Path: utzoo!attcan!uunet!decwrl!sdd.hp.com!hplabs!hpfcso!hpldola!hp-lsd!was
From: was@hp-lsd.COS.HP.COM (Bill Stubblebine)
Newsgroups: sci.electronics
Subject: Help with electret microphone
Message-ID: <25150001@hp-lsd.COS.HP.COM>
Date: 27 Jul 90 00:05:30 GMT
Organization: HP Logic Systems Division - ColoSpgs, CO
Lines: 50

Some time ago at a swap meet, I picked up a nifty ICOM (familiar ham radio
brand) headset containing a microphone and earpiece.  I wanted to use it
for hands-free dictation with my VOX controlled microcassette recorder.

The earpiece in the headset worked as I expected, but the microphone
generated only the faintest sound for recording.  Later, I spoke with a ham
friend familiar with ICOM equipment, and he said that these headsets use an
electret microphone.  All of a sudden the light bulb went on:  tape
recorders expect microphones to transduce acoustic energy directly into
electrical energy without an external power source, as crystal, ceramic or
dynamic microphones do.

Now, I remember reading a long time ago that an electret microphone is
basically a capacitor whose value varies in proportion to incident acoustic
energy.  Therefore, one would expect a circuit to make an electret
microphone do its thing would hold the electret capacitor at a relatively
fixed voltage and detect the charge shuffling in and out of the capacitor
as its capacity varies.  Intuition tells me such a circuit might look
something like this:


                             |<--audio-->|
                             |           |
			   -----       -----   decoupling
			   -----       -----   caps
                             |           | 
                   __________|__/\/\/\___|_________
   microphone      |               R              |      DC source
  (variable-C)  -------                        -------  +
                -------                          ---    -
                   |______________________________|


I've never designed a circuit like this, and I'd rather not experiment with
my nifty new headset, so can anyone supply some pointers?  Voltage level
for the DC source?  Is the polarity important?  What's the right value for
the resistor to match the tape recorder mic input?  (The recorder manual
says the mic jack is designed for a nominal mic impedance of 200-600 ohms.
Should the value of R be in that range?)  Do I need the decoupling caps?

Book recommendations would also be welcome.  Thanks.

				"I groove on 1's and 0's; this analog stuff
				 is mind bending"

                                Bill Stubblebine
                                Hewlett-Packard Logic Systems Div.
                                Colorado Springs, CO  80920
                                was@hp-lsd.hp.com
                                (719) 590-5568