Two discrete op-amp overdrive, 1985 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 OD-2 Turbo OverDrive.

A JFET input buffer and two discrete op-amps, each a JFET pair driving a PNP transistor, on a 5.6 V rail of their own. The second stage clips at its rails. Then a passive tone control and Volume.

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. Each discrete op-amp works like an op-amp with a gain set by its feedback, but it runs out of swing softly at its 5.6 V rail. The first has a gain of 1 plus the Drive pot over 4.7 kΩ, up to about 54. It drives the second, with a fixed gain of about 58, so the second stage does most of the clipping. Between them R11 and C6 trim the treble, and C5 and C7 the bass. Tone is a 10 kΩ pot between an 18 nF from the signal and a 22 nF to ground. The OD-2r build adds a two-transistor stage after Tone with a gain of about 7, so it can get much louder. Only the Turbo channel is modeled.

Download.

Parts and what they do

Stock values for the OD-2, 1985 version. Tap a part on the schematic to change it.

R32, 4.7 kΩ
Feeds Q11’s base. With C18 it filters the supply.
C18, 10 µF electrolytic
Holds Q11’s base steady, so VA carries little of the supply’s ripple.
Q11, 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.
C19, 100 µF electrolytic
Filter cap on VA.
R33, 4.7 kΩ
Top of the VB divider.
R34, 4.7 kΩ
Bottom of the VB divider.
C20, 47 µF electrolytic
Filters VB, the bias for the input buffer and the later stages.
R35, 4.7 kΩ
Feeds the zener.
Z1, 6.2 V zener, in breakdown
A 6.2 V zener, 1N5234B. It holds Q12’s base steady.
C21, 10 µF electrolytic
Across the zener, against its noise.
Q12, 2SC1815
An emitter follower making VC, about 5.6 V, the two discrete op-amps’ supply. A 2SC2458-GR in the pedal. The 2SC1815 stands in.
C22, 10 µF electrolytic
Filter cap on VC.
R36, 16 kΩ
Top of the VD divider. 16 kΩ with 14 kΩ under it makes about 2.6 V, where the OD-2’s trimmer was likely set. 10 kΩ on the OD-2r.
R37, 14 kΩ
Bottom of the VD divider. 10 kΩ on the OD-2r.
C23, 47 µF electrolytic
Filters VD.
R1, 10 kΩ
Input series resistor.
C1, 22 nF
Input coupling cap.
R2, 1 MΩ
Biases Q1’s gate at VB and sets the input impedance near 1 MΩ.
Q1, 2SK117-GR
Input buffer, a source follower.
R3, 10 kΩ
Source resistor.
C2, 18 nF
Coupling cap into the first stage. Against R4 it cuts below about 88 Hz.
R4, 100 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.
R6, 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.
R9, 2.2 kΩ
Q4’s collector load.
R7, 4.7 kΩ
The first stage’s gain leg. Gain is 1 + (R8 + Drive) / 4.7 kΩ.
C3, 100 nF
In series with R7, so the gain falls below about 340 Hz.
Drive, 250 kΩ pot, audio taper
Drive, a 250 kΩ audio pot wired as a variable resistor in the first stage’s feedback.
C4, 100 pF
Across the feedback. It rolls the top off as Drive goes up.
C5, 22 nF
Coupling cap. Against R10 it cuts below about 72 Hz.
R10, 100 kΩ
To ground after C5.
R11, 10 kΩ
With C6, a treble cut from about 2.3 kHz ahead of the second stage.
C6, 6.8 nF
The treble cut’s cap.
C7, 10 nF
Coupling cap into the second stage. Against R12 it cuts below about 160 Hz.
R12, 100 kΩ
Biases the second stage’s plus input at VD.
Q5, 2SK117-GR
Plus input of the second discrete op-amp.
Q6, 2SK117-GR
Minus input of the second discrete op-amp.
R13, 4.7 kΩ
Shared source resistor of the pair.
R14, 2.2 kΩ
Q5’s drain load. The drop across it drives Q7.
Q7, 2SA970-GR
The second stage’s output transistor, a 2SA970-GR PNP. Its swing runs out between ground and the 5.6 V rail, which is where the pedal clips.
R16, 270 kΩ
The second stage’s feedback. Gain is 1 + 270 kΩ / 4.7 kΩ, about 58.
C9, 100 pF
Across R16. It rolls the top off from about 5.9 kHz.
R15, 4.7 kΩ
The second stage’s gain leg.
C8, 150 nF
In series with R15, so the gain falls below about 230 Hz.
R17, 1.8 kΩ
Top of a divider after the second stage, and its collector load with R18.
R18, 820 Ω
Bottom of the divider. It drops the level to about a third.
R19, 10 kΩ
Into the tone network.
R20, 4.7 kΩ
With C10, a gentle treble cut to ground.
C10, 10 nF
The treble cut’s cap.
C11, 18 nF
From the signal to the top of Tone.
Tone, 10 kΩ pot, linear taper
Tone, 10 kΩ linear. Toward C11 the treble passes, toward C12 it goes to ground.
C12, 22 nF
From the bottom of Tone to ground. 22 nF on the OD-2, 27 nF on the OD-2r.
Volume, 50 kΩ pot, audio taper
Volume, 50 kΩ audio.
R27, 47 kΩ
In series after Volume.
C16, 22 nF
Coupling cap into the output buffer.
R28, 470 kΩ
Biases the output buffer at VB.
Q10, 2SC1815
Output buffer, an emitter follower. A 2SC2458-GR in the pedal. The 2SC1815 stands in.
R29, 10 kΩ
Output buffer emitter resistor.
C17, 1 µF
Output coupling cap.
R30, 100 kΩ
Output pulldown.
R31, 1 kΩ
Output series resistor.

Versions

OD-2, 1985
The first OD-2, Turbo channel. No recovery stage, so even at full Volume it comes out near the level of the guitar. Its 2.6 V bias was set with a trimmer, assumed at the drawing’s value.
OD-2r, 1994, modeled after the Boss OD-2r Turbo OverDrive
The OD-2r, Turbo channel. A 4.7 kΩ under the Drive pot, 27 nF under Tone, fixed 10 kΩ and 10 kΩ for the bias, and a recovery stage after Tone.

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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