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From: was@hp-lsd.COS.HP.COM (Bill Stubblebine)
Newsgroups: sci.electronics
Subject: Re: Help with electret microphone
Message-ID: <25150002@hp-lsd.COS.HP.COM>
Date: 1 Aug 90 15:20:35 GMT
References: <25150001@hp-lsd.COS.HP.COM>
Organization: HP Logic Systems Division - ColoSpgs, CO
Lines: 38

jws@thumper.mlb.semi.harris.com (James W. Swonger):

> The condenser mic (electret) is usually packaged with a FET gain stage
> because of the extremely high impedance (oo) of the electret element.

        Email from several other people also pointed this out.  It sounds
        plausible, but the boom microphone in the headset is difficult to
        disassemble, so I can't directly verify whether or not there is an
        FET in there.

> These assemblies usually have three wires (+, gnd, sig) and run off +3V.
> Some will work on 1.5V and some only use 2 wires.  Did the headset have
> a battery compartment?

	The microphone connection is two wires.  One suggestion I received
	was to series the microphone with a 3V DC source through ~3K and
	decouple the audio across the mike.  This is almost like my
	original figure, except that the audio is developed across the
	microphone, not the series resistor, with the FET presumably
	supplying the impedance reduction:


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

	Since I posted my original message, I located a schematic for an
	ICOM VHF transceiver that uses this kind of headset.  The circuit
	it uses seems to employ the principle shown above.  Thanks for all
	the suggestions.  I'll report back on the results.

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