Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!ucsd!ucbvax!hplabs!hpfcso!hpldola!hp-lsd!was
From: was@hp-lsd.COS.HP.COM (Bill Stubblebine)
Newsgroups: sci.electronics
Subject: Re: Help with electret microphone
Message-ID: <25150004@hp-lsd.COS.HP.COM>
Date: 13 Aug 90 07:06:05 GMT
References: <25150001@hp-lsd.COS.HP.COM>
Organization: HP Logic Systems Division - ColoSpgs, CO
Lines: 52

A couple of weeks ago I posted a note asking for help coupling the electret
microphone in an ICOM ham transceiver headset to a VOX controlled micro-
cassette recorder.

I received numerous helpful suggestions, mostly via email.  Most were
similar to the one from Mark Zenier (markz@ssc.uucp):

        > Most electret microphones (new & cheap ones) have a built in
        > FET amplifier.  All you need is a pullup resistor to a DC
        > supply (1.5 to 10 V) and a decoupling cap.  See the Digi-Key
        > or Mouser Catalogs for examples.

Indeed, the electret microphone listings in the Digi-Key catalog were quite
helpful, even showing recommended interface circuits.

Anyway, the headset is now working fine.  The interface circuit I finally
settled on was:

                                                     0.1 uF
                 _____________/\/\/\__________________||___
    3V DC        |            R=3.5K            |     ||
    source  + -------               electret -------        audio
            -   ---                    + FET -------
                 |______________________________|_____||___
                                                      ||
                                                     0.1 uF

I packaged the circuit inside a little plastic battery holder I found at
Radio Shack.  The microphone draws ~200 uA DC (as best I can measure) and
produces ~20mv peak-to-peak audio for normal volume speech.  This is about
the same output level produced by a crystal microphone I had lying around,
and more output than the small hand-held ceramic microphone I normally use
with the microcassette recorder.  (The recorder has AGC, so the microphone
output level doesn't seem to matter too much.)  The ICOM headset boom mike
has a distinctively crisp response to voice frequencies, and excellent
noise rejection.

By the way, the battery polarity did matter.  Both polarities produced
audio, but one polarity was 6-10 dB louder than the other.  I used the
louder polarity.

The only thing I'd still like to do is connect the 3V microphone supply to
the recorder's internal 3V supply so that it switches off when the recorder
is off.  As it is now, I have to remember to unplug the microphone from its
supply to stop the current draw.  This should be an easy modification.

Thanks for all the suggestions.  The net is a truly wonderful thing.

                                Bill Stubblebine
                                Hewlett-Packard Logic Systems Div.
                                was@hp-lsd.hp.com  (Internet)
                                (719) 590-5568