Distortion from audio power amp

[Terry Pinnell]

This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Also posted in alt.binaries.schematics.electronic

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Don Pearce]

On Fri, 14 May 2004 13:46:43 +0100, Terry Pinnell
<terrypin@dial.pipex.com> wrote:

This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Also posted in alt.binaries.schematics.electronic

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.

d

--
http://www.pearce.uk.com

 

[Terry Pinnell]

Don Pearce <complete@nonsense.com> 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.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[legg]

On Fri, 14 May 2004 13:46:43 +0100, Terry Pinnell
<terrypin@dial.pipex.com> wrote:

This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Also posted in alt.binaries.schematics.electronic

You could save everyone a lot of time by researching the source of
your original schematic a bit more carefully. As drawn, there are a
host of problems that people have been trying to point out. These may
not have been present in the original source.

This habit of documenting a 'problem' then posting it, without trying
to examine and fix it first, yourself, is becoming tiresome. The
circuit works no better than expected, as drawn, whether it's been
around for 30 years, or not. Problems don't fix themselves. Errors can
easily be carved in stone.

RL

 

[Rich Grise]

"Terry Pinnell" <terrypin@dial.pipex.com> wrote in message
news:i0f9a091bbt0eecqlml91tgufh611r6e2v@4ax.com...

This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Well, probably R4 is to adjust out the crossover distortion, and
R2 sets the closed-loop gain, which you've got set way too high.
Of course, that could be an artifact of the open circuit between
the C2-R7-Q1 node and the R9-R10-R4-Q4-R11-C1 node.

Good Luck!
Rich

 

[Frank Bemelman]

"Terry Pinnell" <terrypin@dial.pipex.com> schreef in bericht
news:i0f9a091bbt0eecqlml91tgufh611r6e2v@4ax.com...

This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Did you move R11 yet, or is it still as shown in the schematic?

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)

 

[normanstrong]

"Terry Pinnell" <terrypin@dial.pipex.com> wrote in message
news:i0f9a091bbt0eecqlml91tgufh611r6e2v@4ax.com...

This continues from my earlier thread. With the output's DC level
now
correctly set at mid-point, this is the sort of result I'm seeing
into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Who designed this amplifier, and when? It looks like one that could
have been designed 20 years ago. There are also several 'mistakes'
in it that have to be fixed just to get it to work at all. IOW, this
is not the way to design an amp.

Norm Strong

 

[Winfield Hill]

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.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[Rick]

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.

Also, the difference between the base of Q4 and Q3 is 3*Vbe, so it
would be better to bias Q3 and Q4 with a Vbe-multiplier rather than D1,
D2 and R4.

--
Rick

 

[Terry Pinnell]

legg <legg@nospam.magma.ca> wrote:

You could save everyone a lot of time by researching the source of
your original schematic a bit more carefully. As drawn, there are a
host of problems that people have been trying to point out. These may
not have been present in the original source.

Have I been ignoring advice that 'people have been trying to point
out'?

The circuit follows the original in all significant respects. And I
studied it rather carefully. Apparently more carefully than you
studied my posts, or you wouldn't have made such an assumption. If
you're seriously interested in the design issues, rather than tossing
out patronising comments, take a look at the originals yourself.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOriginal1.jpg
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOriginalText.gif

This habit of documenting a 'problem' then posting it, without trying
to examine and fix it first, yourself, is becoming tiresome.

Excuse me? If you're tired, I suggest a short nap. As for not trying
'to examine and fix it first', how did you arrive at that second
grossly inaccurate assumption?

The circuit works no better than expected, as drawn, whether it's been
around for 30 years, or not. Problems don't fix themselves.

Wow, now I never would have thought of that!

Errors can easily be carved in stone.

And your point is? Apart from trying (and failing) to appear
important?

If you have nothing constructive to offer, please don't contribute at
all.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

"Frank Bemelman" <f.bemelmanx@planet.invalid.nl> wrote:

"Terry Pinnell" <terrypin@dial.pipex.com> schreef in bericht
news:i0f9a091bbt0eecqlml91tgufh611r6e2v@4ax.com...
This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Did you move R11 yet, or is it still as shown in the schematic?

No, but aim to do so tomorrow. Using current limiting from bench
supply unit too.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

"normanstrong" <normanstrong@comcast.net> wrote:

Who designed this amplifier, and when? It looks like one that could
have been designed 20 years ago. There are also several 'mistakes'
in it that have to be fixed just to get it to work at all. IOW, this
is not the way to design an amp.

As mentioned earlier in

Subject: Re: Trouble shooting push-pull amp
Date: Thu, 13 May 2004 09:21:04 +0100

it's about 30 years old. Comes from Babani Press series, Book #221,
"28 Tested Transistor Projects", page 21. I'll check author's name
(book is still in shed/workshop) and let you know tomorrow.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

"Rich Grise" <null@example.net> wrote:

"Terry Pinnell" <terrypin@dial.pipex.com> wrote in message
news:i0f9a091bbt0eecqlml91tgufh611r6e2v@4ax.com...
This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Well, probably R4 is to adjust out the crossover distortion, and
R2 sets the closed-loop gain, which you've got set way too high.
Of course, that could be an artifact of the open circuit between
the C2-R7-Q1 node and the R9-R10-R4-Q4-R11-C1 node.

