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Distortion from audio power amp
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Jan Panteltje
Guest





PostPosted: Sun May 16, 2004 9:46 pm    Post subject: Re: Distortion from audio power amp

On a sunny day (16 May 2004 09:32:45 -0700) it happened stratus46@yahoo.com
(Glenn Gundlach) wrote in <acb22b57.0405160832.37c76141@posting.google.com>:

Quote:
Marshall Leach took this about as far as I've ever seen. Check this
out.
http://users.ece.gatech.edu/~mleach/lowtim/graphics/ckt.pdf

OK, OK, its complementary again but its a good amp.
GG
Wow, that is the good stuff, others please note the power limiting

circuit with the 2 transistors in the output, this makes the amp
safe at any load.
(it is in fact current foldback).
The DUAL diff amp (completely symmetric) I have never used.....
JP
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Terry Pinnell
Guest





PostPosted: Sun May 16, 2004 10:04 pm    Post subject: Re: Distortion from audio power amp

Stefan Heinzmann <stefan_heinzmann@yahoo.com> wrote:

Quote:
BTW how did you determine the resistor value? I'm curious regarding your
reasoning.

I'd had recommendations of 0.5R (" agood place to start") and,
elsewhere, 0.1R (no rational, but from an electronics lecturer whose
opinion I vale). So my 0.2R was largely a compromise. At 3A (a mere
20% of the 2N3055's rating), there will be about 600mV across them.
How does that seem to you? Anyway, I have yet to try out the result.

--
Terry Pinnell
Hobbyist, West Sussex, UK
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Terry Pinnell
Guest





PostPosted: Sun May 16, 2004 10:06 pm    Post subject: Re: Distortion from audio power amp

Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote:

Quote:
On Sun, 16 May 2004 10:32:50 +0100, Terry Pinnell
terrypin@dial.pipex.com> wrote:

"petrus bitbyter" <p.kralt@reducespamforchello.nl> wrote:

Terry,

IMHO R11 is at the rihgt place, but the emiiter of Q4 has to be connected to
the collector of Q6. Found this and some other tips in an (old? 1973?) book,
on the subject. This type of amplifiers were never known for their
stability. I posted a schematic on alt.binaries.schematics.electronic that
contains eight clues to stabilise an amplifier like this. I consider the
author a specialist as I did a lot of amplifier (re)building using this book
and never failed.

petrus bitbyter

Thanks both. I duly shifted it yesterday (making them 0.2R resistors
instead of 0.5R while I was about it.) But I now see that might have
been hasty!

Let's see what else the earlier recommnders have to say on this. If I
can get a cleanish output the way it is at present (0.2R in Q5E and
Q6E), I'll be happy.

Has the original design been updated yet in light of all these
modifications to it? It's hard to keep track of all the changes. It's
thrown up some interesting points, this thread.

Well, the last one I drew is not too far off. But I'll redraw shortly,
including all changes made so far.

--
Terry Pinnell
Hobbyist, West Sussex, UK
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Jan Panteltje
Guest





PostPosted: Mon May 17, 2004 1:04 am    Post subject: Re: Distortion from audio power amp

On a sunny day (Sun, 16 May 2004 19:04:33 +0100) it happened Terry Pinnell
<terrypin@dial.pipex.com> wrote in
<o1bfa0lvt9epmh4dhdm5g94pra6gievdrd@4ax.com>:

Quote:
Stefan Heinzmann <stefan_heinzmann@yahoo.com> wrote:

BTW how did you determine the resistor value? I'm curious regarding your
reasoning.

I'd had recommendations of 0.5R (" agood place to start") and,
elsewhere, 0.1R (no rational, but from an electronics lecturer whose
opinion I vale). So my 0.2R was largely a compromise. At 3A (a mere
20% of the 2N3055's rating), there will be about 600mV across them.
How does that seem to you? Anyway, I have yet to try out the result.

LOL design by democracy, this one should be in the text books Wink

No, sorry, it is not the best way to do it.
In fact this makes litle sense to me.
You must ask yourself:
What does this resistor do, if you see nothing then what will happen
without it?
And if you think it does something, what voltage will be across it,
and what does that do?
Well, I dunno your background, but it does pay of to take a piece of paper
and try to design an amp yourself.
Build it, see what happens, or goes wrong.
A scope is a big plus, signal generator you can use 50 or 60 cycles mains.
I have bad experieces in my youth building diagrams from magazines etc..
usually it did not work....
Electuur (now elector) the errata would be next month hehe.
So best way to see how it is done is look at some diagrams from commercial
amps perhaps.
Get a good book on electronics design, a quantity of transistors (these are
cheap these days), and try some basic circuits.
Nothing beats practical experience.
So you are on the right way.
As for the resistor, look at the peak current it will carry.
Say (not this example specific) you have a 20 V pp (peak peak) sinewave.
And an 8 Ohm speaker
Now you can calculate the peak current.
(10 / R, the thing is only doing half the work).
From that the peak voltage across that resistor.
Here already you can see, that putting any resistor in series with an
output transistor causes some voltage drop, that in turn reduces the
possible peak to peak output voltage swing, and so max output power, so
you would rather have no resistors at all.
So for the resistor there must be a VERY good reason to have it.
One such reason would be to use the voltage drop across it to open a
transistor to limit output current (protection).
An other reason could be to provide some DC current stabilization as the
thing gets hot (as in this case), or improve linearity.
If the transistor gets hot, then for the same Vbe there will be more Ib,
and you get thermal run away, (more current, more heat, more current,
more heat), and it goes kaput.
So in the top transistor emittor R10 with the voltage created by diodes
diodes D1 and D2 forms quiesent current stabilization circuit.
Putting a resistor in emittor of Q6 would not do much, except perhaps
linearization.
http://www.swtpc.com/mholley/LittleTiger/PE_DEC_1967_P30.jpg
is a lot better already!
No resistors in the output stage, no voltage drops.
But no protection!
This protection is essential for any serious power amp, look at that big pdf
someone posted here:
http://users.ece.gatech.edu/~mleach/lowtim/graphics/ckt.pdf
I am writing all this because I cannot get it together to draw the ASCII
diagram of one with protection, but look at that .pdf.
Maybe to complicated for you, but look at transistor 443 and 444
443, is base is connected to a voltage divider that gets voltage from
the emittor resistors in the output, and very important, diode '8' and
resistor 563 to ground.
When to much current flows in the output pull up, tr 443 opens,
and then pulls the drive voltage low on tr447.
When the output voltage is high (close to +V), the circuit 563 to ground
changes the bias of tr443 so it limits at a higher current.
So you get foldback current limiting.
This implies if the speaker output is shorted, only a low (non destructive)
current will flow in the output.
OK.
JP
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Terry Pinnell
Guest





PostPosted: Mon May 17, 2004 11:26 am    Post subject: Re: Distortion from audio power amp

Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:

Quote:
On a sunny day (Sun, 16 May 2004 19:04:33 +0100) it happened Terry Pinnell
terrypin@dial.pipex.com> wrote in
o1bfa0lvt9epmh4dhdm5g94pra6gievdrd@4ax.com>:

Stefan Heinzmann <stefan_heinzmann@yahoo.com> wrote:

BTW how did you determine the resistor value? I'm curious regarding your
reasoning.

I'd had recommendations of 0.5R (" agood place to start") and,
elsewhere, 0.1R (no rational, but from an electronics lecturer whose
opinion I vale). So my 0.2R was largely a compromise. At 3A (a mere
20% of the 2N3055's rating), there will be about 600mV across them.
How does that seem to you? Anyway, I have yet to try out the result.

