Octave divider, one and two octaves down, 1982 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 OC-2 Octave.

A JFET follower and an op-amp stage feed three paths: the note, a stage whose sign two flip-flops flip once every two cycles, and a second one flipping once every four. Each switched wave is low-passed toward a sine, and three pots mix them.

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. IC1.1 brings the note up about 4.7 times. A copy of it goes through a low-pass near 690 Hz to two peak detectors, IC2 and IC3. A comparator sets the latch near the bottom of each cycle and another resets it near the top, so the latch’s Q is a clean square at the note. IC7’s other half divides it by two and IC6 divides again by two. In each switching stage the signal goes to both op-amp inputs, and a JFET on the + input flips the stage between a gain of about +0.5 and −0.57. Flipped once every two cycles, the note repeats once every two: an octave down. D10 and D11 hold the bottom of the wave about 0.2 V under the reference, so on louder notes the switching also adds a square at the octave, and the two-pole low-pass after each stage rounds it off. IC6 is a Boss divider chip with no model. A CD4013 half divides in its place. The footswitch, its flip-flop, the bypass path and the switching JFETs Q3 and Q4 are left out, so the model is the pedal with the effect on.

Download.

Parts and what they do

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

C9, 100 µF electrolytic
Bulk cap across the supply.
R46, 4.7 kΩ
Top of the reference divider.
R39, 5.6 kΩ
Bottom of the reference divider. Every op-amp works around its 4.9 V.
C25, 220 µF electrolytic
Filters the reference.
R1, 10 kΩ
In series with the input.
C1, 22 nF
Input coupling cap. With R3 it sets a corner near 7 Hz.
R3, 1 MΩ
Biases Q1’s gate at the reference and sets the input impedance near 1 MΩ.
Q1, 2SK30A-GR
Input source follower. Its gain is just under one. 2SK30A typical of the part, not on the drawing.
R2, 10 kΩ
Q1’s source load.
C2, 1 µF electrolytic
Couples the follower into IC1.1.
R4, 1 MΩ
Biases IC1.1’s + input at the reference.
IC1.1, RC4558
Non-inverting gain stage, 1 + 10 k / 2.7 k, about 4.7 times. A 4558-class op-amp, typical, not named on the drawing.
R7, 10 kΩ
Feedback resistor. With R6 it sets the gain.
R6, 2.7 kΩ
Bottom of the gain network.
C3, 10 µF electrolytic
Holds the gain at 1 for DC. With R6 the gain starts to rise above about 6 Hz.
Direct Level, 100 kΩ pot, linear taper
Level of the note itself, from IC1.1’s output into the mix.
RM1, 100 kΩ
Mixing resistor from Direct Level. No number on the factory drawing, which draws it inside the pot’s outline. 100 kΩ from the redrawn schematics.
C24, 1 µF electrolytic
Couples IC1.1 into the first octave stage.
D10, 1N270 germanium
Germanium diode from the reference. It catches the bottom of the wave at about 0.2 V under the reference. 1N270 typical, the drawing says only Ge.
R44, 47 kΩ
Into the − input of the switching stage.
R45, 47 kΩ
Into the + input of the switching stage.
R49, 100 kΩ
With R45 it divides the signal at the + input to about two thirds while Q8 is off.
Q8, 2SK30A-GR
Switch on the + input. On, it shorts the + input to the reference and the stage inverts with a gain of about 0.57. Off, the stage passes the signal upright at about 0.5. 2SK30A typical, not on the drawing.
R47, 1 MΩ
From the first divider to Q8’s gate.
R48, 1 MΩ
With R47 it halves the divider’s swing, so the gate sits at 0 V (Q8 off) or about 4.5 V (on).
IC5.2, RC4558
Switching stage. The signal goes in at both inputs, and Q8 flips its sign on every other cycle of the note, so the output repeats once every two cycles: an octave down.
R43, 27 kΩ
Feedback resistor of the switching stage.
R42, 22 kΩ
First low-pass. With C28 its corner is about 330 Hz.
C28, 22 nF
First low-pass cap.
R41, 330 kΩ
Low-pass filter resistor.
C26, 4.7 nF
Low-pass filter cap from the follower’s output.
R40, 330 kΩ
Low-pass filter resistor.
C27, 470 pF
Low-pass filter cap. With C26, R40 and R41 it makes a two-pole low-pass near 330 Hz with a small peak.
IC4.2, RC4558
Follower of the two-pole low-pass. It rounds the switched wave toward a sine one octave down.
Oct 1, 100 kΩ pot, linear taper
Level of the octave down into the mix.
RM2, 100 kΩ
Mixing resistor from Oct 1. No number on the factory drawing, 100 kΩ from the redrawn schematics.
C29, 1 µF electrolytic
Couples the octave down into the second octave stage.
D11, 1N270 germanium
