hope I understand you here, I get the hum
>with the inputs shorted or with an ipod connected (but not plugged in,
>running on battery power.) I do not have my probes connected or any
>other gear plugged in. The hum is always consistent in volume and
>equal in both channels regardless of volume adjustment. (I have
>removed the balance pot.) Putting the lid on does not effect the hum
>at all. No scope or DMM connected. With the amp already on the hum is
>at full volume as soon as I throw the preamp switch. And again, with
>or without tubes and/or rectifier tube.
Replace your Ipod on the preamp inputs with shorting plugs until hum
is solved. All other unused inputs on the preamp and power amp should
also be shorted.
Where is the previously mentioned isolation transformer located? Only
the preamp and power amp should be plugged into it, with no ground
wire from the isolation transformer casing or third wire socket pin
connected. Bring out this ground wire/isolation transformer case
connection to a lead wire for temporary attachment to hardware.
Leave the power amp on.
At this time:
1) Note effect on hum (sw off / sw on - in each case ) of a ground
wire connection to
0 1
a) b) - the power amp chassis only
c) d) - the preamp chassis only.
If there is a reduction in hum for any of the four conditions, note it
down.
2) Disconnect the preamp outputs from the power amp inputs and plug
your Ipod directly into both inputs of the power amp. ( A realistic
power amp input volume setting should be established that will be
maintained until hum is defeated.) There should be no hum at this
volume level. At maximum levels, without an program signal, you are
likely to hear breathing, hum, clicks and possibly SW radio.
a) b) Leave the ipod connected to the power amp inputs. Switch on/off
the disconnected preamp. There should be no effect.
3) Leave the Ipod connected to the power amp inputs. With the
disconnected preamp on, brush the outer shell (signal return) on the
end of one of the preamp's output audio connectors against the power
amp chassis. Note the effect.
4) Repeat this with the ground wire attached to
a) - the power amp chassis only.
b) - the preamp chassis only.
5) Leave the ipod connected to the power amp left input channel only.
With the disconnected preamp on, brush the outer shell of the power
amp right channel input patch cord against the preamp chassis.
6) Repeat this with the ground wire attached to
a) - the power amp chassis only.
b) - the preamp chassis only.
Share the results and we'll all think about it for a while.
RL
Paul E. Schoen
I still suspect the DC power supply for the filamants. The capacitors all
connect to the ground bus on the preamp PCB, along with the signal grounds.
This circuit should be isolated, or at most connected at one place, such as
the transformer CT. It is important to keep power grounds and signal
grounds separated. Capacitor charging currents can have peaks of 100 amps
or more, and even a 1 milliohm resistance in the ground circuit can create
100 mV of AC noise.
One clue that points to this possibility is that you get the hum with all
the tubes removed, so this circuit is the only one that is still operating.
Disconnect one of these transformer wires and see if the problem goes away.
Then maybe you will need to connect the grounds of all the capacitors to a
separate ground path by cuts and jumpers on the PCB.
I work with very high current equipment, and it can be surprising to read a
significant voltage between two points on a piece of copper bus bar that is
only a few microhms.
Good luck,
Paul
memja...@yahoo.com
this is good, I'll get on this later tonight. A couple questions:
> Replace your Ipod on the preamp inputs with shorting plugs until hum
> is solved. All other unused inputs on the preamp and power amp should
> also be shorted.
Can I just use old rca plugs with the signal and ground shorted or do
I need a resistor in there?
> Where is the previously mentioned isolation transformer located?
On an adjacent shelf to the stereo gear.
> Only the preamp and power amp should be plugged into it, with no ground
> wire from the isolation transformer casing or third wire socket pin
> connected. Bring out this ground wire/isolation transformer case
> connection to a lead wire for temporary attachment to hardware.
> Leave the power amp on.
Do you mean "with a ground wire from the isolation transformer casing
or third wire socket pin connected." ? If so, and using it's chassis,
I should just be able to use one of the exterior screws holding the
chassis together?
I'll get on this as soon as time allows.
Thanks,
gary
memja...@yahoo.com
> I still suspect the DC power supply for the filamants. The capacitors all
> connect to the ground bus on the preamp PCB, along with the signal grounds.
> This circuit should be isolated, or at most connected at one place, such as
> the transformer CT. It is important to keep power grounds and signal
> grounds separated. Capacitor charging currents can have peaks of 100 amps
> or more, and even a 1 milliohm resistance in the ground circuit can create
> 100 mV of AC noise.
>
> One clue that points to this possibility is that you get the hum with all
> the tubes removed, so this circuit is the only one that is still operating.
> Disconnect one of these transformer wires and see if the problem goes away.
> Then maybe you will need to connect the grounds of all the capacitors to a
> separate ground path by cuts and jumpers on the PCB.
>
> I work with very high current equipment, and it can be surprising to read a
> significant voltage between two points on a piece of copper bus bar that is
> only a few microhms.
