Dumitru Stama wrote:
> From the scope pictures you are absolutely right ! I managed to change
> the schematic and i attached it to this message also. I supplied +9v
> to the circuit and i added all the bypass caps plus compensating ones
> to every stage like you and Mark told me, just to see what happens.

10nF is way too big for compensating cap.  These would typically be a few
10s of pF, and maybe aren't needed at your gain anyway.  What I said was to
put the pads there so you can add the caps if you need to.  Your 10nF
compensation caps are so large they will significantly effect the signal.
In the last two stages they are creating low pass filters with around 72Hz
rolloff.

Remove the compensation caps and check the output of each amp with the scope
to make sure it is not oscillating on its own.  If you want, you can just
put about 47pF there.  That will create a LPF pole of about 15KHz, which
won't effect voice but will guaratee these amps will be stable.

> I think what happens is that i shouldn't put compensating caps since
> the noise i see is not the op-amp self-oscilating but mic noise.

Right.  These amps can be a bit frisky, but I don't think they'll oscillate
by themselves at your gains of 10 and 22.

> From what i could read on google the noise has usually higher
> frequency than 2000, 3000Hz and i might be able to cut a lot of it
> using a lowpass filter. Should i also try a highpass filter from 100,
> 200Hz up ?

Get it working first.  Then the next step is to find the gain you really
want, and maybe adjusting the gain of the stages, and maybe adding a volume
control.  If you really only care about voice frequencies, then you can use
larger compensation caps to cause a deliberate LPF.  100pF compensation caps
would give you 3 poles of LPF, the first at 16KHz and the next two at
7.2KHz.  Remember these don't start attenuating suddenly at the rolloff
frequency, so you want some margin above your highest frequency of interest,
especially with multiple poles.  I wouldn't use higher than 100pF for the
compensation caps.  If you want additional LPF, there are other ways to add
it.


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