Hi Gerhard, Olin, Jinx, thank you for your answers.
I know i am trying to understand something without having a solid base
when i am talking about electronics but it is a hobby and i really
enjoy it when i am able to make something work. I don't want to bug
anyone here, you helped me a lot in the past and i am grateful.
I tried replacing the 10K resistors with 100K, this way increasing the
gain of the last two stages but i got almost no improvement at all.
If i watch on the scope putting a 0.1v scale i can see what i am
speaking in the mic but it is still too low. I will try reading a
little about operating amps as you suggested and try again with
another type of amp and another mic. The schematic suggested by Jinx
had just one op-amp with a variable feedback resistor of 1Mohm which
can deliver a huge gain (in theory). Maybe i will have more luck with
that design instead of three stages like in my previous try.
Can it be an impedance problem in the middle or the fact that i am using
an oscilloscope to see some results on the screen ?


>> Is there another preamplifier i could use ? a much better one than
>> opamp solution adopted by the schematic i attached in my previous
>> message ?

GF> What Olin is saying is that you can improve your circuit, in mainly two
GF> respects:

GF> 1- Increase the amplification. I'd start with the last stage (because of
GF> the problem with the DC coupling). You do that basically by increasing the
GF> feedback resistor (the one from the output to the inverting input).
GF> Increasing this resistor up to 100k in the last two stages shouldn't be a
GF> problem -- at least not to see whether you get /anything/ useful.

GF> This may work already. It also may be that the signal at the output of the
GF> 3rd amplifier has a significant DC offset; that is, it is not centered
GF> around 2.5V (which it should be without DC offset). This in itself doesn't
GF> hurt, but it reduces the maximum amplitude. For that Olin suggested a
GF> second improvement:

GF> 2- AC-couple the individual stages. This means basically that you connect
GF> the output of one stage to a capacitor, and that capacitor to the input
GF> resistor of the following stage. The capacitor needs to be big enough so
GF> that the cutoff frequency of this highpass (the cap and the input impedance
GF> of the next stage form a highpass) is lower than the lowest frequency you
GF> want to amplify.


GF> You probably should read up a bit on basic theory of operational
GF> amplifiers. If you find nothing else, National Semiconductors has a number
GF> of quite good application notes, about basic theory, advanced issues, and a
GF> lot of example circuits.

GF> Gerhard

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