Op-amp distortion with resonant emphasis and a swept middle control, 1991 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 MT-2 Metal Zone.

A JFET buffer, a first op-amp stage with the most gain near 950 Hz, a gain stage of up to about 200, two silicon diodes to ground, a stage that lifts about 105 Hz and 4.9 kHz, and a three-band EQ whose middle band sweeps from about 240 Hz to 5 kHz.

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. The tuned branches in the ground legs of the first and third op-amp stages are gyrators: a transistor or op-amp follower with a resistor and cap acting as an inductor, in series with a cap. The first stage’s peak near 950 Hz sets which frequencies hit the diodes hardest. The third stage’s peaks near 105 Hz and 4.9 kHz put back the lows and upper presence after clipping. Probe the first op-amp’s output with the frequency-response panel to see the pre-emphasis, then sweep Freq with Middle up or down to see the parametric band move.

Download.

Parts and what they do

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

R056, 10 kΩ
Top of the half-supply divider.
R057, 10 kΩ
Bottom of the half-supply divider.
C040, 100 µF electrolytic
Holds the half-supply reference steady.
R059, 10 kΩ
Input resistor.
C042, 47 nF
Input coupling cap.
R058, 1 MΩ
Gate bias to half supply. It sets the input impedance near 1 MΩ.
Q011, 2SK30A-GR
Input buffer, a 2SK184-GR JFET source follower, with 2SK30A values standing in.
R060, 10 kΩ
Source resistor.
C039, 1 µF
Coupling cap. The JFET bypass switch that follows is closed with the effect on, drawn here as a wire.
R052, 100 kΩ
Bias for the switch and the next stage.
R047, 1 MΩ
Bias on the far side of the JFET switch.
C033, 15 nF
Coupling cap into the first op-amp. With R6 it rolls the bass off below about 100 Hz.
R043, 100 kΩ
Bias to half supply.
IC3b, M5218 (4558-class values)
First gain stage, non-inverting. Its ground leg is the pre-emphasis branch above it, a series resonance near 950 Hz, so its gain is highest there.
C032, 100 pF
Across the feedback resistor. It rolls off the treble.
R044, 220 kΩ
Feedback resistor. At the resonance the branch below is about 2.2 kΩ, so the gain there is about 100.
C034, 27 nF
Series cap of the 950 Hz branch.
C035, 10 nF
Feeds the 950 Hz branch’s transistor base.
R053, 47 kΩ
Base bias of the 950 Hz branch’s transistor.
R046, 2.2 kΩ
Sets the 950 Hz branch’s resistance at resonance, and so the height of the peak.
Q010, 2SC1815
Emitter follower of the 950 Hz branch. With its cap and resistors it acts as an inductor (about 1 H), so the branch resonates like a series LC.
R054, 10 kΩ
Emitter resistor of the 950 Hz branch.
R045, 10 kΩ
Series resistor.
R042, 10 kΩ
With R8, halves the signal.
C031, 47 nF
With R8 and R9, a low-pass near 680 Hz.
C029, 33 nF
Coupling cap into the gain stage.
R040, 100 kΩ
Bias to half supply.
IC3a, M5218 (4558-class values)
Main gain stage, non-inverting, with a gain of 1 + (1 k + Dist) / 1 k, up to about 200.
R041, 1 kΩ
Ground leg of the gain stage, with C7.
C030, 10 µF electrolytic
Ground leg cap. The gain falls toward 1 below about 16 Hz.
R051, 1 kΩ
Fixed part of the feedback resistance.
VR01, 250 kΩ pot, audio taper
Dist, 250 kΩ audio (A, as marked), as a variable feedback resistor.
C028, 47 pF
Across the feedback. At full Dist it rolls the gain off above about 17 kHz.
C027, 10 µF electrolytic
Coupling cap.
R033, 2.2 kΩ
Series resistor into the diodes.
D003, D004, 1N914 / 1N4148, one each way
Two 1SS133 silicon diodes to ground, one each way, for symmetric clipping at about ±0.6 V.
R032, 10 kΩ
Series resistor after the diodes.
R031, 4.7 kΩ
De-emphasis. With C10 it makes a gentle treble cut.
C023, 15 nF
De-emphasis cap.
C021, 1 µF electrolytic
Coupling cap.
R029, 100 kΩ
Bias to half supply.
IC4b, M5218 (4558-class values)
Post-clipping stage, non-inverting. Its ground leg is two tuned branches, near 105 Hz and 4.9 kHz, so it lifts the lows and the upper presence region.
R030, 3.3 kΩ
Feedback resistor. With the 470 Ω and 1 kΩ branch resistors it sets a gain of about 8 at 105 Hz and 4 at 4.9 kHz.
C022, 47 pF
Across the feedback resistor.
C024, 15 nF
Series cap of the 4.9 kHz branch.
C025, 1.5 nF
Feeds the 4.9 kHz branch’s transistor base.
R036, 47 kΩ
Base bias of the 4.9 kHz branch’s transistor.
R034, 1 kΩ
Sets the 4.9 kHz branch’s resistance at resonance, and so the height of the peak.
Q008, 2SC1815
