Op-amp overdrive with a Spectrum mid control, 2000 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 Nobels ODR-1.

A JFET input buffer, an op-amp with diodes in its feedback, a second pair of diodes to the reference, a filter, a gain stage with the Spectrum control, a low-pass gain stage, and a buffered output.

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. Clipping happens twice. The diodes across U1A hold its output to about 0.6 V either side of its input, with the clean signal riding underneath, as in the 1979 op-amp overdrive. Then 12 kΩ feeds a second pair to the reference, rounding whatever peaks are left. The Spectrum pot sits between U2A’s two inputs. Its wiper goes to 0 V through 27 nF and through a series resonant leg: 100 nF and a one-transistor inductance (about 2.7 H, R29 × R30 × C27), tuned near 300 Hz. Toward the feedback end it raises the treble and the low mids, toward the input end it lowers both. Across its range the output above 2 kHz moves about 25 dB, and the band around 200 Hz about 12 dB. Sweep Spectrum and watch the Frequency response graph to see both.

Download.

Parts and what they do

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

C201, 220 µF electrolytic
Supply filter cap.
R201, 15 kΩ
Top of the 4.5 V reference divider.
R202, 15 kΩ
Bottom of the reference divider.
C202, C204, 147 µF electrolytic
The reference’s filter caps, 100 µF and 47 µF in parallel, drawn as one.
R1, 33 kΩ
Series input resistor. With the JFET’s gate capacitance it filters radio pickup.
R2, 1 MΩ
Gate resistor to 0 V. The input impedance is about 1 MΩ.
Q1, BC264D
Input buffer, a JFET source follower. Its source sits about a pinch-off voltage above 0 V.
R3, 3.3 kΩ
Source resistor.
C1, 220 nF
Couples the buffer to the effect path. The effect-on switch Q5 that follows is assumed to be closed and not drawn.
R4, 330 kΩ
Biases the switch JFET’s channel at the reference. The board has one each side of Q5, and both are in this model.
R5, 330 kΩ
The other bias resistor at the switch.
C10, 100 nF
Coupling cap into U1A. Against R10 + R11 its corner is near 125 Hz, so the bass is trimmed before the clipping stage.
R10, 2.7 kΩ
With R11 it’s a divider that drops the level by about 2 dB. With C11 it’s a low-pass near 3.4 kHz.
R11, 10 kΩ
Biases U1A’s + input at the reference.
C11, 22 nF
Low-pass cap at U1A’s input.
U1A, JRC4558D
The clipping stage, a non-inverting amplifier with diodes in its feedback.
R12, 820 Ω
First ground leg. With C12 it adds gain above about 2.4 kHz.
C12, 82 nF
First ground leg cap.
R13, 1.5 kΩ
Second ground leg. With C13 it sets the gain above about 50 Hz.
C13, 2.2 µF electrolytic
Second ground leg cap.
Drive, 250 kΩ pot, audio taper
The feedback resistance. Lug 7 goes to the minus input and lug 9 to the output, and R14 joins lug 7 to the wiper. Turning it up puts more track between the wiper and the output, up to about 250 kΩ over 530 Ω, or 53 dB, before the diodes limit it.
R14, 1.8 kΩ
Shunts the part of the Drive track between lug 7 and the wiper, so that part never adds more than 1.8 kΩ.
C14, 120 pF
Across the feedback. It limits the top end at high Drive.
D1, D2, 1N914 / 1N4148, one each way
Two 1N4148s across the feedback, one each way. They cap the difference between U1A’s output and input at about 0.6 V.
C20, 2.2 µF electrolytic
Coupling cap out of U1A.
R20, 12 kΩ
Feeds the second diode pair. With C21 it makes a low-pass near 4.9 kHz.
C21, 2.7 nF
Low-pass cap at the second clipping node.
D3, D4, 1N914 / 1N4148, one each way
Two 1N4148s to the reference, one each way. Through 12 kΩ they soften whatever U1A’s output still has above their knee.
R21, 39 kΩ
Part of the filter between the clipping and the Spectrum stage.
C22, 82 nF
With R22 and C23, a leg to ground that cuts highs from the R21 node.
R22, 12 kΩ
Part of the leg to ground.
C23, 1 nF
Part of the leg to ground.
R23, 10 kΩ
Into U2A’s + input.
R24, 43 kΩ
Biases U2A’s + input at the reference.
C24, 22 nF
Joins the input end of the Spectrum pot to U2A’s + input, in parallel with R25.
R25, 5.1 kΩ
In parallel with C24.
Spectrum, 30 kΩ pot, linear taper
Between U2A’s feedback (lug 4, through R26) and its + input (lug 6). The wiper goes to the resonant leg. The drawing doesn’t show which end is clockwise, so the feedback end is assumed to be clockwise.
C28, 27 nF
From the Spectrum wiper to 0 V.
C26, 100 nF
The capacitor of the series resonant leg. With the gyrator’s 2.7 H it tunes near 300 Hz.
C27, 8.2 nF
Gyrator cap, into the transistor’s base.
R30, 150 kΩ
Gyrator base resistor. The simulated inductance is R29 × R30 × C27.
Q2, 2SC1815
An emitter follower that, with R29, R30 and C27, makes the node GP look like an inductor in series with R29.
R31, 3.3 kΩ
Emitter resistor. About 1.1 mA flows through it.
R29, 2.2 kΩ
Gyrator resistor. It’s also the loss of the simulated inductor.
R26, 1.2 kΩ
From U2A’s minus input to the feedback end of the Spectrum pot.
U2A, JRC4558D
The Spectrum stage, non-inverting, with a gain of 3 before the resonant leg acts.
R27, 10 kΩ
U2A’s leg to the reference.
R28, 20 kΩ
U2A’s feedback resistor.
C25, 560 pF
Across R28. Corner near 14 kHz.
R32, 4.7 kΩ
With C31 it makes a low-pass near 4.1 kHz into U2B.
C31, 8.2 nF
Low-pass cap.
U2B, JRC4558D
Output gain stage, non-inverting. Its gain falls from about 26 dB at low frequencies to about 13 dB around 1 kHz and lower above.
R35, 1.2 kΩ
U2B’s ground leg.
C32, 1 µF electrolytic
U2B’s ground-leg cap. Corner near 130 Hz.
R33, 22 kΩ
U2B’s main feedback resistor.
C33, 4.7 nF
Across R33. Corner near 1.5 kHz.
R34, 5.1 kΩ
In series with C34 across the feedback. It lowers the gain above about 380 Hz.
C34, 82 nF
In series with R34.
C30, 2.2 µF electrolytic
Coupling cap to the Level pot.
Level, 50 kΩ pot, audio taper
Output level.
C41, 1 µF electrolytic
Coupling cap from the Level wiper. The effect-on switch Q7 that follows is assumed to be closed and not drawn.
R41, 330 kΩ
Bias resistor at the switch (one each side of Q7 on the board).
R42, 330 kΩ
The other bias resistor at the switch.
C42, 1 µF electrolytic
Coupling cap into the output buffer.
R43, 150 kΩ
Biases the output buffer at the reference.
U1B, JRC4558D
Output buffer, a voltage follower. It also serves the bypass path on the board, which isn’t modeled.
C43, 3.3 µF electrolytic
Output coupling cap.
R45, 150 kΩ
Output pulldown.

About this version

As drawn
Negative ground, 9 V, with a 4.5 V reference. The drawing shows 4558S op-amps, modeled as the JRC4558D, and a 2SC2362G gyrator transistor, with the 2SC1815 standing in since the library has no 2SC2362 model.

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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