I have a question I'm hoping you may be able to answer based on a
Kitplanes article (Dec 1997) I came across concerning the topic.

As part of an upgrade program in the Flight Simulation Lab here at the
University of Toronto, we purchased a new helmet mounted display system.
This helmet includes an integral mic and speakers. Oddly enough, even
though it is designed for aviation simulators, they used 32 ohm speakers
and a non-amplified microphone (and a DB9 connector). The problem is we
use an aviation (Telex, actually) intercom for communications between the
pilot(s) and instructor station. I have acquired a set of GA style plugs,
but have been unable to find a suitable circuit to make an adapter (i.e.
to amplify the microphone), and radios/headsets is not exactly our
area of expertise.

Based on the above article, in which you talk about radios that can
accept either microphone as input, I'm guessing you've done this exact
conversion. As the helmet manufacturer has not been able to suggest a
solution, any help you could provide would be greatly appreciated (a
circuit diagram? an adapter for sale? where to find the answer if there
is one?)

Thanks,

Bruce Haycock
University of Toronto
Institute for Aerospace Studies