John, MW1FGQ wrote:
"I used to carry a pair of old military headphonees in my kit when
building broadcast installations until they were nicked by some ****."

Several models of telephone receivers were used in WW-2, including the
TS-10 (sound powered) unit. This was probably the most efficient
transducer except for the R-13 and other resonant models designed for
morse code reception. The HS-33 with its leather-covered headband is the
model I saw most often. I don`t remember it being particularly sensitive
but it did have pretty good fidelity.

I seem to recall seeing the most valuable patent ever issued by the U.
S. Patent Office. It`s the Alexander G. Bell telephone patent. The
microphone was dynamic, not carbon, so his receiver had to be sensitive
as the instrument was sound powered.

I used to carry around a surplus TS-10 unit in my kit. Although sound
powered, it is not sharply resonant. The fidelity is not too bad. Aboard
my ship in WW-2, I had a spare TS-10 unit wired with an attenuator and
connected to the ship`s entertainment and information line. There was an
almost 24-hour music feed from radio or records. We had a V-disk
transcription library too. Nobody complained and the zero dBm level was
plenty loud if I turned up the attenuator. Little electrical power is
needed for considerable acoustical power when using the right
transducer. As long as I kept the movie projector running, the skipper
would let me get away with about anything.

Best Regards, Richard Harrison, KB5WZI