I was assembling another version of Don Tillman's remote cable preamp (
http://www.till.com/articles/PreampCable/) when it occured to me that the battery box I built for this would also power an electret microphone. So I dug into the bag of Panasonic electrets I recently found surplus and knocked this together:
The end result- even with that questionable mounting- works superbly into my Contra. Sounds better than even the best PZT crystal pickup I've built. Granted, it's not as good for high volume situtations where feedback is a consideration, but at a total cost of about $4 (plus the battery box- another $10) it's amazingly good.
I was going to modify some of these capsules for better headroom and higher SPL (a modification Siegfried Linkwitz came up with:
http://sound.westhost.com/project93.htm) but it doesn't seem necessary. (I'll probably do it anyway.)
You can find simple electret microphone capsules at your local Radio Shack for under $3. Add some coax, a little heat shrink tubing, and one resistor and capacitor and you can make something as good as microphones that sell for much, much tmore. The best part is that they're so cheap, you can carry a half-dozen spares ;-)