Good Luck!
Rich

Thanks. Actually, R2 sets DC level of output, intended to be half
supply voltage, i.e. about 12V here. Gain is set by R1.

Open circuit? Not sure I follow. I fixed the hair-line track, if
that's what you mean, as reported in other thread yesterday:
Subject: Re: Trouble shooting push-pull amp
Date: Thu, 13 May 2004 20:49:37 +0100
Message-ID: <22k7a0952nmtucvvnvhsnh47829mt2s22k@4ax.com>

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Winfield Hill]

Terry Pinnell wrote...

"Rich Grise" <null@example.net> wrote:

R2 sets the closed-loop gain, which you've got set way too high.
Of course, that could be an artifact of the open circuit between
the C2-R7-Q1 node and the R9-R10-R4-Q4-R11-C1 node.

Thanks. Actually, R2 sets DC level of output, intended to be half
supply voltage, i.e. about 12V here. Gain is set by R1.

Terry, read my post and think about it. If R1 is set for low gain,
yes it does attentuate the input, BUT it thereafter programs a very
high ac gain, nearly open loop. The gain for an inverting opamp,
which this acts like, is R_feedback / R_inout. R_feedback is R7 in
your case. Note there's no R_in, so whenever the gain pot is set to
zero, R_in = 0 and the amp's "gain" is actually *very* high. Weird.

That's just one of a half-dozen serious problems with your circuit.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[Frank Bemelman]

"Terry Pinnell" <terrypin@dial.pipex.com> schreef in bericht
news:8r5aa01e8vhlcgqnlj2np6nbrl93a1s60f@4ax.com...

"Frank Bemelman" <f.bemelmanx@planet.invalid.nl> wrote:

"Terry Pinnell" <terrypin@dial.pipex.com> schreef in bericht
news:i0f9a091bbt0eecqlml91tgufh611r6e2v@4ax.com...
This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Did you move R11 yet, or is it still as shown in the schematic?

No, but aim to do so tomorrow. Using current limiting from bench
supply unit too.

It's not so much the current limiting I was concerned about, but
with R11 sitting in the collector makes it, in my view, a sort
of an unbalanced output stage. Since your output also shows a
rather unbalanced behaviour, with the negative half periods of the signal
being much better that the positive ones, my first try
would be to give it as much balanced behaviour as possible, by
moving that R11 to the emitter side. 2N3055 or cheap no brand
look alikes have a lousy Hfe, sometimes as low as 15-20, but
sometimes as high as 140. At least that is what I remember from
testing a batch of cheap 2N3055's two decades ago ;-)

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)

 

[Frank Bemelman]

"Winfield Hill" <Winfield_member@newsguy.com> schreef in bericht
news:c837ak02715@drn.newsguy.com...

[snip]

That's just one of a half-dozen serious problems with your circuit.

Oliver Hardy would have said "Here's another fine mess
you've gotten yourself into!"

;-)

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)

 

[legg]

On Fri, 14 May 2004 19:58:09 +0100, Terry Pinnell
<terrypin@dial.pipex.com> wrote:

legg <legg@nospam.magma.ca> wrote:

You could save everyone a lot of time by researching the source of
your original schematic a bit more carefully. As drawn, there are a
host of problems that people have been trying to point out. These may
not have been present in the original source.

Have I been ignoring advice that 'people have been trying to point
out'?

The circuit follows the original in all significant respects. And I
studied it rather carefully. Apparently more carefully than you
studied my posts, or you wouldn't have made such an assumption. If
you're seriously interested in the design issues, rather than tossing
out patronising comments, take a look at the originals yourself.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOriginal1.jpg
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOriginalText.gif

This habit of documenting a 'problem' then posting it, without trying
to examine and fix it first, yourself, is becoming tiresome.

Excuse me? If you're tired, I suggest a short nap. As for not trying
'to examine and fix it first', how did you arrive at that second
grossly inaccurate assumption?

The circuit works no better than expected, as drawn, whether it's been
around for 30 years, or not. Problems don't fix themselves.

Wow, now I never would have thought of that!

Errors can easily be carved in stone.

And your point is? Apart from trying (and failing) to appear
important?

If you have nothing constructive to offer, please don't contribute at
all.

Re-examine the circuit.

Construct the circuit as indicated.

this should include-

Installing R1, as drawn, or placing it in series with old C1(new C3),
to swamp out variability of source impedance or vol control setting on
the gain of circuit.

Used fixed value resistors as indicated for R2,R5, R6 and R7 in the
documentation column addressing 24V operation.

Only make changes after familiarizing yourself with the original
circuit's function.

Most of the suggestions posted wouldn't hurt.

