Dumitru Stama wrote:
> 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.

I don't remember the circuit, but you have to increase only one of the
resistors around an opamp.  It's the ratio between the two that matters.  It
would be a good idea to show your new circuit.

> 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.

Then your gain is still too low.  What is the voltage output for a "normal"
voice input?

> and try again with
> another type of amp and another mic.

If I remember right, you were using 741 opamps.  They aren't great, but
should be capable of a gain of 100 at voice frequencies.  There is also
nothing wrong with a electret mic, but you have to read the data sheet and
take care to drive it the right way.  Give us a link to the datasheet for
the mic you are using.

> 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).

Microphones typically need a voltage gain over 1000.  You aren't going to
get that in one stage with reasonable opamp, and certainly not with a 741 at
voice frequencies.  I think multiple stages is the right way to go.

> Can it be an impedance problem in the middle

Again, I don't remember the details of your circuit, but I don't remember
such a problem between stages.  As I said though, I think the impedence
match of the microphone to the input of the first stage is suspect.

> or the fact that i am using
> an oscilloscope to see some results on the screen ?

Oscilloscopes typically have 1Mohm in parallel with 10-30pF input impedence.
This won't effect the output of your opamp in any practical way.


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