Question about converting military helmet

Originally Posted by

Hello I've not posted on rah before, but have lurked long enough to
realize that you guys have
a few electronics experts among you. What I have is a Gentex sph-4
military flyers helmet.
This is a single plug helicopter helmet. I've been told that the
microphone impedence for
military helmets is different (sorry guys I don't have the actual
values) from civilian helmets,
and I have tested this helmet in a helicopter. The earphone portion
works fine. My question is what must be done to this helmet to make
use of it in a civilian helicopter? Is it as simple as
replacing the existing mic with a proper resistance civilian mic? Or
is it something more tricky
and electronically sinister? Any thoughts would be appreciated, thanks
in advance!



Get yourself in the air, no matter how you get up there!

Wayne

Hi Wayne,

I am afraid that the impedance has to be matched to the station boxes fitted to your helicopter type. Normally civilain helicopters work on a tels impedance of 600 ohms and 5 ohm impedance mic for one headset, two headsets connected to the same box in parallel will be halved to 300 ohm. This is for LO impedance systems. HI impedance systems use 600 ohms and a 150 ohm impedance mic. Yours may well work but if someone else connects into the system the whole intercom will deteriorate. You will find the impedance information in the component manual of the station box or ICS box for the aircraft you are flying. The SPH 5 I believe is the civilain equivalent to your military SPH4.

Regards

lectricity