I can attest to the capabilities of the old Trimm headphones. In my
pre-novice days (1963-1964) of building code practice oscillators, my
dad came up with a set for the project. They were old even then,
probably dating to the late 40's or early 50's. They look sort of like
the headphones that Gregory Peck wore in "12 O'clock High". I don't know
where or when my dad got them. I used them in my SWL and Novice days,
although the hard bakelite shells hurt my ears. After I upgraded to
"better" headphones, the old Trimms were seldom used, until this past
winter of horrible 80m band conditions. Could the headphones that helped
win WWII, help win the war on QRM and QRN? The magnetic phones with
their steel diaphragms form a built-in CW filter. They have a noticeable
low and high rolloff with a peak around 850 Hz. Amazingly, they also
reduce the effects of QRN static. They are now my weapon of choice for
CW work.
I wired the elements in parallel. With my Corsair and Paragon, I use an
8 ohm to 500 ohm matching transformer built into a 35 mm film canister,
but with the Scout they work better without the transformer. I'd like to
find another pair, but I'll probably settle for building an outboard
passive filter to emulate the bandwidth or the Trimms with modern
headphones.
Trimm is still in business, but they now make telephone switchgear, I think.
73,
Bob WB2VUF
Stuart Rohre wrote:
If you are so lucky as to have some military high impedance, 15,000 ohm
headphones, you will have such good sensitivity to audio, you can run at a
low level of audio and never hear the hiss.
2000 ohms Trimm headsets that many hams had in the 1950's are also OK, and
more common.
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