Op-amp distortion with a two-octave-down square, 1994 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 DOD FX33 Buzz Box.

Two op-amp gain stages, diodes to ground and a bass-heavy op-amp stage, beside a squarer and two CD4013 flip-flops that divide the note by four. Heavy picks between the two, and an active Saw control sets the highs.

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. Q1 buffers the guitar and feeds two paths. In the distortion path U1A’s gain climbs with frequency, about 12 dB at 110 Hz and 38 dB near 3.8 kHz. U1B adds a gain of 22 to 234, set by Buzz, and D2 and D3 clip what comes out to about 0.6 V either way. U2B then lifts about 13 dB near 40 Hz and about 11 dB near 6.2 kHz, with transistor gyrators in place of coils. In the octave path U4B and U4A square the note, and the two 4013 flip-flops divide it by four into a 0 to 9 V square two octaves down. Heavy picks between the two paths. Even fully down, some of the square gets through the pot, about 11 dB under the note on a steady A2. Saw cuts or lifts the treble, about 15 dB at 5 kHz at either end and flat at noon. Probe CK, OCT1 and OCT2 to see each division. The model leaves out the electronic bypass and its footswitch latch. DOD’s W pot taper is assumed linear.

Download.

Parts and what they do

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

R36, 10 kΩ
Top of the bias divider.
R35, 10 kΩ
Bottom of the bias divider. Every op-amp and transistor stage works around half the battery.
C34, 330 µF electrolytic
Filters the bias.
R1, 10 kΩ
Input series resistor.
C1, 120 pF
With R1 it trims radio hash far above the guitar.
C2, 47 nF
Input coupling cap. With R2 it cuts below about 3 Hz.
R2, 1 MΩ
Holds Q1’s gate at the bias and sets the input impedance near 1 MΩ.
Q1, J201
J201 input buffer, a source follower.
R3, 10 kΩ
Q1’s source resistor.
C3, 1 µF electrolytic
Coupling cap from the buffer to both paths.
R5, 100 kΩ
Holds this side of the input switch Q2 at the bias.
R7, 100 kΩ
Holds the other side of Q2 at the bias. Q2 is on with the effect on, so it’s drawn as a wire and R5 and R7 sit on one node.
C4, 33 nF
Coupling cap into U1A. With R8 it cuts below about 48 Hz.
R8, 100 kΩ
Biases U1A’s + input.
U1A, JRC4558D
First gain stage, non-inverting, with a DC gain of 1. Through C6 and the Q5 gyrator its gain climbs with frequency, about 12 dB at 110 Hz and 38 dB near 3.8 kHz.
R12, 220 kΩ
Feedback resistor.
C5, 100 pF
Across R12. It rolls the gain off above about 7 kHz.
C6, 27 nF
In series with the gain leg, so the gain falls toward the bass.
R11, 2 kΩ
The gyrator’s series resistance, the least the gain leg can be.
C7, 220 pF
Sets the gyrator’s inductance with R9 and R11, about 21 mH.
R9, 47 kΩ
Biases Q5’s base.
Q5, 2N5088
Gyrator: an emitter follower that makes R11 act like a 21 mH coil in series with 2 kΩ.
R10, 10 kΩ
Q5’s emitter resistor.
R13, 10 kΩ
Top of a divider and treble cut between the gain stages.
R14, 10 kΩ
With R13 it halves the signal.
C8, 47 nF
Across R14. With R13 and R14 it cuts the treble above about 680 Hz.
C9, 33 nF
Coupling cap into U1B. With R15 it cuts below about 48 Hz.
R15, 100 kΩ
Biases U1B’s + input.
U1B, JRC4558D
Second gain stage, non-inverting. Buzz sets its gain, 1 plus (10 kΩ plus Buzz) over 470 Ω.
C10, 50 pF
Across the feedback. It rolls off the gain far up.
R16, 10 kΩ
Fixed part of the feedback. It sets the least gain, about 22.
Buzz, 100 kΩ pot, linear taper
Buzz, 100 kΩ as a variable resistor in the feedback. Turned up it adds resistance and raises the gain, from about 22 to 235.
R17, 470 Ω
Bottom of the gain network.
C11, 10 µF electrolytic
Keeps the DC gain at 1. With R17 the gain starts to fall below about 34 Hz.
C12, 10 µF electrolytic
Coupling cap to the diodes.
R18, 2 kΩ
Feeds the diodes and limits their current.
D2, D3, 1N914 / 1N4148, one each way
Two 1N4148s to ground, one each way. They hold the signal to about 0.6 V either way.
