[simpits-tech] Civil and military headsets

[Fred Mahone]

Since I worked with some avionics, I will try to remember all about the 
differences in headsets.

A military headset has a very low impedance microphone, in the order of 5 
ohms. The reason is
a very low impedance microphone has a very low output, on the order of 5 
millivolts a.c. This keeps cross
talk in the cabling down. The earphones are 19 ohms each and are wired in 
parallel, giving a total
impedance of 9.5 ohms. They are in parallel so if one opens up, you still 
have one earphone
working.

Civilian microphones are about 600 ohms output impedance and are transistor 
amplified.
The civilian microphones started with carbon units just like a telephone. 
Occasionally,
you had to give them a sharp thump to unpack the carbon granules. Then they 
went to
a dynamic element but to have the same output level, approximately 150-170 
millivolts,
an amplifier had to be placed at the element. Now they used electret condenser
elements, which still have the amplifier. A bias voltage of about 6 volts 
is fed to the
amplifier via the audio line. The earphones started as two 300 ohm units in 
series
to give the civilian headphones a 600 ohm impedance, but was changed to a
parallel configuration so if a earphone failed, you still had audio. But, they
kept the 300 ohm impedance which now gave the total impedance of 150 ohms.
This causes problems when a number of headsets are used at the same time as
the audio panels are designed to drive a 600 ohm load.

By the civilian headsets having higher impedance, crosstalk (audio going where
it is not supposed to go) occurs and to correct this a shielded cable is 
usually installed
with the shield grounded at one end only.

A military headset microphone does not match the computer sound card in 
either level
or impedance and loads the supply voltage that is produced to power a normal
electret condenser computer microphone. Earphone impedance is about a 
speaker impedance
so they work.

If a newer style civilian headset is used, the microphone will probably 
work as the supply voltage
is there and the impedance and level are pretty close. The earphones are 
higher than ones for
a computer headphone, so the volume will have to be turned down.

I want to use my David Clark military headset for my simulator so I have 
been trying to
come up with an adapter design. A commercial adapter is quite costly as it 
will have
FAA approval which drives the cost up.

I hope I have helped and not confused everyone.

Fred Mahone