Octave fuzz with a transistor phase splitter, 1980 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 Roger Mayer Octavia.

A PNP and a Darlington in a feedback pair make the fuzz. A Darlington phase splitter drives two diodes in opposite phase into one load, so the output is the wave folded up, an octave above the note. A Darlington follower drives the output.

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 and Q2 make a feedback pair. R9 sets their bias, and for the signal the gain is about R9 over R7 and Drive, from about 5 to about 200. Q3 splits the result into two copies in opposite phase. Each diode passes the positive half of its copy, so between them they pass both halves of the wave, turned the same way up. That folded wave repeats at twice the note. R15 and R16 hold the diodes just at the edge of conducting, so even small signals fold. It’s clearest on single notes up the neck. Probe C3N and E3, then OCT, to see the fold. The model leaves out the gain spread between real transistors.

Download.

Parts and what they do

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

R, 2.2 MΩ
Input pulldown at the jack.
C1, 10 nF
Input coupling cap. Against the bias network it cuts below about 40 Hz.
C2, 470 pF
From Q1’s base to ground against radio pickup. It trims the very top.
R2, 180 kΩ
Feeds Q1’s bias through a filter.
C3, 22 µF electrolytic
Filters the bias supply.
R1, 820 kΩ
Top of Q1’s bias divider.
R3, 680 kΩ
Bottom of Q1’s bias divider.
Q1, 2N3906
PNP input stage. Its emitter takes the feedback from Q2’s collector, its collector drives Q2.
R7, 220 Ω
Between Q1’s emitter and the feedback junction.
R9, 47 kΩ
Feedback from Q2’s collector. It sets the pair’s bias, and against R7 and Drive the signal gain.
C6, 22 µF electrolytic
Couples the Drive pot to the feedback junction, so Drive changes the gain without moving the bias.
Drive, 10 kΩ pot, reverse audio taper
A 10 kΩ reverse-audio pot, a variable resistor to ground. Turning it up drops the resistance, shunts more of the feedback away and raises the gain.
R4, 100 kΩ
Q1’s collector load.
R5, 220 Ω
In series with C4.
C4, 1 nF
With R5, a treble shunt on Q2’s base that keeps the pair stable.
Q2a, MPSA13 (half)
First transistor of the Q2 Darlington, direct coupled from Q1.
Q2b, MPSA13 (half)
Second transistor of the Q2 Darlington, the gain stage. The pair has two base-emitter drops, about 1.2 V.
R6, 39 kΩ
Q2’s collector load.
R10, 39 kΩ
Q2’s emitter resistor.
C5, 100 nF
Bypasses R10 above about 40 Hz.
C8, 10 nF
Coupling cap into the phase splitter.
R11, 470 kΩ
Top of Q3’s base bias.
R12, 330 kΩ
Bottom of Q3’s base bias.
Q3a, MPSA13 (half)
First transistor of the Q3 Darlington.
Q3b, MPSA13 (half)
Second transistor of the Q3 Darlington, the phase splitter. Equal resistors on collector and emitter, so the two outputs are the same size and opposite in phase.
R13, 10 kΩ
Q3’s collector load.
R14, 10 kΩ
Q3’s emitter load.
C10, 22 µF electrolytic
Couples the collector’s copy to the top diode.
R15, 2.2 MΩ
Across C10. It holds the top diode near the collector’s voltage, so the diode sits just short of conducting.
D2, 1N914 / 1N4148
Top diode. It passes the collector’s positive half.
C11, 22 µF electrolytic
Couples the emitter’s copy to the bottom diode.
R16, 820 kΩ
Across C11, for the bottom diode’s bias.
D3, 1N914 / 1N4148
Bottom diode. It passes the emitter’s positive half, which comes while the collector swings down.
R17, 100 kΩ
The diodes’ shared load to ground.
C12, 10 nF
Coupling cap into the Volume pot.
Volume, 1 MΩ pot, audio taper
Output level, a 1 MΩ audio pot.
C13, 10 nF
Coupling cap into the output buffer.
R18, 2.2 MΩ
Top of Q4’s base bias.
R19, 2.2 MΩ
Bottom of Q4’s base bias.
Q4a, MPSA13 (half)
First transistor of the Q4 Darlington.
Q4b, MPSA13 (half)
Second transistor of the Q4 Darlington, the output buffer, an emitter follower.
R20, 10 kΩ
Output buffer emitter resistor.
C14, 100 nF
Output coupling cap.
R21, 150 kΩ
Output pulldown.

About this version

2N3906 and MPSA13
Negative ground, 9 V. A 2N3906 in the first stage, MPSA13 Darlingtons in the rest, as the drawing lists them. Each Darlington is drawn and modeled as its two transistors.

Sources

  • L.A. Holt, drawing of the circuit (2006)

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.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

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