Two discrete-op-amp overdrive with one gain knob for both stages, 1995 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.
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.
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.
3Press Play
Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.
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.
5Tap a wire
Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.
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
- Not sure what a control does? Hover over it, or hold a finger on it, for a short note.
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- The graphs use a steady test tone. Play sounds a picked note that dies away.
- Part values are typical of their type, not measured from one unit. Each circuit’s notes say which schematic it’s based on.
Modeled after the Boss BD-2 Blues Driver.
Two gain stages, each a small op-amp built from a JFET pair and a PNP transistor, their gains set together by a dual Gain pot, with silicon diodes to ground between them. Then a passive tone network and an op-amp output stage with a small fixed lift in the low end.
Try this
Common mods
Changes people often make to this circuit. Try it plays the note before and after, at the same loudness. Keep it adds the mod to Your changes, where it can go on a switch.
Input
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.
Your build
Stack blocks from the guitar down to the output. Tap + Add a block wherever you want one. The coupling caps and the supply are wired in for you.
Your blocks. These are blocks you saved with Save block, values and part changes included. Add one to any build of the same kind with + Add a block. They stay in this browser.
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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.
Tap where each leg goes. Tap both ends of a part to put the new one across that 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.
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.
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.
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 clips softly on its own when pushed, as its output transistor runs out of room, so most of the overdrive comes from the stages, not the diodes. The diodes between them hold the clipping node to about ±0.8 V, which only matters at high Gain. One knob raises both stages’ gain, so low settings stay clean and high ones stack two clipping stages. Probe the first stage’s output, then the clipping node, then the second stage, to see where it clips at each Gain setting.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- R18, 10 kΩ
- Input resistor.
- C14, 47 nF
- Input coupling cap.
- R15, 1 MΩ
- Gate bias to the 4 V reference. It sets the input impedance near 1 MΩ.
- Q3, 2SK184-GR
- Input buffer, a 2SK184 JFET source follower.
- R19, 10 kΩ
- Source resistor.
- C15, 10 µF electrolytic
- Coupling cap to the bypass switch (Q6, closed with the effect on and drawn as a wire).
- R17, 100 kΩ
- Bias on the input side of the switch.
- R22, 100 kΩ
- Bias on the output side of the switch.
- C18, 100 nF
- Coupling cap into the first stage.
- R23, 220 kΩ
- Bias for the first stage’s plus input.
- Q11, 2SK184-GR
- Minus input of the stage’s discrete op-amp: one half of a 2SK184 differential pair.
- Q10, 2SK184-GR
- Plus input of the stage’s discrete op-amp: the other half of the pair.
- R30, 4.7 kΩ
- Tail resistor of the differential pair.
- R28, 2.2 kΩ
- Drain load of the plus-input JFET. Its drain drives the PNP output transistor’s base.
- Q9, 2SA1335-GR
- PNP output transistor of the discrete op-amp (2SA1335).
- C21, 47 pF
- Compensation cap from the output transistor’s base to its collector.
- R32, 2.2 kΩ
- Output load resistor.
- R31, 1.5 kΩ
- Ground-leg resistor of the stage. With the cap below it, it sets the gain.
- C22, 150 nF electrolytic
- Ground-leg cap. The gain falls to 1 at low frequencies.
- R29, 22 kΩ
- Fixed part of the feedback resistance.
- Gain (VR1A), 250 kΩ pot, audio taper
- Gain, first half: VR1A, 250 kΩ audio, a variable feedback resistor. The stage’s gain is 1 + (22 k + VR1A) / 1.5 k, about 16 to 180.
- C23, 47 pF
- Feedback cap across the whole feedback path.
- C26, 220 pF
- Treble path around the interstage network (with R37).
- R37, 330 kΩ
- In series with C26.
- R38, 100 kΩ
- Main interstage path.
- C34, 100 nF
- Couples the main path to the clipping node.
- C35, 47 nF
- With R51, part of the interstage filter.
- R51, 18 kΩ
- Interstage filter resistor to ground.
- R50, 1 MΩ
- Keeps the clipping node at 0 V DC.
- D7–D10, 2 × 1N914 / 1N4148, one each way
- D7 and D8 in series one way, D9 and D10 the other (1SS133), clipping to ground. At full Gain they hold the node to about ±0.8 V.
- C27, 2.2 nF
- Coupling cap into the second stage. With R35 it makes a high-pass near 70 Hz.
- R35, 1 MΩ
- Bias for the second stage’s plus input.
- Q13, 2SK184-GR
- Minus input of the stage’s discrete op-amp: one half of a 2SK184 differential pair.
- Q14, 2SK184-GR
- Plus input of the stage’s discrete op-amp: the other half of the pair.
- R36, 4.7 kΩ
- Tail resistor of the differential pair.
