Discrete op-amp overdrive with diode feedback, 1997 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-3.

Two JFET followers, a bridged-T filter, then a discrete op-amp (a JFET pair and a PNP) with diodes and the Drive pot in its feedback. A JFET stage with diodes from drain to gate clips again, and two op-amp stages shape the tone before the Tone and Volume pots.

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. Two JFET followers load the guitar with about 1 MΩ. The bridged-T after them dips the lows and middle: at the discrete op-amp’s input, 100 Hz to 1 kHz comes out about 5 to 7 dB under 8 kHz. The discrete op-amp, a JFET pair and a PNP, is a non-inverting amp with a gain of 1 + (3.3 k + Drive) / 470 Ω above about 720 Hz, with D4 and D5 across its feedback. D2 and D3 ahead of it limit what reaches it. Q6 clips again, with D6 and D7 returning its drain’s swing to its gate. IC1a adds about 18 dB, IC1b shapes the treble, and the 10 kΩ Tone pot with 22 nF cuts treble as it turns down. A 2SA1015 and a 2SC1815 stand in for the 2SA1048 and 2SC2458.

Download.

Parts and what they do

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

R34, 10 kΩ
Top of the 4.5 V reference divider.
R35, 10 kΩ
Bottom of the 4.5 V reference divider.
C22, 47 µF electrolytic
Filter cap on the 4.5 V reference.
R1, 10 kΩ
Input series resistor.
C1, 47 nF
Input coupling cap.
R2, 1 MΩ
Biases Q1’s gate, and sets the input impedance, about 1 MΩ.
Q1, 2SK184-GR
Input buffer, a JFET source follower.
R3, 10 kΩ
Q1’s source load.
C2, 18 nF
Coupling cap into Q2. Against R4’s 100 kΩ it cuts below about 90 Hz.
R4, 100 kΩ
Biases Q2’s gate.
Q2, 2SK184-GR
Second follower, to drive the bridged-T filter.
R5, 10 kΩ
Q2’s source load.
C3, 1 µF
Coupling cap into the bridged-T filter.
C4, 10 nF
The bridged-T’s series cap.
R6, 10 kΩ
One leg of the bridged-T.
R8, 10 kΩ
The other leg of the bridged-T.
R7, 3.3 kΩ
From the bridged-T’s middle toward ground.
C5, 56 nF
In series with R7 to ground.
D2, D3, 1N914 / 1N4148, one each way
One 1N914 each way from the bridged-T’s output to C6, a limit on the swing into the discrete op-amp.
C6, 1 µF
Takes D2 and D3 to ground for signal only.
R9, 220 kΩ
Upper leg of the discrete op-amp’s input bias.
R10, 220 kΩ
Lower leg. With R9 it sets the plus input at about 2.25 V.
Q3, 2SK184-GR
Plus input of the discrete op-amp, one half of a JFET pair.
Q4, 2SK184-GR
Minus input of the discrete op-amp, the other half of the pair.
R12, 6.8 kΩ
The pair’s shared source resistor.
R11, 2.7 kΩ
Q3’s drain load. The drop across it drives Q5.
Q5, 2SA1015-GR
The discrete op-amp’s output transistor, a PNP with its emitter on the supply.
R15, 2.7 kΩ
Q5’s collector load.
D4, D5, 1N914 / 1N4148, one each way
One 1N914 each way across the feedback, so the stage clips softly about 0.6 V either side.
R14, 3.3 kΩ
Minimum feedback resistance.
Drive, 100 kΩ pot, audio taper
A 100 kΩ audio pot wired as a variable resistor in the feedback. Gain is 1 + (3.3 k + Drive) / 470 Ω, about 8 to 220.
C7, 470 nF
In series with R13, so the gain is full only above about 720 Hz.
R13, 470 Ω
The gain leg to ground.
C9, 56 nF
Coupling cap into the JFET clipping stage.
R16, 100 kΩ
Series resistor into Q6’s gate.
R17, 33 kΩ
Q6’s gate to ground. With R16 it divides the signal by about 4.
C10, 1.2 nF
Gate to ground. With R16 and R17 it rolls off above about 5 kHz.
D6, D7, 1N914 / 1N4148, one each way
One 1N914 each way from Q6’s gate to C11, which goes to its drain: they clip the drain’s swing back into the gate.
C11, 470 nF
Couples Q6’s drain to D6 and D7.
Q6, 2SK184-GR
A common-source JFET gain stage.
R18, 10 kΩ
Q6’s drain load.
R20, 2.2 kΩ
Q6’s source resistor.
R19, 1 kΩ
With C12, part-bypasses R20 for signal.
C12, 10 µF electrolytic
The source bypass cap.
R21, 4.7 kΩ
With C13, a low-pass at about 2.8 kHz.
C13, 12 nF
The low-pass cap.
C14, 1 µF
Coupling cap into IC1a.
R22, 47 kΩ
Series resistor into IC1a’s plus input.
R23, 150 kΩ
Biases IC1a’s plus input.
IC1a, M5218 (4558-class values)
Non-inverting gain stage, gain 1 + 6.8 k / 1 k, about 7.8.
R24, 1 kΩ
IC1a’s gain leg.
C15, 10 µF electrolytic
In series with R24.
R25, 6.8 kΩ
IC1a’s feedback resistor.
R26, 12 kΩ
IC1b’s input resistor.
C16, 6.8 nF
With R27, a second input path that lifts the treble.
R27, 4.7 kΩ
In series with C16.
IC1b, M5218 (4558-class values)
Inverting tone-shaping stage, gain of one at low frequencies.
R28, 12 kΩ
IC1b’s feedback resistor.
R29, 4.7 kΩ
With C17, a second feedback path that cuts the treble back.
C17, 18 nF
In series with R29.
C18, 33 nF
Coupling cap to the Tone pot.
Tone, 10 kΩ pot, linear taper
A 10 kΩ linear pot with 22 nF from its far end to ground. Turned down, the wiper moves toward the cap and cuts treble.
C19, 22 nF
The Tone pot’s treble cut cap.
Volume, 100 kΩ pot, linear taper
Output level, a 100 kΩ linear pot.
C20, 1 µF
Coupling cap into the output follower.
R30, 100 kΩ
Biases the output follower.
Q7, 2SC1815
Output buffer, an emitter follower.
R31, 10 kΩ
Q7’s emitter load.
C21, 1 µF
Output coupling cap.
R32, 100 kΩ
Output pulldown.
R33, 1 kΩ
Output series resistor.

About this version

Factory values
Negative ground. 2SK184-GR JFETs, a 2SA1015 in place of the 2SA1048 and a 2SC1815 in place of the 2SC2458, all typical of the parts.

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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