Headset connections (attn: Jim Weir)

The only description of the mic in the documentation is the frequency
badwidth (100Hz to 3kHz). Mic resistance is 5.82kOhms one way,
5.56kOhms with reversed polarity, and yes I already tired to hook up
the mic to the intercom, so its definitely a bare electret mic.

(John) wrote in message ...
Yes the amp in the david Clark mic "Should" work however it is most
likely that this is a hybrid circuit and is so small you would need
micro sized tools and a microscope to work with it.

First you need to determine what kind of Mic it is. It should say in
the documentation for the helmet. If not you need to use an ohmmeter
to determine what it is. Use a DVM and measure the mic resistance in
both directions. If it is the same when you swap the probe polarity
and is less than 1000 ohms then you have a bare dynamic element. If
it changes with the polarity of the meter leads then you have an
eletrofet or a amplified electrofet or an amplified dynamic mic.
I assume that you have already tried to hook the mic to the intercom
and it does not work so that leaves the bare electrofet element as the
logical choice for the element. ( A bare electrofet element puts out
only a few milivolts into about a 1k bias resistor.) If so you can
go the National semiconductor web site
http://www.nationalsemiconductor.com/ and pick a audio amp in the one
to two watt range to power the relative low impedance headphones if
your intercom will not drive the 32 ohm headset to the level desired
and a pre amp that is available from parts suppliers like
www.digikey.com. If you power these from an separate 12 volt
regulated wall wart power supply then you do not have to worry about
many of the power and stability issues where the amp is powered by the
same line that puts out the audio. You MUST use a regulated power
supply or include a three terminal regulator as part of your interface
or the 60 hz hum will be very bad. National has application notes
that give typical designs. Most bare electrofet elements need a 1K
to 4.7K pull up to 6 to 10 volts as the bias power supply to make them
work. AC couple the electrofet mic audio to the preamp input then use
the preamp IC to get the gain necessary to get about 1/4 to one volt
of audio into the intercom. AC couple the preamp output into the
intercom mic audio input through at least a 1 UF cap and it will work
fine.
I cannot tell you exactly what you need since you do not have any
specifications and you must measure what you have as a starting point.