[ARC5] Crystal Headphones

[Richard Knoppow]

     My dad and I built a crystal set around the late 1940s. I 
think this is when he took a class in radio somewhere. It was a 
breadboard on a piece of plywood. Coil wound on a toilet paper 
roll. Fixed crystal diode, have no idea what kind and a pair of 
headphones. Long wire antenna from a second floor porch to the 
garage. Got all the local stations (Detroit) very loudly.
    There were many loudspeakers made based on phonograph horns 
and, in fact, some radio adaptors that fit in place of the pick 
up and attached to the radio.
    Typical radios of the time used the speaker or phones 
connected directly to the plate of the output tube. That's right 
kids, full B+ running right over your head. Typical plate load 
impedance about 5K ohms. Some of these were bi-pole magnetic but 
others, like the Baldwin speaker, were balanced armature.
    I can imagine a sensitive driver on a horn being loud enough 
to listen to directly but I can't imagine it being very loud.
    There are many circuits for simple crystal receivers, the 
more complex ones attempts to improve the selectivity and 
sensitivity by using multiple tuned circuits. Probably the place 
to look is at circuits for lifeboat and ship emergency receivers. 
I passed up a loose coupler at a swap meet several years ago and 
have regretted ever since. No problem here with a crystal set 
since I am maybe two miles from a site with three radio stations 
duplexed on the same antennas, one a 50KW.
    It would be interesting to know the actual source impedance 
of a crystal set.
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