Phaser Guitar Pedal

Intro: 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.

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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Comments

[TheDeadRat]

it would really be great if you had the actual schematics along with the one illustrated type you included. i see the 100k trimpot in your pictures, but nowhere does it mention what the heck to do with it or why its there. im really wanting to give this one a go today, and i follow all your pedal instructable things, but itd be good to help people understand that these things might be there for, or what they are doing, or how to set or use them.

merely a minor gripe, and a personal one. just not sure we know what we are supposed to do with the trimpot. i cant be the only one. anyway thanks for these, brother! i truly appreciate these

[johnrottingliver]

Press the arrows on the side of the picture... I know this is old but maybe it will help others. I agreed until i figured it out

[txdm]

I would love to hear a sound demo from anyone who's made this project.

[TDHofstetter]

"The LFO is comprised of the 5th LM741 op-amp in the circuit (and surrounding circuitry)" should correctly be either:

"The LFO is COMPOSED of the 5th LM741 op-amp in the circuit (and surrounding circuitry)"

or (preferred):

"The 5th LM741 op-amp in the circuit (and surrounding circuitry) comprise the LFO"

The parts comprise the whole, which is composed of the parts.

BTW... tToo bad you didn't include an actual schematic. That would have been nice.

[ccncs517]

Looks like a really cool project!!! I'm not sure what the voltage ratings next to the pair of 2N3904 transistors mean. Sorry if the answer is obvious (I'm still quite new to pedal building).

[TheDeadRat]

im betting you already figured this out since its been two years since you asked this question, but just in case anyone else has the same one...

those are the voltages (approximately) that should appear on those nets/links/nodes/junctions/etc. that theyre on in the schematic.

i.e.)) the cyan link is about 7V, and the purple one is about 4V. theyre listed in an attempt to help you troubleshoot or check your circuit against the original's voltages.

[DavidP820]

I’m on it for over a year on and off. I think I finally got everything right, but it still doesn’t work for me. When I turn the wet/dry pot all the way to wet I can sometimes hear the sound changes a bit, but stays constant (maybe shifts phase but without the LFO working, I don’t know), sometimes it just lower the volume by about half and sometimes I can’t hear anything at all... When the pot is all the way dry the sound is indeed dry without changes. By the way when I turn the LFO’s knob nothing happens either way (wet or dry of course). Help?
would’ve uploaded an image but it looks, well, bad... I don’t think anyone but me would understand what’s going on there :)

[randofo]

Maybe it's the LFO. Try disconnecting the output of the LFO ouput from the transistors and driving an LED instead to see visually if it is working.

(In full disclosure, I don't know the PWM rate of the circuit off the top of my head, so it might just appear that the LED is always on. That said, I would think you should be able to see some sort of change as you adjust the potentiometer.)

Also, make sure the output jack is grounded with a wire. I didn't draw in a ground wire because in my circuit the jack gets grounded through the metal case.

[AGUS2508]

Everything seems to be working fine in the circuit except for the LFO section. Could you check if ive made a mistake anywhere?
All the voltages are correct and when testing it i can hear a rise in volume when turning the top pot but the lfo pot does nothing.
I apologise for the mess in advance.

[randofo]

I opened mine up and compared it against the schematic. I realized I drew the schematic a little bit dumb. The 510K resistor that goes to pin 3 should also go to pin 6 (in addition to the 5.1K resistor and 0.022 cap). Your wiring there seems wrong.

[AGUS2508]

Is there a reason why in the parts list a dpdt switch is listed but in the schematic a spst switch is used instead?

[randofo]

Yes. This project was originally sponsored by Radioshack and I recently updated the parts list for Amazon and elsewhere (since most of the links for Radioshack were broken). I assumed it was a DPDT switch since that is what I normally used, but I suppose SPST (as outlined in the schematic) is also fine. Either will work.

[sumeyraduraak]

I am building this circuit right now but it seems LFO part is not working. I couldnt observe any oscillation on output so i decided to simulate it and it does not work on simulation either. Do you have any idea why that might be happening? Also how does LM741 working with single supply? Doesn't it have to be biased with differential supplies (like v+=4V and v-=-4V)?

[randofo]

The LM741 should work fine without a true split rail supply.

I'm afraid I don't really have enough information here to debug your circuit.

[JunmarF1]

does the transistors need to be matched? like other phaser pedal builds?

[randofo]

I didn't and I was pleased with the results, and couldn't hear anything wrong with the effect. However, I suppose you would get a more pure signal if you did.

[green3ggsandhams]

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

[randofo]

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.

[green3ggsandhams]

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

[randofo]

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.