Hi all:
I'm responding to this rather old thread, since I need a good electret AGC preamp. After much checking, I think I'll go with the circuit developed by Audioguru in post # 15.
I have developed a small circuit board (with the modifications shown), using the TL071. I'm ready to etch the board, but don't want to make something no better than what I presently have. I am interfacing the mic to a speech recognition chip, (HM2007), and have been using a small pre amp I made using a LM358, which works OK, but does not have AGC. I'm not looking for "super sensitivity", just something that will allow me to speak with slightly different voice levels. As I said, the circuit I am using works OK, but it's hard to speak in a consistent volume, which can sometimes result in recognition errors. I plan on using a POT for R3 (100K fixed resistor in the circuit as it is shown), to adjust and "tune" the output to the HM2007. The Mic was also picking up a lot of electrical interference before I added a shielded cable, and that seemed to cure most of those problems. I have to add a 9 volt regulator to the circuit, since I do not have that voltage available. I will heavily filter and de-couple the regulator part, so hopefully that will not present problems.
Just looking for any other advice from the experts. Digital I can handle, but I'm not the best at analog.
Ken