On 7/17/06, Olin Lathrop wrote: > > It still bugs me though, just like the OP's circuit, that the DC bias > resistor to the mic comes straight from the supply. Unless the mic has a > much lower impedence than I think, I would want to see the top end of that > resistor go to a filter from the supply voltage. Otherwise supply noise > will couple directly into the input of the amp. > What the original circuit shows is common. But these mics are usually battery powered. > > As I mentioned, > > I've never needed more than one stage for amplification, either > > opamp or transistor > > So can you tell us what the output impedence of a properly biased electret > is, and what a typical output voltage level is for normal voice input? > I snagged a random datasheet off digikey for a cheap chinese mic "EM9765-422" Zout=2.2k (Seems typical of this kind of mic) Sensitivity=42 dB below 1V/Pa (Other mics are in the -30 dB to -45 dB ballpark.) Not really sure how loud 1 Pa is. The datasheet used 1ubar as a test condition, so I'll assuming this is normal voice input. 1 ubar is 0.1 Pa. So 42 dB below 0.1V is about 800uV. I'm not sure if the datasheet figure is RMS, peak, or peak-to-peak. Assuming its RMS and we want 5V peak-to-peak, the desired gain would be about 2000. Regards, Mark markrages@gmail -- You think that it is a secret, but it never has been one. - fortune cookie -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist