Two-transistor, op-amp distortion with a two-band resonant EQ, 1983 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-2 Heavy Metal.

A JFET buffer, two transistor gain stages and an op-amp with lopsided diodes in its feedback. Then diodes in series with the signal, silicon diodes to ground, and an EQ stage whose Low and High controls boost or cut three tuned branches near 90 Hz, 960 Hz and 1.3 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. Three gain stages in a row, so even the lowest Dist setting clips on a hard pick. The Dist pot works on two stages at once: one side sets the op-amp’s gain, the other loads the first transistor. The EQ’s tuned branches are gyrators, op-amps that act like inductors, each in series with a cap. With both EQ controls up, the low branch lifts the bass near 90 Hz and the two high branches lift a band around 1 kHz, while the frequencies between stay put. Probe the output and turn Low and High full up to see the HM-2’s scooped shape pushed to the extreme.

Download.

Parts and what they do

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

R10, 1 kΩ
Feeds the two gain transistors, Q6 and Q7, from the battery. With the 47 µF it filters their supply. The op-amps and the other transistors run straight off the battery rail.
C5, 47 µF electrolytic
Filter cap on the supply for Q6 and Q7.
R6, 10 kΩ
Top of the 4.5 V bias divider.
R5, 10 kΩ
Bottom of the bias divider.
C25, 47 µF electrolytic
Holds the 4.5 V bias steady.
R1, 10 kΩ
Input resistor.
C1, 47 nF
Input coupling cap.
R8, 1 MΩ
Gate bias to 4.5 V. It sets the input impedance near 1 MΩ.
Q1, 2SK30A-GR
Input buffer, a 2SK30A JFET as a source follower.
R7, 10 kΩ
Source resistor.
C4, 1 µF electrolytic
Coupling cap.
R11, 1 MΩ
Bias on one side of the JFET bypass switch (closed with the effect on, drawn as a wire).
R15, 1 MΩ
Bias on the other side of the bypass switch.
C6, 47 nF
Coupling cap into the first gain stage.
R19, 22 kΩ
Series input resistor of the first gain stage.
R16, 470 kΩ
Collector-to-base feedback. It sets Q6’s bias and, with the 22 kΩ in front, limits its gain.
C10, 100 pF
Across the feedback resistor. It rolls off the treble above about 3.4 kHz.
R21, 100 kΩ
Base to ground, the other half of Q6’s bias.
Q6, 2SC2240-GR
First gain stage, a 2SC2240 in common emitter.
R18, 22 Ω
Emitter resistor, 22 Ω.
R17, 10 kΩ
Collector load.
C11, 47 nF
Coupling cap.
R22, 22 kΩ
Series input resistor of the second gain stage.
R26, 100 kΩ
Base bias to the supply.
R29, 470 kΩ
Collector-to-base feedback. It sets the bias, and with the 22 kΩ in front it limits the gain to about 20.
C14, 100 pF
Across the feedback resistor. It rolls off the treble above about 3.4 kHz.
Q7, 2SA970-GR
Second gain stage, a 2SA970 PNP in common emitter.
R28, 120 Ω
Emitter resistor, 120 Ω.
R27, 10 kΩ
Collector load to ground.
R25, 68 kΩ
From the 9 V supply. With the collector load it sets the op-amp input DC level.
IC1b, M5216 (4558-class values)
Third gain stage, non-inverting, with diodes in its feedback.
D3, D4, D5, 1N914 / 1N4148 one way, 2 × 1N914 / 1N4148 the other way
Diodes across the feedback: D3 one way, D4 and D5 in series the other, so the clipping is lopsided. Their type isn’t marked on the sheet, so silicon is assumed.
C9, 100 pF
Across the feedback. The op-amp gain rolls off above about 7 kHz.
R20, 220 kΩ
Feedback resistor.
C31, 10 µF electrolytic
Couples Q6’s collector to the Dist pot’s low end.
R49, 150 Ω
In series with the low end of the Dist pot.
Dist, 250 kΩ pot, audio taper
Distortion, 250 kΩ (VR4, marked D, modeled as audio), wiper grounded. One side is the op-amp’s gain-setting resistor to ground. The other loads Q6’s collector. Turning it up raises the op-amp’s gain and lightens the load on Q6, so both stages have more gain.
R42, 47 kΩ
Sets the op-amp’s maximum gain: 1 + 220 k / 47 k, about 5.7.
