Message 16, 18 November 2003
Hi Bluelight,
Thanks for sharing this information on interphone/headset impedances. The subject has been confusing to many for a considerable period of time, so your details and background on the developmental status of these systems (vis-a-vis impedance ratings) is most timely and quite appreciated. Another excellent example of why the history of the gear is so important as a requisite backdrop for gaining greater understanding of the relevance of current gear technical specs.
Cheers, DocBoink

Message 12, 17 November 2003
Hi all,
In some publications headsets and earphones are described as "high impedance" or "low impedance", but in relation to what? It was especially confusing with the HS-38/HS-38A headset in the early P-series helmets. To begin with they were described as "low impedance" and later as "high impedance". When I finally succeeded in digging out some figures it all made sense. Here goes (everything relates to USAAF/USAF standards):
The early WW2 standard headset used in helmets was the HS-18 with R-14 receivers, and when configured as a real headset with an HB-7 headband it was an HS-23. They had an impedance of 2000 ohms.
Around 1943 a so-called "low impedance" headset was introduced based on the ANB-H-1 receiver with an impedance of "only" 600 ohms. The proper headset version was designated HS-33 and the helmet version HS-38. When equipped with a boom mike the HS-33 was designated H-46. For a period the HS-18/HS-23 and HS-33/HS-38 were used in parallel and in order not to confuse one from the other the HS-18/HS-23 was equipped with a black PL-54 jack and the HS-33/HS-38 with a red PL-354 jack. It was possible to use the HS-33/HS-38 in a high impedance aircraft comms installation by connecting through an MC-385-( ) adapter. These adapters appear at intervals on eBay. The HS-33/HS-38 and later HS-38A headsets were used into the 1950s and the associated aircraft intercom systems were designated AIC-2 or AIC-3.
In the mid-1950s a new standard was introduced for aircraft equipped with the AIC-10 intercom system. The new H-75/AIC headsets installed in P-4 helmets and retrofit in earlier helmets was only 5 ohms, and suddenly the HS-38/HS-38A headsets were "high impedance". The 5 ohms is still the USAF standard today so technically it would be possible to use a P-4 helmet in an F-16.
BTW, the term "intercom" is also used in single-seat aircraft even if the pilot has no other crewmembers to communicate with. The intercom system is used to connect the transmit/receivers with control boxes and the helmet communications system and in most aircraft there is also an external plug e.g. in the nose gear well where a crew chief can plug in his headset to communicate with the pilot during pre-flight inspection.
Cheers, Bluelight

H-154/AIC headset with the typical triangular MX3678
earcups and spring loaded arms.

H-46 headset with M-6A/UR boommike. Both pictures
are thumbnails.