Do you know the mic level you need for the Garmin GNC255? Turning your pot mostly towards to zero means, that you are using a low level close to the 70mV you specified?
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Sorry, I meant I turned the pot mostly towards zero resistance.
I've not measured the output amplitude properly during typical use with the foam on, but I'd shot for about 300mV peak-to-peak at zero pot resistance on the oscilloscope to close proximity normal speech when it was in pieces on my bench. The Garmin GNC255 requires 70mV - 1000mV depending on what its mic input gain has been set to.
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There's plenty of scope to increase the amplitude on my mic setup. The second op-amp stage is just a buffer so could easily be given gain via a feedback resistor.
For reference, the Bose A20 mic outputs 600mV at 114dB SPL.
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