The PastFx I.C. Mufnstuf is an opamp fuzz from PastFX, described by the maker as "an opamp fuzz with a bunch of extra stuff"1. It is based on the 1978 EHX OpAmp Muff circuit, and Guitar Pedal X's Stefan Karlsson called it "a cool take on a 1978 OpAmp Muff - with all the smart extras onboard"2. Those extras are the point: where the original was a straightforward opamp fuzz, the Mufnstuf adds a clean/fuzz blend, a three-way mids control, a bass cut button, a tone bypass and a voice switch that covers the variations found in the original circuits.
Controls
The pedal carries more controls than the circuit it is based on. PastFx lists them as follows1:
| Control | What it does |
|---|---|
| Mix | Clean/fuzz blend; there will always be a tad of fuzz even at the cleanest settings1 |
| Mids | Three-way switch: Stock (down) / Boost (middle) / Cut (up)1 |
| Tone | Tone shaping2 |
| Level | Output level2 |
| Shape/Voice | Three-way Rough/Raw/Smooth switch, representing the variances of the original circuits and shaping the fuzz texture1 |
| Sustain | Fuzz sustain2 |
| Bass Cut button | Set at frequencies suited for bass guitar, a subtle cut representing the variances of the circuit it is based on1 |
| Tone Bypass button | Removes the tone stage; a blue LED lights up in place of red2 |
The Shape/Voice switch positions are Rough (up), Raw (middle) and Smooth (down)1. Guitar Pedal X's control rundown matches the maker's, listing "Clean / Fuzz Blend, Mids : Stock / Hi / Lo, Tone, Level, Shape/Voice : Rough / Raw / Smooth, Sustain, Bass Cut" and the tone bypass button2.
The bass cut button is worth noting for bass players: rather than a fixed sweepable EQ, it is a single button set at frequencies suited for bass guitar, giving a subtle cut that reflects the variances of the original circuit1.
Power
The Mufnstuf runs on 9V DC through a 2.1mm Boss style negative tip jack, the standard center-negative connector used by most pedalboard supplies1. PastFx lists the current draw as TBA, so no figure has been published yet1.
PastFx has not published a current draw figure for the I.C. Mufnstuf, listing it as TBA1. Until a figure is available, treat it as unknown when planning a daisy chain or battery-free supply budget.
Price and availability
Guitar Pedal X's reviewer gave the I.C. Mufnstuf a warm reception, calling it "a great catch" and writing: "I always have space for another great Big Muff, and I don't have too many of the OpAmp variety so far - so this is for sure one I would like to hear more of"2.
PastFX has built its range around reproductions and extensions of classic circuits, including the Jet Phaser, the M-1-A Stacked Sheep's Head Fuzz and the Patriarch. The I.C. Mufnstuf sits in the fuzz lane alongside those, and more broadly with other fuzz pedals built on opamp clipping12.
Questions
What circuit is the PastFx I.C. Mufnstuf based on?
It is based on the 1978 Electro-Harmonix OpAmp Muff. Guitar Pedal X described the pedal as a cool take on that circuit with smart extras onboard, and PastFx adds controls the original did not have, including a clean/fuzz blend, a three-way mids control, a bass cut button and a Rough/Raw/Smooth voice switch that represents the variances of the original circuits.
What does the Rough/Raw/Smooth switch do?
The three-way Shape/Voice switch shapes the fuzz texture, representing the variances of the original OpAmp Muff circuits. Rough is the up position, Raw is the middle and Smooth is down. It lets you move between different flavors of the base fuzz without changing any other setting.
Does the Mufnstuf have a bass cut for bass guitar?
Yes. A bass cut button is set at frequencies suited for bass guitar, giving a subtle cut that represents the variances of the circuit it is based on. It is one of several additions PastFx made to the original opamp fuzz design, alongside the mids control and the clean/fuzz blend.
What power supply does the I.C. Mufnstuf need?
It runs on 9V DC through a 2.1mm Boss style negative tip jack. PastFx lists the current draw as TBA, so no figure has been published yet. Use a center-negative supply as with standard Boss pedals.
How much does the I.C. Mufnstuf cost in the US?
Guitar Pedal X listed it at $237 USD as of July 17, 2026, down from $339 USD, with an Australian price of $299 AUD, reduced from $339 AUD. Availability is limited because the pedal uses rare components, but it was actively available direct from the PastFX webstore at that date.
Sources (2)
- I.C. Mufnstuf, PastFx, 2026
- Guitar Pedal X, Guitar Pedal X, 2026-07-17
