RE: Clara Rockmore and the Tragedy of Leon Theremin

Robert Peter Oakes — Wed, 09 Mar 2016 16:55:45 GMT

Time really does fly, I remember when they were all the rage back then too, at the end of the day though, musical instruments have not changed much in hundreds of years... they still need to wobble the air in funny ways to make a pleasurable or not experience :)

RE: Clara Rockmore and the Tragedy of Leon Theremin

jc2048 — Wed, 09 Mar 2016 12:58:52 GMT

Thanks, a fascinating piece.

I remember long ago (early 1970s) seeing a design in a magazine for a transistor Theramin [curiously, it was a magazine for tape recorder enthusiasts, rather than one of the electronic hobby magazines that were on the go then]. The magazine itself must have dated from the late 1960s. The design was by someone called Robert Moog, who I'd never heard of then, and the circuit was beautifully economic - I think it had just 4 transistors [in those days transistors were expensive, made of germanium rather than silicon, and would have been the dominant cost of the device]. I didn't know much about electronics then and can remember studying the circuit diagram trying to figure out how it worked. The tone was generated by two oscillators (can't quite remember, but it was somewhere in the range 100-150kHz, or something that sort of order) the outputs of which were mixed together with a simple diode mixer. The difference signal was the audio (there was a simple passive filter to stop the higher oscillator frequencies from contaminating the audio). The capacitance of the hand to antenna (a few tens of pF) pulled the frequency of one of the oscillators and the difference signal would then vary. Hugh problems with temperature of course - they must have wandered all over the place, even when they'd warmed up. There would also have been a tendency for the oscillators to try to lock together when they were close in frequency, both directly capacitatively and through the supply. The volume control, in contrast, was just a single oscillator. I'm a bit tentative about how that worked. The hand would have changed the frequency of oscillation and I suspect it was fed to a filter so that it was running up and down the slope of the filter and the resulting signal could be rectified and the level used to control the gain of the output transistor. Something like that. Interestingly, at the time, even though I didn't know what I was looking at, I knew it was the work of a good designer. Wish now I'd built one, but I didn't have the money for the set of coils, which was how Moog was making money from it. Something that puzzled me at the time, and I only understood later, were the coils between the oscillator and antenna. They were wound with lots of turns on ferrite rods. That was to try and match the aerial to increase the range. [Same principle as a bottom-loaded monopole.]

Much later on, of course, I learnt that it was based on Theramin's original design with valves (vacuum tubes). Strange world back then when you couldn't just go online and instantly find out things.