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From: kube@ucbvax.ARPA (Paul Richard Kube)
Newsgroups: net.aviation
Subject: M-87 microphone preamp circuit?
Message-ID: <10352@ucbvax.ARPA>
Date: Wed, 11-Sep-85 03:00:23 EDT
Article-I.D.: ucbvax.10352
Posted: Wed Sep 11 03:00:23 1985
Date-Received: Thu, 12-Sep-85 23:33:47 EDT
Reply-To: kube@ucbvax.UUCP (Paul Richard Kube)
Organization: University of California at Berkeley
Lines: 20

Now that it's seeing a lot of airtime near civilization, the old C120
is finally getting a radio.  I would like to use a headset with this
radio (those who have tried to hear themselves think inside a C120
above idle will understand).  I have a headset, an Astrocom with
a 5 ohm M-87 mike.  The noise cancelling properties of the M-87 are
wonderful, but at 5 ohms it is of course useless with civilian
avionics.  

Question:  What does it take to get the M-87 to live with standard
radios?  Just a transformer?  If so, where does one get a tiny 
5:50000 ohm audio transformer---or whatever the ratio is supposed to be?
Or does it take some active circuitry?  If so, does anyone have a
circuit, or the story on what the parameters are supposed to be
(input and output impedance and peak-to-peak voltage would do)?

	Paul Kube
	kube@berkeley
	ucbvax!kube