On 7/17/06, Dumitru Stama <list@mirosat.com> wrote:
>
> 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 ?
>

You've got a scope?  You should be able to troubleshoot this.

(Olin:  the circuit was attached to the first message.  Scroll back in
your threaded MUA and you'll see it.)

What op-amp are you using?

Try this:

Set a constant-amplitude noise source in front of a microphone.  An
oscillator connected to a speaker would be ideal.  (other ideas:  Play
a sine wave on your computer speakers. Some electronic guitar tuners
have a tone output.  Put an amplified speaker on the 1 kHz test output
of your scope.)

Now measure along the circuit:
1) Peak-to-peak voltage at the last op-amp
2) Peak-to-peak voltage at the middle op-amp
3) Peak-to-peak voltage at the first op-amp
4) Peak-to-peak voltage at the microphone. (Set scope on AC coupling).

My opinion:  The input impedance is too low.  1k*1uF = 1kHz
low-frequency cutoff.  Switch the first stage to a buffer or scale the
1k and 100k resistors to 10k and 1M instead.

If you enjoy electronics as a hobby, I'd recommend you find a copy of
"The Art of Electronics".  It's not too mathematical and it will help
you solve problems like this.  (By the way, does anyone know when the
third edition is coming out?)

Regards,
Mark
markrages@gmail
-- 
You think that it is a secret, but it never has been one.
  - fortune cookie
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist