Two op-amp distortion with LED clipping and three-band tone, 1988 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 Marshall Guv’nor.

A non-inverting op-amp stage that boosts mostly treble, an inverting stage with a fixed gain near 68, red LEDs to ground, and a passive bass, middle and treble network.

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. One turn of the Gain pot changes both stages. Its wiper is the first op-amp’s output. The track between the wiper and the minus input is the feedback resistance, and the rest sits in series with the second stage’s 10 kΩ input resistor. Turning Gain up raises the first stage’s gain and cuts the resistance ahead of the second, so the second stage’s gain rises too. The first stage amplifies only above about 720 Hz, the corner of R1 and C1, so the low strings drive the LEDs less than the high ones. Red LEDs clip near 1.6 to 1.8 V, more than twice silicon, so the signal into the tone network is large. Probe the LED node and sweep Gain. Middle cuts a band around 1 kHz as it turns down.

Download.

Parts and what they do

Stock values for the TL072 version. Tap a part on the schematic to change it.

R4, 47 kΩ
Top of the 4.5 V divider.
R5, 47 kΩ
Bottom of the 4.5 V divider.
C7, 10 µF electrolytic
Filters the 4.5 V reference.
R16, 2.2 MΩ
Input pulldown, against switching pops.
C5, 9.6 nF
Input coupling cap. With R3 its corner is near 17 Hz.
R3, 1 MΩ
Biases U1’s input at 4.5 V and sets the input impedance, about 690 kΩ with R16.
IC1a, TL072
First gain stage, non-inverting. The gain is 1 plus the Gain pot’s feedback part over R1, above R1 and C1’s corner only. Down low it’s 1.
Gain, 100 kΩ pot, linear taper
The op-amp output is the wiper. Between the wiper and the minus input is U1’s feedback resistance, and the rest of the track is in series with R2 into the second stage. U1’s gain reaches about 33 dB above 720 Hz, and the second stage’s gain rises as the series part shrinks.
C2, 120 pF
Across the feedback part of the Gain pot. It rolls off U1 above about 13 kHz at full gain.
R1, 2.2 kΩ
U1’s ground leg. With the Gain pot it sets the maximum gain.
C1, 100 nF
U1’s ground-leg cap. With R1 its corner is near 720 Hz, so U1 boosts treble and leaves bass at unity. Burgundy measured 94 nF on his unit.
C3, 220 nF
Coupling cap from the Gain pot’s far end. Measured 195 nF.
R2, 10 kΩ
Input resistor of the second stage, in series with the unused part of the Gain pot.
C4, 100 nF
Second coupling cap. Against R2 and the pot, it sets a bass corner between about 15 and 160 Hz depending on Gain. Measured 89 nF.
IC1b, TL072
Second gain stage, inverting. Its gain is R6 over R2 plus the series part of the Gain pot, from about 6 up to 68.
R6, 680 kΩ
U2’s feedback resistor.
C8, 220 pF
Across R6. Its corner is near 1 kHz, so U2 cuts the treble that U1 boosted, at 6 dB per octave.
C9, 220 nF
Output coupling cap from U2. Measured 183 nF.
R7, 1 kΩ
Current-limiting resistor for the LEDs.
LEDs, Red LED, one each way
Two red 3 mm LEDs to ground, one each way. They clip at about 1.6 to 1.8 V. Swap them for 1N914s to hear how much quieter and harder silicon clipping is.
C13, 4 nF
The treble path into the tone network.
R9, 1.5 kΩ
The bass and middle path into the tone network.
C12, 10 nF
Part of the middle network.
R11, 100 Ω
Part of the middle network.
Middle, 10 kΩ pot, audio taper
Middle. Turning it down moves its wiper toward ground, so C11 shunts more of the mids at the R9 node to ground.
C11, 220 nF
Couples the Middle pot’s wiper to the R9 node. Measured 183 nF.
R10, 680 Ω
Carries the lows from the R9 node to the bottom of the Treble pot.
C10, 100 nF
From the R9 node to the Bass wiper.
R8, 680 Ω
Bass wiper to ground.
Bass, 10 kΩ pot, audio taper
Bass. Turned up, the wiper moves toward ground and more of the pot sits between the bottom of the Treble pot and ground, so less of the low path is shunted away. Turned down, the wiper nears the Treble pot, and C10 and R8 pull that point toward ground. The drawing doesn’t show which end is clockwise, so clockwise is assumed to be more bass.
C6, 68 nF
From the bottom of the Treble pot to ground. It rolls off the treble in the low path.
Treble, 10 kΩ pot, audio taper
Treble. Its wiper is the tone network’s output: toward the top more comes from C13’s treble path, toward the bottom more from the bass path.
Level, 100 kΩ pot, linear taper
Output level.
R12, 22 kΩ
Output resistor. With C14 it forms a low-pass near 15 kHz, and it isolates the output from the cable.
C14, 470 pF
Output filter cap, at the effects loop send, which is normalled to the output.

Versions

TL072
Negative ground, 9 V, with a 4.5 V reference from two 47 kΩ resistors. The drawing shows a TL072 for both stages.
RC4558
A bipolar dual op-amp in place of the TL072, a common swap. Its input bias current through the 1 MΩ bias resistor shifts the first stage’s DC slightly.

Sources

  • Michael Burgundy, traced schematic, 22 June 1999

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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