Two-stage op-amp distortion with gyrator Low and High controls, 1993 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 HM-3 Hyper Metal.

A JFET buffer, an op-amp boost with the Dist pot, a second op-amp with lopsided diodes in its feedback, diodes to ground, then Low and High controls built from transistor gyrators, and a transistor 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. IC1.1’s gain is 1 plus R6 and Dist over 220 Ω, up to about 230. IC1.2 has a gain of about 11 until its diodes conduct: one on one side, two on the other, so the halves clip unevenly and even harmonics come in. D4 and D5 then clip hard to ground. Low lifts or cuts a band near 87 Hz. High works two bands at once, near 890 Hz and 5.4 kHz. Each gyrator, a transistor with three parts around it, acts as a coil. The model leaves out the bypass switching.

Download.

Parts and what they do

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

R101, 10 kΩ
Top of the 4.5 V reference divider.
R102, 10 kΩ
Bottom of the reference divider.
C101, 100 µF electrolytic
Filters the reference. The gyrators’ resistors return to it.
R1, 10 kΩ
Input series resistor.
C1, 47 nF
Input coupling cap.
R2, 1 MΩ
Biases Q1’s gate at 4.5 V and sets the input impedance near 1 MΩ.
Q1, 2N5457
Input buffer, a source follower.
R3, 10 kΩ
Source resistor.
R4, 4.7 kΩ
With C2, a treble cut from about 5 kHz.
C2, 6.8 nF
The treble cut’s cap.
C3, 47 nF
Coupling cap into IC1.1. Against R5 it cuts below about 72 Hz.
R5, 47 kΩ
Biases IC1.1’s plus input at 4.5 V.
IC1.1, JRC4558D
First gain stage, non-inverting, with the Dist pot in its feedback.
Dist, 50 kΩ pot, audio taper
Dist, a 50 kΩ audio pot wired as a variable resistor in IC1.1’s feedback. Gain is 1 + (R6 + Dist) / 220 Ω.
R6, 220 Ω
In series with Dist, so with Dist at 0 the gain is 2.
C4, 47 pF
Across the feedback. With Dist up it rolls the top off above a few kHz.
R7, 220 Ω
The gain leg.
C5, 10 µF electrolytic
In series with R7, so the gain falls below about 72 Hz and is 1 at DC.
R8, 10 kΩ
With C6, a treble cut from about 1.6 kHz between the stages.
C6, 10 nF
The treble cut’s cap.
IC1.2, JRC4558D
Second gain stage, non-inverting, with diodes across its feedback.
R9, 220 kΩ
Feedback resistor. With R10 the gain is about 11 until the diodes conduct.
C7, 100 pF
Across the feedback, against oscillation.
D1, D2, D3, 1N914 / 1N4148 one way, 2 × 1N914 / 1N4148 the other way
1N4148s across the feedback: D1 one way, D2 and D3 in series the other, so the two halves of the wave clip at different levels.
R10, 22 kΩ
The gain leg.
C8, 47 nF
In series with R10, so the gain falls below about 150 Hz.
C9, 1 µF electrolytic
Coupling cap to the diodes.
R11, 2.2 kΩ
Feeds the diodes to ground.
D4, D5, 1N914 / 1N4148, one each way
Two 1N4148s back to back to ground. They hold the signal to about 0.6 V either way.
R12, 10 kΩ
With C10 across it and R13, C11 below, a gentle tilt.
C10, 10 nF
Across R12, so the treble passes it.
R13, 5.6 kΩ
To ground through C11.
C11, 47 nF
With R13, pulls down the mids and treble at the point after R12.
C12, 1 µF electrolytic
Coupling cap into IC2.1.
R14, 1 MΩ
Biases IC2.1 at 4.5 V.
IC2.1, JRC4558D
A follower, so the EQ stage sees a low impedance.
R15, 3.3 kΩ
Into the EQ stage’s plus input. The EQ legs pull against it.
IC2.2, JRC4558D
The EQ stage. With Low and High at the middle its gain is about 1.
R16, 3.3 kΩ
Feedback, equal to R15.
C13, 10 nF
Across R16, a treble roll-off from about 4.8 kHz.
Low, 10 kΩ pot, linear taper
Low, 10 kΩ linear, from the plus input to the minus input. Toward the minus input it lifts the band near 87 Hz, toward the plus input it cuts it.
C14B, 470 nF
In parallel with C14.
High, 10 kΩ pot, linear taper
High, 10 kΩ linear. Its wiper carries two gyrators, so it lifts or cuts bands near 890 Hz and 5.4 kHz together.
C14, 1 µF
With C14B, in series with the Low gyrator, which acts as a coil of about 2.2 H. Together they tune the Low band near 87 Hz.
C15, 68 nF
The gyrator’s cap.
R17, 100 kΩ
Biases the gyrator’s transistor from the reference. With its cap and series resistor it sets the coil the gyrator acts like.
Q2, 2N3904
The gyrator’s follower.
R18, 330 Ω
The gyrator’s series resistor. It sets how broad the band is.
R19, 10 kΩ
The follower’s emitter resistor.
C16, 39 nF
In series with the first High gyrator, a coil of about 0.82 H, tuning a band near 890 Hz.
C17, 8.2 nF
The gyrator’s cap.
R20, 100 kΩ
Biases the gyrator’s transistor from the reference. With its cap and series resistor it sets the coil the gyrator acts like.
Q3, 2N3904
The gyrator’s follower.
R21, 1 kΩ
The gyrator’s series resistor. It sets how broad the band is.
R22, 10 kΩ
The follower’s emitter resistor.
C18, 22 nF
In series with the second High gyrator, a coil of about 40 mH, tuning a band near 5.4 kHz.
C19, 1.2 nF
The gyrator’s cap.
R23, 100 kΩ
Biases the gyrator’s transistor from the reference. With its cap and series resistor it sets the coil the gyrator acts like.
Q4, 2N3904
The gyrator’s follower.
R24, 330 Ω
The gyrator’s series resistor. It sets how broad the band is.
R25, 10 kΩ
The follower’s emitter resistor.
C20, 10 µF electrolytic
Coupling cap to Level.
Level, 50 kΩ pot, audio taper
Level, 50 kΩ audio.
C21, 1 µF electrolytic
Coupling cap into the output buffer.
R26, 100 kΩ
Biases the output buffer at 4.5 V.
Q5, 2N3904
Output buffer, an emitter follower.
R27, 10 kΩ
Output buffer emitter resistor.
C22, 10 µF electrolytic
Output coupling cap.
R28, 100 kΩ
Output pulldown.
R29, 1 kΩ
Output series resistor.

About this version

HM-3
Values from PedalPCB’s clone. 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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