MR> On 7/19/06, Olin Lathrop <olin_piclist@embedinc.com> wrote:
>> Mark Rages wrote:
>> >> 1 - There should be a cap to ground from the node between R1 and R2.
>> >> Otherwise any supply noise and anything that node picks up is going to
>> >> end up on the signal.  There should be at least a 100nF ceramic, and
>> >> maybe a 10uF electrolytic too.  A single 1uF ceramic is probably good
>> >> enough.
>> > But then it won't cancel supply noise from the mic.
>> Right, but then it wasn't doing that before either.
>>
MR> Why not?

Because i didn't know how to make it. After two days of having the
op-amp in my head and google in front of my eyes i think a bandpass
filter is the answer. I re-made the schematics and a new eagle library
containing TL072 since i could not find it in the official suite nor
on the cadsoft site.
http://electronica.asm.ro/electret/scope.html
After removing the 10nF compensating caps and putting 100pF ones
instead the circuit works very well and the gain can be seen. Now i
have that mic-noise problem :)
I was thinking about a RC low-pass filter at the end of the third
stage to cut off everything above 3KHz. A resistor in series with the
output of the last op-amp and then a capacitor tied to the ground.
Taking 1nF capacitor the resistor should be of about 55K after
computing using R = 1/(2*PI*C*F)
Can this be the answer ?

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