Phaser Guitar Pedal

A phaser guitar pedal is a guitar effect that splits a signal, send one path through the circuit cleanly and shifts the phase of the second. The two signals are then mixed back together and when out of phase, cancel each other out. This creates a sound somewhat akin to a flanger or an auto-wah.

This effect pedal first hit the scene very hard in the 1970s and added a special brand of outerspaciness to a pretty funky decade. Looking to revive this original vintage sound, I have built a classic 4-stage phaser. This particular pedal is pretty basic and allows you to adjust depth and the rate of the phasing. While the controls are rather bare, you can still dial it in to produce a subtle fullness to the guitar, or crank the dials all the way up for full-on slippery sounding funk.

Materials

(x5) LM741
(x4) 2N5457 FET
(x3) 2N3904 transistor
(x1) 100K trim potentiometer
(x1) General purpose PCB
(x1) DPDT pushbutton
(x2) Aluminum knobs
(x2) 50K potentiometers
(x2) 510K resistors *
(x1) 390K resistor
(x2) 150K resistors *
(x11) 100K resistors *
(x1) 47K resistors *
(x1) 43K resistor
(x4) 22K resistors *
(x2) 10K resistors *
(x1) 5.1K resistor *
(x2) 2.2K resistors *
(x1) 220uF capacitor **
(x1) 22uF capacitor **
(x1) 10uF capacitor **
(x1) 0.33uF capacitor
(x3) 0.15uF capacitors
(x1) 0.022uF capacitor ***
(x4) 0.01uF capacitors ***
(x1) 0.001uF capacitor ***
(x1) 7.5V Zener Diode
(x2) Stereo audio jacks
(x1) 9V battery snap
(x1) 9V battery
(x1) BB-sized enclosure

* Carbon film resistor kit. Only kit necessary for all labeled parts.
** Electrolytic capacitor kit. Only one kit necessary for all labeled parts
*** Ceramic capacitor kit. Only one kit necessary for all labeled parts.

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Phaser Schematic

Build the circuit as specified in the schematic. Do not worry about the potentiometers, audio jacks, or toggle switch for the time being. These will be installed later.

Keep in mind that you are squeezing a lot of components into a small space, so lay the parts out and plan carefully before you begin soldering.

About the Circuit

While this may seem like a very large mess of analog electronics, the circuit is somewhat simple.

The guitar signal first enters through a preamp stage. It is then split such that the clean signal goes straight to the output jack and the signal to be phase shifted goes to a series of 4 LM741 op-amps that form an all-pass filter. This filter is essentially what shifts the phase based on the signal from the LFO (low frequency oscillator).

The LFO is comprised of the 5th LM741 op-amp in the circuit (and surrounding circuitry). The rate of the LFO is controlled by a 50K potentiometer. The LFO then provides a CV (control voltage) to the all-pass filter by way of the 2N5457 FETs. This modulation then causes the signal in the filter to shift phase at the rate of the LFO.

The audio signal from the all-pass filter then goes to the foot switch. If the switch is open, only the clean signal makes it to the output jack. If the switch is closed the phase shifted signal is allowed to pass through to the output and get mixed with the clean signal. However, before the altered signal gets mixed in with the clean signal it passes through a 50K potentiometer which determines how much of the signals get mixed together.

From there, it goes out to the amp and the rest is history.