LOL design by democracy, this one should be in the text books Wink
No, sorry, it is not the best way to do it.
In fact this makes litle sense to me.
You must ask yourself:
What does this resistor do, if you see nothing then what will happen
without it?
And if you think it does something, what voltage will be across it,
and what does that do?
Well, I dunno your background, but it does pay of to take a piece of paper
and try to design an amp yourself.
Build it, see what happens, or goes wrong.
A scope is a big plus, signal generator you can use 50 or 60 cycles mains.
I have bad experieces in my youth building diagrams from magazines etc..
usually it did not work....
Electuur (now elector) the errata would be next month hehe.
So best way to see how it is done is look at some diagrams from commercial
amps perhaps.
Get a good book on electronics design, a quantity of transistors (these are
cheap these days), and try some basic circuits.
Nothing beats practical experience.
So you are on the right way.
As for the resistor, look at the peak current it will carry.
Say (not this example specific) you have a 20 V pp (peak peak) sinewave.
And an 8 Ohm speaker
Now you can calculate the peak current.
(10 / R, the thing is only doing half the work).
From that the peak voltage across that resistor.
Here already you can see, that putting any resistor in series with an
output transistor causes some voltage drop, that in turn reduces the
possible peak to peak output voltage swing, and so max output power, so
you would rather have no resistors at all.
So for the resistor there must be a VERY good reason to have it.
One such reason would be to use the voltage drop across it to open a
transistor to limit output current (protection).
An other reason could be to provide some DC current stabilization as the
thing gets hot (as in this case), or improve linearity.
If the transistor gets hot, then for the same Vbe there will be more Ib,
and you get thermal run away, (more current, more heat, more current,
more heat), and it goes kaput.
So in the top transistor emittor R10 with the voltage created by diodes
diodes D1 and D2 forms quiesent current stabilization circuit.
Putting a resistor in emittor of Q6 would not do much, except perhaps
linearization.
http://www.swtpc.com/mholley/LittleTiger/PE_DEC_1967_P30.jpg
is a lot better already!
No resistors in the output stage, no voltage drops.
But no protection!
This protection is essential for any serious power amp, look at that big pdf
someone posted here:
http://users.ece.gatech.edu/~mleach/lowtim/graphics/ckt.pdf
I am writing all this because I cannot get it together to draw the ASCII
diagram of one with protection, but look at that .pdf.
Maybe to complicated for you, but look at transistor 443 and 444
443, is base is connected to a voltage divider that gets voltage from
the emittor resistors in the output, and very important, diode '8' and
resistor 563 to ground.
When to much current flows in the output pull up, tr 443 opens,
and then pulls the drive voltage low on tr447.
When the output voltage is high (close to +V), the circuit 563 to ground
changes the bias of tr443 so it limits at a higher current.
So you get foldback current limiting.
This implies if the speaker output is shorted, only a low (non destructive)
current will flow in the output.
OK.
JP


Thanks for all the suggestions. But you need to bear in mind the basic
premise explained clearly at the outset. Maybe you haven't studied the
whole thread? This is a 23-year old project using a 28+ year old
circuit which I have dusted off and am trying to get working. I would
inded like to incorporate improvements at the same time. But I don't
plan a major redesign (which would require a rebuild). Even adding R10
and R11 was physically difficult. If I was starting from scratch it
would be an entirely different matter.

It's not unknown for me to design circuits from scratch. And I do know
how to use pen, paper and calculator, read a PDF, etc, etc; perhaps
even deploy Spice when appropriate. But in this situation I feel no
shame whatsoever in taking the shortcut of accepting informed advice.
And if others' estimates vary, I also feel it reasonable to make my
own judgements.

Getting back to what I thought was your simple original question: what
value of R10 and R11 do *you* propose, if neither 0.5 nor 0.2R?

--
Terry Pinnell
Hobbyist, West Sussex, UK
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Jan Panteltje
Guest





PostPosted: Mon May 17, 2004 10:15 pm    Post subject: Re: Distortion from audio power amp

On a sunny day (Mon, 17 May 2004 08:26:25 +0100) it happened Terry Pinnell
<terrypin@dial.pipex.com> wrote in
<tfpga0hhguopf2756na7erpda94440n497@4ax.com>:
Quote:

It's not unknown for me to design circuits from scratch. And I do know
how to use pen, paper and calculator, read a PDF, etc, etc; perhaps
even deploy Spice when appropriate. But in this situation I feel no
shame whatsoever in taking the shortcut of accepting informed advice.
And if others' estimates vary, I also feel it reasonable to make my
own judgements.

Getting back to what I thought was your simple original question: what
value of R10 and R11 do *you* propose, if neither 0.5 nor 0.2R?
I cannot answer a question of 'how would you pump limonade in the

ultrasonic plane'.
It makes no sense, you do not even specidy where those resistors are now.
And building a veroboard with this circuit takes about an hour.
JP
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Terry Pinnell
Guest





PostPosted: Tue May 18, 2004 11:49 am    Post subject: Re: Distortion from audio power amp

Winfield Hill <Winfield_member@newsguy.com> wrote:

Quote:
Terry Pinnell wrote...