C22, 47 nF
With the 47 kΩ, the op-amp’s gain falls below about 70 Hz.
C12, 1 µF electrolytic
Coupling cap.
R23, 10 kΩ
Series resistor.
D6, D7, 1N914 / 1N4148, one each way
Two diodes back to back, in series with the signal. Signals below their turn-on don’t get through, so they cut off quiet decays and add crossover distortion. Their type isn’t marked on the factory sheet. Silicon is assumed, though a hobbyist redraw shows them as germanium.
R30, 10 kΩ
Series resistor into the diodes to ground.
D8, D9, 1N914 / 1N4148, one each way
Two diodes to ground, one each way, clipping evenly at about ±0.6 V. Their type isn’t marked on the sheet, so silicon is assumed.
C16, 1 nF
With the 10 kΩ before it, a low-pass near 16 kHz.
C15, 1 µF electrolytic
Coupling cap to the buffer.
R24, 68 kΩ
Biases the buffer at 4.5 V.
IC1a, M5216 (4558-class values)
Buffer (voltage follower) into the EQ.
R54, 3.3 kΩ
Input resistor of the EQ stage.
IC3a, M5216 (4558-class values)
EQ stage, non-inverting, with unity gain when both EQ pots are centered.
R52, 3.3 kΩ
Feedback resistor of the EQ stage.
C33, 470 pF
Across the feedback resistor.
High, 10 kΩ pot, linear taper
High, 10 kΩ (marked G, modeled as linear, flat at center). It slides two tuned branches, near 960 Hz and 1.3 kHz, between the op-amp’s two inputs. Toward the minus input it boosts them, toward the plus input it cuts.
Low, 10 kΩ pot, linear taper
Low, 10 kΩ (marked G, modeled as linear, flat at center). It works the same way with one tuned branch near 90 Hz.
C35, 1.5 µF
Series cap of the tuned branch.
R47, 330 Ω
Sets the branch’s resistance at resonance, and so how sharp the boost or cut is.
IC3b, M5216 (4558-class values)
Op-amp follower of a gyrator: with its resistor and cap it acts like an inductor, so the branch resonates like a series LC. In the low branch it’s about 2.2 H with 1.5 µF, resonant near 87 Hz.
C30, 68 nF
Gyrator cap.
R51, 100 kΩ
Gyrator resistor, from the 4.5 V bias.
C29, 100 nF
Series cap of the tuned branch.
R45, 330 Ω
Sets the branch’s resistance at resonance, and so how sharp the boost or cut is.
IC2b, M5216 (4558-class values)
Op-amp follower of a gyrator: with its resistor and cap it acts like an inductor, so the branch resonates like a series LC. In this high branch it’s about 0.16 H with 0.1 µF, resonant near 1.3 kHz.
C26, 4.7 nF
Gyrator cap.
R44, 100 kΩ
Gyrator resistor, from the 4.5 V bias.
C28, 150 nF
Series cap of the tuned branch.
R41, 330 Ω
Sets the branch’s resistance at resonance, and so how sharp the boost or cut is.
IC2a, M5216 (4558-class values)
Op-amp follower of a gyrator: with its resistor and cap it acts like an inductor, so the branch resonates like a series LC. In this high branch it’s about 0.18 H with 0.15 µF, resonant near 960 Hz.
C27, 6.8 nF
Gyrator cap.
R43, 82 kΩ
Gyrator resistor, from the 4.5 V bias.
C34, 10 µF electrolytic
Output coupling cap.
R53, 10 kΩ
Output resistor.
Level, 10 kΩ pot, audio taper
Level, 10 kΩ audio (VR1). With R53 10 kΩ in series in front of it, the most it passes is about half of the EQ stage’s output.
C32, 1 µF electrolytic
Coupling cap to the output bypass switch.
R50, 1 MΩ
Bias on one side of the output switch (closed with the effect on, drawn as a wire).
R48, 1 MΩ
Bias on the other side of the output switch.
C7, 47 nF
Coupling cap to the output transistor.
R12, 470 kΩ
Base bias of the output follower.
Q3, 2SC2240-GR
Output emitter follower.
R13, 10 kΩ
Emitter resistor.
R4, 1 kΩ
Output resistor.
C11, 1 µF electrolytic
Output coupling cap.
R3, 100 kΩ
Output pull-down.

About this version

Factory values
The transistor and JFET models are typical parts, not measured ones. The op-amps use 4558-class values in place of the M5216.

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.

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