>
> Good luck,
>
> Paul
One of the steps that contributed to our victory over the 120 Hz
component of this hum problem was scratching out 3 sections of the
ground tracks of the PCB around the filter caps. I have previously
asked the question (which remains unanswered) why there are 2 ground
tracks on the PCB, top and bottom, that are then joined back together
at every solder point and standoff connection. I can see that the
mounting holes are tinned from one side through to the other. Why not
then just make the board with one ground track? What could I be
missing here? Anyway, it wouldn't take much effort to separate the two
circuits, I might need some advice on how and where to start and stop
the two busses. I'll put together a diagram of the PCB with index
numbers near the components, that way one could say "cut at 3 and 11,
jumper that to 15..." and so on.
thanks,
gary
legg
>RL,
>
>this is good, I'll get on this later tonight. A couple questions:
>
>> Replace your Ipod on the preamp inputs with shorting plugs until hum
>> is solved. All other unused inputs on the preamp and power amp should
>> also be shorted.
>
>Can I just use old rca plugs with the signal and ground shorted or do
>I need a resistor in there?
A short will function. If you have some biult with resistors less than
50K, they'll probably work here. Commercial terminators will likely be
50, 75 or 600 ohms.
>
>Do you mean "with a ground wire from the isolation transformer casing
>or third wire socket pin connected." ? If so, and using it's chassis,
>I should just be able to use one of the exterior screws holding the
>chassis together?
A purchased isolation transformer will bring the ground wire through
unbroken, with contact to the transformer body and housing. A
connection to a screw terminal should work. If in Doubt check with an
ohmmeter.
>I'll get on this as soon as time allows.
Notify me at the from address, with nospam removed, any time. The
thread is getting buried.
RL
Paul E. Schoen
<memja...@yahoo.com> wrote in message
news:645ae339-63ee-475a...@e6g2000prf.googlegroups.com...
Mostly I suspect the capacitor labeled C106 on the schematic, which appears
to be actually C304 and C305 on the PCB layout. There's also a C306 in the
schematic that does not seem to be on the PCB. But it looks like the
filaments are tied to a ground strip that is also used as the reference
ground for the input and the output, and there will be current flow there
that will generate a voltage and create hum.
The tracks to the filamants should be separated from the other ground
tracks, and tied back directly to the negative side of the big filter caps,
along with the anodes of D103 and D104. There should be nothing else
connected to this separate ground. However, you might want to connect
something like a 1k resistor from this filament return circuit to ground
somewhere.
It's very confusing to have multiple parts with the same ref des. Usually
for circuits with left and right channels, you mi8ght have R101, R102,
C101, etc for the left, and R201, R202, C201, etc for the right. Components
that are shared, like the power supply, could be labeled R1, R2, C1, etc,
or R301, R302, C301, etc. Unfortunately the tubes are shared as A and B for
left and right channel, so they seem to be labeled V301 and V302. It may
have been better to have used one tube for the left and the other for the
right. Actually i still don't see the reason for V301A.
I think just a resistor would do as well, if not better. If tubes act as
transconductance, with output current proportional to grid voltage, then a
resistor would provide a linear voltage. If V301A acts as a constant
current source, I think you might get nonlinearity. But I have not really
analyzed the circuit completely. By now I would have just replaced the
whole thing with a wall-wart DC supply and a couple of ICs.
Paul
Eeyore
"Paul E. Schoen" wrote:
> Mostly I suspect the capacitor labeled C106 on the schematic, which appears
> to be actually C304 and C305 on the PCB layout. There's also a C306 in the
> schematic that does not seem to be on the PCB. But it looks like the
> filaments are tied to a ground strip that is also used as the reference
> ground for the input and the output, and there will be current flow there
> that will generate a voltage and create hum.
Ah, you spotted tha too !
Yes, it's rubbish. Typical sadly of homebrew circuits.
> The tracks to the filamants should be separated from the other ground
> tracks, and tied back directly to the negative side of the big filter caps,
> along with the anodes of D103 and D104. There should be nothing else
> connected to this separate ground. However, you might want to connect
> something like a 1k resistor from this filament return circuit to ground
> somewhere.
I agree 100%.
Graham
JosephKK
Alas it is based on the notion that the A and B parts of a dual tube
are better matched than two tubes of the same kind. There is no truth
in it though.
memja...@yahoo.com
I suspect that this preamp saw several evolutionary iterations before
it reached me in kit form. I have also wondered about C306, which
doesn't exist. Neither does C106. If you look on the line section
schematic you will see an H+ schematic on the bottom that shows C304
and C305. As you have noticed some of this doesn't reflect what I was
given to assemble, and the picture of the advertised preamp displays
quite a few different components. For instance, I recently replaced
the axial C301s and 303s with a radial version. It was suggested that
the axial ones having a coil construction would be inducing
current...needless to say I have replaced much of this thing already,
including the tubes.
I made a little diagram of how I interpret your proposed changes here:
http://home.comcast.net/~garyworld/pcblayout2.pdf
Is this correct?
gary
Paul E. Schoen
<memja...@yahoo.com> wrote in message
news:ec42a338-1149-4e26...@i12g2000prf.googlegroups.com...
Well, it's one step in the right direction, but now there is no current
path for the filaments back to the negative supply. So, remove the tracks
on V300 and V301, pin 5, and run wires from these terminals back to the
negative side of the big capacitors C304 and C305. The other grounds are
probably OK.
What's with the crazy tube pin numbers? Each tube should be numbered 1
through 9.
See my other more recent post on an LT Spice simulation of this circuit
using JFETs. For all the trouble with this circuit, you could build a
decent preamp with about $5 worth of parts and have it fit in a matchbox.
Why tubes? And if you want to be a tube purist, you really need to get rid
of the silicon diodes :)
Paul
memja...@yahoo.com
> <memjaych...@yahoo.com> wrote in message
> What's with the crazy tube pin numbers? Each tube should be numbered 1
> through 9.
sorry about that, as I was starting to understand more about this
stuff I found it easier to number the tube pins according to channel.
I have since learned that this is not common practice but haven't
corrected the drawing yet.
> See my other more recent post on an LT Spice simulation of this circuit
> using JFETs. For all the trouble with this circuit, you could build a
> decent preamp with about $5 worth of parts and have it fit in a matchbox.
> Why tubes? And if you want to be a tube purist, you really need to get rid
> of the silicon diodes :)
>
> Paul
I'll check that out. I'd like to build this $5 preamp and compare. The
honest truth is that I love music, I build my own speakers and I can't
afford high end gear so I thought I could learn to build it. This has
proven to be naive to say the least. Still, I am determined to go
until I either give up or lose interest in listening to great sounding
music. This preamp was an experiment, I wanted to know what the fuss
over tube gear is all about. I thought that I might build an all tube
system over a few years, some effecient speakers and give it a try.
Those plans have changed a bit, I'm thinking of building the Linkwitz
Orions next (hoping dipoles will get my neighbors off my back a bit,)
so I'll first be building 8 50W chip amps to power each driver. I've
seen several designs using the LM3875 and LM3886 chips. Do you have
any experience with these amps?
gary
Paul E. Schoen
<memja...@yahoo.com> wrote in message
news:2f2f3d22-0713-4c0b...@e23g2000prf.googlegroups.com...
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
I found some interesting links with a dogpile search for tube preamp:
http://www.sweetwater.com/shop/studio/preamps/buying-guide.php
http://www.hometheaterhifi.com/volume_5_4/99$preamplifier.html
http://www.vacuumstate.com/preamp.htm
http://hometheater.about.com/library/weekly/aa010599.htm
The more reasonable sites (particularly sweetwater) contend that the appeal
of vacuum tube circuits is mostly the "pleasant sounding" even harmonic
distortion that gradually appears when a tube circuit is overloaded, rather
than the abrupt and harsh odd harmonic square wave distortion that occurs
with solid state circuits.
More of the vacuum tube cultism is evident in the $99 preamp site, and the
bottlehead site, although I must admit that there is a serious "cool"
factor with the finely handcrafted enclosures and exotic glass tubes that
glow and emit comforting infrared rays. But the description of the sound
"qualities" seems more like wine tasting, where minuscule impurities are
actually celebrated, and the ability to detect such nuances is the big
deal.
For myself, I have some hearing loss, so extreme hi-fi is wasted on me. I
have seen this thread as a learning experience, and it was interesting to
refresh my knowledge of tube circuits. I also enjoyed translating the
circuit to JFETs and running a simulation. If I had a real need for a
preamp, I would probably put together the simple circuit and use it. But I
have a lot of perfectly good audio gear that I have never (or very rarely)
used, and I have other projects that demand my time.
I don't have any experience with the LM3875, but I see it has made it into
the Wiki. It is a linear amplifier, which is inherently inefficient, and I
have more interest in PWM type "Class D" amplifiers. See
http://www.irf.com/technical-info/appnotes/an-1071.pdf.
I designed a 35 watt amplifier for a project some years ago, using
push-pull darlington output transistors and an op-amp. It was for 50/60 Hz
only, but it had to produce 120 VAC as well as 24 VAC, with short circuit
protection. For another application, we used a 100 watt monolithic power
Op-Amp, which had to operate up to 500 Hz. These were linear amplifiers,
and I was working on various ways to increase efficiency. My early efforts
to design a PWM amplifier (in the early 1980s) was unsuccessful, but I
learned a lot.
For raw power, I remember seeing an Unholtz-Dickie shaker amplifier at the
place where my father worked in the mid 1960s, and it had vacuum tubes that
were probably 3 feet high and a foot in diameter. This was for testing
components of missiles for survival under extremes of mechanical vibration
and shock. The amplifier was rated at several kW, and was water-cooled. I
thought it was a shame to use boring sine waves of fixed frequencies. I
said they should just play some rock and roll! The company still exists,
and they have a 480 kW shaker. Now that's some amplifier!
http://www.udco.com/largetseries.shtml
Let me know how your project turns out. I think many of us learned
something from this discussion.
Paul