[Boatanchors] Aviation Headset

[Richard Knoppow]

----- Original Message ----- 
From: "David C. Hallam" <dhallam at knology.net>
To: "Boatanchors-qth" <boatanchors at mailman.qth.net>
Sent: Thursday, December 26, 2013 8:23 AM
Subject: [Boatanchors] Aviation Headset


>I have an Aviall microphone/earphone headset and can't seem 
>to find any information about the microphone or earphone 
>impedance.  It seem that the company is now owned by Boeing 
>and don't sell this type of item anymore.  Is there some 
>kind of standard impedance for aviation headsets?  I also 
>don't seem to see any model number on the headset either. 
>Does anyone know where I can find the information?
>
> David
> KW4DH

    Aviall appears to be a marketing organization run by 
Boeing Aircraft.  Its likely the phones are made by someone 
else and marketed by Aviall under their own name.  I checked 
David Clark Company for the specs on their military aviation 
headphone sets.  The phones are very low impedance, about 30 
ohms each and the microphones are moving coil dynamic types 
with an impedance of around 5 ohms.  I don't know if this is 
a defactor standard.  Very old head sets with microphones 
may have higher impedance, on the order of 300 ohms per 
phone and possibly carbon microphones. Typical carbon 
microphones have a resistance on the order of 25 to 50 ohms 
and usually work with about 4 volts across the mic. The old 
standard AT&T telephone microphones worked at 4.5 volts. If 
your phones have a very low Z dynamic mic and must work into 
a HI-Z input a speaker output transformer may work. Usually 
commercial dynamic microphones had voice coil impedances of 
anywhere from 8 to 30 ohms and built-in transformers for 
matching higher Z.  Some had 150 ohm voice coils for direct 
use with medium-impedance broadcast type amplifiers. (I 
think some EV mics have direct voice coils like this).  The 
headphones will work off any loudspeaker amplfier but the 
mic may require some if its really low-Z since its output 
voltage will be very low.
    A lot of aviation type headsets have various 
noise-cancelling arrangements both for microphone and 
headphone.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com