An On-board Amplifier Addition for the 2006 Minimum Theremin
(September 7, 2006)

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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. The circuit purposely distorts the output of the Minimum Theremin by converting it to a square wave, which is harmonically rich and easy to hear with a small loudspeaker, even when driven at conservative power levels. As a compromise, the theremin's relatively soft tone is exchanged for a loud, buzzy, kazoo-like tone. Another compromise results from the amplifier producing an AC load current that couples back into the theremin's oscillators. This effect may reduce the sensing distance of the instrument, alter the linearity of the pitch-position relationship, and cause the sound to be more spurious in quality. Regardless of these compromises, however, the on-board amplifier addition may be useful when a completely self-contained instrument is desired.

Schematic and Circuit Description

Refer to the above schematic. The theremin's output is applied to operational amplifier U3A input pin 3, and operational amplifier U3B input pin 6, via DC blocking capacitor C14. Resistors R16 and R14 form a voltage divider to provide a 2.5V bias to these input pins, via R13. The 2.5V also serves as a reference for the other operational amplifier 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 2.5V reference. Capacitors C10 and C11 provide AC coupling so that no speaker current is drawn when the input signal is absent. Rectifier CR4 provides protection from inadvertent battery reversal, and C13 provides decoupling for the integrated circuit. Resistor R15 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 CR4.

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 R15 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 9V battery is about 7mA when no tone is present, and about 35mA 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 ©2006 by Arthur Harrison

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(Back to "The 2006 Minimum Theremin")