4-Pole Phaser Guitar Pedal

Phsae Shift Circuit Schematic

4 POLE PHASER GUITAR EFFECTS PEDAL

This is one of my ongoing endevours into audio hardware. A 4 pole phaser unit which utilizes 8 phase shifting stages, each utilizing an active filter with a 90 degree phase shift at each stage. Two of these stages couple to produce a 180 degree phase shift, thus cancelling out any sound at that particular frequency. By changing the resistances of the filter, we determine the frequency at which there is a 180 degree shift. Most phaser pedals have an automated way of shifting through various frequencies across the spectrum where this phase shift occurs. How do we implement this?

In the bigger picture, this effects unit employs a LFO circuit, which is why I have decided to implement a Weinbridge oscillator that is connected to all the JFETs for each filter (which act like a variable resistor). The user can control the oscillation frequency using the variable resistor, which in turn determines the speed of where the dip in frequency occurs. This is where the well known "wish-washy" sound of the phaser pedal comes from.

This circuit was developed and simulated using SPICE.

Weinbridge Oscillator Circuit Schematic

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