Noise and osci in audio amp

[Dominik Werder]

Hello!

Maybe you can help me with this:

My audio amplifier sounds bad: it sounds like it is overdriven, but not
always, meaning not with every music.

And the other thing: It oscillates on 66kHz.

Can this oscillation on such a not audible frequency be the cause for
the distortion?

I would guess that it could be a bad capacitor but I don't know a way to
check if a capacitor is still healthy without removing it from the
circuit.. Do you?

Is it possible that a semiconductor is only "a bit" broken so that the
amp still works but just sounds bad?

Thank you very much!
Dominik

 

[Eeyore]

Hello!

Maybe you can help me with this:

My audio amplifier sounds bad: it sounds like it is overdriven, but not
always, meaning not with every music.

And the other thing: It oscillates on 66kHz.

Can this oscillation on such a not audible frequency be the cause for
the distortion?
Yes.


I would guess that it could be a bad capacitor but I don't know a way to
check if a capacitor is still healthy without removing it from the
circuit.. Do you?

Do you have decent power supply decoupling ?

Is it possible that a semiconductor is only "a bit" broken so that the
amp still works but just sounds bad?

No.

It could be many, many things including poor layout. Power amplifiers are
noriously tricky for the inexperienced to get right.

Graham

 

[Dominik Werder]

I would guess that it could be a bad capacitor but I don't know a way to

Do you have decent power supply decoupling ?

Power supply has a torodial transformer and big capacitors, there is no
oscillation on power supply lines. Is it that what you mean?

No.

That's good..

It could be many, many things including poor layout. Power amplifiers are
noriously tricky for the inexperienced to get right.

I simulated the whole thing with spice and the real thing behaves like
the simulated one, at least as far as you look at the operating point.

I just don't know where's the best place to start debugging.. Maybe you
have some idea so that I don't have to disassemble the whole thing and
check each component..


bye!
Dominik

 

[Steffen Buehler]

I just don't know where's the best place to start debugging.. Maybe
you have some idea so that I don't have to disassemble the whole
thing and check each component..

Sometimes already a little re-soldering helps.

Regards
Steffen

 

[Abstract Dissonance]

Hello!

Maybe you can help me with this:

My audio amplifier sounds bad: it sounds like it is overdriven, but not
always, meaning not with every music.

And the other thing: It oscillates on 66kHz.

Can this oscillation on such a not audible frequency be the cause for
the distortion?

I would guess that it could be a bad capacitor but I don't know a way to
check if a capacitor is still healthy without removing it from the
circuit.. Do you?

Is it possible that a semiconductor is only "a bit" broken so that the
amp still works but just sounds bad?

Are you sure your amp is working in the linear region? Cause some music is
more dynamic then others and can distort the amp even though they seem to
sound at the same level.

 

[Michael A. Terrell]

Are you sure your amp is working in the linear region? Cause some music is
more dynamic then others and can distort the amp even though they seem to
sound at the same level.

He already told you that it is oscillating.


--
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

 

[Michael A. Terrell]

Hello!

Maybe you can help me with this:

My audio amplifier sounds bad: it sounds like it is overdriven, but not
always, meaning not with every music.

And the other thing: It oscillates on 66kHz.

Can this oscillation on such a not audible frequency be the cause for
the distortion?

I would guess that it could be a bad capacitor but I don't know a way to
check if a capacitor is still healthy without removing it from the
circuit.. Do you?

Is it possible that a semiconductor is only "a bit" broken so that the
amp still works but just sounds bad?

Thank you very much!
Dominik

You need to find out what is causing the oscillation before you
destroy the amp. Add some .01 uF capacitors across the electrolytics,
and check your layout. If there are any Op Amps in there, make sure to
bypass the two power rails to ground with .01 uF capacitors.
Electrolytics can have a high ESL, which means that they will not pass
the higher frequencies to ground properly.


--
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

 

[phaeton]

Add some .01 uF capacitors across the electrolytics,
and check your layout.

Capacitors across the capacitors, or is that a typo?

If there are any Op Amps in there, make sure to
bypass the two power rails to ground with .01 uF capacitors.

You mean bypassing the power rails of the Op Amps, right?

