Building a Wah-Wah Effect Guitar Pedal

My niece plays electric guitar, she asked me if I could make her a "Wah Wah" effect pedal.

So I accepted the challenge, that's why I built this one!

It is based pretty much on (reverse engineering of) current commercial products, nothing original, but with my personal touch.

The design and operating principle of this effects pedal is well-proven and existed for about sixty years, so there is plenty of data to support it.

Most parts are common and easy to obtain, except for the cast aluminium pedal case kit (easy to get online, but specialized for the purpose). Initially I wanted to build a custom made pedal body using a mix of old school method with aluminium sheet and 3D printing, but I decided for the quicker buy-ready-to-use way...

Electronic components come from my components bin, some from my junk boxes and a few ad-hoc purchased.

The tools are the usual: Wire cutter, pliers, soldering iron, drill, wrenches, etc. Nothing special.

Here some materials:

1. Generic Wah Wah pedal case kit. https://www.mercadolibre.com.ar/pedal-de-efecto-wah-wha-kit-gabinete-diy-vox-cry-baby-dunlop/up/MLAU3249602169?pdp_filters=item_id%3AMLA1506529401#polycard_client=gift_registry&wid=MLA1506529401&sid=navigation
2. Foot switch 3PDT. https://www.mercadolibre.com.ar/footswitch-3pdt-pro-pedal-de-efecto-cretencion-guitarra/up/MLAU3047676338?pdp_filters=seller_id%3A481000532#polycard_client=recommendations_vip-seller_items-above&reco_backend=ranker-retsys-same-seller&reco_model=rk_entity_sameseller&reco_client=vip-seller_items-above&reco_item_pos=1&reco_backend_type=low_level&reco_id=50b49219-962d-4e50-b186-91123f9f94c4&wid=MLA1479822031&sid=recos
3. 10x10cm single sided perforated board.
4. Self-adhesive felt pad.
5. Two-sided adhesive tape.
6. Knurled knob for potentiometer.
7. Plastic LED holder 5mm
8. Black primer spray can.
9. White spray can.
10. Clear satin varnish spray can.
11. 1" masking tape.
12. 1/4" masking tape.
13. 9V alkaline battery.
14. 14 DIP IC socket
15. Some cut of 2mm eva foam.
16. Contact glue.
17. Assorted 0,5mm2 color wires
18. 1/8" shrink tube.
19. Assorted screws from my "tacho de los tornillos" (the ones that came with the pedal kit were pretty useless... :-/ ).
20. Grease for plastic mechanisms.

The bill of materials of the circuit can be found in the attached pdf file.

The history tells that the wah wah pedal was invented in the mid-1960s by some engineers from the Vox company, famous by their guitar amplifiers.

The legendary guitar star Jimi Hendrix made this pedal an essential part of his sound. Then, legions of guitarist adopted the wah wah for their effects setup all around the world.

In the early 1980s the Scottish manufacturer Jim Dunlop began the production of the pedal based on the same classic design and expanding the line offering several models and sizes.

The operation principle is relatively simple:The first photo of this step shows the most classic (and basic) scheme.

Q0 is just an emitter follower that reduces the load on the instrument side.

The magic is made by Q1 (common emitter amplifier), Q2 (emitter follower) and their associated components. They act as a variable frequency band-pass filter.

The clean signal from the guitar (buffered by Q0) enters the base of Q1 via R1 and C1 and the processed signal outs from the collector of Q1 via C5.

The output signal enters potentiometer VR1. Its shaft is driven by the pedal through a rack and pinion mechanism to achieve the wah wah effect. Its wiper goes to the base of Q2 through C4, so the signal amplitude varies as the cursor moves. The output of this stage is the emitter of Q2.

An LC filter is inserted into the feedback loop; it consists of the inductor L1 (which is also part of the bias path) and C2 that receives the variable signal from emitter of Q2.

The variable feedback through C2 has the effect of a "variable reactance" which affects the resonance of the active filter built around these components, and this what is commonly called "Wah Wah"!

Something about the inductor:

The inductor is like the heart of any wah wah pedal; it plays a central role in the pedal's sonic features.

The components used are very common and easy to get. The inductor, however, is somewhat more difficult to obtain, but not impossible.

My research into the operation and construction of the pedal provided answers about the inductor, its characteristics, and how to obtain it. DIYers often use small transformers for transistor radios or current sensing coils or anything suitable they can find in their junk boxes (this is the easy and cheap way).

The most sophisticated modders use the Fasel inductor supplied by the manufacturer (nice, buy very expensive).

I selected two candidates from my (many) junk boxes; An output transformer from an old germanium transistor radio and a ferrite core horizontal driver transformer from a tv set that time forgot. According to the LCR meter, the radio transformer has an inductance of 476mHy and the ferrite transformer of 606mHy. Both have a series resistance of about 100ohm. The actual inductance value is not that crytical (within a certanin margin). The value of the radio transformer is closer to the nominal value, but is somewhat bulky and I observed that it exhibits some microfonics during the experiments on the breadborard. Meanwhile, the ferrite transformer is smaller and easy to install on the board compared to the other one.

Also there is the possibility of building it (provided that the necessary materials and equipment are available...).

The potentiometer:

The other key component is the potentiometer; it acts a transducer that translates the movement of the pedal into an electric signal.

It need to be robust enough to withstand the mechanical demand of pedal movement for a long time.

The resistive element has a logarithmic curve (type A), giving a progressive change in the tone as the pedal moves. However, a linear pot can also be used, bearing in mind that the response to pedal movement will be more aggressive.

It's a matter of taste; some prefer a progressive response and other a faster or snappy response to the pedal.

