Moog Music Standard Etherwave Theremin
The Theremin is one of the oldest electronic instruments, and the only one known that you play without touching.
By moving your hands in the space around its antennas, you control pitch and volume. Theremins have been used on great classic recordings like Good Vibrations and '50s sci-fi soundtracks, and add amazing coloration to any performance or recording or can be played as a serious solo instrument. Like the Ethervox, the Etherwave is an authentic adaptation of inventor Leon Theremin's original design.
The Etherwave is a quality theremin with a design that retains many characteristics of the original theremins, including a five-octave pitch range and reliable spacing between notes for sophisticated playability. Rotary pitch, volume, waveform, and brightness controls let you fine-tune your performance. Antennas are nickel-plated 3/8 inch brass tube and cabinets are furniture-grade hardwood, finished with a custom Moog stain. The Etherwave is fitted with an adapter for mounting on a standard microphone stand and is designed for use with a musical instrument amplifier such as the Moog TB-15.
All Etherwaves include a power supply and two video tutorials on one DVD: Clara Rockmore: The Greatest Theremin Virtuosa, and Mastering The Theremin, featuring Lydia Kavina.
Understanding the Etherwave
How the Etherwave Works
There are several resonant circuits, or tuned circuits, in the Etherwave theremin. Since resonant circuits are not as common or accessible in today’s electronic gear as they used to be, we’ll define some basic terms and concepts that will help you understand how the theremin circuit works.
A resonant circuit consists of a capacitor (sometimes called a condenser) and an inductor (sometimes called a coil). A capacitor is a device consisting of two conductive plates separated by insulating material such as air or polyester. The capacitance of a capacitor depends on the size of the plates and the distance between them. An inductor is a device consisting of a coil of wire, sometimes wrapped around an iron or ferrite core. The inductance of an inductor depends on the number and size of the turns of wire and the material of the core. A resonant circuit has the property that its electrical impedance changes radically within a narrow frequency band, the middle of which is called the resonant frequency of the circuit.
When you bring your hand near a theremin antenna, you are actually forming a variable capacitor in which the antenna is one ‘plate’ and your hand is the other. For the high frequencies and very low currents that we’re talking about, your hand is effectively grounded by being attached to your body, so the antenna and your hand form a variable capacitor to ground. We call this variable capacitance hand capacitance. You increase the hand capacitance by bringing your hand nearer to the antenna. In normal theremin playing, hand capacitance is less than one picofarad, a very small capacitance change indeed! In addition to hand capacitance, a theremin antenna has a fixed capacitance to ground, which we’ll call the antenna capacitance. Antenna capacitance depends mostly on the size of the antenna, and is typically 10-15 picofarads.
A large inductor, called the antenna coil, is connected to each antenna inside the theremin. The antenna coil, antenna capacitance, and hand capacitance form a resonant circuit. In this Theremin's design, the resonant frequencies are about 285 kHz for the pitch antenna, and about 450 kHz for the volume antenna. 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.
The Etherwave Circuit
Before looking at the schematic diagram itself, we’ll review the functions of the Etherwave’s circuit.
Tone Production and Pitch Control
The volume antenna circuit consists of the volume antenna itself, in series with several inductors. It’s connected to the output of the volume oscillator, which provides a high frequency signal. When the antenna circuit resonant frequency is at or near the volume oscillator frequency, a high frequency current flows through the inductors, which induces a high frequency voltage across each of the inductors. These voltages are at a maximum when the antenna circuit’s resonant frequency is exactly the same as the frequency of the volume oscillator, and decrease when the antenna circuit’s resonant frequency is decreased by the addition of hand capacitance to the volume antenna. The volume antenna circuit also includes a detector, which converts the high frequency voltage across one of the inductors to a direct (DC) voltage. This voltage, which is called the volume control voltage, controls the gain of a voltage-controlled amplifier (VCA). Thus, as the player brings his left hand near the volume antenna, the volume control voltage decreases, the VCA gain decreases, and the audio output signal goes from loud to complete silence. The audio output signal is line level, and may be fed to a line input of a power amplifier or mixing console.
The pitch antenna circuit is connected to the variable pitch oscillator in such a way that increases in hand capacitance will decrease the variable pitch frequency as much as 3 kHz. This is how the pitch antenna circuit, in conjunction with the beat frequency oscillator circuit, enables the player to cover a usable pitch range of some five octaves (two octaves below to three octaves above middle C) simply by moving their right hand through a distance of two feet or so.
Volume Control and Timbre Control
The Etherwave’s tone is produced in a circuit configuration called a beat frequency oscillator. It consists of two high-frequency oscillators, plus a detector circuit which extracts the difference frequency, or beat frequency. One of the high-frequency oscillators (called the fixed pitch oscillator) operates at about 285 kHz, while the other high-frequency oscillator (called the variable pitch oscillator) operates over a range of about 282 - 285 kHz. The difference frequency ranges from zero to about 3 kHz, which is three and a half octaves above middle C.
Front panel controls include four potentiometers: two for antenna tuning and two for timbre control. The Pitch Tuning potentiometer is connected to the pitch tuning circuit, which adjusts the frequency of the fixed pitch oscillator over a small range. Similarly, the Volume Tuning potentiometer is connected to the volume tuning circuit, which adjusts the frequency of the volume oscillator over a small range. These circuits provide the player with a way of fine-tuning the antenna responses during performance. In earlier theremin designs these tuning functions were implemented with large variable capacitors. Such variable capacitors are no longer generally available at reasonable prices.
The VCA is deliberately designed to distort the difference frequency waveform, thereby adding desirable harmonic content. The Brightness and Waveform potentiometers vary the biases on the VCA input, which change the way in which the audio waveform is distorted. The Brightness potentiometer determines how much the waveform is distorted, and therefore the amount of the total harmonic content. The Waveform potentiometer determines which harmonics will be strong, and which will be weak. It is similar to a Rectangular Width control on analog synthesizers.
The entire theremin circuit runs on 12 volts, which is supplied by a simple, small power supply. Total current consumption is about 30 milliamperes.