theremin circuit history

Hi all,

I'm currently building a theremin for A2 level coursework, and was wondering whether anybody knew what the stages in developing the theremin circuit were. I have to design a modern instrument, but should look at the old circuits first, but I have little knowledge of them. I know that there are loads of schematics on this website, but I don't know what kind came first.

Thanks,

ukebert

The first production Theremin was the RCA Theremin.

Prior to that, there were a number of hand-made Theremins that Lev Termin built.

Prior to the Theremin was the Aetherphone which featured gestural control of pitch only.

Of course, these instruments use vacuum tubes and discreet components on hand-wired chassis.

The tone of these early instruments, as well as modern instruments, are produced by hetrodyning two RF oscillators, namely fixed and variable. The pitch antenna is part of the variable oscillator's tank circuit -- the variable oscillator's frequency decreases as the performer moves closer to the antenna.

The early Theremins used Armstrong oscillators -- wherein the feedback is coupled through a coil. Many people attribute the "sound" of the early Theremins to this oscillator design (along with numerous other factors). Anyway, you will want to check out the Armstrong oscillator so that you can better follow the tube schematics.

You may want to look at Paia.com -- the schematic to the "Theremax" is available. Of course, it is a modern design however the circuits are straightforward and will will familiarize you with the building blocks of a Theremin.

Good luck to you.

Thanks very much for replying, I will certainly take a look at the circuits you suggested.

At the moment I'm having some trouble with radio interference, but hopefully I can get the system working before April.

Another question:

The heterodyne principle states that the signal from two oscillators is mixed, producing a trace with a frequency equal to the difference in frequency of the two oscillators. Or something. Anyway, as part of my coursework I need to look at old designs as well as modern alternatives. How was this mixing done for the original instruments?
At the moment I'm using a simple non-linear feedback amplifiery thingummy. It's working fairly well. Thing is, I've got to design all of the systems, so nothing can be too complicated.

Regards,

ukebert

I will do my best to answer your question -- I am by no means an expert on tube circuits...

There are two basic methods of extracting the difference frequency, namely mixer/detector or ring modulation.

As you know, if you mix two frequencies together the resultant amplitude will modulate at a rate that is equal to the difference frequencies. This modulation is the result of the two different frequencies alternately cancelling and reenforcing each other. Anyway, this signal can then be sent to a detector circuit that detects the peaks and, as such, extracts the difference frequency.

Another method is to use a ring modulator which accepts two signals at its inputs and produce a signal that consists of the sum and difference frequencies of the input signals. A low pass filter generally follows the ring modulator to filter out the high frequency (which would be way above the audio range, anyway).

Again, some folks attribute that "magical" sound of the tube theremins to the Armstrong oscillators and the tube detector circuitry. I don't speak from any personal hands-on experience, though. So, if someone wishes to discuss tube Theremins, please jump in!

hi,

i haven't been able to retrieve ANY info about
lev termen's flavour of the armstrong oscillator
design. i say 'flavour' because all the circuits
labelled 'armstrong oscillator' i found so far
are rather different from the RCA implementation
or the similar one in clara rockmore's custom
instrument.

i'd appreciate any more detailed insights!

best

vern-