Electrolytics can have a high ESL, which means that they will not pass
the higher frequencies to ground properly.

Sorry to butt in. I'm curious too, because I have troubles with
oscillation in some of my audio circuits.

thx

-phaeton

 

[Michael Black]

I just don't know where's the best place to start debugging.. Maybe you
have some idea so that I don't have to disassemble the whole thing and
check each component..

But that's some of your problem. You assume it's a bad component, when
as some have explained, there are far more obvious things to check before
bad components.

As has been said, layout is important. If the output gets too close
to the input, that will make it oscillate. If the amplifier IC (I forget
whether an IC is involved) isn't properly bypassed at the power terminal,
then it may oscillate. If you're wiring isn't so great, leads too long
that go all over the place, then that may cause oscillation. If you're
trying this on a breadboard, the use of the breadboard may cause the
oscillation (because it places the components too close together, or because
it may not provide enough pathway for the power). It may be a design
that actually needs some sort of circuitry to prevent oscillation; it
used to be common to see IC amplifiers with a resistor and capacitor in
series from the output to ground to ensure there'd be no oscillation.


It would have to be some very odd set of circumstances for a semiconductor
device to fail in such a way that it would cause oscillation.

A bad bypass capacitor might cause oscillation, because then it can't
do it's job, but that really falls under the category of proper bypassing.

Michael

 

[Michael Black]

Capacitors across the capacitors, or is that a typo?


You mean bypassing the power rails of the Op Amps, right?


Sorry to butt in. I'm curious too, because I have troubles with
oscillation in some of my audio circuits.

It's because the electrolytics have too much inductance (due
to construction methods) to be effective at higher frequencies. So
the electrolytic will take care of low frequency matters, but start
to lose effectiveness at higher frequencies. Hence, you put a low
value capacitor in parallel. That low value capacitor will be
lousy to deal with low frequencies (because it's a low value capacitor),
but at higher frequencies it will fine since it will it be a .01uF or
whatever the value through the range and up further.

Michael

 

[phaeton]

It's because the electrolytics have too much inductance (due
to construction methods) to be effective at higher frequencies. So
the electrolytic will take care of low frequency matters, but start
to lose effectiveness at higher frequencies. Hence, you put a low
value capacitor in parallel. That low value capacitor will be
lousy to deal with low frequencies (because it's a low value capacitor),
but at higher frequencies it will fine since it will it be a .01uF or
whatever the value through the range and up further.

Michael

I see....

Does it matter so much if the .01uf is ceramic or mylar?

thx

-phaeton

 

[Michael A. Terrell]

I see....

Does it matter so much if the .01uf is ceramic or mylar?

thx

-phaeton

Not a whole lot. What do you have on hand? The mylar have a little
more inductance, but they can be used in most places. These days SMD
parts are used, and ceramic is the preferred type.


--
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

 

[Abstract Dissonance]

He already told you that it is oscillating.

Is it necessarily true that the oscillations are equivilent to the amp(I'm
thinking of an op amp or transistor amp used int the amplification and not
the whole amp circuit) not working in the linear region?

i.e., we can have oscillations that still allow the amp to work in the
linear region? and we can the amp working in the non-linear region and have
no oscillations? It seems to me that there are more things possible like he
could have set is q point in the wrong place and its causing distortion but
the oscillations could be caused from something else?

Note, I'm not claiming I know the answer and just know a little about
amps(mostly the basics. I do understand how oscillations can cause
distortion as they increase the overall amplitude of the signal. I just
don't if oscillations are almost always the cause of designing an amp to
work in the non-linear region?

 

[phaeton]

Is it necessarily true that the oscillations are equivilent to the amp(I'm
thinking of an op amp or transistor amp used int the amplification and not
the whole amp circuit) not working in the linear region?

Lesee, how was the phrase put to me a little bit ago....

"All amplifiers oscillate, and all oscillators amplify".


To answer your question, yes you can have oscillations when the amp is
in linear mode, and AFAIK it would be more prone to oscillate when
everything's biased right, etc. I've built guitar pedals that whistle
dixie so effing loud that you can use them to saw a tree in half at
fifty paces- yet i can still play through them.

