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Old February 19th 05, 07:49 PM
Brian Whatcott
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On Sat, 19 Feb 2005 08:58:50 -0600, "aol" wrote:

I have a couple of Davd Clarks given to me by my nephew, a former Air Force
loadmaster. I want to convert to use on civil aviation radios. As a former
engineer and ham radio operator I understand radio principles very well and
I am aware of the impedances of military com systems (5 ohm mics, 19 ohm
speaker). I am not that savy with civil aviation radios, however. I have
managed to easily convert the speaker side with impedance matching
transformer. I am also aware that the mic side is not just a simple
impedance matching issue. Does david clark have a mic element replacement
for the M87A/C boom mic that I can order without having to order a complete
conversion kit (for over $100 dollars). I know I would be just as well off
to get a headset off ebay but I want to waste my time with this useless
project for those rainy days when I can't fly the plane.
Cheers all
Bruce Stafford (KE5NT)

[I compacted the following from an "Avionics" article]

" There are applicable standards for cockpit audio, namely TSO-C50,
and DO-214 for signal guidelines.

Once upon a time, a microphone was a carbon element, with a 150-ohm
impedance (borrowed from the telephone industry). It required external
DC excitation of at least 8 to 12 volts DC to produce audio of about
150 millivolts (mV). Headsets and radio outputs had a 600-ohm
impedance and required at least 50 milliwatts (mW) of power to produce
acceptable levels. Both microphone and headset signals were normally
referenced to the airframe ground. If a speaker existed, it was
understood to have an impedance of 4 ohms and be driven at 5 watts.

Military operators adopted "noise canceling" microphones like the M87
and M101, which had a dynamic microphone (mic) element built like a
studio microphone. The M87 had an active diaphragm with balanced
openings on both sides of the mic housing. The concept has the voice,
mixed with ambient noise, entering one side of the diaphragm and
noise-only entering the other side. The noise cancels mechanically in
the diaphragm, and the resulting signal is voice only.

Dynamic self-excited microphones offered low impedance (about 5 to 8
ohms) but operated at low signal level (often under 1 mV).
A low-level microphone must be accompanied by a low-impedance headset
to keep drive voltages low. This established the 8- to 20-ohm standard
headset/dynamic microphone combination commonly used by the military.

.. Military users also adopted amplified dynamic microphones and
600-ohm headsets as an alternative. (The amplified dynamic mic has low
impedance but also a small outboard amplifier to make it work like a
"carbon equivalent" microphone.)
Military audio systems often had to support different mic and headset
standards within a single station (or audio box for each pilot). And
each user typically had his own station control within the aircraft."

Hope this helps

Brian Whatcott Altus OK