Germanium diode from the reference, as D10. 1N270 typical, the drawing says only Ge.
R54, 47 kΩ
Into the − input of the second switching stage.
R53, 47 kΩ
Into the + input of the second switching stage.
R52, 100 kΩ
With R53 it divides the signal at the + input while Q7 is off.
Q7, 2SK30A-GR
Switch on the + input, run by the second divider at a quarter of the note. 2SK30A typical, not on the drawing.
R50, 1 MΩ
From the second divider to Q7’s gate.
R51, 1 MΩ
With R50 it halves the divider’s swing at Q7’s gate.
IC5.1, RC4558
Second switching stage. It flips the octave down’s sign every two of its cycles, so its output repeats once every four cycles of the note: two octaves down.
R55, 27 kΩ
Feedback resistor of the second switching stage.
R56, 22 kΩ
First low-pass. With C32 its corner is about 150 Hz.
C32, 47 nF
First low-pass cap.
R57, 330 kΩ
Low-pass filter resistor.
C31, 10 nF
Low-pass filter cap from the follower’s output.
R58, 330 kΩ
Low-pass filter resistor.
C30, 1 nF
Low-pass filter cap. With C31, R57 and R58 it makes a two-pole low-pass near 150 Hz with a small peak.
IC4.1, RC4558
Follower of the second two-pole low-pass.
Oct 2, 100 kΩ pot, linear taper
Level of the two octaves down into the mix.
RM3, 100 kΩ
Mixing resistor from Oct 2. No number on the factory drawing, 100 kΩ from the redrawn schematics.
C6, 1 µF electrolytic
Couples the mix to the output buffer. Q3, the electronic switch after it, is drawn as a wire: it is on with the effect on.
R12, 1 MΩ
Holds Q3’s side of C6 at the reference.
R13, 1 MΩ
Holds Q3’s other side at the reference. With Q3 on, R12 and R13 are in parallel.
C8, 47 nF
Couples the mix into Q2.
R20, 1 MΩ
Biases Q2’s base at the reference.
Q2, 2SC1815
Output emitter follower. 2SC1815 typical, the drawing shows only an NPN.
R15, 10 kΩ
Q2’s emitter load.
C11, 1 µF electrolytic
Output coupling cap.
R14, 100 kΩ
Holds the output at 0 V so the jack doesn’t pop.
R16, 1 kΩ
In series with the output.
R8, 22 kΩ
Feeds the trigger side. With C21 its corner is about 720 Hz.
C21, 10 nF
First treble cut on the trigger side.
R9, 330 kΩ
Low-pass filter resistor.
C5, 2.2 nF
Low-pass filter cap from the follower’s output.
R5, 330 kΩ
Low-pass filter resistor.
C4, 220 pF
Low-pass filter cap. With C5, R5 and R9 it makes a two-pole low-pass near 690 Hz with a peak, so the trigger sees mostly the note’s fundamental.
IC1.2, RC4558
Follower of the trigger side’s low-pass.
R10, 33 kΩ
With C20 a treble cut near 15 kHz.
C20, 330 pF
Treble cut cap.
R32, 33 kΩ
Into the peak detectors. With C18 it delays the wave there slightly against T3.
C18, 10 nF
With R31 and R32 a shelving low-pass ahead of the peak detectors.
R31, 68 kΩ
In series with C18.
IC2.2, RC4558
Positive peak detector. Through D7 and R34 it charges C22 to the top of the wave.
D6, 1N914 / 1N4148
Closes the loop while the wave falls, so the op-amp’s output doesn’t swing to the rail. 1S1588-class, typical.
D7, 1N914 / 1N4148
Lets the op-amp charge C22 upward only.
R33, 10 kΩ
Feedback from after D7. While D7 is off, C22 discharges through R34 and R33 toward the wave.
R34, 1 kΩ
In series with C22.
C22, 1 µF electrolytic
Holds the positive peak.
IC2.1, RC4558
Comparator. Its output goes high when the wave at T3 rises over the held peak, near the top of each cycle.
R35, 22 kΩ
To the latch’s reset pin.
IC3.2, RC4558
Negative peak detector. Through D9 and R37 it pulls C23 to the bottom of the wave.
D8, 1N914 / 1N4148
Closes the loop while the wave rises. 1S1588-class, typical.
D9, 1N914 / 1N4148
Lets the op-amp pull C23 downward only.
R38, 10 kΩ
Feedback from after D9.
R37, 1 kΩ
In series with C23.
C23, 1 µF electrolytic
Holds the negative peak.
IC3.1, RC4558
Comparator. Its output goes high when the wave at T3 falls under the held bottom, near the bottom of each cycle.
R36, 22 kΩ
To the latch’s set pin.
IC7 (8 to 13), CD4013B
Half a CD4013 used as a set-reset latch, clock and D grounded. The bottom of each cycle sets it and the top resets it, so its Q is a clean square at the note.
IC7 (1 to 6), CD4013B
Divider: D tied to Q bar, so each rising edge from the latch flips it. Its Q bar is a square one octave down. It runs Q8 and clocks IC6.
IC6, CD4013B
IC6 is a Boss divider chip (BA634, no longer made), drawn as a box with IN and OUT. Modeled as half a CD4013 wired to divide by two. Its output, two octaves down, runs Q7.

About this version

As drawn
Negative ground. The op-amps work around 4.9 V from R46 and R39. The flip-flops run 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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