Op-amp distortion with a two-band gyrator EQ, 2001 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 Maxon DS-830 Distortion Master.

A JFET input buffer, an op-amp gain stage with diodes in its feedback, two more diodes to ground, then bass and treble controls built from two transistor gyrators, 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. IC1A’s gain is 1 plus R6 and the Gain pot over 390 Ω, from about 6 to over 600. D2 and D3 across its feedback round the clipping off, and D4 and D5 to ground then hold what’s left near 0.6 V either way. IC1B carries the EQ. Each control is a pot from its plus input to its minus input with a tuned leg on the wiper: a transistor gyrator acting as a coil, in series with a cap. Treble is tuned near 2.5 kHz, Bass near 126 Hz. Turned toward the minus input, a control lifts its band, and toward the plus input it cuts it. Bass is a reverse audio pot, so at noon it already lifts 100 Hz by about 7 dB. The model leaves out the bypass switching.

Download.

Parts and what they do

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

R23, 10 kΩ
Top of the 4.5 V reference divider.
R24, 10 kΩ
Bottom of the reference divider.
C16, 220 µF electrolytic
Filters the 4.5 V reference. The gyrators’ resistors return to it.
C1, 100 nF
Input coupling cap.
R1, 10 kΩ
Input series resistor.
R2, 1 MΩ
Biases the JFET’s gate at 4.5 V and sets the input impedance near 1 MΩ.
Q1, 2N5457
Input buffer, a source follower. A 2SK256-Y in the pedal. The 2N5457 stands in.
R3, 10 kΩ
Source resistor.
R4, 10 kΩ
With C2, a treble cut from about 4 kHz ahead of the gain stage.
C2, 3.9 nF
The treble cut’s cap, to the 4.5 V reference.
C3, 1 µF
Coupling cap into the gain stage.
R5, 100 kΩ
Biases IC1A’s plus input at 4.5 V.
IC1A, JRC4558D
Gain stage, non-inverting, with diodes across its feedback.
R7, 390 Ω
The gain leg. Gain is 1 + (R6 + Gain) / 390 Ω.
C5, 1 µF
In series with R7, so the gain falls below about 410 Hz and is 1 at DC.
C4, 10 pF
Across the feedback, against oscillation.
D2, D3, 1N914 / 1N4148, one each way
Two 1N914s back to back across the feedback. Past about 0.6 V either way they pull the gain down, which rounds the clipping.
R6, 2 kΩ
Least feedback resistance, so the gain never drops below about 6.
Gain, 250 kΩ pot, audio taper
Gain, a 250 kΩ audio pot wired as a variable resistor in the feedback.
C6, 1 µF
Coupling cap to the diodes.
R8, 1 kΩ
Feeds the diodes to ground and limits their current.
D4, D5, 1N914 / 1N4148, one each way
Two 1N914s back to back to ground. They hold the signal to about 0.6 V either way.
C7, 470 nF
Coupling cap to the EQ stage.
R9, 10 kΩ
Into the EQ stage’s plus input. The EQ legs pull against it.
R10, 100 kΩ
Biases IC1B’s plus input at 4.5 V.
IC1B, JRC4558D
The EQ stage. With both controls at the middle its gain is about 1.
R18, 3 kΩ
Feedback of the EQ stage.
C12, 47 nF
Across R18. The treble falls from about 1.1 kHz, on top of what the controls do.
Treble, 100 kΩ pot, linear taper
Treble, 100 kΩ, from the plus input to the minus input. A W taper in the pedal, flat near the middle. Linear stands in. Toward the minus input it lifts the band near 2.5 kHz, toward the plus input it cuts it.
C8, 100 nF
In series with the treble gyrator. With the gyrator’s coil it tunes the band.
C9, 3.3 nF
The treble gyrator’s cap.
R11, 22 kΩ
Biases Q2 from the 4.5 V reference. With C9 and R13 it sets the coil the gyrator acts like.
Q2, 2SC1815
The treble gyrator’s follower.
R13, 560 Ω
The treble gyrator’s series resistor. It sets how broad the band is.
R12, 10 kΩ
Q2’s emitter resistor.
Bass, 5 kΩ pot, reverse audio taper
Bass, 5 kΩ reverse audio. With R14 it runs from the plus input to the minus input. Its wiper carries the bass gyrator.
R14, 3.9 kΩ
In series with Bass on the plus side, so the bass cut reaches less far than the lift.
C10, 3.3 µF
In series with the bass gyrator. With the gyrator’s coil it tunes the band.
C11, 220 nF
The bass gyrator’s cap.
R15, 10 kΩ
Biases Q3 from the 4.5 V reference.
Q3, 2SC1815
The bass gyrator’s follower.
R17, 220 Ω
The bass gyrator’s series resistor.
R16, 10 kΩ
Q3’s emitter resistor.
C13, 1 µF
Coupling cap to Level.
Level, 100 kΩ pot, audio taper
Level, 100 kΩ audio.
C14, 100 nF
Coupling cap into the output buffer.
R19, 510 kΩ
Biases the output buffer at 4.5 V.
Q4, 2SC1815
Output buffer, an emitter follower.
R20, 10 kΩ
Output buffer emitter resistor.
C15, 1 µF
Output coupling cap.
R21, 100 kΩ
Output pulldown.
R22, 470 Ω
Output series resistor.

About this version

DS-830
Values from Aion FX’s clone with its clipping toggle in the stock position, both diode pairs in. No factory drawing was at hand.

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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