An On-board Amplifier Addition for the 1999 Minimum Theremin

(Updated December 7, 2002)

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Introduction

This battery-powered amplifier is designed to be added directly to the Minimum Theremin circuit board. It uses the same 9-volt battery as the theremin, and can drive a small speaker, providing added portability.

About Sine Waves and Amplifiers

When comparing equal-amplitude sine and harmonically-rich waves in the same speaker, the sine wave will have a much lower volume. To produce reasonable volume, a comparatively large sine wave voltage amplitude must be delivered to the loudspeaker. Often, amplifiers lack the voltage "overhead" required to produce these large amplitudes, and "clipping" occurs. Even with adequate amplification, most loudspeakers are not equipped for such large amplitudes, and may distort before sufficient loudness can be achieved. Harmonically-rich sounds, such as those produced by many conventional instruments, contain a combination of fundamentals and harmonics that seem comparatively loud at lower amplitudes.

The Minimum Theremin produces sine-waves over most of its range, and is therefore hard to amplify sufficiently, especially when restricted by a small internal speaker and limited power from a 9-volt battery. To address these problems, the circuit described here purposely distorts the output of the instrument by converting the sine wave to a square wave, which is harmonically rich and easy to hear with a small loudspeaker, when driven at conservative power levels. As a compromise, the theremin's soft, lustrous, sine-wave tone is exchanged for a loud, buzzy, kazoo-like tone. Another compromise lies in the fact that the amplifier produces a relatively large AC electrical current, some of which couples back into the theremin's oscillators, causing an "injection locking" effect. This effect may subsequently reduce the sensing distance of the instrument, alter the linearity of the pitch-position relationship, and cause the timbre to be more spurious in quality. Regardless of these compromises, however, the on-board amplifier addition may be useful for instances where a completely self-contained instrument is desired for demonstrations of the theremin principle.

Schematic and Circuit Description

Refer to the above schematic. The theremin's output is applied to DC blocking capacitor C14. The signal's DC offset is established at 2.5 volts via R15 and voltage divider R18-R16. The 5 volts for the divider is obtained from the theremin's voltage regulator. The signal is applied to one input of each operational amplifier, and the 2.5 volt reference is applied to the other inputs. The operational amplifiers are operated open-loop so that their outputs saturate, producing a square wave. They are also 180 degrees out of phase, so that the bridge-tied speaker load is effectively driven at approximately twice the available rail voltage. The NE5532N dual operational amplifier is employed for its high output-drive capability.

Capacitor C12 provides decoupling for the 2.5 volt reference. Capacitors C10 and C11 provide AC coupling so that no speaker current is drawn when the input signal is absent. Rectifier CR2 provides protection from inadvertent battery reversal, and C13 provides decoupling for the integrated circuit. Resistor R17 limits the output current in the operational amplifiers to a safe value. Important:  Observe the polarity for the four electrolytic capacitors, C10, C11, C12, and C13, and be sure they are connected in the correct direction. Also observe the polarity for rectifier CR2.

Practically any small loudspeaker may be used with this circuit. The prototype uses a 3"-diameter speaker with a 45 ohm impedance. If a higher-impedance speaker is used, then R17 may be reduced accordingly to increase the voltage through the speaker. (A 2", 100 ohm unit available from Mouser is shown in the Parts Table.) Commonly-available 8 ohm speakers will work, but not as efficiently as the higher-impedance units. With the amplifier addition, the total current drawn from the 9 volt battery is about 7 milliamperes when no tone is present, and about 35 milliamperes with the tone present.

Lay-out

The amplifier may be added to the original Minimum Theremin circuit board as shown in the following drawing. Only the applicable portion of the board is shown; refer to the Minimum Theremin lay-out drawing for reference.

Parts Table

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Text and drawings ©1999, 2001, 2002 by Arthur Harrison

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