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TVOX Stories
Of the Theremin and Futurist Stringed Instrument Making
Page 4

The primary requisite of a good theremin is therefore that the quality of the sound: it must be the most natural (or the least electronic) possible.
When we rank theremins on the basis of their sounds, we find that the best have tubes based circuits. I have already said the mixer is the heart of the instrument insofar as it is responsible for what we hear. Both Theremin and RCA used a tetrode, a type of tube that has two control grids positioned inside the electron flow developed between the anode and cathode. The beat is made available by connecting the signals transmitted from the two oscillators to the control grids on the transformer of the anode charge.

Diagram of a tube mixer.
Diagram of a tube mixer.

If we assume that the oscillator produce perfectly sinusoidal harmonic signals, theory teaches us that the beat of the mixer will also be sinusoidal: although this would undoubtedly provide a pure audio signal, it would be of little musical interest.
However, by giving the two oscillators similar (but not identical) frequencies, one of the two tends to assume the working frequency of the other. One important result of this behaviour is that the wave form of the signal produced by the mixer tends to become distorted in such a way as to enrich the harmonics of the signal to the point at which it acquires more personality and a greater depth of tone. Furthermore, the wave form gradually changes as the frequencies of the two oscillators move further apart.
Unfortunately, there is also a contraindication: if the two signals bind too early, the two oscillators tend to assume the same working frequency. This limits any excursion into the lower octaves, which is brusquely interrupted at the level of a few hundred Hertz. It is the task of the designer to limit this phenomenon simply by carefully isolating the oscillatory stages from the mixer: the final result must be a distorted but integral audio signal down to the lower thresholds of audibility (i.e. 16 Hz).

Today’s theremins have transistor or integrated circuits, and the mixers are usually normal pre-amplifiers/separators whose only entry receives both high-frequency signals.

Diagram of a transistor mixer.
Diagram of a transistor mixer.

However, even in this case, the signal produced by the mixer may be adequately distorted, but the generated harmonies (which are mainly of the odd type) tend to be unpleasant.

It would therefore seem that tetrode tubes are the best means of generating the beat inside a theremin, although it must be said that electronics is a science of infinite solutions and so it cannot be excluded that it may find a way of designing a transistor circuit that fully reproduces tube behaviour. However, this would probably destroy the simplicity of the circuitry that characterises the instrument conceived by Léon.

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