At 07:57 PM 3/8/00 -0700, you wrote: >I'm working on a 16f877 a/d project that reads audio from an electret(sp?) >microphone which has been amplified by an lm386 set for a gain of 100. The >mic itself seems to be low on noise, but the 10k resistor to V+ is a >standard carbon 1/4W variety and it seems to be producing excessive noise. >I conclude this based on the fact that I can short the mic and the noise >persists, but disconnecting the resistors will stop the noise. Test >circuit is below (use fixed-pitch font to view). Would a metal-oxide >resistor help, or are there other options? 1. Yes. Carbon composition resistors are noise generators. The noise is proportional to the current. Use a 10k resistor to bias the FET. Carbon film is better than composition, metal film is best. 2. You should use a low noise pre-amp. Either a low noise op-amp or a low noise transistor. Why are you trying to do all the gain in the LM386? If it's a minimal parts count, you're going to have to live with the amplified noise of the LM386. 3. Bypass all power supplies (+/-) to ground with 0.1 uF capacitors. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causae scire" ================================================================