Don Pearce wrote:

The amp is oscillating because there is no dominant pole compensation.
A small cap from collector to base of Q2 (the voltage amplifier)
should make things much nicer.

Thanks, I'll try that tomorrow.

I think Don meant to place the cap from Q1's base to Q2's collector.
But first change R7 to 47k, and R2 to 2.7k plus a 5k pot. Then use
3.3pF compensation (for 1 MHz loop bandwidth). There should also be
a 1k resistor in series with C3 for stability over the gain range.

Simulations with those changes appear to prevent setting DC output
level anything close to the desired 12V. (This is regardless of R4
setting.) It seems R7 must be much higher than you suggest. Perhaps
the original author's 470k might be a tad high, but 330k looks good.

--
Terry Pinnell
Hobbyist, West Sussex, UK
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Winfield Hill
Guest





PostPosted: Tue May 18, 2004 2:17 pm    Post subject: Re: Distortion from audio power amp

Terry Pinnell wrote...
Quote:

Winfield Hill <Winfield_member@newsguy.com> wrote:

Terry Pinnell wrote...

Don Pearce wrote:

The amp is oscillating because there is no dominant pole compensation.
A small cap from collector to base of Q2 (the voltage amplifier)
should make things much nicer.

Thanks, I'll try that tomorrow.

I think Don meant to place the cap from Q1's base to Q2's collector.
But first change R7 to 47k, and R2 to 2.7k plus a 5k pot. Then use
3.3pF compensation (for 1 MHz loop bandwidth). There should also be
a 1k resistor in series with C3 for stability over the gain range.

Simulations with those changes appear to prevent setting DC output
level anything close to the desired 12V. (This is regardless of R4
setting.) It seems R7 must be much higher than you suggest. Perhaps
the original author's 470k might be a tad high, but 330k looks good.

I suggested 47k for R7 because it's easier to establish the bandwidth
with a feedback-stabilizing capacitor, and to reduce the output dc-
bias voltage uncertainty from Q1's base current. (R4 is meant to help
establish the output dc bias current, it has little or no effect on the
output's dc bias voltage.) Q1 and Q2 need about 1.3V at Q1's base (the
R2 R7 junction) to operate, so we want R2 = 1.3/(12-1.3) x R7. So if
R7 = 47k, then R2 = 0.12 R7 = 5.71k more or less. Or a 2.7k + 5k pot
for a 2.7k to 7.7k adjustment range. That should work out just fine.
Don't forget the 1k resistor in series with C3.

With a say 50kHz overall bandwidth we'd want a 68pF cap from Q1's base
to Q2's collector (sorry for my earlier miscalculation).

If you want to keep R7 = 470k, then add a 10k resistor in series with
C3, and a 5 to 10pF cap from Q1's base to Q2's collector.

BTW, I'd return R8 directly to Q6's emitter.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
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Winfield Hill
Guest





PostPosted: Tue May 18, 2004 4:35 pm    Post subject: Re: Distortion from audio power amp

Terry Pinnell wrote...
Quote:

Simulations with those changes appear to prevent setting DC output
level anything close to the desired 12V. ...

Terry, are you setting R2 to the middle of its range? It appears
that you often have the R2 pot = 0 (or full) in your simulations.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
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Jim Thompson
Guest





PostPosted: Tue May 18, 2004 6:18 pm    Post subject: Re: Distortion from audio power amp

On Tue, 18 May 2004 08:49:08 +0100, Terry Pinnell
<terrypinDELETE@THISdial.pipex.com> wrote:

[snip]
Quote:

Simulations with those changes appear to prevent setting DC output
level anything close to the desired 12V. (This is regardless of R4
setting.) It seems R7 must be much higher than you suggest. Perhaps
the original author's 470k might be a tad high, but 330k looks good.

You are placing too much reliance on simulations without any support
from understanding how it functions.

Why don't you try a little pencil-to-paper analysis?

