Circuit Bending

My First Pedal Mod (part 1)

Let’s Mod Your Big Muff

Do you or a friend you love have a Big Muff in need of tremolo effect on the Sustain/Distortion stage? Read on.

Prerequisite Knowledge

Before attempting these modifications, you may find it advantageous to have a little background knowledge. Here are things you may want to know.

Let’s Begin

In a previous post I mentioned being astonished by an episode of Collin’s Lab. In this episode, Collin opens a Big Muff Pi fuzz pedal and modulates the sustain (which is actually the distortion) stage of the pedal using an Arduino that controls an MCP41100 digital pot which essentially creates a tremolo effect. Now, it just so happened that I possessed a Big Muff and several Arduinos and have been agonizing over a good use for them. This is better than a good use for them; this is a perfect use!

In this first installment, I will simply break the pedal down, wire up the digital pot, and implement Collin’s Arduino sketch. If you have already gotten that far in this project or you would rather work through that part with the Makezine project (who would blame you?), then you may want to skip to my next, yet-to-be-published installment of My First Pedal Mod. In the interest of full transparency, I am writing this after several weeks of breaking things, researching the fixing of things (like transistor bias) and ultimately repairing the broken things. One of the joys found in these old Russian pedals is that they used crap components that wore out, so I found myself completely replacing the wires and the footswitch in mine.  You will notice that many of the images will look like an abomination with wires all over the place. Well, there is ample reason for that and I am asking now that you recognize that I am a hobbyist and am literally learning this stuff as I go along.

Background on Big Muff


To begin, I want to explain a little about the Big Muff as I understand it. It seems extremely simple on the surface, but is actually a complex stack of several stages that ultimately deliver a big sound that is pretty recognizable. I’m not going to dig too deep here, mainly because numerous sites are devoted to that discussion and do a far better job than I could. It basically breaks down to four stages:

  1. Buffer Stage – Provides a little boost to the input signal before driving it into fuzz heaven.
  2. Clipper Stage – This is the cool one. This stage takes the boosted signal and runs it through two Common Emitter amplification circuits, both of which are using diode loops to increase sustain. The transistors are hit hard enough to clip the peaks of the signal thus creating that beefy distortion that we all know and love. This is the first stage I will work on, just like Collin demonstrated in the video link above.
  3. Tone Stage – This is the second stage I will be working on. I’m essentially taking the second channel of the MCP42100 (did I mention that I opted for the dual channel model of the digital pot?) and controlling the tone sweep with the Arduino.
  4. Final Amplification Stage – This is a basic transistor amplifier and just about the only part of this pedal that I am not altering.

BM - circuit

For more details about the circuit and great background on the pedal as well, I found this site to be a terrific resource. Note that the circuit changed a bit over the years, so if your pedal differs (mine did considerably from the ElectroSmash article) be very careful about making any alterations using that schematic as a guide.

Wiring the MCP42100

Here is the basic idea for wiring up the digital pot. It really doesn’t get easier than this.
…and here is the ugly monster i created. Ignore the buttons and things for now.

To begin, I wired up the MCP42100 to the Arduino as described in Collin’s video. Notice that I am not using the MCP41100 that the video mentions. I paid an extra 30 cents and got the 2 channel chip. This really just means that I can control two pots discretely using a single set of commands. Set up was easy: I wired up 5 volts and ground along with the three control pins for sending commands. Once that is set, you are ready to start controlling the pot via serial commands from the Arduino. You will need to find Collin’s sketch on your own if you want his clean code. Otherwise, download mine and prepare to be overwhelmed by the rat nest!

Download Collin’s Original Sketch: WavePot-090819a

The important bits of the sketch include the pin connections and variable definitions:

Collin Cunningham /
 Wiring connections:
 Arduino pin MCP41100 pin
 10 --> 1 
 11 --> 3
 13 --> 2
 GND --> 4 
 5V --> 8
 connect 3 distortion control wires to MCP41100 pins 5,6,7
 in the order they were originally connected to back of distortion pot
 a 100K pot is connected to Arduino as a rate control - connect the
 pot's left terminal to GND, center terminal to Analog 1 pin, and right
 terminal to 5V.
#define DATAOUT 11//MOSI
#define DATAIN 12//MISO - not used, but part of builtin SPI
#define SPICLOCK 13//sck
#define SLAVESELECT 10//ss

Before going much further, I would recommend familiarizing yourself with how this code works. Especially the delays and how the oscillation values are being fed into the pot.

Ok, now that the digital pot has controls, I placed it in line for Sustain control. That required me to desolder the existing 100K potentiometer and move its wires to the appropriate positions on the MCP chip. In the diagram above this is referred to as:

  • A – this receives the wires previously connected to the left-most lug of the pot.
  • B – this receives the wires previously connected to the right-most lug of the pot.
  • Wiper – this receives the wires previously connected to the center lug on the pot.

If you are a genius like me, you may find that the pot got wired in backwards. This will manifest as low values sent to the MCP increasing the Sustain (distortion). You don’t want this! You want


to result in no distortion and


to result in full distortion. If you see the opposite happening, switch A and B now to correct this issue. In the next installment, having the positions reversed will cause trouble.

Closing Remarks

Again, this post is really designed to build the background for the next step in the build. Next, we set a tempo input for the tremolo effect. I hesitated to even post this build because I personally receive so much gratification in seeing teasers like Collin’s video and taking it in a direction that is personal and special to me. I really don’t want to color your creativity and curiosity. I ended up creating the post after all because I realized that it is possible that someone out there might stumble across it and take it in a crazy new direction. PLEASE! If you are inspired and have some results you want to share, let me know. If you are confused and stuck, let me know. I love to hear from you guys and really want to encourage you to get your hands in it. Remember: There are no stupid questions; just stupid people.Until next time…

One reply on “My First Pedal Mod (part 1)”

Wow P, I am completely “blown out” by all this and probably would be more so if I had a better understanding of what actually took place here, Can’t wait to hear it man! Thanks for sharing.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.