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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Phase modulator vibrato for musical instruments

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The term “vibrato” is often used mistakenly instead of “tremolo” which describes a regular variation in signal or loudness. Vibrato, on the other hand, is modulation of pitch or frequency

In this circuit, vibrato is achieved by using two cascaded phase shift networks. The two different phase shift networks are connected in parallel and are fed the same audio signal. The forward gains of the two networks are varied by two transconductance amplifiers which are controlled by a single low-frequency oscillator.

Following the transconductance amplifiers, the two separately phase-shifted signals are resistively mixed to obtain a vibrato output.

In essence, each phase shift stage is a low-pass filter with a different corner frequency, as marked. When signal has passed through the two paralleled phase shift networks, the two outputs are 90° out of phase. Dual op amps IC1, IC2 & IC3 provide the six phase shift stages. Op amp IC1a acts as a unity gain buffer. Its non-inverting input is connected to 0V via a 1MΩ resistor to provide a high impedance – important if an electric guitar is the signal source.

The two phase-shifted signals are applied to LM13700 transconductance amplifiers IC4a & IC4b. Their individual gains are controlled by the DC voltage fed to IC4’s pins 1 & 16.

The slowly varying control voltage is generated by op amp IC1b which is configured as a low frequency sinewave oscillator. Its output at pin 7 is fed to a phase splitter stage comprising op amps IC2c & IC2d. IC2c is a unity gain inverter while IC2d is a unity gain non-inverting buffer. Hence the gain control signals applied to pins 1 & 16 of IC4 will be in anti-phase. Hence, IC4a will be at maximum gain when IC4b is at minimum and vice versa.

The net result of the common signal at the junction of the two 10kΩ resistors is that its amplitude is essentially constant while its phase is continually shifted back and forth over a 90° range. The overall gain of the entire circuit is unity.

The six phase-shift stages need to be calibrated one at a time and the easiest way to do this is with an oscilloscope connected in XY mode. For example, for IC2a, first connect your scope to the input and output of the phase shift stage and set your audio oscillator to 206Hz. You then adjust trimpot VR1 until a circle pattern appears on the scope screen.

You then do the same thing for IC3a at 1657Hz and so on until all stages have been adjusted.

Craig Sellen,
Philadelphia, USA. ($65)

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