Phaser Guitar Pedal

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Introduction: Phaser Guitar Pedal

About: My name is Randy and I am a Community Manager in these here parts. In a previous life I had founded and run the Instructables Design Studio (RIP) @ Autodesk's Pier 9 Technology Center. I'm also the author ...

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
(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.


Please note that some of the links on this page contain Amazon affiliate links. This does not change the price of any of the items for sale. However, I earn a small commission if you click on any of those links and buy anything. I reinvest this money into materials and tools for future projects. If you would like an alternate suggestion for a supplier of any of the parts, please let me know.

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.

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53 Comments

I have been testing my circuit with an oscilloscope... I know that the input signal should be a clean sine wave, but what should the output look like after it goes through the op amps? And how will the wave change when turning the pot dial? Thanks

5 replies

If you view the 'clean' signal at the same time as the phase shifted signal, they should be the same signal, but the phase shifted one should be moving in and out of phase (at the rate determined by the LFO).

One potentiometer controls the LFO

The other potentiometer mixes the two signals together before being output to the amp.

Thanks so much for getting back so soon. I see what you are saying with the output signal beings shifting in and out of phase, which then gets mixed with the original signal. Currently, the signal at the drain of my 5457s to my op amps is a sine wave that has been shifted up. When the 50k pot is turned, this signals amplited changes...should this be expected? My altered output waves looks similar to the input wave, but it no longer is a perfect wave, and it does not shift in or out of phase. Which part of the circuit could potentially be the issue? (I'm generating my input frequency with a function generator at 80hz) thanks again man

Honestly, I have never looked at the output wave. What you are describing sounds feasibly 'about right' (I think).

The pot is basically mixing the two signals together into one signal. That wave is going to be some combination of the two.

Being that it is sort of summing the two waves, I would think its probably correct that the amplitude increases.

Okay, that makes sense with what I have been getting so far. I'm pretty sure that it's doing what it should be doing now. I am using ceramic caps instead of the film ones... Do you think that they shouldnt be a problem in your design?

Should be fine so long as the value are the same.

Hello!

I am about to built this pedal and your tutorial is very helpful. Thank you so much!! I was wondering why do we put a 100K pot rather than just a usual resistor? We are not going to change this setting anyway. Also, what setting are we meant to put it on?

Thanks!

The output jack has only one wire conected??

1 reply

No, the tip should be connected to the wire as mentioned, the sleeve should be wired to ground.

What is the recommended wattage on the 7.5v zener diode? I'm having issues sourcing one through radioshack (likely due to their restructure sale), and am turning to amazon (they are also not common on any of the component sites I've been looking at)

3 replies

What ratings are you finding? It probably does not to be very much.

Additionally, I don't think I see any instructions for adjusting the trimpot. Am I missing them?

Thanks for the reply! I found a 7.5V .5w Zener for about $1.75 and free shipping, so I'm going to order a handful of those if they are stout enough for this application.

What do you use to create the graphical schematic? I really like the look and readability of it, and would like to be able to make my own. Thanks!

3 replies

I custom draw each one in Adobe Illustrator. It's not exactly the fastest way, but at this point I have generated a lot of different component graphics I can copy and paste from one to the next.

Luckily illustrator is something I already know how to use... ??

Good to know - thank you!

Just a quick question: the 5th op-amp, in the bottom right, is connected with some green wires in the schedule. Some greens cross each other but it seems odd to have them connected with each other, am I right?

It didn't work. I was never able to get the 510k Res. And, what's with
the video? It doesn't even show the pedal working. It'd be nice if there
was a detailed explanation on the schematic.

BOM

x1 9v Battery Holder

x2 Audio Jack

x2 50k Pot

x1 100k Trim Pot

x1 SPST Switch

x2 2.2k Res

x1 5.1k Res

x2 10k Res

x4 22k Res

x1 43k Res

x1 47k Res

x11 100k Res

x2 150k Res

x1 390k Res

x2 510k Res

x7 .01uF Cap

x1 .001uF Cap

x1 .22uF Cap

x1 .33uF Cap

x1 10uF Cap

x1 22uF Cap

x1 220uF Cap

x1 7.5v Zener Diode

x3 2N3904 Trans

x4 2N5457 Trans

x5 LM741 Op-Amp