Two-channel op-amp overdrive, 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 SD-2 Dual Overdrive.

Two pedals in one box with a Channel switch. Crunch is one op-amp stage with LEDs in its feedback. Lead is five op-amp stages: a mid boost, a gain stage, a fixed gain of about 67, LEDs to ground, diodes in feedback, and a treble filter.

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. Both channels take the input and run all the time, and the Channel switch picks one for the output. Crunch is one op-amp stage with a red LED one way and a red LED plus a silicon diode the other way in its feedback, so it clips softly and a little lopsided once its output passes about 1.7 V. Lead lifts the mids about 4 dB near 1 kHz with a gyrator in its first stage, then gains up to 54 in the Gain stage and about 67 in a fixed inverting stage, which runs into its rails at most settings. Red LEDs to ground clip what is left at about ±1.7 V, and a diode stage with one diode one way and two the other rounds it again. A treble filter and a gain of 3 follow. Probe each Lead stage in turn to see where it clips. The model leaves out the SD-2’s switching, and the way the two channels share the jacks is assumed.

Download.

Parts and what they do

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

C24, 100 µF electrolytic
Across the 9 V supply.
R29, 100 kΩ
Top of the reference divider.
R30, 100 kΩ
Bottom of the reference divider.
C25, 47 µF electrolytic
Filter cap on the reference.
IC3A, JRC4558D
Buffers the 4.5 V reference for both channels.
R1, 10 kΩ
Lead: series resistor at the input.
C1, 47 nF
Lead: input coupling cap.
R2, 1 MΩ
Lead: biases the input JFET’s gate at 4.5 V.
Q1, 2SK184-GR
Lead: input buffer, a JFET source follower.
R3, 4.7 kΩ
Lead: source resistor.
C2, 47 nF
Lead: coupling cap into the mid stage.
R4, 220 kΩ
Lead: biases the mid stage at 4.5 V.
IC1B, JRC4558D
Lead: the mid stage. Its gain is 1 plus R5 over the gyrator’s impedance, which is lowest near 1 kHz.
R5, 2.2 kΩ
Lead: the mid stage’s feedback resistor.
C3, 100 pF
Lead: across R5, trims the very top.
C4, 27 nF
Lead: in series with the gyrator. With its inductance of about 0.85 H, it tunes the boost to about 1 kHz.
R6, 2.2 kΩ
Lead: the gyrator’s series resistance. It sets how much the mids lift, about 4 dB at 1 kHz.
C5, 8.2 nF
Lead: the gyrator’s cap. R6 times R8 times C5 makes about 0.85 H.
R8, 47 kΩ
Lead: the gyrator’s base resistor to the reference.
Q2, 2SC1815
Lead: the gyrator’s transistor, an emitter follower.
R7, 10 kΩ
Lead: Q2’s emitter resistor.
C6, 47 nF
Lead: coupling cap into the gain stage.
R9, 220 kΩ
Lead: biases the gain stage at 4.5 V.
IC1A, JRC4558D
Lead: the gain stage, non-inverting, 1 plus Gain over R10, up to about 54.
Gain, 250 kΩ pot, audio taper
Lead Gain, a 250 kΩ audio pot as a variable resistor in the feedback.
C7, 68 pF
Lead: across the Gain pot, trims the top at high gain.
R10, 4.7 kΩ
Lead: the gain leg’s resistor.
C8, 82 nF
Lead: in series with R10. Below about 410 Hz the gain falls.
C9, 68 nF
Lead: coupling cap into the fixed stage. With R11, a high-pass near 160 Hz.
R11, 15 kΩ
Lead: the fixed stage’s input resistor.
IC2A, JRC4558D
Lead: inverting stage, a fixed gain of about 67 (R12 over R11). It clips at the rails at most settings.
R12, 1 MΩ
Lead: the fixed stage’s feedback resistor.
C10, 180 pF
Lead: across R12. It rolls off above about 880 Hz.
C11, 10 µF electrolytic
Lead: coupling cap toward the LEDs.
R13, 1 kΩ
Lead: feeds the LEDs.
D2, D3, Red LED, one each way
Lead: two red LEDs to ground, one each way. They clip at about ±1.7 V.
R14, 1 kΩ
Lead: with R15, a divider after the LEDs.
C12, 22 nF
Lead: across R14, lets the treble around it.
R15, 10 kΩ
Lead: bottom of the divider after the LEDs.
C13, 10 µF electrolytic
Lead: coupling cap into the diode stage.
R16, 100 kΩ
Lead: biases the diode stage at 4.5 V.
IC2B, JRC4558D
