JFET-input silicon fuzz with a feedback pair, 1997 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-3 Fuzz.

A JFET buffer and a transistor stage, then a direct-coupled pair with feedback from the second emitter to the first base, as in a Fuzz Face. A Big Muff style Tone and two more transistors 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. Q1 buffers the guitar and Q2 brings it up about 3 times, with C4 and R9 trimming the treble. Q3 then drives the fuzz pair, Q4 and Q5, wired much like a Fuzz Face but in silicon, with R12 setting their bias. Fuzz sets how much of Q5’s emitter resistance C7 bypasses, so the pair’s gain runs from about 40 dB to about 67 dB near 700 Hz. Q5 clips against Va on one side first, so the second harmonic is strong. Q6, the Tone network and Q7 follow. Q8 filters the supply.

Download.

Parts and what they do

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

R30, 2.2 kΩ
Feeds Q8’s base.
C18, 47 µF electrolytic
Filters Q8’s base. Q8 copies it to Va at a much lower impedance.
Q8, 2SC1815
A filter for the supply: an emitter follower on a filtered base.
C19, 100 µF electrolytic
Filter cap on Va.
R29, 220 Ω
Feeds Vc from Va.
C16, 47 µF electrolytic
Filter cap on Vc.
R27, 22 kΩ
Top of the Vb divider.
R28, 22 kΩ
Bottom of the Vb divider.
C15, 47 µF electrolytic
Filters Vb.
R1, 10 kΩ
Input series resistor.
C1, 47 nF
Input coupling cap.
R2, 1 MΩ
Holds Q1’s gate at Vb and sets the input impedance near 1 MΩ.
Q1, 2SK184-GR
Input buffer, a JFET source follower.
R3, 10 kΩ
Q1’s source resistor.
C2, 248 nF
1 µF and 330 nF in series on the original, 248 nF. Couples Q1 into Q2’s divider.
R5, 47 kΩ
Top of Q2’s base divider.
R4, 12 kΩ
Bottom of Q2’s base divider, about 1.7 V.
R6, 220 Ω
Base stopper on Q2.
Q2, 2SC1815
First gain stage, about 3 times, with an unbypassed emitter resistor.
R8, 33 kΩ
Q2’s collector load.
R7, 10 kΩ
Q2’s emitter resistor, unbypassed.
C4, 10 nF
With R9, from Q2’s collector to ground. It trims the treble going on.
R9, 2.2 kΩ
In series with C4.
Q3, 2SC1815
Emitter follower, so Q2 drives the fuzz pair from a low impedance.
R10, 15 kΩ
Q3’s emitter resistor.
R11, 6.8 kΩ
Into the fuzz pair. Q4 and Q5’s feedback works against it.
C5, 1 µF
Coupling cap into Q4.
Q4, 2SC1815
First transistor of the fuzz pair, emitter grounded.
R13, 22 kΩ
Q4’s collector load.
Q5, 2SC1815
Second transistor of the pair, its base on Q4’s collector.
R14, 3.3 kΩ
Lower half of Q5’s collector load. The output comes off its top.
R15, 3.3 kΩ
Upper half of Q5’s collector load.
R12, 100 kΩ
From Q5’s emitter back to Q4’s base. It sets the pair’s bias.
C6, 1 nF
Across R12.
Fuzz, 1 kΩ pot, reverse audio taper
Fuzz, a 1 kΩ reverse-audio pot from Q5’s emitter to ground. Turned up, more of it is bypassed by C7 and the pair’s gain rises.
C7, 10 µF electrolytic
From the Fuzz wiper to ground.
C8, 18 nF
Coupling cap into Q6. Against the divider it trims the low bass.
R17, 330 kΩ
Top of Q6’s base divider.
R16, 150 kΩ
Bottom of Q6’s base divider.
Q6, 2SC1815
Gain stage ahead of the tone network, a gain near one.
R19, 10 kΩ
Q6’s collector load.
C9, 8.2 nF
Across R19. It rolls off the treble.
R18, 10 kΩ
Q6’s emitter resistor, unbypassed.
C10, 47 nF
Coupling cap into the tone network.
C11, 10 nF
Tone, treble side: in series to the top of the Tone pot.
R20, 47 kΩ
Tone, treble side: to ground.
R21, 47 kΩ
Tone, bass side: first resistor.
C12, 10 nF
Tone, bass side: to ground.
R22, 47 kΩ
Tone, bass side: to the bottom of the Tone pot.
Tone, 100 kΩ pot, linear taper
Tone, 100 kΩ, modeled linear. It fades from the bass side to the treble side.
Volume, 100 kΩ pot, audio taper
Volume, a 100 kΩ audio pot on the Tone wiper.
C13, 1 µF
Coupling cap into the output buffer.
R23, 100 kΩ
Biases the output buffer at Vb.
Q7, 2SC1815
Output buffer, an emitter follower.
R24, 10 kΩ
Output buffer emitter resistor.
C14, 10 µF electrolytic
Output coupling cap.
R25, 100 kΩ
Holds the output at ground.

About this version

2SC1815, 2SK184
Negative ground, 9 V. The 2SC1815 stands in for the 2SC2458s, and the library’s 2SK184 is the input JFET.

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.

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© 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.

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