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Technical Description
Richard H. Dorf

Perhaps the earliest electronic musical instrument to gain fame, if not general acceptance, was the invention of Dr. Leon Thérémin and was called by his name. It was manufactured commercially for a time by RCA Victor but is no longer anything but a curiosity.

The Thérémin is basically a simple beat-frequency audio oscillator containing two superaudio-frequency L-C oscillators. One is fixed in frequency and the other is varied. The outputs of the two oscillators are mixed in a detector and filtered. The resultant detector output is the difference frequency and is in the audio range.

Any variable audio oscillator can be used to produce music by simply manipulating the tuning knob with enough skill. It is an awkward trick, however, and the singular distinction of Dr. There'- min's method was his method of controlling the variable-frequency oscillator. It is indicated in the circuit of Fig. 1. This diagram is that of a simple home-built version of the Thérémin, taken from a 1935 issue of Radio-Craft magazine.
Fig 1. One version of the Theremin

Fixed and variable oscillators are shown. The variable oscillator has a metal rod connected to its plate circuit and this rod projects upward out of the chassis or case of the instrument. When the player's hand is brought near it, hand capacitance changes the tuning of the oscillator, the amount of change depending on the proximity of the hand. The oscillator trimmers are initially adjusted so that the two oscillators are on the same frequency - at zero beat - and there is no audio output. As the hand is brought near the rod the variable oscillator frequency decreases, so that an audible beat is heard. The highest audio tone is produced when the hand is closest to the rod.

It requires a good deal of practice to achieve any kind of musical results. There is no form of "keying," so the jumps between pitches are always glides unless the volume control is operated to shut off the sound between notes. The vibrato is produced by a rhythmic fluttering of the hand. The tone of the instrument is an eerie sort of wail, both because of the gliding tone and the purity of the waveform. No form of the instrument of which the author knows had any method of varying the approximately sine shape of the detected beat note, though it is not hard to see how it might have been passed through distorting and filtering circuits without much trouble.

The circuit of Fig. 1 shows an ordinary manual volume control, which some of the inventor's earlier instruments had. He found this awkward, however, for doing any sort of keying and later developed a volume control which, like the pitch-determining system, depended on the proximity of the hand to a rod. The RCA version of the Thérémin, diagrammed in Fig. 2, shows this modification.
Fig 2. A Theremin made in former years by RCA.
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V1 is the variable oscillator, controlled by the pitch-control rod coupled to the tuned circuit, and V3 is the fixed-frequency oscillator. V2 is a detector, the plate of which passes audio through a transformer to V4, the first audio amplifier, thence to V6, the output tube. A PLAYOFF switch shorts the grid of V6 to ground to silence it.

Volume control is made possible by V5 and V7. Plate current for V4 passes through V5, which is connected as a rectifier. The filament of V5 is not heated by the power supply of the instrument, but by power taken from oscillator V7. Under normal conditions the oscillator resonant coil (so marked on the diagram) is not resonant at the oscillator frequency. It does not, therefore, transfer any appreciable power to the filament coil coupled magnetically to it. To raise the volume the operator brings his hand near the volume control loop. The hand capacitance brings the oscillator resonant coil nearer resonance at the oscillator frequency and some energy is coupled to the filament coil. This heats the filament of V5, allowing some plate current to pass through V4 so that V4 can amplify. When the hand is nearest the loop, the coil is nearest to resonance, maximum filament heat is obtained, and maximum V4 plate current allows maximum volume.

With this arrangement, a practiced player can produce some interesting effects not only by varying volume in the usual way, but by giving varying attack and decay characteristics to the keying of tones. It takes a particular adroitness, however, and this, coupled with the rather limited musical interest of the tone, has prevented any really widespread use of the Thérémin.

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