Two discrete op-amp distortion with a split filter, 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 MZ-2 Digital Metalizer.

The analog drive inside a digital multi-effect: a JFET buffer, a discrete op-amp, a mid path and a full-range path mixed into a second discrete op-amp with the Drive pot in its feedback, then Tone and Level.

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 first discrete op-amp has a fixed gain of about 58. Its output splits. One path goes through a two-pole low-pass near 1.6 kHz built around Q5, then C9 and R14, which cut its bass, so it adds mids. The other goes through Q6 at full range. Both meet at the second stage’s minus input through 4.7 kΩ and 6.8 kΩ. That stage is inverting, its gain the Drive pot and 10 kΩ over those resistors, up to about 50, and it clips at its 5 V rail. Tone is a 10 kΩ pot between a 10 nF from the signal and a 10 nF to ground. The pedal’s chorus and delay, and the op-amp that mixes them in after Level, aren’t on the bench.

Download.

Parts and what they do

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

R28, 2.2 kΩ
Feeds Q12’s base. With C19 it filters the supply.
C19, 47 µF electrolytic
Holds Q12’s base steady.
Q12, 2SC1815
An emitter follower making VA, a filtered 9 V minus about 0.7 V. A 2SC2458-GR in the pedal. The 2SC1815 stands in.
C20, 47 µF electrolytic
Filter cap on VA.
R30, 4.7 kΩ
Top of the VB divider.
R31, 4.7 kΩ
Bottom of the VB divider.
C23, 47 µF electrolytic
Filters VB.
R29, 2.2 kΩ
Feeds the zener.
Z1, 5.6 V zener, in breakdown
A 5.6 V zener, 1N5232B. It holds Q13’s base steady.
C21, 47 µF electrolytic
Across the zener, against its noise.
Q13, 2SC1815
An emitter follower making VC, about 5 V, the two discrete op-amps’ supply. A 2SC2458-GR in the pedal. The 2SC1815 stands in.
C22, 47 µF electrolytic
Filter cap on VC.
R32, 4.7 kΩ
Top of the VD divider.
R33, 4.7 kΩ
Bottom of the VD divider.
C24, 47 µF electrolytic
Filters VD, the discrete op-amps’ bias.
R34, 10 kΩ
Input series resistor.
C1, 22 nF
Input coupling cap.
R1, 1 MΩ
Biases Q1’s gate at VB and sets the input impedance near 1 MΩ.
Q1, 2SK117-GR
Input buffer, a source follower.
R2, 4.7 kΩ
Source resistor.
C2, 3.9 nF
Coupling cap into the first stage. Against R3 it cuts below about 87 Hz.
R3, 470 kΩ
Biases the first stage’s plus input at VD.
Q2, 2SK117-GR
Plus input of the first discrete op-amp, one of a JFET pair.
Q3, 2SK117-GR
Minus input of the first discrete op-amp.
R5, 4.7 kΩ
Shared source resistor of the pair.
R4, 2.2 kΩ
Q2’s drain load. The drop across it drives Q4.
Q4, 2SA970-GR
The first stage’s output transistor, a 2SA970-GR PNP.
R8, 2.2 kΩ
Q4’s collector load.
R7, 270 kΩ
The first stage’s feedback. Gain is 1 + 270 kΩ / 4.7 kΩ, about 58.
C4, 100 pF
Across R7. It rolls the top off from about 5.9 kHz.
R6, 4.7 kΩ
The first stage’s gain leg.
C3, 100 nF
In series with R6, so the gain falls below about 340 Hz.
C5, 10 nF
Coupling cap into the mid path.
R9, 470 kΩ
Biases Q5’s gate at VB through R10 and R11.
R10, 33 kΩ
First resistor of the low-pass.
R11, 33 kΩ
Second resistor of the low-pass.
C6, 22 nF
From between R10 and R11 to Q5’s source. With C7 it makes a two-pole low-pass near 1.6 kHz.
C7, 1 nF
From Q5’s gate to ground, the low-pass’s second cap.
Q5, 2SK117-GR
A source follower, the low-pass’s buffer.
R12, 4.7 kΩ
Q5’s source resistor.
C8, 18 nF
Coupling cap after Q5.
R13, 470 kΩ
Holds the point between C8 and C9 at VB.
C9, 68 nF
Coupling cap to the mixing resistor. Against R14 it cuts the mid path below about 500 Hz.
R14, 4.7 kΩ
The mid path’s mixing resistor, the second stage’s input resistor for it.
C10, 4.7 nF
Into Q6. Against R15 it passes from about 72 Hz.
R15, 470 kΩ
Biases Q6 at VB.
Q6, 2SC1815
An emitter follower for the full-range path. A 2SC2240-GR in the pedal. The 2SC1815 stands in.
R16, 4.7 kΩ
Q6’s emitter resistor.
C11, 1 µF
Coupling cap from Q6.
R17, 6.8 kΩ
The full-range path’s mixing resistor.
Q8, 2SK117-GR
Minus input of the second discrete op-amp, where both paths meet.
Q7, 2SK117-GR
Plus input of the second discrete op-amp, held at VD.
R19, 4.7 kΩ
Shared source resistor of the pair.
R18, 2.2 kΩ
Q7’s drain load. The drop across it drives Q9.
Q9, 2SA970-GR
The second stage’s output transistor, a 2SA970-GR PNP. Its swing runs out between ground and the 5 V rail, which is where the pedal clips.
R21, 2.2 kΩ
Q9’s collector load.
C12, 100 pF
Across the feedback. It rolls the top off, more as Drive goes up.
R20, 10 kΩ
Least feedback resistance, so the gain never drops below about 2.
Drive, 250 kΩ pot, audio taper
Drive, a 250 kΩ audio pot wired as a variable resistor in the second stage’s feedback.
R22, 47 kΩ
With C13 a treble cut from about 340 Hz, shelved by R23.
C13, 10 nF
The treble cut’s cap, in series with R23 so the cut stops at about a quarter.
R23, 15 kΩ
Sets where the treble cut levels off.
C14, 47 nF
Coupling cap into Q10.
R24, 470 kΩ
Biases Q10 at VB.
Q10, 2SC1815
An emitter follower driving the tone and level pots. A 2SC2240-GR in the pedal. The 2SC1815 stands in.
R25, 4.7 kΩ
Q10’s emitter resistor.
C15, 10 nF
From Q10 to the top of Tone.
Tone, 10 kΩ pot, linear taper
Tone, 10 kΩ linear. Toward C15 the treble passes, toward C16 it goes to ground.
C16, 10 nF
From the bottom of Tone to ground.
Level, 100 kΩ pot, audio taper
Level, 100 kΩ audio. Its wiper is the output here. In the pedal it goes on to the op-amp that mixes in the digital effects.

About this version

MZ-2
The analog section, values from Aion FX’s clone. The Level wiper is the output, since the pedal’s own mixer and buffer aren’t on the drawing.

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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