Transistor-boosted hard-clipping distortion, 1994 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-1 (DS1-A revision).

An input buffer, a transistor stage with about 35 dB of gain, an op-amp gain stage whose output is clipped by two diodes to ground, a passive scooped tone control, level, and an output buffer.

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. Most of the gain is in the transistor stage. With 22 Ω in its emitter it gains about 56, so an ordinary pickup already swings it most of the way to the rails, and it clips softly and slightly lopsided. The op-amp adds up to 22 times more, and the diodes after it square the result off at about ±0.6 V. Probe the transistor’s collector, then the diodes, to see where each kind of clipping happens. The tone control blends a 234 Hz low-pass with a 1 kHz high-pass, leaving a notch around 500 Hz in the middle, the opposite of the 1979 overdrive’s mid hump. Try 1 kΩ for R9, a popular mod, to bring the transistor stage down to about 7.

Download.

Parts and what they do

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

R24, 10 kΩ
Top of the 4.5 V reference divider.
R25, 10 kΩ
Bottom of the 4.5 V reference divider.
C15, 47 µF electrolytic
Filter cap on the 4.5 V reference.
C23, 100 µF electrolytic
Supply filter cap.
R1, 1 kΩ
Input series resistor, protection against static from the jack.
C1, 47 nF
Input coupling cap. Against R2 the corner is about 7 Hz.
R2, 470 kΩ
Biases the input buffer and sets the input impedance, about 470 kΩ.
Q1, 2SC2240-GR
Input buffer, an emitter follower.
R3, 10 kΩ
Input buffer emitter resistor.
C2, 470 nF
First coupling cap into the booster.
R4, 100 kΩ
With C2, a high-pass at about 3.4 Hz.
C3, 47 nF
Second coupling cap. The booster’s feedback makes its input impedance low, a few kΩ, so this cap also cuts bass ahead of the gain: about 16 dB down at 100 Hz and 5 dB at 400 Hz, against the treble.
R5, 100 kΩ
Booster base resistor to ground. With R7 it sets the booster’s bias.
Q2, 2SC2240-GR
The booster, a common-emitter stage with shunt feedback from collector to base, gaining about 56. Its bias depends on the transistor’s current gain, so different parts idle at different collector voltages.
R8, 10 kΩ
Booster collector resistor.
R9, 22 Ω
Booster emitter resistor, only 22 Ω. 1 kΩ here is a common mod that brings the gain down to about 7.
R7, 470 kΩ
Feedback from collector to base. It biases the transistor and pulls its gain down from several hundred to about 56.
C4, 250 pF
Across R7. It increases the feedback at high frequencies, which rolls off the treble in the booster.
C5, 68 nF
Coupling cap into the op-amp stage.
R39, 47 kΩ
Limits current into the op-amp input when the booster swings hard.
D8, 1N914 / 1N4148
Protects the op-amp input from being pulled below ground. It only conducts once the op-amp is already clipping.
R10, 100 kΩ
Biases the op-amp input at 4.5 V through R39, and with C5 forms a 23 Hz high-pass.
U1, LM358
Gain stage, non-inverting, with a gain of 1 + distortion pot / 4.7 kΩ, up to about 22. Later units used different dual op-amps.
Dist, 100 kΩ pot, linear taper
Wired as a variable resistor in the feedback path. More resistance, more gain.
C7, 100 pF
Across the feedback. It rolls off the very top.
R13, 4.7 kΩ
Sets the maximum gain with the distortion pot.
C8, 470 nF
In series with the gain leg. Full gain above about 72 Hz at full distortion.
C9, 470 nF
Output coupling cap.
R14, 2.2 kΩ
Feeds the diodes and, with C10, filters above about 7 kHz while they are not conducting.
D4, D5, 1N914 / 1N4148, one each way
The hard-clipping diodes, one each way to AC ground. Adding a diode to one side is the common asymmetry mod.
C10, 10 nF
Across the diodes. It rounds the edges of the clipped wave.
C11, 22 nF
Tone high-pass cap. With R17 the corner is about 1 kHz.
R17, 6.8 kΩ
Tone high-pass resistor.
R16, 6.8 kΩ
Tone low-pass resistor. With C12 the corner is about 234 Hz.
C12, 100 nF
Tone low-pass cap.
Tone, 20 kΩ pot, linear taper
Blends the low-pass and high-pass sides. In the middle, both are down around 500 Hz. That’s the scoop.
Level, 100 kΩ pot, linear taper
Output level.
C13, 47 nF
Coupling cap into the output buffer.
R19, 1 MΩ
Top of the output buffer’s bias divider.
R20, 1 MΩ
Bottom of the output buffer’s bias divider.
Q3, 2SC2240-GR
Output buffer, an emitter follower.
R21, 10 kΩ
Output buffer emitter resistor.
C14, 1 µF
Output coupling cap.
R23, 100 kΩ
Output pulldown.

About this version

DS1-A
Negative ground, 9 V. The 1994-on circuit with an LM358-class dual op-amp. The 1978 original used a different chip and circuit.

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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