Overdrive and distortion paths with a blend knob, 1990 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 OS-2 OverDrive/Distortion.

One buffer feeds two op-amp clipping stages at once: diodes to ground after one, diodes in the feedback of the other. A Color pot blends the two, then a Tone stage and a follower drive the 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. The J201 buffer feeds two paths at once. The overdrive path is IC1.1 with a lot of gain, then two diodes to ground after R9, a filter with a high branch and a low branch, and Q2, a follower. The distortion path takes less bass through C13 and clips in IC1.2’s feedback, with two diodes one way and one the other, so it clips lopsided. The Drive knob turns a pot in each path together. The Color pot’s wiper sits on the reference, which C22 makes a ground for audio. Turned to one end it shorts one path, and in between both come through. IC2.1 sums them through R31 and R21 and adds gain, IC2.2 carries the Tone control, and Q3 drives the output.

Download.

Parts and what they do

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

C100, 100 µF electrolytic
Across the 9 V supply.
R100, 10 kΩ
Top of the reference divider.
R101, 10 kΩ
Bottom of the reference divider.
C101, 100 µF electrolytic
Filter cap on the reference.
R1, 10 kΩ
Series resistor at the input.
C1, 47 nF
Input coupling cap.
R2, 1 MΩ
Biases the gate at the reference. The input impedance is about 1 MΩ.
Q1, J201
Input buffer, a JFET source follower (J201). It feeds both paths.
R3, 10 kΩ
Source resistor.
C2, 22 nF
Couples the buffer to the overdrive path. With R4 it cuts below about 33 Hz.
R4, 220 kΩ
Biases IC1.1’s + input at the reference.
IC1.1, JRC4558D
The overdrive path’s gain stage, a non-inverting amplifier with a lot of gain. It clips at its own rails when pushed.
R5, 4.7 MΩ
Fixed feedback resistor. With the Drive pot at 0 it still leaves a lot of gain.
C3, 100 pF
Rolls off the gain at high frequencies.
R7, 100 Ω
With C4, the main leg to ground. Its 100 Ω sets the gain above about 3.4 kHz.
C4, 470 nF
In series with R7. The gain falls below the frequency it sets with R7.
R6, 1.2 kΩ
With C5, a second leg to ground that holds up the lower mids.
C5, 2.2 µF electrolytic
In series with R6. With it the gain holds up down to about 60 Hz.
R8, 12 kΩ
In series with the Drive pot in the feedback, so the gain never falls too far.
Drive, 250 kΩ pot, audio taper
Drive, one half of a dual 250 kΩ audio pot, as a variable resistor in the overdrive path’s feedback beside R5. Turned up, more resistance and more gain.
C6, 10 µF electrolytic
Keeps DC off the Drive pot.
C7, 10 µF electrolytic
Couples IC1.1 to the diodes.
R9, 1 kΩ
Feeds the diodes. The op-amp drives the diodes through it.
D1, D2, 1N914 / 1N4148, one each way
Two 1SS133 diodes to ground, one each way. They hold the overdrive path near ±0.6 V.
C8, 18 nF
The upper branch of the filter after the diodes passes the highs through it.
R10, 2.2 kΩ
In series with C8.
R11, 6.8 kΩ
Divides the upper branch down.
R12, 12 kΩ
Joins the upper branch to the output of the filter.
R14, 6.8 kΩ
The lower branch: with C9 a low-pass for the lows and mids.
C9, 100 nF
With R14, rolls off the lower branch above about 230 Hz.
R13, 8.2 kΩ
Joins the lower branch to the output of the filter.
C10, 1 µF electrolytic
Couples the filter to Q2.
R15, 1 MΩ
Biases Q2’s base at the reference.
Q2, 2N5088
Emitter follower (2N5088) for the overdrive path.
R16, 10 kΩ
Q2 emitter resistor.
C11, 10 µF electrolytic
Couples Q2 to the blend.
R17, 20 kΩ
With C12, a last treble cut on the overdrive path, and the resistance the Color pot works against.
C12, 820 pF
With R17, cuts above about 9.7 kHz.
C13, 4.7 nF
Couples the buffer to the distortion path. Small: with R18 it cuts below about 340 Hz, so the lows don’t mush the clipping.
R18, 100 kΩ
Biases IC1.2’s + input at the reference.
IC1.2, JRC4558D
The distortion path’s gain stage, with the diodes in its feedback.
D3, D4, D5, 1N914 / 1N4148 one way, 2 × 1N914 / 1N4148 the other way
Three 1SS133 diodes in the feedback, D3 and D4 in series one way and D5 the other way, so the clipping is lopsided: about 0.6 V one way and 1.2 V the other.
C14, 100 pF
Rolls off the gain at high frequencies.
R32, 100 Ω
With C23, the leg to ground. The gain is the feedback resistance over 100 Ω, so it is very high.
C23, 4.7 µF electrolytic
In series with R32.
R19, 1 kΩ
In series with the Drive pot in the feedback.
Drive (B), 250 kΩ pot, audio taper
The other half of the dual Drive pot, turned by the same knob, as a variable resistor in the distortion path’s feedback.
C15, 10 µF electrolytic
Couples IC1.2 to the blend.
R20, 20 kΩ
With C16, a treble cut on the distortion path, and the resistance the Color pot works against.
C16, 10 nF
With R20, cuts above about 800 Hz. It softens the distortion path’s fizz.
Color, 20 kΩ pot, linear taper
Color, 20 kΩ linear, wiper on the reference (C22 makes it ground for audio). At 0 the wiper grounds the distortion path and only the overdrive is left. At 10 it grounds the overdrive path. In between both come through. Direction assumed.
C22, 47 µF electrolytic
Makes the Color pot’s wiper a ground for audio.
R21, 100 kΩ
Mixes the distortion path into IC2.1.
R31, 33 kΩ
Mixes the overdrive path into IC2.1.
IC2.1, JRC4558D
Makes up the level after the mix, a non-inverting gain of about 7.8.
R22, 150 kΩ
IC2.1 feedback resistor.
R23, 22 kΩ
IC2.1 − input to the reference. With R22 it sets the gain.
R24, 10 kΩ
Feeds the Tone stage.
Tone, 20 kΩ pot, linear taper
Tone, 20 kΩ (marked G, assumed linear), between IC2.2’s two inputs, its wiper to ground through C21 and R30. Toward 10 the treble path grounds the − input and IC2.2 boosts the highs. Toward 0 it grounds the + input and cuts them.
C21, 10 nF
The Tone wiper’s cap to ground.
R30, 4.7 kΩ
Limits how far the Tone control goes either way.
IC2.2, JRC4558D
The Tone stage. With the knob at noon its gain is about 1.
R25, 10 kΩ
IC2.2 feedback resistor.
C17, 1 nF
Rolls off IC2.2’s gain above about 16 kHz.
C18, 10 µF electrolytic
Couples the Tone stage to the Level pot.
Level, 50 kΩ pot, audio taper
Level, 50 kΩ audio, a divider to ground.
C19, 1 µF electrolytic
Couples the Level wiper to Q3.
R28, 1 MΩ
Biases Q3’s base at the reference.
Q3, 2N5088
Output buffer, an emitter follower (2N5088).
R26, 10 kΩ
Q3 emitter resistor.
C20, 1 µF electrolytic
Output coupling cap.
R27, 100 kΩ
Output pulldown.
R29, 1 kΩ
Series resistor at the output jack.

About this version

Stock
Negative ground, 9 V. J201 buffer, JRC4558 op-amps, 2N5088 followers, as in the kit 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.

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