Two-transistor silicon fuzz with a mid scoop, 1971 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 Shin-ei Companion FY-2.

Two silicon stages, each with feedback from collector to base. The Fuzz pot blends the first stage’s output with the second’s, and a fixed scoop network cuts the middle before the Volume pot.

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. Each stage biases itself with a resistor from collector to base, so the collectors sit low: Q1 near 3 V, and Q2 near 0.8 V, where it clips almost at once. The Fuzz pot mixes Q1’s output, through 2.2 nF, with Q2’s, through 3.3 nF. Both caps cut the lows. The scoop network passes lows through the 10 kΩ and 15 kΩ and highs through the 1 nF, and the 100 nF to ground cuts the middle between them: 1 kHz comes out about 18 dB under 300 Hz. The output is low, about 34 mV rms at a 200 mV pick with Volume full. The model leaves out the gain spread between real transistors, which moves Q2’s collector and how hard it clips.

Download.

Parts and what they do

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

C1, 47 nF
Input coupling cap.
R1, 2.2 MΩ
Feedback from Q1’s collector to its base. It sets Q1’s bias.
C2, 1 nF
Across R1. It lowers Q1’s gain at high frequencies.
Q1, 2SC536F
First stage, emitter to ground.
R2, 22 kΩ
Q1’s collector load.
C3, 47 nF
Coupling cap into Q2.
R3, 1.2 MΩ
Feedback from Q2’s collector to its base (2.2 MΩ on one kit maker’s drawing).
Q2, 2SC536F
Second stage, emitter to ground. It runs close to saturation, so it clips first.
R4, 47 kΩ
Q2’s collector load.
R5, 100 kΩ
In series with R4 to the supply. At DC Q2’s load is 147 kΩ.
C4, 47 nF
Across R5. Above about 34 Hz it shorts R5, so the load the signal sees is 47 kΩ.
C5, 3.3 nF
From Q2’s collector to one end of the Fuzz pot.
C6, 2.2 nF
From Q1’s collector to the Fuzz pot’s wiper.
Fuzz, 50 kΩ pot, linear taper
A 50 kΩ linear pot. Its wiper carries Q1’s signal. Turned up, the wiper moves toward Q2’s end and puts 50 kΩ between Q1 and the output.
R6, 10 kΩ
Lows path of the scoop network.
C7, 1 nF
Highs path of the scoop network.
R7, 15 kΩ
Joins the output to the lows path.
C8, 100 nF
Shunts the middle of the lows path to ground.
Volume, 50 kΩ pot, linear taper
Output level, a 50 kΩ pot.

About this version

2SC536F
Negative ground. Two 2SC536F transistors, as the kit maker lists for the original.

Sources

  • S. Castledine, redrawn FY-2 Fuzzmaster schematic, 2002

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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