- R33, 2.2 kΩ
- Drain load of the plus-input JFET. Its drain drives the PNP output transistor’s base.
- Q12, 2SA1335-GR
- PNP output transistor of the discrete op-amp (2SA1335).
- C20, 100 pF
- Compensation cap from the output transistor’s base to its collector.
- R25, 2.2 kΩ
- Output load resistor.
- R34, 2.2 kΩ
- Ground-leg resistor of the stage. With the cap below it, it sets the gain.
- C24, 1 µF electrolytic
- Ground-leg cap. The gain falls to 1 at low frequencies.
- R27, 33 kΩ
- Fixed part of the feedback resistance.
- Gain (VR1B), 250 kΩ pot, audio taper
- Gain, second half: VR1B, 250 kΩ audio, turning with VR1A. The stage’s gain is 1 + (33 k + VR1B) / 2.2 k, about 16 to 130.
- C25, 100 pF
- Feedback cap across the whole feedback path.
- R26, 5.6 kΩ
- Tone-shaping network: series resistor.
- C17, 5.6 nF
- Across R26. It passes the treble around it.
- C19, 5.6 nF
- Tone-shaping network: cap to ground. With R26 a low-pass near 5 kHz, partly bypassed by C17.
- C100, 18 nF
- Tone network: series cap to the top of the Tone pot.
- Tone (VR2), 10 kΩ pot, linear taper
- Tone, 10 kΩ linear (VR2). Its ends go to C100 from the signal and C101 to ground, and the wiper sets a brighter or darker blend. The direction is assumed: clockwise brighter.
- C101, 18 nF
- Tone network: cap from the bottom of the Tone pot to ground.
- Level (VR3), 100 kΩ pot, audio taper
- Level, 100 kΩ audio (VR3), ahead of the output op-amp.
- C10, 47 nF
- Coupling cap into the output op-amp.
- R13, 470 kΩ
- Bias to the 4 V reference.
- IC1B, M5218 (4558-class values)
- Output stage, half of an M5218, non-inverting. A fixed tone-shaping network (R8, C8 and the Q7 network) lifts about 6 dB around 130 Hz.
- D1, D3, 1N914 / 1N4148, one each way
- D1 and D3 (1SS133) across the op-amp’s inputs. They only conduct if the inputs are driven more than a diode drop apart.
- R8, 6.8 kΩ
- Feedback resistor.
- C8, 2.2 nF
- Across R8. It rolls off the treble above about 10 kHz.
- C9, 56 nF
- Ground-leg network of the output op-amp: series cap.
- C16, 56 nF
- Ground-leg network: cap to the Q7 base.
- R21, 1.2 kΩ
- Ground-leg network: resistor to the Q7 emitter.
- R10, 470 kΩ
- Q7 base bias to the 4 V reference.
- Q7, 2SC1815
- Emitter follower (2SC2458) in the output op-amp’s ground leg. With C9, C16, R21 and R10 it makes the ground leg’s impedance dip in a band around 130 Hz, so the op-amp lifts that band by about 6 dB and stays close to unity gain elsewhere.
- R20, 10 kΩ
- Q7 emitter resistor.
- C7, 10 µF electrolytic
- Coupling cap to the output switch (Q4, closed with the effect on and drawn as a wire).
- R11, 100 kΩ
- Bias on one side of the output switch.
- R8B, 100 kΩ
- Bias on the other side of the output switch.
- C6, 10 µF electrolytic
- Coupling cap to the output transistor.
- R7, 100 kΩ
- Base bias of the output follower.
- Q1, 2SC1815
- Output emitter follower (2SC2458).
- R2, 10 kΩ
- Emitter resistor.
- C1, 10 µF electrolytic
- Output coupling cap.
- R3, 100 kΩ
- Output pull-down.
- R1, 1 kΩ
- Output resistor.
- R6, 2.2 kΩ
- Base feed of the supply filter transistor.
- C4, 47 µF electrolytic
- With R6, filters the base of Q2. Q2 acts as a capacitance multiplier, for a clean supply about a diode drop below the battery.
- Q2, 2SC1815
- Supply filter transistor (2SC2458), emitter follower from the filtered base.
- C5, 100 µF electrolytic
- Filter cap on the 8 V supply.
- R5, 10 kΩ
- Top of the 4 V divider.
- R4, 10 kΩ
- Bottom of the 4 V divider.
- C2, 47 µF electrolytic
- Divider filter cap.
- IC1A, M5218 (4558-class values)
- Buffers the 4 V reference (the other half of the M5218).
- C11, 47 µF electrolytic
- Filter cap on the 4 V reference.
About this version
- Factory values
- The 2SC2458 transistors use 2SC1815 values. The 2SK184 and 2SA1335 models are typical mid-range parts, not measured. The M5218 uses 4558-class values.
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.
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