As well as other good advice here, you should decouple the mic supply as
power supply noise will enter via the 10K to Vcc.
Split the 10 K into 2 x 4k7 and add an electrolytic (say about 10uF to
100uF) from the junction to ground.
Chances are you can in fact use 2 x 10k with OK results with most electrets.
This isolates the filter action from the low impedance of the supply
itself - you are then not trying to pass all the supply noise present at
this point.

Also, IF you are using a differential amplifier try taking an equal sized
capacitor from the BOTTOM of the mic to the "other" amplifier input.

The bottom 4K7 you show really doesn't do much UNLESS you do something like
the differential input mentioned above.



regards


      Russell McMahon
_____________________________

>From other worlds - www.easttimor.com
                                www.sudan.com

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))


-----Original Message-----
From: JB <jb@CI.GLENWOOD-SPRINGS.CO.US>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Thursday, 9 March 2000 15:57
Subject: [OT] Noise in discrete components


>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?
>
>Lots of noise:
>
> __________>> +9V
>|
>\
>/
>\ 10k
>/
>|
>|-| |--->> amp (lm386 gain ~100)
>|
>[ ] mic
>|
>\
>/
>\ 4.7k
>/
>|
>|________________>> GND
>
>Lots of noise:
>
> __________>> +9V
>|
>\
>/
>\ 10k
>/
>|
>|-| |-->> amp (lm386 gain ~100)
>|
>|
>|
>\
>/
>\ 4.7k
>/
>|
>|________________>> GND
>
>Minimal noise:
>
>|-| |--->> amp (lm386 gain ~100)
>|
>|
>|
>|
>|________________>> GND
>