Octave fuzz with a discrete booster and active EQ, 1993 style

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Boss FZ-2 Hyper Fuzz.

A JFET buffer, a discrete op-amp booster, then a split-phase transistor pair into one collector load for the octave. Two filter stages make the Fuzz I and Fuzz II voices. Gain Boost skips the fuzz. A two-band active EQ and a buffer follow.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. The booster is an op-amp built from parts, a JFET pair and a PNP, with Gain setting it from about 2.5 to 36. A 480 Hz low-pass follows, then Q5 splits the signal into two copies in opposite phase for Q6 and Q7. They share a collector load, so the note cancels and its octave adds, and two diodes clip what comes out. Fuzz II filters it with a dip near 1 kHz into a gain of 12. Fuzz I uses one 1 kHz low-pass into a gain of 2.7. Gain Boost sends the booster straight on. The EQ stage lifts or cuts a 100 Hz band, tuned by a gyrator, and a treble shelf. Probe the pair’s collectors to see the octave. The model’s octave pair is matched, real units vary.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

R45, 2.2 kΩ
Feeds Q9’s base.
C26, 47 µF electrolytic
Smooths Q9’s base. Q9 multiplies its effect, so the 9 V line after Q9 is very quiet.
Q9, 2SC1815
Capacitance multiplier, an emitter follower whose base sits on C26.
C27, 100 µF electrolytic
Filter cap on the line after Q9.
R43, 10 kΩ
Top of the reference divider.
R44, 10 kΩ
Bottom of the reference divider.
C28, 47 µF electrolytic
Filter cap on the reference.
R1, 10 kΩ
Series resistor at the input.
C1, 47 nF
Input coupling cap.
R2, 1 MΩ
Biases the input JFET’s gate at the reference.
Q1, 2SK184-GR
Input buffer, a JFET source follower.
R3, 10 kΩ
Source resistor of the input buffer.
C2, 1 µF
Coupling cap into the booster.
R4, 22 kΩ
Biases the booster’s + input at the reference.
Q2, 2SK184-GR
The booster’s + input, one JFET of a pair.
Q3, 2SK184-GR
The booster’s − input, the other JFET of the pair.
R5, 4.7 kΩ
Shared source resistor of the pair.
R6, 2.2 kΩ
Q2’s drain resistor.
Q4, 2SA1335-GR
PNP gain stage driven from Q2’s drain. Its collector is the booster’s output.
R9, 1 kΩ
Q4’s collector resistor.
C4, 47 pF
Across the feedback. It trims the top and keeps the booster stable.
R8, 2.2 kΩ
In series with the Gain pot.
Gain, 50 kΩ pot, audio taper
A 50 kΩ audio pot as a variable resistor in the feedback. The booster’s gain is 1 plus this and R8 over R7, about 2.5 to 36.
R7, 1.5 kΩ
The booster’s gain leg.
C3, 2.2 µF electrolytic
In series with R7. Below about 50 Hz the gain falls.
R10, 10 kΩ
With C5, a low-pass at about 480 Hz before the fuzz.
C5, 33 nF
Low-pass cap before the fuzz.
C6, 1 µF
Coupling cap into the phase splitter.
R11, 220 kΩ
Top of Q5’s base bias.
R12, 100 kΩ
Bottom of Q5’s base bias.
Q5, 2SC1815
Phase splitter. Its collector and emitter carry the signal in opposite phase.
R13, 4.7 kΩ
Q5’s collector resistor. Half of R14, so the collector’s copy is about half the emitter’s.
R14, 10 kΩ
Q5’s emitter resistor.
R15, 1 kΩ
Series resistor toward Q6.
C7, 1 µF
Couples Q5’s collector to Q6.
R18, 1 kΩ
Series resistor toward Q7.
C8, 1 µF
Couples Q5’s emitter to Q7.
R16, 100 kΩ
Top of Q6’s base bias.
R17, 27 kΩ
Bottom of Q6’s base bias.
R19, 100 kΩ
