Art's Theremin Page: Transposable Chord Generator

Transposable Chord Generator

This circuit consists of three precise triangle-wave oscillators, each using an Analog Devices type AD654 voltage controlled oscillator (VCO) and an AD620 instrumentation amplifier (INAMP). Each oscillator frequency (F1, F2, F3) is adjusted with a ten-turn potentiometer. The three triangle waves are summed at U1B to produce a triad chord. Transposition of the triad over a wide range is achieved by dividing each of the input voltages by a common factor, set by the TRANSPOSE potentiometer and buffered by U1A.

Voltage regulator VR1 provides 7.0V for the rest of the circuit. A voltage divider consisting of two 10.0K resistors provides 3.5V applied to the INAMP REF inputs to remove the common-mode DC that appears at the Ct terminals of the VCOs. The 3.5V is also applied to positive input of summing amplifier U1B so that it can operate with a single supply. The VCOs have their open-collector square-wave outputs grounded, since the output waveforms are obtained from the voltage across their timing capacitors. The VCO scale factors are selected with the 10.0K resistors at their Rt inputs and 0.01uF timing capacitors. VCO response is defined by f out = 1/10RC, which, for the given values, is 1000Hz/V. The full scale frequency for each oscillator, therefore, is approximately 3kHz, defined by the maximum voltage obtained at the output of U1A. The 10.0K resistors at the VCO +Vin inputs approximately compensate offset currents at the Rt inputs. A 100uF capacitor blocks the DC offset voltage from the audio output, and a 220-Ohm resistor ensures that U2B will remain stable with capacitive loads.

The resistors with one-percent accuracy are 50PPM/°C metal-film types and the 0.01uF capacitors temperature-stable film types. The circuit's estimated current consumption is about 10mA, so it may conveniently be run from a single 9V alkaline battery (Energizer 522 or equal) for at least 20 hours before VR1 drops out.

A simplified version of the circuit may be constructed without the INAMPS by summing the square waves directly from the VCO outputs. However, square waves contain odd harmonics that are more prevalent than those found in triangle waves, resulting in a courser-sounding output. A more elaborate circuit with triangle-to-sine converters would provide fundamental tones.

My thanks to Mike Mattheiss for his assistance in breadboarding and evaluating this circuit.

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