Op-amp overdrive, 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 Ibanez TS808 Tube Screamer.

An input buffer, an op-amp with two diodes in its feedback loop, an active tone stage, and an output buffer. The diodes limit how far the output can get from the input, so the clean signal always rides under the clipped part.

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 4.7 kΩ and 47 nF on the op-amp’s minus input mean the clipping stage has full gain only above about 720 Hz, so the low strings clip least. The tone stage then rolls the top off. Together they make the mid hump this pedal is known for. Probe the clipper output: the diodes hold it within about half a volt of the input, so the input signal is still in there and the pick attack comes through.

Download.

Parts and what they do

Stock values for the Symmetric, 1979 version. Tap a part on the schematic to change it.

R32, 10 kΩ
Top of the 4.5 V reference divider.
R33, 10 kΩ
Bottom of the 4.5 V reference divider.
C17, 100 µF electrolytic
Filter cap on the 4.5 V reference, so everything biased from it sits at a quiet AC ground.
C16, 47 µF electrolytic
Supply filter cap.
R1, 1 kΩ
Input series resistor.
C1, 20 nF
Input coupling cap. Against the 510 kΩ bias resistor it sets a corner near 16 Hz.
R2, 510 kΩ
Biases the input buffer from the reference, and sets most of the pedal’s input impedance.
Q1, 2SC1815
Input buffer, an emitter follower, with a gain of one and a high input impedance.
R3, 10 kΩ
Input buffer emitter resistor.
C2, 1 µF
Coupling cap into the clipping stage.
R5, 10 kΩ
Biases the clipper’s plus input at 4.5 V.
U1, JRC4558D
Clipping stage. A non-inverting amp with its gain set by the drive pot and the diodes across its feedback.
R4, 4.7 kΩ
With R6 and the drive pot it sets the gain: 1 + (51 k + drive) / 4.7 k.
C3, 47 nF
In series with R4, so the stage has full gain only above about 720 Hz. Below that the gain falls toward one, and the bass clips least.
R6, 51 kΩ
Minimum feedback resistance, so the gain never drops below about 12.
Drive, 500 kΩ pot, linear taper
Wired as a variable resistor in the feedback path. More resistance means more gain before the diodes take over.
C4, 51 pF
Across the feedback. It rolls off the top of the clipped signal, more so as the drive goes up.
D1, D2, 1N914 / 1N4148, one each way
The clipping diodes across the feedback, a series string each way. More diodes, or higher-voltage parts, let the output swing further before they conduct, and different strings each way make the clipping lopsided. Empty both sides for a clean boost with the same filtering.
R7, 1 kΩ
With C5, a low-pass filter at about 720 Hz ahead of the tone stage.
C5, 220 nF
The low-pass cap. Together with the clipper’s high-pass, it makes the mid hump.
R9, 10 kΩ
Biases the tone stage’s plus input.
Tone, 20 kΩ pot, linear taper
Slides the R8/C6 network from the plus input, where it cuts treble, to the minus input, where it boosts it.
R8, 220 Ω
Tone network resistor.
C6, 220 nF
Tone network cap.
U2, JRC4558D
Tone stage.
R10, 1 kΩ
Tone stage feedback resistor.
R11, 1 kΩ
Output series resistor.
C7, 1 µF
Coupling cap to the level pot.
Level, 100 kΩ pot, audio taper
Output level.
C8, 100 nF
Coupling cap into the output buffer.
R13, 510 kΩ
Biases the output buffer.
Q2, 2SC1815
Output buffer, an emitter follower.
R14, 10 kΩ
Output buffer emitter resistor.
R15, 100 Ω
Output series resistor.
C9, 10 µF electrolytic
Output coupling cap.
R16, 10 kΩ
Output pulldown. With the buffer it keeps the output impedance low.

Versions

Symmetric, 1979
Negative ground. The 4.5 V reference biases both op-amps and both buffers. One diode each way clips both halves evenly, so the harmonics are odd. The maker’s own Maxon OD808 is the same circuit.
TS9, 1982, modeled after the Ibanez TS9 Tube Screamer
The same circuit with a different output network after the output buffer: 470 Ω in series and 100 kΩ to ground, instead of 100 Ω and 10 kΩ. The output impedance goes from about 100 Ω to about 470 Ω, which only matters into a long cable or a low-impedance load. The difference people often hear between the two pedals is more likely the op-amp. Many TS9s used other 4558 types. This build keeps the JRC4558D.
Asymmetric, 1981, modeled after the Boss SD-1
The asymmetric Boss version: two diodes one way and one the other, and a 1 MΩ drive pot with 33 kΩ minimum feedback, so the gain runs about 8 to 220 instead of 12 to 118. That pedal’s tone-stage values, buffers and gain-leg return (to 4.5 V instead of ground) differ slightly too. This build leaves them at the 1979 values.

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.

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