...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 |

I love to cook with wine. Sometimes I even put it in the food.
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Terry Pinnell
Guest





PostPosted: Thu May 20, 2004 7:18 pm    Post subject: Re: Distortion from audio power amp

Winfield Hill <Winfield_member@newsguy.com> wrote:

Quote:
Terry Pinnell wrote...

Simulations with those changes appear to prevent setting DC output
level anything close to the desired 12V. ...

Terry, are you setting R2 to the middle of its range? It appears
that you often have the R2 pot = 0 (or full) in your simulations.

Not sure why you say that? Yes, I've been setting it around 50%,
corresponding to the original spec of 47k, with occasional experiments
at 95% and 5%. BTW, Circuitmaker won't allow either 0% or 100%; has to
be 1%<R<99%.

In present circuit
http://www.terrypin.dial.pipex.com/Images/PushPullBreadboardNow.gif
I've reverted to fixed resistor of 47k. I had that working fine.
Swapping to various other fixed, 39k - 82k (with Vcc = 24.0 V) appears
to make little visible difference.

As you see from my new thread
Subject: Are these 2N3055s borderline spec?
my most exasperating problem now is the lack of repeatability! My best
result today as you see in
http://www.terrypin.dial.pipex.com/Images/PushPullBreadboardPuzzle.gif
has some crossover distortion, and I'm sure I wasn't seeing that in my
best result yesterday... Hmm, I was working outside in the garden, in
bright sunshine; wonder if somehow that affected it?

--
Terry Pinnell
Hobbyist, West Sussex, UK
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Jan Panteltje
Guest





PostPosted: Thu May 20, 2004 8:47 pm    Post subject: Re: Distortion from audio power amp

On a sunny day (Thu, 20 May 2004 16:18:09 +0100) it happened Terry Pinnell
<terrypinDELETE@THISdial.pipex.com> wrote in
<jaipa0ls61v88jeh866h5d15j104ie4cmv@4ax.com>:

Quote:
In present circuit
http://www.terrypin.dial.pipex.com/Images/PushPullBreadboardNow.gif
I would connect left side C4 to base Q2, and make it 330pF at least.

(Without spice, without testing without thinking).

Quote:
I've reverted to fixed resistor of 47k. I had that working fine.
Swapping to various other fixed, 39k - 82k (with Vcc = 24.0 V) appears
to make little visible difference.
You should adjust for a few mA quiecent current.

The cross-over should beless, and, since you have 4 be junctions, why not
use 4 diodes, and a trimpot.

or like this
--------------- base q3
| |
/ | c
\ |/
/<---------|
\ I0 |\/ NPN e
/ |
| |
-----------------base q4

Quote:
http://www.terrypin.dial.pipex.com/Images/PushPullBreadboardPuzzle.gif
You did not reverse emittor base did you?

JP
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John Popelish
Guest





PostPosted: Thu May 20, 2004 9:27 pm    Post subject: Re: Distortion from audio power amp

Terry Pinnell wrote:
Quote:

Winfield Hill <Winfield_member@newsguy.com> wrote:

Terry Pinnell wrote...

Simulations with those changes appear to prevent setting DC output
level anything close to the desired 12V. ...

Terry, are you setting R2 to the middle of its range? It appears
that you often have the R2 pot = 0 (or full) in your simulations.

Not sure why you say that? Yes, I've been setting it around 50%,
corresponding to the original spec of 47k, with occasional experiments
at 95% and 5%. BTW, Circuitmaker won't allow either 0% or 100%; has to
be 1%<R<99%.

In present circuit
http://www.terrypin.dial.pipex.com/Images/PushPullBreadboardNow.gif
I've reverted to fixed resistor of 47k. I had that working fine.
Swapping to various other fixed, 39k - 82k (with Vcc = 24.0 V) appears
to make little visible difference.

As you see from my new thread
Subject: Are these 2N3055s borderline spec?
my most exasperating problem now is the lack of repeatability! My best
result today as you see in
http://www.terrypin.dial.pipex.com/Images/PushPullBreadboardPuzzle.gif
has some crossover distortion, and I'm sure I wasn't seeing that in my
best result yesterday... Hmm, I was working outside in the garden, in
bright sunshine; wonder if somehow that affected it?

If it is shining on your bias diodes, D1 and D2, it will generate
photo voltaic effects that will shift the output bias current up
(hotter but lower distortion).