R19, 47 kΩ
Series resistor into the last stage.
C17, 1 µF electrolytic
Coupling cap into U2B.
R20, 100 kΩ
Biases U2B’s + input.
U2B, JRC4558D
Boost stage after the diodes. Its gain is 1 plus 3.3 kΩ over the two gyrator legs to ground. That makes a peak of about 13 dB near 40 Hz and one of about 11 dB near 6.2 kHz, with about 1.4 dB between.
R21, 3.3 kΩ
Feedback resistor.
C18, 50 pF
Across R21. It rolls off the gain far up.
C19, 13 nF
With the Q7 gyrator it tunes the treble peak near 6.2 kHz. DOD lists it at 3 %.
R24, 1 kΩ
The treble leg’s series resistance. It sets that peak’s height, about 11 dB.
C20, 1 nF
Sets the gyrator’s inductance with R22 and R24, about 47 mH.
R22, 47 kΩ
Biases Q7’s base.
Q7, 2N5088
Gyrator for the treble peak.
R23, 10 kΩ
Q7’s emitter resistor.
C21, 4.7 µF electrolytic
Tantalum. With the Q8 gyrator it tunes the bass peak near 40 Hz.
R27, 100 Ω
The bass leg’s series resistance. Only 100 Ω, so that peak is the taller one, about 13 dB.
C22, 47 nF
Sets the gyrator’s inductance with R25 and R27, about 2.2 H.
R25, 470 kΩ
Biases Q8’s base.
Q8, 2N5088
Gyrator for the bass peak.
R26, 10 kΩ
Q8’s emitter resistor.
C13, 1 µF electrolytic
Coupling cap to the distortion end of Heavy.
R34, 2 kΩ
Input resistor of the squarer. It sets U4B’s gain with R33.
U4B, JRC4558D
Inverting stage. Its gain is about 560 at 110 Hz and 77 at 1 kHz, falling above about 80 Hz through C32, so the low notes square first.
R33, 2 MΩ
Feedback resistor, a gain of 1000 with R34.
C32, 1 nF
Across R33. With it the stage cuts treble above about 80 Hz.
R32, 10 kΩ
Input resistor of the second squaring stage.
U4A, JRC4558D
Inverting stage with a gain of 200. Any played note drives it rail to rail, so its output is a square that clocks the flip-flops.
R31, 2 MΩ
Feedback resistor, a gain of 200 with R32.
D5, 1N914 / 1N4148
Keeps the clock from going below ground.
R29, 100 kΩ
Clock line to ground.
U5A, CD4013B
Half a 4013, D tied to Q bar, so each rising clock flips it. Its Q is a square one octave down.
U5B, CD4013B
The other half, clocked by U5A. Its Q is a 0 to 9 V square two octaves down.
C29, 1 µF
Non-polar coupling cap from the divider to the octave end of Heavy.
Heavy, 100 kΩ pot, linear taper
Heavy, 100 kΩ from the distortion to the octave square. Turned up the wiper moves toward the square.
R48, 100 kΩ
Biases Q11’s base and Heavy’s wiper.
Q11, 2N5088
Emitter follower after Heavy.
R50, 10 kΩ
Q11’s emitter resistor.
R49, 22 kΩ
Feeds U2A’s + input. Saw’s treble cut works against it.
Saw, 100 kΩ pot, linear taper
Saw, 100 kΩ between U2A’s inputs, its wiper to R51 and C24. Down, they shunt treble from the + input. Up, from the − input, which raises the treble gain.
R51, 2 kΩ
Limits how far Saw lifts or cuts the treble.
C24, 8.2 nF
Sets where Saw starts working, about 10 kHz with R51 alone.
U2A, JRC4558D
Active treble stage around Saw. Its gain is 1 in the bass.
R52, 22 kΩ
Feedback resistor.
C23, 50 pF
Across R52. It rolls off the gain far up.
C25, 10 µF electrolytic
Coupling cap to Thrust.
Thrust, 100 kΩ pot, linear taper
Thrust, the output level.
C26, 220 nF electrolytic
Coupling cap from Thrust to the output switch.
R53, 100 kΩ
Holds this side of the output switch Q10 at the bias.
R54, 100 kΩ
Holds the other side of Q10 at the bias. Q10 is on with the effect on, so it’s drawn as a wire.
C27, 220 nF electrolytic
Coupling cap into the output buffer.
R55, 100 kΩ
Biases Q9’s base.
Q9, 2N5088
Output buffer, an emitter follower.
R56, 10 kΩ
Q9’s emitter resistor.
C28, 10 µF electrolytic
Output coupling cap.
R57, 100 kΩ
Output pulldown, so the jack doesn’t pop.
R58, 100 Ω
In series at the output jack.

About this version

Rev A5, 1996
Negative ground. Every op-amp sits at half the battery on R35 and R36. The flip-flops run straight from the battery.

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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