Top of Q7’s base bias.
R20, 27 kΩ
Bottom of Q7’s base bias.
Q6, 2SC1815
One half of the octave pair.
Q7, 2SC1815
The other half, fed the opposite phase.
R22, 10 kΩ
The pair’s shared collector load. Both halves pull on it, so the wave here repeats at twice the note.
R21, 1.8 kΩ
The pair’s shared emitter resistor.
C9, 47 µF electrolytic
Bypasses R21.
C10, 1 µF
Coupling cap into the diodes.
D2, D3, 1N914 / 1N4148, one each way
Two 1N914 diodes to ground, one each way. They clip at about ±0.6 V.
C11, 1 nF
Fuzz II: with R23, a treble path around the two low-passes.
R23, 10 kΩ
Fuzz II: in series with C11.
R24, 47 kΩ
Fuzz II: first low-pass resistor.
C12, 47 nF
Fuzz II: first low-pass cap, about 70 Hz with R24.
R25, 10 kΩ
Fuzz II: second low-pass resistor.
C13, 4.7 nF
Fuzz II: second low-pass cap.
C14, 1 µF
Fuzz II: coupling cap into the stage.
R26, 10 kΩ
Fuzz II: input resistor of the stage.
IC1A, JRC4558D
Fuzz II: inverting stage, a gain of 12 (R27 over R26).
R27, 120 kΩ
Fuzz II: the stage’s feedback resistor.
C15, 47 pF
Fuzz II: across R27, rolls off above about 28 kHz.
R28, 1 kΩ
Fuzz II: series resistor to the Mode switch.
R29, 10 kΩ
Fuzz I: low-pass resistor.
C16, 15 nF
Fuzz I: low-pass cap, about 1 kHz with R29.
C17, 1 µF
Fuzz I: coupling cap into the stage.
R30, 10 kΩ
Fuzz I: input resistor of the stage.
IC1B, JRC4558D
Fuzz I: inverting stage, a gain of 2.7 (R31 over R30).
R31, 27 kΩ
Fuzz I: the stage’s feedback resistor.
C18, 47 pF
Fuzz I: across R31.
R32, 1 kΩ
Fuzz I: series resistor to the Mode switch.
R33, 100 kΩ
Holds the Mode switch’s common at the reference.
R34, 10 kΩ
Feeds the EQ stage’s + input and the pots’ input ends.
IC2A, JRC4558D
The EQ stage. Each pot sits between its + and − inputs, with a tuned leg to ground on its wiper.
Treble, 50 kΩ pot, linear taper
A 50 kΩ linear pot. Toward the − input it lifts the treble around 3.2 kHz and up, toward the + input it cuts it.
C19, 15 nF
The treble leg’s cap.
R35, 3.3 kΩ
The treble leg’s resistor, to ground.
Bass, 50 kΩ pot, linear taper
A 50 kΩ linear pot. Toward the − input it lifts around 100 Hz, toward the + input it cuts there.
C20, 150 nF
The bass leg’s cap. With the gyrator’s 15 H it tunes to about 100 Hz.
C21, 47 nF
The gyrator’s cap. R36 times R37 times C21 makes about 15 H.
R37, 100 kΩ
The gyrator’s resistor to the reference.
IC2B, JRC4558D
The gyrator’s follower.
R36, 3.3 kΩ
The gyrator’s series resistance. It sets how sharp the bass band is.
R38, 10 kΩ
The EQ stage’s feedback resistor.
C22, 47 pF
Across R38, trims the very top.
C23, 1 µF
Coupling cap into the Level pot.
Level, 50 kΩ pot, audio taper
Output level, a 50 kΩ audio pot. Gain Boost shorts it out.
C24, 1 µF
Coupling cap into the output buffer.
R39, 100 kΩ
Biases the output buffer at the reference.
Q8, 2SC1815
Output buffer, an emitter follower.
R40, 10 kΩ
Output buffer emitter resistor.
C25, 10 µF electrolytic
Output coupling cap.
R41, 100 kΩ
Output pulldown.
R42, 1 kΩ
Series resistor at the output jack.

About this version

Stock
Negative ground, 9 V. 2SC1815 values stand in for the 2SC3378, 2SK184 values for the JFETs.

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type. Caps are ideal, and the battery or HT is a voltage behind a resistance.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.