I've built an Electret Microphone preamp based on this application note from the OPA1692 datasheet (page 27):
The circuit seems to work fantastic, but I've realized that I'd ideally need a bit more gain than the proposed 24.8 (~28 dB). Ideally I'd like to aim at something around 40 dB gain.
The problem I'm facing now is that I'm not quite certain what gain the OPA1692 can sustain. In the data sheet it says that it has a GBW of 5.1 MHz. 40 dB would equal to a gain of 100, which would narrow my bandwidth down to 50 kHz. I obviously only need 40 dB for 20 kHz, but I've heard things about not going too close to the amplifiers limits.
So how far is too far? How much headroom do I have to consider?
Oh and one more thing: The 2.2nF capacitor in the circuit limits the gain of the amplifier to a corner frequency of ~20 kHz. I've realized too late that I should have better selected a corner frequency higher than that so that my passband (20 kHz) actually has a reasonably flat response (do they really not consider this when designing these circuits or am I missing something?). I could simply reduce the capacitor size to fix that, but since I'd be already close to my amplifiers limits, would the extra amplification outside the audio band cause any problems?
PS: Sadly I've already built this circuit on a PCB. Changing components would be a trivial thing, but before I redo the board I'd rather just live with not enough gain.