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.
Step 1: Materials
(x5) LM741 (RadioShack #276-007)
(x4) 2N5457 FET (RadioShack #55050922 - online only)
(x3) 2N3904 transistor (RadioShack #276-2016)
(x1) 100K trim potentiometer (RadioShack #271-0284)
(x1) General purpose PCB (RadioShack #276-150)
(x1) ON/OFF pushbutton (RadioShack #55050535 - online only)
(x2) Aluminum knobs (RadioShack #55048882 - online only)
(x2) 50K potentiometers (RadioShack #271-1716)
(x2) 510K resistors (RadioShack #55049227 -online only)
(x1) 390K resistor (RadioShack #55049555 - online only)
(x2) 150K resistors (RadioShack #55049550 - online only)
(x11) 100K resistors (RadioShack #271-1347)
(x1) 47K resistors (RadioShack #271-1342)
(x1) 43K resistor (RadioShack #55050326 - online only)
(x4) 22K resistors (RadioShack #271-1128)
(x2) 10K resistors (RadioShack #271-006)
(x1) 5.1K resistor (RadioShack #55048038 - online only)
(x2) 2.2K resistors (RadioShack #271-1325)
(x1) 220uF capacitor (RadioShack #272-1029)
(x1) 22uF capacitor (RadioShack #55047796 - online only)
(x1) 10uF capacitor (RadioShack #55047762 - online only)
(x1) 0.33uF capacitor (RadioShack #55047258 - online only)
(x3) 0.15uF capacitors (RadioShack #55047730 - online only)
(x1) 0.022uF capacitor (RadioShack #272-1066)
(x4) 0.01uF capacitors (RadioShack #272-131)
(x1) 0.001uF capacitor (RadioShack #55046626 - online only)
(x1) 7.5V Zener Diode (RadioShack #55051160 - online only)
(x2) Stereo audio jacks (RadioShack #274-312)
(x1) 9V battery snap (RadioShack #270-324)
(x1) 9V battery (RadioShack #23-2211)
(x1) BB-sized enclosure
Step 2: 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.
Step 3: Attach Wire
Attach 6" wires for the two potentiometer connections to the circuit board.
Also, connect 6" wires to the circuit board for the audio jacks.
Finally, connect the red power wire from the power jack to the appropriate place on the circuit board.
Step 4: Guitar Pedal Template
Print out and afix the attached template to the the outside of the guitar pedal closure in preperation for drilling.
Step 5: Drill
Drill 9/32" holes for each of the potentiometers.
Drill a 1/2" hole to house the foto switch.
Drill a 3/8" hole for each audio jack.
Step 6: Insulate the Case
Cut out a 1/8" sheet of cork using the attached template.
Apply spray adhesive to one side of the cork and stick it to the inside of the enclosure's lid.
Step 7: Rubber
Cut a rubber spacer out of 1/8" thick adhesive rubber sheeting using the attached template.
Attach the rubber spacer to the inside of the enclosure where the potentiometer mounting holes are.
Step 8: Install
Mount the potentiometers and foot switch in their respective mounting holes.
Step 9: Wire it Up
Wire the circuit board to the audio jacks, potentiometers, foot switch and the 9V battery snap as defined in the schematic.
Step 10: Audio
Mount the audio jacks into the body of the case.
Step 11: Power
Connect the battery to the 9V battery snap.
Step 12: Close the Case
Close the enclosure using the appropriate hardware.
Step 13: Knobs
Press the knobs onto the potentiometer shaft.
Step 14: Plug and Play
Plug the guitar into the audio-in jack and the amp into the audio out jack.
You should now be ready to rock out.