Some of them even quiet down when you hit them with signal, but my most
recent high-gain distortion pedal would actually get the 2nd stage
transistor biasing disrupted by the oscillations- blatty gated sound.

I don't know if you mentioned it and I missed it, or not- Is this amp
your design, or something you've purchased?

-phaeton

 

[Eeyore]

Power supply has a torodial transformer and big capacitors, there is no
oscillation on power supply lines.

I bet there is that 66 kHz there if you look closely actually. Crank up the
oscilloscope gain.

Is it that what you mean?

Not exactly - no.

The impedance of the supply needs to be low at high frequencies too. This almost
invariably requires 'local decoupling' of the power supply with film caps
typically close to the active circuitry.

That's good..


I simulated the whole thing with spice and the real thing behaves like
the simulated one, at least as far as you look at the operating point.

Spice is *completely useless* for simulating the real world.

I just don't know where's the best place to start debugging.. Maybe you
have some idea so that I don't have to disassemble the whole thing and
check each component..

You don't need to do that. You *do* however need experience. It's quite unlilely
to be a component and very likely to be layout or a design oversight. You should
check your 'earth' paths too ( which have finite resistance ) to make sure
you're not coupling the output back to the input.

Graham

 

[Eeyore]

Is it necessarily true that the oscillations are equivilent to the amp(I'm
thinking of an op amp or transistor amp used int the amplification and not
the whole amp circuit) not working in the linear region?

The oscillation is quite likely close to the amplifier's slew rate limit thus
making the amp much less linear than it should be.

Graham

 

[Michael A. Terrell]

Is it necessarily true that the oscillations are equivilent to the amp(I'm
thinking of an op amp or transistor amp used int the amplification and not
the whole amp circuit) not working in the linear region?

i.e., we can have oscillations that still allow the amp to work in the
linear region? and we can the amp working in the non-linear region and have
no oscillations? It seems to me that there are more things possible like he
could have set is q point in the wrong place and its causing distortion but
the oscillations could be caused from something else?

Note, I'm not claiming I know the answer and just know a little about
amps(mostly the basics. I do understand how oscillations can cause
distortion as they increase the overall amplitude of the signal. I just
don't if oscillations are almost always the cause of designing an amp to
work in the non-linear region?

When a power amplifier is oscillating, it is running near, or at the
full output level. This causes heating in the outputs, and the
ultrasonics can burn out the speaker(s). When you add a lower frequency
signal, you are modulating the oscillation which may or may not sound
OK, but it is not the proper way to run any amplifier.


--
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

 

[Eeyore]

Not a whole lot. What do you have on hand? The mylar have a little
more inductance, but they can be used in most places. These days SMD
parts are used, and ceramic is the preferred type.

My experience with ceramics at higher voltages is not so good. They tend to
self-combust.

Use any plastic film cap and it should really be at least 0.1uF.

Graham

 

[Michael A. Terrell]

My experience with ceramics at higher voltages is not so good. They tend to
self-combust.

Use any plastic film cap and it should really be at least 0.1uF.

Graham

How much high voltage surface mount work do you see?


--
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

 

[Dominik Werder]

As has been said, layout is important. If the output gets too close

to the input, that will make it oscillate. If the amplifier IC (I forget
whether an IC is involved) isn't properly bypassed at the power terminal,
then it may oscillate. If you're wiring isn't so great, leads too long
that go all over the place, then that may cause oscillation. If you're
trying this on a breadboard, the use of the breadboard may cause the
oscillation (because it places the components too close together, or because
it may not provide enough pathway for the power). It may be a design
that actually needs some sort of circuitry to prevent oscillation; it
used to be common to see IC amplifiers with a resistor and capacitor in
series from the output to ground to ensure there'd be no oscillation.

Many thanks for your tips!!
I checked the layout but from my unexperienced point of view it should
be ok, output is at least 5cm away from input.

I tried several things like resoldering nodes that didn't looked that
good and exchanging capacitors, but I couldn't find the real cause.

Debugging is especially painful because I have no own oscilloscope,
therefore I need to carry the amp to the lab at monday..

But again, thank you very much!
Dominik