On Fri, Oct 10, 1997 at 08:48:33AM -0700, Scott Walsh wrote:

>      Also, is anybody familiar with the term 'condensor microphone' that
>      used to be on old tape recorders?

Yes, it was a precursor to the current electret microphone. It consisted of
a pair of plates arranged as an air-spaced capacitor (aka condensor)
biased by a high voltage. One plate vibrated with the sound waves, resulting
in a AC signal superimposed on the bias voltage.

The output level was very low, but the quality was very high.

An electret microphone (as universally used in modern cassette recorders)
works on the same principle, except that the bias is provided by a
permanently charged "electret" analogous to a permanent magnet. An amplifier
is usually included inside the microphone, usually a single FET. This
is why electret microphones require a DC supply. One neat thing about
this is that they can be substituted for carbon microphones, which also
require a low-voltage DC supply. Hence electret mikes are also widely used
in telephones and communication radios where carbon mikes were once used.


--
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