[Boatanchors] Microphones

[Glen Zook]

Actually, carbon microphones don't require a transformer in tube type equipment.  There are all sorts of examples in amateur radio equipment and in commercial FM two-way equipment where the carbon microphone feeds the 1st audio amplifier as part of the grid resistor or in series with the cathode.

Even with solid-state equipment, transformers are not needed.  But, there is a requirement for a DC voltage to be present.

In telephones, a transformer was used in many applications.  However, the earliest telephones did not use a transformer.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: Richard Knoppow <1oldlens1 at ix.netcom.com>
To: Glen Zook <gzook at yahoo.com>; 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 10:50 PM
Subject: Re: [Boatanchors] Microphones
 

----- Original Message ----- From: "Glen Zook" <gzook at yahoo.com>
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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.