Keeley Electronics Psi Fuzz

The Psi Fuzz is a fuzz pedal from Keeley Electronics, designed by Robert Keeley and Craighton Hale¹. It is Keeley's take on the classic Big Muff Pi circuit, reworked with a germanium transistor gain stage and LED clipping, and Robert Keeley summed up its character on the company's blog as "Creamy, thick, fuzz n' muff. Psi Fuzz."¹

At the time of Keeley's own write-up, the design was still being fine tuned and the pedal was listed as coming soon¹. The page below covers what the maker has said about the circuit, how its controls came to be, and how the Psi Fuzz concept later grew into the Keeley OCTA PSI Transfigurating Fuzz.

A Big Muff Pi-based fuzz with a germanium stage

The Psi Fuzz starts from the Electro-Harmonix Big Muff Pi, the fuzz circuit Robert Keeley acknowledged head-on in his blog post on the pedal, titled with a nod to the many Muff variants on the market¹. The Big Muff Pi itself is a fuzz pedal from Electro-Harmonix, and the company still sells expanded versions such as the Deluxe Big Muff Pi, which adds a Bass Boost, Noise Gate and adjustable Mids section to the original².

Keeley's version is not a straight copy. Where the stock Muff circuit uses silicon transistors throughout, the Psi Fuzz adds a germanium stage: an AC187 germanium transistor in a common emitter amplifier. Keeley says the collector sits around 4.5V and the stage delivers about 10dB of gain¹.

Germanium has a history in the Big Muff line beyond this pedal. Electro-Harmonix's own Germanium 4 Big Muff Pi, released in 2010, was not based on the common four-stage Big Muff circuit at all but on a pair of fuzz-style amplifier circuits from Bob Myer's original designs, each with two germanium transistors³⁴. The Psi Fuzz takes a different route, keeping the Muff-style op-amp framework and threading germanium gain into it.

Circuit details: transistor, op-amp and LEDs

Robert Keeley described the Psi Fuzz's build in plain terms on the company blog¹:

The single transistor is the AC187 germanium described above. The clipping stage is where the pedal departs most visibly from a stock Muff. Keeley replaced the six silicon diodes in the feedback loop of the third stage with two LEDs, and one of those LEDs pokes through a hole in the top of the enclosure, so the clipping action is visible while you play¹.

The parts list as the maker describes it:

The Cream and Voltage controls

The Psi Fuzz's two headline controls, Cream and Voltage, exist because of a wiring mistake. Keeley wired 10K audio-taper pots into the positions meant for 50K pots, and the 50K pots where the 10Ks belonged. In his words¹:

Rather than undo the error, he found the swapped values sounded exceptional, and the controls kept their accidental taper and range¹. For a player, that means the Cream and Voltage knobs respond across their sweep differently than a conventionally specified Muff clone would, which is part of what separates this pedal from the many stock-spec Big Muff variants on the market.

The Octa Psi Transfigurating Fuzz

The Psi Fuzz name resurfaced in September 2024, when Keeley Electronics released the Octa Psi Transfigurating Fuzz, a fuzz pedal with pitch shifting and octave generation that MusicRadar reported took Robert Keeley and his team over three years to make⁵⁶. It launched at an introductory price of $249⁷.

The Octa Psi is a three-in-one: a pitch-shifter, an octave generator and an analog fuzz in one box⁵⁶. Its fuzz side is a 100 percent analog, multi-stage transistor-based circuit with Fuzz, Level and Tone controls, plus a three-way toggle for three tonal shapes: Punch, Scoop and Psi⁷⁸. The Psi position is the one that delivers what Premier Guitar calls massive undertones⁹, and Pedal of the Day described the fuzz as "Super thick, analog transistor fuzz and distortion"⁵.

The pitch side offers Up, Dual and Down pitch-shifting modes selected by a three-way toggle, covering multiple octaves, wide intervals, chords and even chorus modulation⁹. A rotary switch gives eight pitch modes, and there is an adjustable ramp speed for the octave and pitch mode that produces sounds akin to an expression pedal sweeping⁶¹⁰. Expression control is also available on the rear of the pedal¹¹.

The Octa Psi's controls at a glance:

Reviewers took to it. Pedal of the Day called the pedal "FUN" with "wonderful, warm, fuzzy octave tones and real magic when experimenting"⁵. Guitar Pedal X's Stefan Karlsson gave it Bronze, third place, in his 2024 fuzz category roundup⁷.

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