Dumitru Stama wrote:
> I increased the resistors between the inverting input and the output
> which in conjunction with the one between stages should create the
> gain. I will start again today with the circuit, with AC-coupling
> between stages (inserting a capacitor like Gerhard said in his previous
> message)

When you're done update the schematic and post it so we're all on the same
page.  This time remember to include component designators so we can talk
about the individual components without a lot of confusing hand waving (like
your first sentence above).

> and maybe use the other type of op-amp i have available :
> TL072CP Previously i was using LM358N.

The TL072 is a nice opamp with essentially infinite input impedence for your
purposes.  The thing to watch with the TL072 is the min/max supply range,
the fact that it needs a +/- supply, and that it needs 2V or more headroom
from each rail.  Once you take care of that, however, it's a very nice
opamp.  Put pads in for compenstation caps just in case.  The TL072 is
notorious for oscillating even when you think you've done everything
according to the datasheet to make it stable.  Actually leaving pads for a
compenstation cap is good practice for any opamp until you're sure they
aren't needed.

> The only problem i have now is choosing the right capacitor for AC
> coupling so i will not create an unwanted highpass filter but i believe
> there is a formula somewhere

Of course there is.  You might try looking it up.

Let's say you use 10Kohm input resitors in place of the 1K now (I always
thought 1K was too low).  Let's also say you care about frequencies down to
20Hz.  20Hz with 10Kohms comes out to 800nF, so 1uF will work fine.  Make
sure this is a bipolar cap like ceramic, not polarized like electrolytic or
tantalum.

> I can see on the scope a straight line at ~2.5v but it is not oscilating
> unles i speak very hard straight into the microphone

You definitely need more gain.  I think you are loosing a lot of signal due
to impedence mismatch at the input.


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