Lead: the diode stage, a gain of about 3 with diodes in its feedback.
D4–D6, 2 × 1N914 / 1N4148 one way, 1N914 / 1N4148 the other way
Lead: diodes in the feedback, two in series one way (D5, D6) and one the other (D4), so the clipping is lopsided.
C14, 470 pF
Lead: across the feedback, rolls off above about 3.4 kHz.
R17, 100 kΩ
Lead: the diode stage’s feedback resistor.
R18, 47 kΩ
Lead: the diode stage’s gain leg.
C15, 22 nF
Lead: in series with R18. Below about 150 Hz the gain falls to 1.
R19, 2.2 kΩ
Lead: first resistor of the treble filter.
C16, 15 nF
Lead: first filter cap.
R20, 22 kΩ
Lead: second resistor of the filter.
C17, 47 nF
Lead: with R21, a shelf that cuts the treble about 6 dB.
R21, 12 kΩ
Lead: sets the shelf’s depth.
C18, 10 µF electrolytic
Lead: coupling cap into the output stage.
R22, 33 kΩ
Lead: the output stage’s input resistor.
IC3B, JRC4558D
Lead: inverting output stage, a gain of about 3 (R23 over R22).
R23, 100 kΩ
Lead: the output stage’s feedback resistor.
C19, 220 pF
Lead: across R23, rolls off above about 7 kHz.
C20, 100 nF
Lead: coupling cap into the Tone pot.
Tone, 10 kΩ pot, linear taper
Lead Tone, a 10 kΩ linear pot. Toward the dark end its wiper sees more of C21 to ground.
C21, 27 nF
Lead: the Tone pot’s treble cap to ground.
Level, 100 kΩ pot, audio taper
Lead Level, a 100 kΩ audio pot.
R24, 4.7 kΩ
Lead: series resistor into the output buffer.
C22, 47 nF
Lead: coupling cap into the output buffer.
R25, 470 kΩ
Lead: biases the output buffer at 4.5 V.
Q3, 2SC1815
Lead: output buffer, an emitter follower.
R26, 10 kΩ
Lead: output buffer emitter resistor.
C23, 1 µF
Lead: output coupling cap.
R27, 100 kΩ
Lead: output pulldown.
R28, 1 kΩ
Lead: series resistor toward the jack.
R2, 10 kΩ
Crunch: series resistor at the input.
C1, 47 nF
Crunch: input coupling cap.
R3, 1 MΩ
Crunch: biases the input JFET’s gate at 4.5 V.
Q1, 2SK184-GR
Crunch: input buffer, a JFET source follower (a J201 in the kit).
R4, 4.7 kΩ
Crunch: source resistor.
C2, 2.7 nF
Crunch: with C3 and R5, a treble shelf. The lows drop to half through C2 over C3, the treble goes through C2 into R5.
C3, 2.7 nF
Crunch: the shelf’s cap to ground.
R5, 22 kΩ
Crunch: the shelf’s resistor to ground.
C4, 47 nF
Crunch: coupling cap into the op-amp.
R7, 1 MΩ
Crunch: biases the op-amp at 4.5 V.
IC1.1, JRC4558D
Crunch: the one gain stage, non-inverting, 1 plus R8 and Gain over R6, up to about 380.
D1–D3, Red LED one way, Red LED + 1N914 / 1N4148 the other way
Crunch: in the feedback, a red LED one way (D2) and a red LED with a 1SS133 the other (D1, D3), so the clipping is soft and lopsided.
C6, 22 pF
Crunch: across the feedback, trims the very top.
R8, 10 kΩ
Crunch: in series with the Gain pot.
Gain, 250 kΩ pot, audio taper
Crunch Gain, a 250 kΩ audio pot as a variable resistor in the feedback.
C5, 4.7 µF electrolytic
Crunch: in the gain leg. Below about 50 Hz the gain falls.
R6, 680 Ω
Crunch: the gain leg’s resistor.
R9, 560 Ω
Crunch: first resistor of the treble cut after the op-amp.
C7, 39 nF
Crunch: with R9, a fixed treble cut above about 7 kHz.
R10, 2.2 kΩ
Crunch: series resistor into the Tone pot.
C8, 22 nF
Crunch: coupling cap into the Tone pot.
Tone, 10 kΩ pot, linear taper
Crunch Tone, a 10 kΩ linear pot. Toward the dark end its wiper sees more of C9 to ground.
C9, 22 nF
Crunch: the Tone pot’s treble cap to ground.
Level, 100 kΩ pot, audio taper
Crunch Level, a 100 kΩ audio pot.
C10, 47 nF
Crunch: coupling cap into the output buffer.
R11, 470 kΩ
Crunch: biases the output buffer at 4.5 V.
Q2, 2SC1815
Crunch: output buffer, an emitter follower (a 2N5088 in the kit).
R12, 10 kΩ
Crunch: output buffer emitter resistor.
C11, 1 µF electrolytic
Crunch: output coupling cap.
R13, 100 kΩ
Crunch: output pulldown.
R14, 1 kΩ
Crunch: series resistor toward the 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.

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