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?
Power supply has a torodial transformer and big capacitors, there is no oscillation on power supply lines. Is it that what you mean?
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.
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..
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.
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.
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Michael A. Terrell
Central Florida
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.
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.
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.
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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
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?
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?
I bet there is that 66 kHz there if you look closely actually. Crank up the oscilloscope gain.
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.
Spice is *completely useless* for simulating the real world.
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.
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
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