On 7/18/06, Dumitru Stama <list@mirosat.com> wrote:
>
> Hi Olin,
> I attached the updated schematics.
> I managed to build it and it seems it works. On paper it has a gain of
> 10*22*22 which is more than 4000. I can speak further away from the
> mic and i can see big changes on the scope but i think i have a
> problem since i see a lot of free oscilations when i am not making any
> noise. I guess is the problem you mentioned already with
> self-oscilations of this particular type of op-amp.
> I also found the formula for highpass cutoff freq : f=1/(2*PI*R*C)
> Thanks for the hint :-)
> There is only one thing i really don't know now and this is where
> should i put the caps for stoping the self-oscilations of op-amp ?
>

The new schematic looks a lot more plausible.  But it looks like
you're running the TL072 off of a 5V supply.  This won't work well at
all.  See the TL072 homepage at TI's website: Vs(Min)(V) = 7V.  The
LM358 you mentioned is a better choice for 5Voperation.

Compensation capacitors are often parallel to the feedback resistor
(your R7, R8, R9).  But first make sure the power supply is bypassed
with a few uF near the part.

Regards,
Mark
markrages@gmail


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