Two-mode distortion with a discrete op-amp, 1987 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 DS-2 Turbo Distortion.

A JFET input buffer, a transistor booster, a discrete op-amp gain stage and two diodes to ground. The Turbo switch picks a plain path into the gain stage (mode I) or a path with a peak near 1 kHz (mode II), and an active Tone stage follows.

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. Q22 boosts the guitar about 12 times, after two diodes that hold its input under about ±0.6 V. The gain stage is an op-amp built from parts: a JFET pair, a PNP gain stage and an emitter follower. Dist sets its gain from about 2 to about 250, and C33 under the gain leg lets the bass through with less gain below about 720 Hz. Two diodes after it clip to ground. In mode I two more diodes in the feedback round off the upper mids, and C21 makes a notch about 6 dB deep near 1 kHz after the Tone stage. Mode II feeds the gain stage through a filter that peaks about 5 dB near 1 kHz, with a dip near 2 kHz where its two paths cancel. Probe the gain stage output, then the diodes, in each mode. The model leaves out the electronic switching and the footswitch for Turbo.

Download.

Parts and what they do

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

C11, 100 µF electrolytic
Across the 9 V supply.
R15, 4.7 kΩ
Feeds Q7’s base from the 9 V supply.
C14, 10 µF electrolytic
Smooths Q7’s base, so ripple on the 9 V supply doesn’t reach the 8 V line.
Q7, 2SC1815
Emitter follower that makes the 8 V line, about 0.7 V under the battery.
C16, 100 µF electrolytic
Filter cap on the 8 V line.
R20, 10 kΩ
Top of the 4 V divider.
R19, 10 kΩ
Bottom of the 4 V divider.
C17, 47 µF electrolytic
Filter cap on the 4 V line.
R13, 10 kΩ
Series resistor at the input.
C12, 47 nF
Input coupling cap.
R16, 1 MΩ
Biases the input JFET’s gate at 4 V. The input impedance is about 1 MΩ.
Q6, 2SK184-GR
Input buffer, a JFET source follower (2SK184).
R17, 4.7 kΩ
Source resistor of the input buffer.
C15, 1 µF electrolytic
Coupling cap after the input buffer.
R18, 100 kΩ
Holds this side of the bypass switch at 4 V.
R60, 100 kΩ
Holds the other side of the bypass switch at 4 V. The switch, a JFET, is modeled as a closed wire, so R18 and R60 sit on one node.
C38, 1 µF electrolytic
Coupling cap toward the input diodes.
R62, 4.7 kΩ
Feeds the input diodes.
D14, D15, 1N914 / 1N4148, one each way
Two diodes to ground, one each way (1SS-188FM, modeled as 1N914-class silicon, type not checked). They hold the booster’s input under about ±0.6 V.
C40, 47 nF
Coupling cap into the booster.
R63, 100 kΩ
Biases the booster’s base at 4 V.
Q22, 2SC1815
The booster, a common-emitter stage. Its gain is about 2.2 kΩ over the 220 Ω and 1.5 kΩ in parallel, about 12.
R65, 2.2 kΩ
Booster collector resistor.
R66, 1.5 kΩ
Booster emitter resistor. It sets the current, about 2.3 mA.
R67, 220 Ω
With C42, sets the booster’s gain for the signal.
C42, 10 µF electrolytic
Bypass cap under R67.
C39, 47 nF
Mode I: couples the booster straight to the gain stage.
R61, 100 kΩ
Holds the mode I path at 4 V.
C41, 2.2 nF
Mode II: the high-pass half. With R64 and R68 it passes the treble.
R64, 10 kΩ
Mixes the high-pass half into the mode II path.
R69, 22 kΩ
First resistor of the mode II filter.
R70, 22 kΩ
Second resistor of the mode II filter.
C44, 1 nF
Filter cap to ground at Q23’s base.
C43, 47 nF
Feeds Q23’s output back to the filter’s middle. With the 47 to 1 ratio to C44, the filter peaks near 1 kHz.
Q23, 2SC1815
Emitter follower in the mode II filter.
R72, 4.7 kΩ
Q23’s emitter resistor.
C45, 22 nF
Couples the filter’s output into R68.
R71, 100 kΩ
Holds the mode II path at 4 V.
R68, 10 kΩ
Mixes the filtered half into the mode II path.
R49, 100 kΩ
Biases the gain stage’s + input at 4 V.
Q16, 2SK184-GR
The gain stage’s + input, one JFET of a pair.
Q19, 2SK184-GR
The gain stage’s − input, the other JFET of the pair.
R53, 4.7 kΩ
Shared source resistor of the input pair.
R50, 2.2 kΩ
Q16’s drain resistor.
