Headset Incompatability

General aviation Aircraft mic inputs are designed for resistive CARBON
microphones.
Now most modern aircraft mics are NOT carbon mics but they still
emulate the old carbon mics and are powered by the bias current that
is supplied by the radio to power the old carbon mic.
The reason that some mics are not compatible is the electronic
amplifiers in the new NON carbon mics seek to bias them selves at some
voltage level between 3 and 8 volts. If you connect two of the
amplifiers together directly without DC isolation resistors and caps
the mic amp that has the lowest bias point wins and only it works
well.
Good intercoms have independent bias resistors for EACH microphone
input. The cheap intercoms have only one bias resistor and tie the
microphones together directly. This will only work if the mics are
truly carbon elements but not if you use a modern mic that emulates a
carbon mic.

On Wed, 18 Jun 2003 22:51:45 +0100, "Andrew"
wrote:

I am trying to get to grips with why a pair of headsets will work fine in
some aircraft but not in others.

Headset suppliers say that this is not uncommon and try to sell a 'Matcher
Unit'

I would like to know what these units do and how they work (they intercept
the mike jack and are powered by a nine volt battery)

Does anybody have any technical info on this problem??

Many thanks