some headphone experiments & crystal radio

[Hue Miller]

Tonite i happened to open some presents wrapped for me in 1943. That is,
several new packed soundpowered headphone elements. I had been told these
were factory defective; i decided to measure them anyway, and was thinking,
worst case, i would maybe try the old spark routine to reconnect the wire
inside. Surprise, they measured continuity at 800 ohms DC. I reckon that
is maybe worth about 4k ohm impedance???
I was unable to send sound from one to the other, and figured the diaphram
was jammed, but before giving up i decided to try reception on a crystal
radio. Lo and behold, they work great for this! In fact, for solid copy
signals, sound level was equivalent to the standard mini piezo hi-impedance
earplug. And, more surprising, for a low level signal, the lowest signal
level that i figured i could actually listen to, low enough that listening
did require some attention, some work -- the soundpowered element worked
better, louder ( at least in this particular setup. )  Also, altho i don't
think i have ever heard the "distortion" some writers report when using the
piezo earplug without a dc return resistor across it, in this test i found
the coil & diaphragm soundpower element clearer, cleaner sounding than the
piezo element (even with the piezo element paralleled by a DC load). The
piezo sound which i had before hailed as "hi fi", i now feel is strangely
"bassy" for such a tiny diaphram it has, and more tiresome to listen to for
a period. ( and maybe i'm interested in more than the rythm section at this
age, compared to my young years listening to "the Golden Age of AM rock".
Or is what i find tiresome in the piezo element, what others report as
distortion? More experiment called for!
Also, i think i now recall that the soundpowered sets had a different element
for transmit.
Good luck on finding such earphones. I think the headset/ microphone sets
are WW2 collectibles now because of standard Navy use as talking equipment
on warships. The headphones can be distinguished from your run of the mill
headsets because the soundpowered ones are noticeably larger and more robust.
Hue Miller

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