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Audio effects: guitar phaser

Diagram: phaser

This circuit is a phaser circuit, as used in guitar foot-pedal effects. It used 11 sections of op-amp such as LM324 but others will do. The whole circuit works of a dual rail supply: +/- 9v. The 0v rail is indicated by a chassis sign. Make sure these supplies are well decoupled. There is a spare section of one of the IC's which you could used to derive a center tap from an 18v supply.

The circuit works by means of an LED illuminating six cadmium sulphide photo sensitive resistors or LDRs: these six should be arranged in a circle around the LED so that they all get illuminated approximately the same. Exact matching does not matter too much. The whole photo assembly should, of course, be shielded from external lighting.

Each CdS cell acts as a variable resistor in a single variable phase-shift circuit and there are six such individual phase shifters arranged in series, each increasing the overall shift.

The LED's brightness is modulated by an oscillator (bottom section of the circuit) whose frequency can be adjusted by the Speed pot. Max and min illumination are adjustable - the LED should not extinguish.

The LDRs control the shift of the six stages of the phase shifter. This phase shifter has overall feedback, varied by the Intensity control, to alter the intensity of the phasing effect by altering the overall phase shift.

Apart from that, there is an input buffer (top right) whose output feeds the phase shifter and also feeds through to the mixer stage where it is mixed with the phased signal.

I hope there are no errors in the circuit but it is given as is, without any obligation but of course we will answer all reasonable queries. Unfortunately I have been out of audio for some time now so cannot give too much assistance.

Modern phasers will use a digital delay line circuit, but this discrete version works and illustrates all the principles involved.

Enjoy it!

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Page's Author: Richard Torrens