Big Muff Pi Mod for More Mid-Range Tone

The Big Muff Pi is a fuzzbox/sustain pedal made by the Electro-Harmonix company. It was introduced around 1970 and came to prominence as it was used by major guitar artists such as David Gilmour. (If you're not familiar with the Big Muff Pi, see the demonstration video at the end of this article.) While many guitarists love the Big Muff Pi there is an oft-heard desire for stronger midrange frequencies. The stock circuit uses a tone control circuit with a "scooped" midrange frequency response that can be too severe for some tastes. In this article I'll show you a simple and inexpensive mod that anyone can do that will flatten out the mid range while retaining the tone control knob in this classic pedal. Let’s take a look at the overall schematic (Figure 1) of the Big Muff Pi. We'll be working on the tone control circuit which is identified here, and enlarged for clarity in Figure 2. Figure 1 - Big Muff Pi Overall Schematic Figure 2 - Big Muff Pi Tone Control Circuit The Big Muff Pi uses a simple passive tone control that is a high-pass filter and a low-pass filter in parallel. A potentiometer allows for adjustment between higher or lower frequencies which are then passed along to the final amplifier stage and then on to the output. The components in the tone circuit are identified as C10, C11, R18, R19, and R20. This is the order that is consistent with most of the Big Muff Pi schematics I have seen, and with which you may find yourself working. To create a flat mid range we'll change capacitor C10 to a 0.01uf capacitor. Also change resistor R18 to a 39k ohm resistor. If 39k is not readily available, you can change both R18 and R19 to 33k. You can use other values for R18 and R19, as long as you use the same value for both. However, this will also change how the tone control responds as you adjust the tone knob. The specific tone control component values have varied throughout the history of the Big Muff Pi. Two of the more popular versions are referred to as the Triangle and Ram's Head. Here are the values from those eras, along with the values for a flat mid range:

VERSION	C10	C11	R18	R19
Triangle	0.0039uf	0.01uf	39k	39k
Ram’s Head	0.0039uf	0.01uf	22k	39k
Flat Mid Range	0.01uf	0.01uf	39k	39k

The figure below shows a plot of the frequency responses of the Big Muff tone circuit. The pink line with the noticeable scoop in the center is the stock Ram’s Head version. The darker, flat line above it is the response using the values for a flat mid range in the table above. Both plots were made with the tone control knob set at its center of travel. As you can see, the modification results in significantly more mid-range frequencies and adds back the tone that many guitarists feel has been missing. Frequency Response of Stock Tone Control vs. Modified Circuit You might be wondering about the type of capacitor to use in this mod. Metal film capacitors are often suggested in other mods to help reduce noise. There are also those who claim that using ceramic capacitors helps give the Big Muff Pi a better, warmer tone. In my experience you can use whichever kind you have on hand, or costs less and you will be happy with the result. Anyone interested in a deeper look into Big Muff tone control mods will want to try a piece of free software, the Duncan Tone Stack Calculator, from Duncan Amplification. It can be downloaded here. This software will allow you to change component values in not only the Big Muff tone circuit, but a few other tone control circuits as well. You can then see graphically what the frequency response of the circuit will be. Happy modding, and here is the demonstration video of the stock Big Muff Pi.