I assume you are fixing something made a long time ago. Better
references existed at that time.

The quasi-complementary output circuit (Q2 and onwards) was shown as a
standard circuit in the GE and RCA transistor application manuals as
early as 1960, using pnp germanium power transistors. These were
possibly based on articles by H.C.Lin published around 1956.

http://203.44.53.131/QuasiComp/Lin.zip

other articles on same basic circuit at

http://www.alphalink.com.au/~cambie/#Quasi

The common references continued in the same manuals over the decades,
to include the silicon npn variations in the same circuit. The parts
used in the circuit from ETI'76 are identifiable in the basic
references, by function, as are the functions of those parts that are
omitted.

RL

 

[Jan Panteltje]

On a sunny day (Fri, 14 May 2004 14:18:43 +0100) it happened Don Pearce
<complete@nonsense.com> wrote in <imh9a0lgk27pl0flv9sfqi3e4197p6rvf7@4ax.com>:

On Fri, 14 May 2004 13:46:43 +0100, Terry Pinnell
terrypin@dial.pipex.com> wrote:

This continues from my earlier thread. With the output's DC level now
correctly set at mid-point, this is the sort of result I'm seeing into
an 8 ohm resistor.
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmpOut1.gif

The circuit is as shown, at
http://www.terrypin.dial.pipex.com/Images/AudioPowerAmp1.gif

Any help in removing the distortion would again be much appreciated
please.

Also posted in alt.binaries.schematics.electronic

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.

Yes, but what is the reason for R11 (in collector bottom 2N3055).

That transistor has high Z collector out, why add .5 Ohm? It would do
nothing?
JP

 

[Terry Pinnell]

legg <legg@nospam.magma.ca> wrote:

Re-examine the circuit.

Construct the circuit as indicated.

this should include-

Installing R1, as drawn, or placing it in series with old C1(new C3),
to swamp out variability of source impedance or vol control setting on
the gain of circuit.

OK, thanks, I missed that. Win made same point.

Used fixed value resistors as indicated for R2,R5, R6 and R7 in the
documentation column addressing 24V operation.

The text suggests a 220k preset as an alternative to a fixed resistor.
That option seemed more appropriate when I built this project, as
exact supply voltage was not precisely 24V, and somewhat
indeterminate. R5 and R6 *are* fixed. Original R7 is my R4, which
again I made variable for same reasons of flexibiliy mentioned above.

Only make changes after familiarizing yourself with the original
circuit's function.

Most of the suggestions posted wouldn't hurt.

I assume you are fixing something made a long time ago.

Yes, 1981 according to my notes.

Better references existed at that time.

The quasi-complementary output circuit (Q2 and onwards) was shown as a
standard circuit in the GE and RCA transistor application manuals as
early as 1960, using pnp germanium power transistors. These were
possibly based on articles by H.C.Lin published around 1956.

http://203.44.53.131/QuasiComp/Lin.zip

other articles on same basic circuit at

http://www.alphalink.com.au/~cambie/#Quasi

Interesting stuff, but couldn't find any close similarity in any of
those circuits to this one.

The common references continued in the same manuals over the decades,
to include the silicon npn variations in the same circuit. The parts
used in the circuit from ETI'76 are identifiable in the basic
references, by function, as are the functions of those parts that are
omitted.

I'm impressed by your archives - my ETI mags were dumped many years
ago!

I do intend to implement most of the suggestions made, and will report
back on the results.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

Winfield Hill <Winfield_member@newsguy.com> wrote:

Terry Pinnell wrote...

"Rich Grise" <null@example.net> wrote:

R2 sets the closed-loop gain, which you've got set way too high.
Of course, that could be an artifact of the open circuit between
the C2-R7-Q1 node and the R9-R10-R4-Q4-R11-C1 node.

Thanks. Actually, R2 sets DC level of output, intended to be half
supply voltage, i.e. about 12V here. Gain is set by R1.

Terry, read my post and think about it. If R1 is set for low gain,
yes it does attentuate the input, BUT it thereafter programs a very
high ac gain, nearly open loop. The gain for an inverting opamp,
which this acts like, is R_feedback / R_inout. R_feedback is R7 in
your case. Note there's no R_in, so whenever the gain pot is set to
zero, R_in = 0 and the amp's "gain" is actually *very* high. Weird.

OK, I'll add a small series R to the R1 preset. But I still don't
understand how Rin can ever be zero in practice. I had assumed it
would be sufficient to prevent the sort of 'runaway gain' you warn
about. Even on my bench test, my function generator (the PM5134 we've
been discussing in another thread), has output impedance of either 50
or 600R. I've been using the lower setting, but a gain of 1k/50 = 20
hardly seems anything to get too excited about? And, although I'd have
to check this, I think the actual previous stage (dog bark synthesised
sound), would have Rs > 50.

That's just one of a half-dozen serious problems with your circuit.

So - you don't like this circuit too much then? <g>

--
Terry Pinnell
Hobbyist, West Sussex, UK