We have been asked by Playgroup, local club promoters, to provide a 'sweetner' to perusade electronica djs [Evil Nine] to take part in an event they are promoting. This item is to be a Theremin. We therefore need to design and build a theremin to be presented to Evil Nine on 30th October 2009 at The Komedia (where we will also be hosting the pre-party from 6-10pm).
This is a lot of work to be done in a short amount of time, and the results must be professional. Volunteers wanted!
How a Theremin works
The theremin is unique among musical instruments in that it is played without physical contact. The musician stands in front of the instrument and moves his or her hands in the proximity of two metal antennas. The distance from one antenna determines frequency (pitch), and the distance from the other controls amplitude (volume). Most frequently, the right hand controls the pitch and the left controls the volume, although some performers reverse this arrangement. Some low-cost theremins use a conventional, knob operated volume control and have only the pitch antenna.
The theremin uses the heterodyne principle to generate an audio signal. The instrument's pitch circuitry includes two radio frequency oscillators. One oscillator operates at a fixed frequency. The frequency of the other oscillator is controlled by the performer's distance from the pitch control antenna. The performer's hand acts as the grounded plate (the performer's body being the connection to ground) of a variable capacitor in an L-C (inductance-capacitance) circuit. The difference between the frequencies of the two oscillators at each moment allows the creation of a difference tone in the audio frequency range, resulting in audio signals that are amplified and sent to a loudspeaker.
To control volume, the performer's other hand acts as the grounded plate of another variable capacitor. In this case, the capacitor detunes another oscillator, which affects the amplifier circuit. The distance between the performer's hand and the volume control antenna determines the capacitor's value, which regulates the theremin's volume.
Modern circuit designs often simplify this circuit and avoid the complexity of two heterodyne oscillators by having a single pitch oscillator, akin to the original theremin's volume circuit. This approach is usually less stable and cannot generate the low frequencies that a heterodyne oscillator can. Better designs (e.g. Moog, Theremax) may use two pairs of heterodyne oscillators, for both pitch and volume.
Links to source materials/ideas
- How to Build a Theremin Circuit diagrams and discussion on building a true analogue theremin.