Emitter follower of the 4.9 kHz branch. With its cap and resistors it acts as an inductor (about 70 mH), so the branch resonates like a series LC.
R037, 10 kΩ
Emitter resistor of the 4.9 kHz branch.
C020, 220 nF electrolytic
Series cap of the 105 Hz branch.
C017, 47 nF
Feeds the 105 Hz branch’s transistor base.
R025, 470 kΩ
Base bias of the 105 Hz branch’s transistor.
R027, 470 Ω
Sets the 105 Hz branch’s resistance at resonance, and so the height of the peak.
Q007, 2SC1815
Emitter follower of the 105 Hz branch. With its cap and resistors it acts as an inductor (about 10 H), so the branch resonates like a series LC.
R024, 10 kΩ
Emitter resistor of the 105 Hz branch.
R028, 22 kΩ
Input resistor of the inverting buffer.
IC4a, M5218 (4558-class values)
Inverting buffer, gain 1.
R026, 22 kΩ
Feedback resistor: gain 22 k / 22 k = 1.
C018, 10 pF
Across the feedback resistor.
R014, 22 kΩ
Input resistor of the bass and treble stage.
IC1a, M5218 (4558-class values)
Bass and treble stage, non-inverting, unity gain with both controls centered.
Low, 100 kΩ pot, linear taper
Low, 100 kΩ (marked G, modeled as linear, flat at noon). Its wiper feeds a branch tuned near 105 Hz. Toward the minus input it boosts, toward the plus input it cuts. The direction is assumed.
High, 100 kΩ pot, linear taper
High, 100 kΩ (marked G, modeled as linear, flat at noon). It works the same way with a 10 nF and 2.2 kΩ branch, a shelf above about 7 kHz. The direction is assumed.
R015, 22 kΩ
Feedback resistor.
C010, 10 pF
Across the feedback resistor.
C044, 10 nF
High branch cap.
R061, 2.2 kΩ
High branch resistor.
C008, 220 nF
Series cap of the low branch.
R012, 2.2 kΩ
Sets the low branch’s resistance at resonance.
IC1b, M5218 (4558-class values)
Follower of the low-branch gyrator. With 2.2 kΩ, 47 nF and 100 kΩ it acts as about 10 H, resonating with 220 nF near 105 Hz.
C009, 47 nF
Gyrator cap.
R013, 100 kΩ
Gyrator resistor, from half supply.
C011, 1 µF
Coupling cap to the middle stage.
R038, 47 kΩ
Input resistor of the middle stage.
IC2a, M5218 (4558-class values)
Middle stage. It inverts at unity gain for the direct signal, while its plus input adds the output of the swept band-pass network below. Depending on the Middle control the band is added or subtracted.
R035, 47 kΩ
Feedback resistor.
C026, 100 pF
Across the feedback resistor.
R050, 330 Ω
From the stage input to one end of the Middle pot.
C037, 1 µF electrolytic
From the stage output to the other end of the Middle pot.
R049, 330 Ω
Series resistor at the output end of the Middle pot.
Middle, 100 kΩ pot, linear taper
Middle, 100 kΩ (marked G, modeled as linear, flat at noon). It sets what drives the band-pass network, the stage’s input (cut) or its output (boost). The direction is assumed.
IC2b, M5218 (4558-class values)
Buffer driving the band-pass network.
C036, 22 nF
Band-pass network: series cap.
R048, 2.2 kΩ
Band-pass network: minimum series resistance.
Freq (VR02a), 50 kΩ pot, reverse audio taper
Freq, one half of a dual 50 kΩ pot (reverse audio, C, as marked), as a variable resistor in series.
Freq (VR02a), 50 kΩ pot, reverse audio taper
Freq, the other half, as a variable resistor to ground. Both halves turn together: turning Freq up lowers both resistances and moves the band from about 240 Hz up to about 5 kHz.
R062, 2.2 kΩ
Band-pass network: minimum resistance to ground.
C043, 8.2 nF
Shunt cap in the band-pass network.
C038, 100 nF
Couples the band-pass output to the middle stage’s plus input.
R039, 1 MΩ
Bias to half supply.
C005, 10 µF electrolytic
Coupling cap to the Level pot.
Level, 50 kΩ pot, audio taper
Level, 50 kΩ audio.
C004, 1 µF electrolytic
Coupling cap to the output switch.
R006, 1 MΩ
Bias on one side of the output JFET switch (closed with the effect on, drawn as a wire).
R005, 1 MΩ
Bias on the other side of the output switch.
C002, 10 µF electrolytic
Coupling cap to the output transistor.
R004, 100 kΩ
Base bias to half supply.
Q001, 2SC1815
Output emitter follower.
R002, 10 kΩ
Emitter resistor.
C001, 10 µF electrolytic
Output coupling cap.
R003, 100 kΩ
Output pull-down.
R001, 1 kΩ
Output resistor.

About this version

Factory values
The transistors use 2SC1815 values in place of the 2SC3378-GR, the op-amps 4558-class values for the M5218, and the input JFET 2SK30A values. Typical parts, not measured.

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, not measured parts. 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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