How it Works
How the Classic RCA Theremin Works (modern theremins that employ heterodyning work in a similar way)
A low-level electromagnetic field, or radio wave, is generated within the circuits of the instrument. This field, which extends to an active distance of a few feet, emanates from the two antennas; the vertical pitch control rod on the top of the cabinet, and the horizontal volume control loop on the side. The electromagnetic fields are relatively weak; a tiny fraction of a mobile phone in use.
The theremin’s audible note is created by mixing together or heterodyning, two dedicated, nearly identical sets of radio waves, which are likewise produced by a pair of nearly identical pitch circuits called oscillators. Each oscillator creates a predictable, undulating radio wave that is supersonic (beyond human hearing).
One of the two pitch oscillators is connected to the pitch antenna, and the frequency it produces is variable, responding to how close or far the performer’s hand is to the pitch antenna. The other pitch oscillator remains at a set, fixed frequency, and is unaffected by the performer’s hand. The vertical pitch antenna on the cabinet is connected only to the variable pitch oscillator, through a coil consisting of over a thousand turns of very fine wire wrapped around a foot tall, 3” hollow tube, the pitch control resonant coil. Both of the pitch oscillators are pre-set to oscillate at approximately 175 kilohertz, which is in the radio frequency, or RF range. This is well below the AM radio band, and yet still about ten times beyond human hearing. Oscillator circuits employ a basic electronic component called a capacitor to help determine the frequency at which they oscillate. A simple capacitor is nothing more than two small, conductive metal plates separated from each other by an insulator, which can be paper or even air. We can easily change the frequency at which the oscillator oscillates by simply changing the amount of capacitance (for example substituting a larger capacitor in place of a smaller one or vice-versa). When a performer places her hand within the pitch antenna’s field, her body capacitance causes just the variable oscillator’s frequency to shift or change, with the greatest change occurring when the performer’s hand is closest to the antenna. The resulting altered frequency of the variablepitch oscillator is then blended with the fixedpitch oscillator’s unchanging frequency. The outputs of the variable and fixed oscillators are then fed into a special circuit called the mixer, which blends the output of their two frequencies together. Similar to algebraic equation, the supersonic like frequencies cancel each other out, leaving just the difference frequency, which is well within the range of human hearing. Thus, if the performer moves her hand close to the pitch antenna and stops when the blending of the pitch oscillators results in a difference frequency of 440 cycles (for example), an audible note corresponding to “A” on a piano keyboard will be heard.
Space control of volume is produced by an oscillator as well, but this one is a single oscillator whose only function is to produce an RF energy field across a large volume control resonant coil that’s made up of hundreds of turns of very fine wire wrapped around a 3” diameter hollow tube. As with the variable pitch oscillator, the volume oscillator’s frequency is above the range of hearing, and is altered when the performer’s other hand approaches the volume loop antenna that’s mounted to the left side of the cabinet. Around the finely-wound volume control resonant coil, there’s a short induction coil, comprised of only a few turns of heavier gauge wire. This short coil is connected directly to the filament of a special vacuum tube (a “UX-120”), which has a rapid-heating, rapid-cooling filament. When the performer’s hand is away from the volume antenna, the volume resonant coil and induction coil are working at their most efficient energy transfer (think of an optimally tuned Tesla coil), and the filament of the UX-120 remains brightly lit. However when the performer’s hand approaches the volume loop antenna, the RF energy in the volume oscillator decreases, resulting in a corresponding decrease in the efficiency of energy being transferred between the two coils, and the UX-120’s filament dims. When the performer’s hand is closest to the volume antenna, the UX-120 filament extinguishes entirely. To complete the picture of how this affects the volume, the current flowing through the UX-120 is utilized to act as a valve of sorts, allowing a greater or lesser amount of current to flow through an audio amplifier that’s connected to the UX-120 and directly controlled by it. The output of the audio amplifier is connected to the speaker, from which we hear the sounds that the performer is controlling by moving one hand to change the pitch, and the other to change the volume.
Text Generously Provided by RCATheremin.com