Build the EM Theremin
This classic electronic instrument gives good vibrations and excitations.
By Robert Moogost electronic musical instruments are sonic chameleons that try to sound like a wide variety of other things. However, there is one electronic instrument that makes no l apologies for its single, immediately recognizable sound: the theremin. This monophonic instrument has added its distinctive, melodic character to the scores of many horror and suspense movies and made its pop debut on theBeach Boys' "Good Vibrations." It has also appeared on many concert stages, including Carnegie Hall. The theremin was named after its inventor, Russian physicist and musician Leon Theremin, who developed the instrument in the 1920s. Unlike most musical instruments, the theremin is played with absolutely no physical contact. Players wave their hands in the air near two antennas. As one hand getscloser to the straight vertical tube (called the pitch antenna), the pitch rises; as the other hand gets closer to the horizontal tubular loop (called the volume antenna), the volume decreases. Because the theremin's pitch and volume are intimately tied to the playi
86 Electronic Musician February 1996
er's hand motions, the tone has a vibrant, wavering quality, not unlike a human voice or aviolin. Among the requests for DIY projects that EM receives, by far the most common is a do-it-yourself theremin. The instrument presented here is an authentic theremin, with antenna response characteristics, pitch range, and tone color that closely emulate Leon Theremin's original designs. However, it is reasonably easy to build. It uses currently available components and
When you bring your handnear a theremin antenna, you are actually forming a variable capacitor: the antenna is one "plate" and your hand is the other. With the high frequencies and very low currents used by the instrument, your hand is effectively grounded by being attached to your body, so the antenna and your hand form a variable capacitor to ground. This variable capacitance is called hand capacitance. You increase thehand capacitance by bringing your hand nearer to the antenna. During normal operation, the hand capacitance is less than one picofarad, which is a very small capacitance indeed! Each antenna forms a resonant circuit with a group of inductors collectively called an antenna coil. In this design, the resonant frequencies are about 260 kHz for the pitch antenna and about 450 kHz for the volumeantenna. At or near the resonant frequency, a tiny change in hand capacitance results in a larger change in the impedance of the antenna circuit as a whole. Refer to Figure 1, the functional block diagram, and Figure 2, the schematic diagram of the entire circuit. The variablepitch oscillator (VPO), fixed-pitch oscillator (FPO), and detector sections form a beatfrequency oscillator. Q1, Q2, and theirassociated components constitute the VPO, the frequency of which is set
The theremin's main circuitry is mounted on a single prototyping board, and the two antenna circuits are mounted on their own smaller boards.
to make fine adjustments to the volume-oscillator frequency during performance. The audio waveform is applied to pin 3 of U3-A at a level high enough to clip it. Thishas the effect of reshaping the waveform from a skewed sine to a quasirectangular wave, which is very similar to the waveform of Professor Theremin's original instruments. P3 varies the input resistance of U3-A, which influences the amount by which the audio waveform is clipped. P4 shifts the bias at the input of U3-A, which changes the waveform width and therefore the output's harmonic spectrum....