So, just to confirm,  here is what you suggest. The total gain of the
differential amp (three opamps total) should be 20db. Then a voltage
divider, and a simple single opamp set at 20db gain. Should it be
inverting or non inverting? I could do this with two SSM2135 chips as
they are dual opamps per chip. I'll put 10K pulldown resistors on each
input of the ultimate input. Do I need them on subsequent stages as
well? I'll put 22pF caps from the true input to the complement input.
If I use DC blocking series caps, how do I figure out the value? I
will also have a 2K resistor across the put (in parallel with the 22pF
cap).

How does that sound? I'm going to try to draw this up so I can post an
image. And just out of curiousity, why is varying the Rgain resistor
to change the gain a bad idea?

Thanks,

Josh
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On Thu, 19 Aug 2004 09:11:40 -0700, Bob Blick <bblick@sonic.net> wrote:
> Typical gain you will need is 20 to 40 db.
>
> Do not use a potentiometer where you suggested. Run your differential amp
> at 20 db fixed gain, put the pot afterwards as a voltage divider, and
> follow that with a single opamp with 20 db more gain.
>
> Using input coupling capacitors gives you a small measure of protection
> against DC into your opamps.
>
> You do need to set a DC level on the inputs of the opamps - pull them down
> with resistors. 10K is fine.
>
> And load the mic with 1 or 2k.

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