[Boatanchors] Microphones

[Richard Knoppow]

----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: <W4AWM at aol.com>; <bluegrassdakine at hotmail.com>; 
<boatanchors at mailman.qth.net>; <boatanchors at theporch.com>; 
<collins_radios at yahoogroups.com>
Sent: Saturday, October 13, 2012 6:32 PM
Subject: Re: [Boatanchors] Microphones


Actually, most microphones on "modern" equipment are "medium 
impedance". A true "low impedance" microphone generally has 
an impedance below 50-ohms and most have an impedance of 
under 20-ohms. Almost all of these microphones are carbon, 
or "carbon compatible" types.

Glen, K9STH

     You are right about current practice but in the past 
many broadcast microphones of the moving coil type were made 
with impedances of around 20 to 50 ohms. These did not 
contain an internal matching transformer.  Most of these 
were made by Western Electric, who liked this impedance, but 
there were many other microphones of this sort and many 
broadcast microphones can be wired for 20 to 50 ohm output 
as well as some medium impedance of around 200 ohms.
     Carbon microphones can be made with a variety of 
impedances but all must be used with some sort of 
transformer input. The resistance of most is somewhere 
around 20 to perhaps a couple of hundred ohms. I don't have 
one handy to measure but I think the resistance of the T-1 
transmitter used in Bell System 500 series phones was about 
200 ohms.
     Carbon microphones were perhaps the single most 
numerous type since they were used in virtually every 
telephone in the world.  Carbon mics have the advantage of 
being _amplifiers_ since they operate with an external power 
supply and can deliver power well in excess of the power in 
the acoustical wave that actuates them.  Modern electret 
microphones with solid state amplifiers have all but 
supplanted carbon microphones.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com