Eric Prebys <pre…@viper.princeton.edu> wrote:
>OK, admittedly I’m being a cheapskate, but I was trying to adapt
>a standard telephone headset for use in a airplane. The headphone
>part of course works fine, but after finding the elusive
>.206" plug to connect the microphone, it still does not work.
>Does anyone know if there is a fundamental difference between the
>microphone types? Also, I noticed there were three contacts
>on the mic plug. I assumed that two were for the mic and
>the third was for a remote ptt button. Do aircraft headsets perhaps
>use a powered mic?
Well, I must be a cheapskate too, because I did the same thing
(successfully). First off, the headphones may seem to work, but the
aviation headsets have a much higher impedance than the hi-fi
variety. Some intercoms can’t drive that low impedance, and they
load down the output so much that the other headphones (my
instructor’s, in this case) are almost impossible to hear. When I
discovered this, I attached a small impedance-matching audio
transformer to the side of one of the ear cups (looks a bit
Frankensteinish). As for the microphone, aviation mics are designed
to emulate a carbon mic. That is, they present a variable
resistance to a current flow driven by +12 volts. I was able to rig
up an electret mic to the base of a transistor, and by playing
around with the emitter resistor and collector resistor, I got
something that ATC could hear. It was not good quality, though, and
I now use Marv Golden’s $89.95 marvel. You’re right about the PTT
switch. It switches to ground.
-Bob Scott
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