On Nov 15, 2007, at 9:47 PM, Dr. Gerald N. Johnson wrote:
On Thu, 2007-11-15 at 21:41 +0200, Geoffrey S. Mendelson wrote:
I recently purchased (for $5) a headset designed for computer use.
It's
the "closed" design, so while to me it eventually becomes
uncomfortable,
before that it makes listening easier by reducing external noise.
The microphone, is a low voltage condenser unit that requires DC.
A computer sound card puts out five volts, but I think that much
less will do.
As a first shot, supply 5 volts (maybe 4.5 from three AAA cells) to
the
microphone through a 680 series resistor. Couple to the radio
through a
0.1 mf disc ceramic capacitor. Probably don't go lower than 330
ohms or
higher than 1K, capacitor between .01 and .5 mf.
If it's a standard electret capsule a 2k2 pull up to 3v to 12v DC is
usually standard and coupling out via electrolytic or tantalum of a
few microfarads is typical.
Changing the load resistor varies the sensitivity.
The output impedance is about the same as the pull up resistor so it
expects to feed a high-ish impedance amp (a few k or more in this case).
e.g.
<http://www.epanorama.net/circuits/microphone_powering.html>
--
73 DE N7WIM / G8UDP
Kevin Purcell
kevinpurcell@pobox.com
BIRRGRMP BRU ROTARY GE GE UGERUMF OM WITH MY SET RETTYSNITCH SPITTY
TONE HIT IN POTIMUS? BK
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