On Fri, 26 Sep 2003 05:00:26 -0400, Michael Horowitz
wrote:

Folks - I'm having difficulty getting audio from my Avcomm headset to
my Icom A-21 handheld.
The Icom requires a condenser microphone and the Avcomm has an
'electret' element, which as I understand is a variety of condenser
mic.
Has anyone had a similar problem and how was it solved? - Mike

PS- I see a variety of circuitry on the 'net describing how to power
the microphone, but I was hoping not to have to go off building
another box.

The trap for players in this equation is that the electret (or
condenser) microphone in a standard GA headset is not connected
directly to the mic plug. The microphone input circuit on (standard)
aviation radios and intercoms (despite this day and age) is designed
to accommodate a CARBON capsule microphone by supplying a DC bias
voltage though a suitable input load resistance. Genuine carbon
microphones are rare these days but the electret (or dynamic)
microphone and a small transistor amplifier built inside the plastic
housing makes the whole assembly appear electrically as a
pseudo-carbon microphone to the GA radio.

The ICOM A-20 handheld , and I suspect the A-21, has an external
microphone input that accommodates a 'pure' electret microphone, and
*not* the pseudo-carbon of a GA headset. ICOM produced an outboard
adaptor for GA headsets to be used with the A-20 and the circuit was
custom designed to provide the correct DC bias for a pseudo-carbon GA
microphone and then to attenuate the high level voice signal down to
the much lower equivalent of what comes out of a 'pure' electret
capsule, which then goes into the ICOM microphone circuit. The adaptor
also had an amplifier to produce transmit sidetone

The other tricky part is the PTT. In GA this is easily accomplished by
grounding or earthing an independent PTT line which either engages a
relay or some form of solid state switching in the GA radio. In the
ICOM the PTT is activated by a DC voltage level shift when the
microphone is connected by a series PTT switch. The ICOM GA headset
adaptor achieves this with fixed value resistances that emulate the
same effect as switching in the 'pure' electret capsule.

The key to this whole exercise is getting the circuit for the innards
of the ICOM adaptor! The picture then becomes pretty clear.

I believe that later versions of the ICOM have worked around the need
for an 'electrical' adaptor with GA headsets by building the
microphone input circuit ready to accommodate pseudo-carbon in the
first place. Any outboard cables are now just 'plug size' and gender
changers rather than there being any amplifiers or matching circuitry.

A safe bet in this adapting and matching stuff is to always think of
the (standard) GA headset as having a CARBON microphone regardless of
what is written on the box. It's carbon as far as the electrics are
concerned, anyway!