Military Headset Adaptor
Brett Leyshon
after spending a while on the Internet my research has led me to this
newsgroup and I am looking for a person named Jim Weir.
I am trying to locate a circuit diagram for a military to civilian headset
adaptor.
Can anyone help me locate one?
Brett
Edward Schrom
Good luck.
http://www.marvgolden.com/adapters.htm
http://www.fly-bluesky.com/headsetparts.htm
Ed
"Brett Leyshon" <b_le...@optusnet.com.au> wrote in message
news:3c42cc02$0$19541$afc3...@news.optusnet.com.au...
Mr.TwistAir
www.rst-engr.com
Cheers,
Alex
http://galsaero.al.ru - General Aviation in Russia
Brett Leyshon <b_le...@optusnet.com.au> wrote in message
news:3c42cc02$0$19541$afc3...@news.optusnet.com.au...
Jim Weir
"Mr.TwistAir" <jjheli...@mtu-net.ru> wrote in message
news:a20cas$278$1...@gavrilo.mtu.ru...
Thanks for the referral, but I CAN NOT and I WILL NOT be drawn into the
military headset adapter fray. I want absolutely nothing to do with them.
Jim
Phil Smith
Looks like Jimbo has had a dummy spit on this topic...surprising given
the number of avionics devices that he has created over three-odd
decades of enterprise and that such a device has the complexity of one
or two 20cent transistors and can be knocked up by a hobbyist in an
afternoon on a bit of 'veroboard' [or whatever you know as the
pre-punched copper-tracked experimenter's circuit board].
[Just brainstorming] Perhaps he gets a zillion phone calls from
enquirers who 'expect' him put something on the market to adapt every
military headset ever made but aren't prepared to pay commercial
market price for development, production and certification costs; and
then they start getting pissed off at him...who knows? Anyway that's
Jim's call, I guess.
I've had success since the 1970s with adaptors for the generic
H-157/AIC series of military headset that uses a 5ohm M-87/AIC dynamic
microphone and two (in parallel) 19ohm H-143 earphone elements. I'll
be back at work at the end of the month, with access to a document
scanner, and should be able to rustle you up an email attachment of a
circuit.
In the meantime, some general guidelines for collective newsgroup
interest...[some prior electrical or electronic knowledge is
preferrable so that some tech stuff doesn't go over your head!!]
A GA radio / intercom microphone input expects to 'see' a carbon
microphone. It does this by providing a DC bias voltage that [in the
days of yore] 'powers' the carbon granules which change resistance
with the sound pressure variation of speech.
These days, a carbon microphone is rarely used on headsets and is
substituted by a transistor amplifier that is conveniently powered by
the DC bias voltage. The circuit components are chosen so that the
amplifier 'draws' a similar current to a carbon element and also
boosts the weaker signal output of the electret (or dynamic
microphone) to an equivalent level of a genuine carbon microphone.
Such an amplifier is usually embedded into the microphone capsule on
the end of the boom or, in some cases, in one of the earcups.
A problem with adapting the military dynamic microphone, particularly
the M-87/AIC variant, is that its output is very low (a few
millivolts) and the amplifier needs high gain to boost the speech
signal voltage to a level comparable with a carbon mike. With high
gain comes the problem of susceptibility to radio frequency feedback.
This is a phenomenon that occurs if the transistor amplifier
(particularly with in-line adaptors) picks up a radiated VHF
transmission from the aircraft on the headset cables (acting as an
antenna), detects the signal and then feeds it back into the radio
microphone circuit causing a classic feedback squeal. To counter this
problem, there needs to be a filter to block any RF picked up on the
microphone wire pair from getting into the amplifier. Ferrite beads
with a few turns of wire in series with the mike leads and small value
capacitors, say, 100 - 500 picofarads across the leads aid in
'choking' the RF before it gets to the first transistor of the
amplifier.
A further consideration is shielding and earthing / grounding. The
H-157/AIC variant headset (as I remember) uses only unshielded twisted
pair wires for the cable that is (on mine anyway) about as long as
from the earcup to the waistline. In most military intercom systems
the single headset plug connects to a 'fly lead' that usually dangles
from the roof just behind a crew member's seat. The other end of the
fly lead is wired to the particular crew members intercom control box.
The fly lead (as I recall) is overall unshielded but the microphone
pair is wrapped in a conductive impregnated cotton thread and there is
a wire tracer that is bundled against the conductive cotton. The wire
tracer is earthed at the intercom control box. The clincher that
allows a military intercom to survive RF feedback, even with the long
fly lead carried by, say, a loadmaster in the cargo bay of a transport
aircraft is the properly engineered input filters to the intercom
microphone amplifier circuit. There is also a balanced transformer
input which provides further immunity to RF.
In the GA environment, the difference (compared to military) in
shelding / earthing arrangement is that the wiring all the way from
the microphone to the radio / intercom is generally unbalanced
(similar to coaxial cable) and uses shielded cable that is (through
the mike jack) bonded to the airframe. The cable connecting the
military / civil adaptor to a GA microphone jack should also
preferably be shielded to minimise stray RF pickup. The construction
of an outboard in-line adaptor in a metal box rather than, say, a
plastic 'jiffy box' will also have some bearing on the outcome of
success!
The earphone side of the equation is somewhat easier but there are
still a few traps. The low impedance of the H-175 / AIC earphones can
be transformed up to the (nominal for a single headset) 300-odd ohm
output impedance expected by a GA radio / intercom using a small 8 to
500 ohm transformer commonly found in 'six transistor' pocket radios.
Here in Aus, Dick Smith and Jaycar sell them for a few dollars each.
The only problem that I found is that the H-143 earphone capsules are
a bit deaf (electrically) and require somewhat more volume from the
radio / intercom than comparable GA headset earphone elements to get
the same listening level. In a military environment, the intercom
control boxes have an earphone amplifier that is matched to the lower
sensitivity and particular dynamic range of the H-143. At the time
that I was flying a single seat ag aeroplane as a glider tug, the
headset plugged straight into the radio, so it wasn't an issue...I
just cranked up the radio volume. Using the same headset and adaptor
with a simple GA intercom that has a single volume control for all
users at the same time as the co-pilot and passengers were using GA
headsets...it was too loud for them when just comfortable for me!
On a final note, just as an example of experimental development
originating from my own military / civil adaptor saga, my current
preferred headset is actually a custom mix comprising a Bilsom Viking
industrial hearing protector...very comfortable...fitted with an
M-87/AIC boom microphone...still one of the best noise-cancelling
microphones ever built despite some criticism (from purists) of the
distinctive 'nasally' sound that they produce. The microphone
amplifier is built on a piece of veroboard about 3/4 inch square,
potted in epoxy and wrapped in copper foil that is bonded to the
shield braid of the cable. The amplifier sits in one of the earcups
between layers of foam rubber. The earphone elements are a pair of
'rocking armature' earpieces that were common in telephone handsets
years back. These are as robust as old boots, have a frequency
response nicely tailored for speech-over-radio, and have the correct
nominal impedance for a GA earphone radio / intercom output. For the
cable / plugs, rather than stuff around with figure 8 shielded pair,
individual mike and earphone plugs, heatshrink and so on...a much
tidier arrangement was to simply buy a David Clarke spare part
replacement cable from a pilot's shop...it has much more flex than the
average general purpose stereo shielded cable and already has moulded
plugs. The shield braid and centre conductors also look a bit more
meaty than than the brand X figure 8!!
Phil Smith