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Originally Posted by Tim Farrant  For gain you could put a pot between the 2 -ve inputs of the opamps to create gain. Less than x1 gain is how it is drawn. With a pot and limiting resistor between the opamp -ve inputs this becomes the traditional instrumentation input with the transfomer doing the summing/difference passively. Our Elixir mic preamp has the same design idea except it's only 1 input with a lot more gain, but still the siganal is re-balanced by the transformer.. |
Actually, I'm wrong (too many glasses of wine when I wrote this). The best way to change gain is to vary the feedback resistors around the opamps. You need to change both resistors at the same time and keep them equal in value. You will only be able to achieve a limited range, say +/- 10dB.