--
John Popelish
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Terry Pinnell
Guest





PostPosted: Fri May 21, 2004 2:01 am    Post subject: Re: Distortion from audio power amp

John Popelish <jpopelish@rica.net> wrote:

Quote:
If it is shining on your bias diodes, D1 and D2, it will generate
photo voltaic effects that will shift the output bias current up
(hotter but lower distortion).

Yes, sun was shining directly onto them, so that might indeed have
contributed.

But I seem to have made progress this evening. It looks as if Q4 and
one of the 2N3055s were *both* 'borderline'. Although Q4 was testing
OK on my transistor tester, and showing a reasonable gain, in
exasperation I swapped it for an alternative and immediately started
getting better results.

I now have a combination that delivers a clean-looking sine of about
6.9V rms into 8 ohms. That's on breadboard (with 6 croc-clip
connections to the case-mounted 2N3055s), and with a regulated 24.0 V
bench supply. It remains to be seen if I can retain that good
performance when I rebuild the stripboard circuit and try alternative
supplies.

I intend to replace the two presets for setting the output DC level
and quiescent current respectively. The first is obviously easy to set
(half supply voltage), but how do I determine optimum setting for
quiescent current please?

I may well also experiment with other improvements and design changes
later. But I want to get this working robustly first. Its author, R
Torrens, did so with a variety of supply voltages from 12 to 36 V. His
tests showed distortion was "...around 0.5% or better, which is
approaching hi-fi levels." But I wonder if he had to almost
'hand-pick' his transistors as I seem to be doing for Q4, Q5 and Q6?


--
Terry Pinnell
Hobbyist, West Sussex, UK
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Terry Pinnell
Guest





PostPosted: Fri May 21, 2004 2:28 am    Post subject: Re: Distortion from audio power amp

Winfield Hill <Winfield_member@newsguy.com> wrote:

Quote:
Terry Pinnell wrote...

Winfield Hill <Winfield_member@newsguy.com> wrote:

Terry Pinnell wrote...

Don Pearce wrote:

The amp is oscillating because there is no dominant pole compensation.
A small cap from collector to base of Q2 (the voltage amplifier)
should make things much nicer.

Thanks, I'll try that tomorrow.

I think Don meant to place the cap from Q1's base to Q2's collector.
But first change R7 to 47k, and R2 to 2.7k plus a 5k pot. Then use
3.3pF compensation (for 1 MHz loop bandwidth). There should also be
a 1k resistor in series with C3 for stability over the gain range.

Simulations with those changes appear to prevent setting DC output
level anything close to the desired 12V. (This is regardless of R4
setting.) It seems R7 must be much higher than you suggest. Perhaps
the original author's 470k might be a tad high, but 330k looks good.

I suggested 47k for R7 because it's easier to establish the bandwidth
with a feedback-stabilizing capacitor, and to reduce the output dc-
bias voltage uncertainty from Q1's base current. (R4 is meant to help
establish the output dc bias current, it has little or no effect on the
output's dc bias voltage.) Q1 and Q2 need about 1.3V at Q1's base (the
R2 R7 junction) to operate, so we want R2 = 1.3/(12-1.3) x R7. So if
R7 = 47k, then R2 = 0.12 R7 = 5.71k more or less. Or a 2.7k + 5k pot
for a 2.7k to 7.7k adjustment range. That should work out just fine.
Don't forget the 1k resistor in series with C3.

Thanks, appreciated. For time being at least I've reverted to a
circuit very close to the original Torrens design, which as you may
have seen was well tested and documented. I have only a 1k preset and
the frequency-limiting cap C4 you recommended. (I see I need to change
that to 68pF.)

Present circuit is at
http://www.terrypin.dial.pipex.com/Images/PushPullBreadboardNow.gif

Quote:
With a say 50kHz overall bandwidth we'd want a 68pF cap from Q1's base
to Q2's collector (sorry for my earlier miscalculation).

If you want to keep R7 = 470k, then add a 10k resistor in series with
C3, and a 5 to 10pF cap from Q1's base to Q2's collector.

BTW, I'd return R8 directly to Q6's emitter.

Will try that change too, when I have the existing circuit working.

--
Terry, West Sussex, UK
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