Op-amp distortion with diodes in the feedback and to ground, 1986 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 DOD FX-55B Supra Distortion.

An input buffer, an op-amp gain stage with diodes in its feedback, a second diode pair to ground, then an op-amp tone stage and an output buffer. One file for four DOD pedals on the same board.

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. U1A buffers the guitar. U2A’s gain is 1 plus the feedback over the gain leg, the Gain pot plus R5: about 23 with Gain at 9 on the FX-55B, and over 8,000 at full, where D1 and D2 take over. Past about 0.6 V either way they pull the gain down. D3 and D4 then clip what’s left to ground. C4 keeps the stage’s gain at 1 for DC. Level comes before the tone stage. The Gain pot’s audio taper is wired so most of the gain comes in the last part of its turn. The model leaves out the electronic bypass.

Download.

Parts and what they do

Stock values for the FX-55B version. Tap a part on the schematic to change it.

R13 (DOD), 22 kΩ
Top of the 4.5 V reference divider.
R14 (DOD), 22 kΩ
Bottom of the reference divider.
C17 (DOD), 10 µF electrolytic
Filters the 4.5 V reference.
C1, 10 nF
Input coupling cap.
C2, 120 pF
From the input to ground against radio pickup.
R2, 10 kΩ
Input series resistor.
R3, 470 kΩ
Biases the input buffer at 4.5 V and sets the input impedance near 470 kΩ.
IC1A, MC1458
Input buffer, a follower.
IC2A, LF353
Gain stage, non-inverting. Its gain is 1 plus the feedback over the gain leg, until the diodes conduct.
R4, 3.9 MΩ
Across the clipping diodes. It sets the most gain the stage has before they conduct.
D1, D2, 1N914 / 1N4148, one each way
Silicon diodes back to back in the feedback. Past about 0.6 V either way they pull the gain down, which rounds the clipping.
C3, 120 pF
Across the feedback. It rolls off the top end, more as the gain goes up.
Gain, 500 kΩ pot, audio taper
Gain, a 500 kΩ audio pot. The part from its wiper to R5 is the gain leg, so turning it up drops that resistance and raises the gain. Its other lug is free except on the FX-50B.
R5, 470 Ω
The least resistance in the gain leg. It sets the top gain.
C4, 1 µF electrolytic
In series with the gain leg, so the stage’s DC gain is 1. With the pot near full it cuts the gain below about 340 Hz.
R6, 10 kΩ
Feeds the diodes to ground and limits their current.
R9 (DOD), 220 kΩ
Across the effect path’s switch, which is open with the effect on. With CM it loads this point with 220 kΩ.
C13 (DOD), 1 µF electrolytic
From the switch to ground.
C5, 1 µF electrolytic
Coupling cap to the diodes.
D3, D4, 1N914 / 1N4148, one each way
Silicon diodes back to back to ground. They hold the signal to about 0.6 V either way.
C6, 1 nF
Across the diodes. With R6 it trims the treble above about 16 kHz.
C7, 100 nF
Coupling cap to the Level pot. 100 nF on the FX-55B, 10 nF on the others, which cuts the bass with R13.
Level, 100 kΩ pot, audio taper
Level, a 100 kΩ audio pot to the reference. It comes before the tone stage.
IC2B, LF353
Tone stage, non-inverting.
R7, 18 kΩ
Feedback resistor of the tone stage.
C8, 120 pF
Across R7. 120 pF on the FX-55B and FX-50B, 3.9 nF on the FX-53, which rolls the treble off from about 2.3 kHz.
Tone, 100 kΩ pot, linear taper
Tone, 100 kΩ linear. Its wiper is the tone stage’s minus input. Turned up, toward R8 and C10, it adds gain in the highs. Turned down, toward C9, it cuts them.
R8, 2 kΩ
With C10, the leg that lifts the treble.
C10, 22 nF
To ground from R8.
C9, 100 nF
From the tone stage’s output to the pot’s other end.
R9, 1 kΩ
With C11, from the pot’s other end to ground.
C11, 10 µF electrolytic
To ground from R9.
R10, 10 kΩ
Into the output buffer.
IC1B, MC1458
Output buffer, a follower.
R11, 1 kΩ
In series at the output.
C12, 1 µF electrolytic
Output coupling cap.
R12, 100 kΩ
Output pulldown.

Versions

FX-55B
DOD’s drawing of 1986. 3.9 MΩ across the diodes, 470 Ω in the gain leg, and no resistor between the clipping diodes and Level. A 22 kΩ and 22 kΩ divider makes the 4.5 V reference.
FX-53, modeled after the DOD FX-53 Classic Tube
The FX-53 from a hand trace of one unit, which agrees with PedalPCB’s values. 680 Ω in the gain leg, 1.5 nF across the clipping diodes, 220 kΩ into Level, and a tone stage with 3.9 nF across its feedback, so it’s darker.
FX-52, modeled after the DOD FX-52 Classic Fuzz
The FX-52 from a traced drawing. The same board with a 100 kΩ Fuzz pot wired as on the FX-50B, its free lug tied to U2A’s output, 3.9 MΩ across the diodes, and 3.3 kΩ in the gain leg.
FX-50B, modeled after the DOD FX-50B Overdrive Plus
From PedalPCB’s table, with no drawing of the pedal itself. No resistor across the diodes, and the Gain pot’s free lug tied to U2A’s output, so turning it up adds feedback resistance as it takes it out of the gain leg.

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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