As this project is based on well known and proven designs, everything began with an internet search to take a look on what DIYers and moders had done. Actually, there are several interesting projects on this topic here on Instructables!

The design adopted is based on my personal interpretation/merge of the most recent version of the "535Q Multi Wah" and "Junior" models.

As an old school guy, I trust testing and measuring on the breadboard more than computer simulations. I made an LT Spice model after I have built the board just for checking and fine tuning purposes and to create the frequency plots shown in the photos.

As I had some time constraints, I decided to assemble the circuit on perforated board instead of designing a proper PCB. The circuit is small and simple, it is no big deal doing it this way.

Compared to the basic circuit shown in Step 1, it may look somewhat overbuilt, but it keeps the same operation principle.

The Multi Wah model incorporates op amp input and output buffers. It also features six selectable frequency ranges and an adjustable output boost up to +16db.

I simplified the scheme just a bit; there are just three frequency ranges, similar to the Junior model, and omitted the extra boost. After all, the pedal already has +16 to +18db of gain on the resonance peaks; let the overdrive to the rest of the pedals of the set. I kept the variable Q control because it was easy to implement, but I'm actually not really convinced of the virtues of this feature.

Schematic and simulation files are available here in photo, PDF and txt formats (just remove the .txt extension ;D).

The foot switch acts as a bypass control; it also controls a pilot light to indicate that the effect is on.

The fore mentioned pilot light is a high bright 5mm blue LED. It draws less than 500uA, but emits a very visible light.

The current draw is about 9mA at nominal 9V.

I tested the circuit by powering the supply from 6V to 12V. It works stably within a wide voltage range. The gain drops slightly below 6V, but it can still be played.

Working on the pedal case was a very straight process since a kit was used.

The original pegs where board should be fitted were ground down. My board is attached with four screws.

I used some aluminium scrap to make a support for the Q control pot and the range selector.

Also, a 5mm LED holder was added.

Since the case is made of cast aluminium, this is the most defying step.

Aluminium is hard to paint in a hobbyist shop if some basic precautions are not taken.

Before painting, sanding is required to remove the surface oxide layer, then a washing a degreasing.

The first paint to be applied must be the primer to ensure good paint adhesion. I used a nice mat black spray primer/paint that works fine.

Most of the pedals of the kind are plain black (boring!), so I tried to do something more or less different, not too flashy, simple but elaborate at the same time.

I decided for a black and white paint scheme with checkered stripes around the sides of the base and rocker plate (it's not original at all, but it looks good!).

The process of masking, cutting, and alternately detaching little square pieces of 1/4" of painter's tape was painstaking, but the end result was very satisfying!.

The masked parts were coated with white paint. After it dried, the whole masking was removed.

Some areas needed to be corrected, so an extra masking was applied exposing only the required areas, then another layer of black paint was sprayed on.

Finally the whole painting was finished with a coat of transparent paint.

Putting the parts together is the most satisfying stage of the project because all the ideas and processes materialize into the real thing.

The rubber parts were glued in place with contact cement.

The circuit, connectors, switches, potentiometers and other accessories were fastened in place.

At this point, the wiring was made using 0.5mm2 cables.

I made a padded space for the 9V battery at the rear end of the case using self-adhesive felt.

Also I made a dust cover using a piece of black EVA foam and double-sided adhesive tape on the top of the case where the potentiometer is linked to the pedal cog.

The pedal plate was attached to the base by inserting the steel axle through the hinge lugs. The gear mechanism was lubricated with a special grease for plastic mechanisms (Super Lube is OK too).

The 9V battery was connected to its clip. The bottom plate of the was installed and screwed in together with the four rubber feet.

The final touch is the stickers on the front end and the bottom plate.

The symbol of a triode is something like my logo of DIYer. I called this pedal "Luli WAH!" after my niece Lucía "Luli". After all, I built it for her.

As Capt. Murphy said almost 80 years ago: "Anything that can go wrong will go wrong", and so it did.

Virtually there are not DIYers all around the world whose projects work on the first try. There is nothing wrong with that, overcoming difficulties and correcting errors are part of the fun of this satisfying activity.

Before delivering the pedal to the end user, i.e. Luli, I handed it to a friend in my office to test it with his gear to get a feedback of how the Wah-Wah performs.

It took not too much time to the problems to show up; after all, this device that just come off the workbench with minimal lab testing:

1. The pedal made an awful pop noise when the pedal switches on and off.

I wired the battery line to the foot switch on the first version of the wiring; that was the cause of the pop noise.

I corrected this problem by replacing the input jack from mono to stereo. Then I disconnected the negative line from the footswitch and wired it to the R pole of the new jack. So, the pedal turns on when the guitar plug is inserted (like most pedals).

1. The wah pot started making scratchy noises shortly after being used.

The original potentiometer I installed was a sealed cermet type due to its durability. Everything alright, except it was a cheap knockoff, not a legit Bourns or any other well-known brand.

The resistive strip was made of some sort of "chinessium" instead of cermet...

I replaced it with a good 40-years-old Mallory NOS unit from my components stock.

1. There was some roll-off at the low end of the frequency response, which made the sound a bit thin in the lower notes.

It was an easy fix: I just corrected a wrong connection on the board and changed a couple of capacitor values in the signal path.

The published BOM and schematic correspond to the final corrected version of the construction.

Now the pedal is really finished, so Luli can enjoy playing music with it!

I thank Kevin for testing the pedal, his constructive feedback and support.

To my furry orange friend for his funny company and his "quality control" :-))))

And to my niece Luli for her passion for music, her talent and for asking me to build the Luli Wah!

I enjoyed the making of this project; it taught me learn a lot about guitar effects pedals.

I hope you like it too.