Q17, 2SA1335-GR
PNP voltage gain stage driven from Q16’s drain (2SA1335).
R51, 2.2 kΩ
Q17’s collector resistor.
C31, 1 nF
From Q17’s base to its collector. It rolls off the gain stage’s top end and keeps it stable.
Q18, 2SC1815
Emitter follower, the gain stage’s output.
R52, 4.7 kΩ
Q18’s emitter resistor.
C32, 100 pF
Across the feedback. It trims the very top.
R55, 1 kΩ
In series with the Dist pot in the feedback.
Dist, 250 kΩ pot, audio taper
A 250 kΩ pot as a variable resistor in the feedback. The gain is 1 plus this and R55 over R54, up to about 250 (marked D taper, assumed audio).
R54, 1 kΩ
The gain leg’s resistor.
C33, 220 nF electrolytic
In series with R54. Below about 720 Hz the gain falls, so the bass gets less gain than the mids.
D8, D9, 1N914 / 1N4148, one each way
Two 1SS-133 diodes, one each way, in the feedback through C34. Mode I only: in mode II R56’s 1 MΩ leaves them out of it.
C34, 2.2 nF
In series with the feedback diodes, so they act on the upper mids and treble.
R56, 1 MΩ
In series with the diodes’ leg. Mode I shorts it.
C35, 10 µF electrolytic
Coupling cap after the gain stage.
R57, 4.7 kΩ
Feeds the clipping diodes.
D11, D12, 1N914 / 1N4148, one each way
Two 1SS-133 diodes to ground, one each way. They square the wave off at about ±0.6 V.
C36, 10 µF electrolytic
Coupling cap into the Tone stage.
R58, 15 kΩ
The Tone stage’s input resistor.
R59, 1 kΩ
Feeds the bright end of the Tone pot from the input.
Tone, 100 kΩ pot, linear taper
A 100 kΩ pot between the stage’s input and output. Toward the bright end its wiper feeds treble in through C37. Toward the dark end it feeds treble back from the output, which cuts it (marked G taper, assumed linear).
C37, 6.8 nF
From the Tone wiper to the summing node. Only the treble goes through it.
R45, 4.7 kΩ
Feeds the dark end of the Tone pot from the output.
R46, 33 kΩ
Feedback from Q13’s collector to the summing node. With R58 it sets the stage’s gain, about 2.2.
C30, 10 µF electrolytic
Couples the summing node to Q13’s base.
R48, 150 kΩ
Top of Q13’s base bias.
R35, 150 kΩ
Bottom of Q13’s base bias.
Q13, 2SC1815
The Tone stage, a common-emitter stage with its feedback through R46 and the Tone pot.
R47, 4.7 kΩ
Q13’s collector resistor.
R34, 4.7 kΩ
Q13’s emitter resistor.
C22, 10 µF electrolytic
Bypasses R34.
C29, 470 pF
From Q13’s collector to its base. It rolls off the very top.
R33, 22 kΩ
First half of the output filter’s series resistance.
R32, 1 MΩ
Keeps C21 charged while its switch is open.
C21, 10 nF
Mode I only: a cap to ground between R33 and R44. With C28 across both resistors it makes a notch, about 6 dB deep near 1 kHz.
R44, 22 kΩ
Second half of the output filter’s series resistance.
C28, 4.7 nF
Across R33 and R44, so the treble goes around them.
Q12, 2SC1815
Emitter follower after the Tone stage. Its base sits at Q13’s collector voltage.
R43, 4.7 kΩ
Q12’s emitter resistor.
R42, 10 kΩ
Series resistor of the last treble filter.
C26, 1 nF
With R42, rolls off above about 16 kHz.
R41, 10 kΩ
With C25, a shelf that trims the treble a few dB.
C25, 6.8 nF
With R41, a shelf that trims the treble a few dB.
C27, 10 µF electrolytic
Coupling cap into the Level pot.
Level, 50 kΩ pot, audio taper
Output level, a 50 kΩ audio pot.
C9, 1 µF electrolytic
Coupling cap toward the output switch.
R11, 100 kΩ
Holds the effect side of the output switch at 4 V.
R36, 100 kΩ
Holds the far side of the output switch at 4 V. The switch is modeled as a closed wire.
C23, 1 µF electrolytic
Coupling cap into the output buffer.
R37, 100 kΩ
Biases the output buffer’s base at 4 V.
Q11, 2SC1815
Output buffer, an emitter follower.
R38, 4.7 kΩ
Output buffer emitter resistor.
C24, 1 µF electrolytic
Output coupling cap.
R39, 100 kΩ
Output pulldown.
R40, 1 kΩ
Series resistor at the output jack.

About this version

Stock
Negative ground, 9 V. 2SC1815 values stand in for the 2SC3378 transistors.

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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