Two op-amp distortion with an amp tone stack and a Contour control, 1991 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 Marshall Shredmaster.

Two op-amp gain stages into one silicon diode each way, an amp-style bass and treble stack, a recovery stage, and a Contour pot that slides between a mid scoop and a treble cut. The last stage rolls off above about 1.6 kHz.

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 first stage’s gain runs up to about 31 above 1 kHz. The Gain pot also sits in series with the second stage’s input, so that stage’s gain runs from about 6 to 72, and turning up raises both. One 1N914 each way to the reference clips the result to about 0.5 V either side. Then come the amp-style stack, with 1 kΩ fixed in place of a middle pot, a recovery stage with a gain of about 5.7 that rolls off above about 330 Hz, the Contour network, and an inverting stage that rolls off above about 1.6 kHz. With the knobs at their default settings a 200 mV pick comes out about 100 mV rms.

Download.

Parts and what they do

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

R16, 47 kΩ
Top of the 4.5 V reference divider.
R17, 47 kΩ
Bottom of the 4.5 V reference divider.
C19, 47 µF electrolytic
Filter cap on the 4.5 V reference.
C1, 10 nF
Input coupling cap. Against R1 it cuts below about 16 Hz.
R1, 1 MΩ
Biases the first stage’s plus input, and sets the input impedance, about 1 MΩ.
IC1b, TL072
First gain stage, non-inverting.
R2, 3.3 kΩ
The first stage’s gain leg: gain is 1 plus the Gain pot’s feedback part over 3.3 kΩ, up to about 31.
C3, 47 nF
In series with R2, so the first stage’s gain is full only above about 1 kHz.
Gain, 100 kΩ pot, linear taper
A 100 kΩ linear pot with the output on its wiper. Lug 1 to the wiper is the first stage’s feedback. The wiper to lug 3 is in series with the second stage’s input, so turning up raises both stages’ gain.
C2, 100 pF
Across the first stage’s feedback. It rolls off the top.
C4, 68 nF
Coupling cap into the second stage (220 nF and 100 nF in series in the original).
R3, 9.4 kΩ
The second stage’s input resistor, two 4.7 kΩ in series in the original.
IC1a, TL072
Second gain stage, inverting.
R4, 680 kΩ
The second stage’s feedback: gain is 680 kΩ over R3 plus part of the Gain pot, about 7 to 72.
C5, 47 pF
Across R4. It rolls the second stage off above about 5 kHz.
C6, 100 nF
Coupling cap to the diodes.
R5, 10 kΩ
Series resistor into the diodes.
D2, D3, 1N914 / 1N4148, one each way
One 1N914 each way to the reference.
C7, 22 nF
The tone stack’s treble cap.
R6, 6.8 kΩ
The tone stack’s slope resistor.
C8, 220 nF
The tone stack’s bass cap.
C9, 22 nF
The tone stack’s middle cap.
Treble, 25 kΩ pot, linear taper
A 25 kΩ linear pot. Its wiper feeds the recovery stage.
Bass, 100 kΩ pot, audio taper
A 100 kΩ audio pot wired as a variable resistor under the Treble pot.
R7, 1 kΩ
Fixed 1 kΩ in place of a middle pot.
IC2a, TL072
Recovery stage, non-inverting.
R9, 220 kΩ
The recovery stage’s feedback: gain 1 + 220 k / 47 k, about 5.7.
C11, 2.2 nF
Across R9. It rolls the recovery stage off above about 330 Hz, toward a gain of one.
R8, 47 kΩ
The recovery stage’s gain leg.
C10, 220 nF
In series with R8, so the gain falls below about 15 Hz.
R10, 100 Ω
Series resistor into the Contour network.
C12, 100 nF
From the first Contour node to lug 1 of the Contour pot.
R11, 33 kΩ
First series resistor of the Contour network.
C13, 47 nF
From the middle Contour node to lug 3 of the Contour pot.
R12, 33 kΩ
Second series resistor of the Contour network.
C14, 1 nF
Across both 33 kΩ resistors, a path for the highs.
Contour, 100 kΩ pot, linear taper
A 100 kΩ linear pot, its wiper to the reference. It sets how much of each Contour cap shunts its node.
C15, 220 nF
Coupling cap into the last stage.
R13, 100 kΩ
The last stage’s input resistor.
IC2b, TL072
Last stage, inverting, gain of one at low frequencies.
R14, 100 kΩ
The last stage’s feedback, equal to R13.
C16, 1 nF
Across R14. The last stage rolls off above about 1.6 kHz, 6 dB per octave.
Volume, 100 kΩ pot, audio taper
Output level, a 100 kΩ audio pot returning to the reference.
C17, 220 nF
Output coupling cap.
R15, 1 MΩ
Output pulldown.

About this version

TL072s
Negative ground. Two TL072 dual op-amps, all four halves biased from the 4.5 V reference.

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.

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