Op-amp fuzz, 1979 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 Electro-Harmonix Muff Fuzz (op-amp version).

Two halves of a dual op-amp: an inverting stage with a gain of 4.7, then a stage with no feedback resistor, only two diodes, its gain set by how little they conduct near zero volts until they switch on. One volume knob.

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 second stage has no feedback resistor. For small signals the diodes barely conduct, and their leakage sets the gain, about 900 with these diode models. Anything above a millivolt or so drives the output until one diode conducts and holds it within about 0.4 V of the input. The result is a near-square wave of fixed height, about 0.8 V peak to peak, whatever the input level, so the guitar volume changes the sound little until the note fades. Probe O2 and turn the guitar volume down. The op-amp choice matters little: the square is so small that even the MC1458’s slower slew finishes each edge in under 2 µs. Changing the diodes (open D1, D2) changes the level more than the character. LEDs roughly quadruple it, and germanium cuts it to about a quarter.

Download.

Parts and what they do

Stock values for the 4558, as drawn version. Tap a part on the schematic to change it.

R6, 680 kΩ
Top of the reference divider.
R7, 680 kΩ
Bottom of the reference divider: both + inputs sit at half the battery.
C3, 10 µF electrolytic
Filters the reference. It returns to the positive rail instead of ground, so the reference follows the battery for fast changes.
C1, 10 nF
Input coupling cap. With R1 it sets a corner near 160 Hz, so the lowest notes reach the op-amps weakened.
R1, 100 kΩ
Input resistor of the first stage, and the pedal’s input impedance. At 100 kΩ it loads a passive pickup slightly.
U1, RC4558
First stage, inverting with a gain of 470 kΩ / 100 kΩ = 4.7.
R2, 470 kΩ
First stage feedback resistor.
R3, 10 kΩ
Input resistor of the second stage. Once a diode conducts, the gain is roughly the diode’s small resistance over this 10 kΩ.
U2, RC4558
Second stage. With no feedback resistor it runs at open-loop gain until a diode conducts.
D1, D2, 1N914 / 1N4148, one each way
The only feedback around the second stage. On the drawing they’re chosen to taste. These are silicon.
C2, 100 nF
Output coupling cap.
Volume, 100 kΩ pot, audio taper
Output level. Even fully up the output is under 1 V peak to peak.

Versions

4558, as drawn
Negative ground. Both stages are halves of one 4558 dual op-amp, biased at half the battery by the 680 kΩ divider.
MC1458
The 1458, a dual 741, as listed for the op-amp Muff Fuzz in Wikipedia’s Big Muff history. It has a third of the 4558’s bandwidth and half its slew rate.

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, not measured parts. 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.

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