Theremin
The Theremin (named after its inventor Leon Theremin) is a musical instrument that dates from the early days of radio and electronics. It is a simple radio frequency device which relies on the interaction of two oscillators to produce a range of musical notes. One oscillator serves as a reference while the second oscillator is free to be influenced by the capacitance of the player’s hand approaching the instrument’s pitch antenna. To achieve control over the volume of the sound produced by the instrument another oscillator arrangement is used.
Playing the instrument is an interesting blend of electronics and theatrics. As the Theremin virtuoso gestures in space, glissandos of sound emanate from the instrument’s loudspeaker. Although the Theremin has been used for serious musical performance, it is more usually recognised for the eerie sound effects in 1950’s sci-fi movies.
I had being toying with the idea of building a Theremin since the late 60’s but never got around to actually doing so until a few of years ago. Here is a description of the results of quite a few hours spent in my backyard shed and a few useful tips for the enthusiast who might want to also have a go.
The Design
Although I had a strong urge to produce a traditional instrument, it is hard to ignore the benefits of the semiconductor when devising a project of this nature. The result of my labour is an instrument using the same oscillator frequencies and a similar functional arrangement as the original RCA Theremin but utilizing transistors instead of the valves (or toobs).
My Theremin is constructed using separate modules representing each functional unit of the instrument. In this way it is a simple matter to build each element of the Theremin, get it operating correctly and then finally assemble the whole instrument. The Theremin’s modules are:
- Pitch reference oscillator
- Pitch control oscillator
- Mixer
- Volume control oscillator
- Volume resonant circuit and voltage controlled amplifier
- Audio amplifier
- Power supply
Each oscillator used in this Theremin is of the Colpits type and uses a miniature ferrite core inductor and three capacitors as the frequency determining components. Two of the oscillators incorporate a means to allow the operator to adjust their frequency with a front panel control.
Theremin Block Diagram
The block diagram shows the general arrangement of the Theremin’s modules. The signals from the two pitch oscillators are fed to the mixer where sum and difference products of these signals are produced. After filtering, only the audio difference signal emerges which is fed to the voltage-controlled amplifier (VCA). The gain of the voltage-controlled amplifier is varied by the DC signal from the detector in the volume resonant circuit. This DC signal is produced as the volume oscillator is pulled away from its set frequency by the capacitance of the players hand as it approaches volume antenna. The audio tone from the VCA is fed to the amplifier and finally to the loudspeaker.The Circuits
Each module can be built, tested and tuned so that the final assembly is the simple process of hooking everything together and performing a few final adjustments. In order to test and tune the modules, an oscilloscope is required.
Note: All capacitor values are in uF unless otherwise noted.
Pitch Reference Oscillator (172kHz)
The pitch reference oscillator operates at around 172kHz and is trimmed to the correct frequency by the operator using the 10k potentiometer. A small ferrite core inductor and several capacitors form the frequency determining network. The 172kHz output signal of this oscillator is fed to the mixer module using a length of screened cable.
Pitch Variable Oscillator (172kHz)
The pitch variable oscillator also runs at 172khz and is influenced by stray capacitance associated with the pitch antenna. The purpose of the four 10mH ferrite inductors is to improve the linearity of the relationship between the player's hand position and pitch. Without these components, the Theremin pitch would rise slowly at first and then suddenly rise through several octaves in just a few millimetres of hand movement, making the instrument difficult to play. As these inductors form a series resonant circuit with stray capacitance associated with the antenna, it is difficult to predict their optimum values. The value of these components is best adjusted by trial and error to achieve the best hand movement/pitch relationship. The output of this oscillator is also fed to the mixer module using a length of screened cable.
Volume Variable Oscillator Circuit
The volume oscillator operates near 220kHz and is strongly influenced by the volume antenna. When correctly tuned, and the operator’s hand is almost touching the antenna, the frequency of this oscillator will match the tuning of the volume resonant circuit. This condition causes maximum signal to appear across the tuned circuit and corresponds to audio cut-off. The 10k trim potentiometer enables the operator to set the oscillator to the correct frequency.
Mixer
The mixer is the simplest module to assemble and requires no adjustment. Its function is to mix the signals from the two pitch oscillators producing an audio signal. The pitch of the audio signal is determined by the difference in frequency of the two input signals.
Volume Resonant Circuit and Voltage Controlled Amplifier
This module alters the volume of the Theremin in
response to the player's hand gestures. It consists of a resonant
circuit tuned to the same frequency as that produced by the volume
oscillator when the player’s hand is close to the antenna. This causes
maximum signal voltage to be developed across the tuned circuit. As the
player’s hand moves away from the volume antenna, the frequency of the
volume oscillator also moves away from the resonant frequency of the
tuned circuit. This causes the DC bias at the base of TR1 to fall away
and its collector voltage to rise. As FET 1 is biased into the ohmic
region, the rising voltage at its gate causes its channel resistance to
increase. This reduces the effect of the shunt across the audio feed to
FET 2 and increases the volume of the audio signal.
The measured characteristics of FET 1 when biased into the ohmic region
Semiconductor Pin-outs
Audio Amplifier
The audio amplifier uses a TDA2003 monolithic amplifier originally designed for car radio applications. The circuit comes directly from the manufacturer’s data sheet.
Power Supply
WARNING - LETHAL VOLTAGE
If you are unsure, use a 12VAC plug-pack type power unit in place of the transformer and switch indicated in the schematic. A separate earth connection will be required.
The power supply uses a 240VAC/12VAC transformer, bridge rectifier and a 3-terminal voltage regulator. Capacitors across the rectifier diodes are required to reduce common-mode induced hum.WARNING - LETHAL VOLTAGE
If you are unsure, use a 12VAC plug-pack type power unit in place of the transformer and switch indicated in the schematic. A separate earth connection will be required.
Assembly and Testing
When each of the modules have been built and tested it is a simple matter to assemble them and perform some final adjustment. The layout of the modules is not especially critical but obviously the pitch variable oscillator should be mounted close to the pitch antenna and the volume oscillator close to the volume antenna.
The Layout of an Early Prototype
The volume trim is adjusted so that the sound reduces to
zero as the operator’s hand almost touches the volume antenna. The
pitch trim is then adjusted so that no tone is heard when the player is standing in the playing position. The Theremin is now ready to play.Technical Notes
I recently received an enquiry asking how the frequency of the oscillators could be shifted by varying the DC bias of the oscillator transistor.
The frequency shift is due to the change in the DC current flowing through the inductor. As the inductor has an iron core (ferrite), the change of DC current alters the operating point along the B-H curve causing a change in oscillator frequency. See the following Wikipedia article on magnetic saturation for an explanation of the B-H curve.
http://en.wikipedia.org/wiki/Saturation_%28magnetic%29
Some Useful Links
Wikipedia article on Leon Theremin
Instructions on how to play the theremin
Thomas Grillo provides some excellent theremin video instruction on YouTube
The following links to Theremin related products available through Amazon might be useful:
Strange Apparatus
