Four-band transistor booster with an op-amp mixer, early 1970s 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 Maestro FRB-1 Full Range Booster.

A follower feeds four transistor stages, each tuned to one band from treble to bass. A switch per band lets it through to the op-amp mixer at full level, and a Fuzz switch puts two diodes across the mixer’s feedback.

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 drives four transistor stages at once. Each has a gain under 7 and its own band: the treble stage’s 2 nF input cap passes only the highs, the bass stage’s 10 nF from the supply to its collector cuts above about 160 Hz, and the two middle ones sit between. With every switch off, each band reaches the 741 through 570 kΩ, and together they add up to a nearly flat response with about 2 dB of gain. A band switch shorts its 470 kΩ, so that band comes in 5 times as loud: the treble switch adds about 8 dB at 5 kHz. Probe O1 and turn the bands on one at a time.

Download.

Parts and what they do

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

C1, 1 µF electrolytic
Input coupling cap.
R1, 390 kΩ
From the supply to Q1’s base. With R2 it sets the base near 9.7 V.
R2, 470 kΩ
Base to ground. With R1 it sets the input impedance near 210 kΩ.
Q1, 2N5088
Input follower. It drives all four bands at once.
R3, 82 kΩ
Q1’s emitter load.
C2, 2 nF
Into the treble stage. Against the base’s low input resistance, 2 nF passes mostly the highs.
R5, 200 kΩ
Collector to base. It biases the stage and, with the 33 kΩ to ground, sets the collector near 6 V.
R6, 33 kΩ
Base to ground.
Q2, 2N5088
The treble band’s amplifier, common emitter. Its gain is about 100 kΩ over 15 kΩ, under 7.
R4, 100 kΩ
Collector load.
R7, 15 kΩ
Emitter resistor, unbypassed.
C3, 10 nF
Output cap of the treble band, 10 nF against 100 kΩ and more, so it passes the highs only.
R20, 100 kΩ
Into the mixer with the S4 switch closed.
R21, 470 kΩ
In series with the 100 kΩ while S4 is open, so the band reaches the mixer at about a fifth of the level.
C4, 10 nF
Into the upper mid stage. It sets the low end of this band.
R9, 200 kΩ
Collector to base, as on the treble stage.
R10, 33 kΩ
Base to ground.
Q3, 2N5088
The upper mid band’s amplifier, common emitter. Its gain is about 100 kΩ over 15 kΩ, under 7.
R8, 100 kΩ
Collector load.
R11, 15 kΩ
Emitter resistor, unbypassed.
C6, 100 nF
Output cap of this band.
R22, 100 kΩ
Into the mixer with the S3 switch closed.
R23, 470 kΩ
In series with the 100 kΩ while S3 is open, so the band reaches the mixer at about a fifth of the level.
C5, 500 pF
From the supply to the collector. It rolls off the top of this band against the 100 kΩ load.
C7, 47 nF
Into the mid stage. It sets the low end of this band.
R13, 200 kΩ
Collector to base, as on the treble stage.
R14, 33 kΩ
Base to ground.
Q4, 2N5088
The mid band’s amplifier, common emitter. Its gain is about 100 kΩ over 15 kΩ, under 7.
R12, 100 kΩ
Collector load.
R15, 15 kΩ
Emitter resistor, unbypassed.
C9, 100 nF
Output cap of this band.
R24, 100 kΩ
Into the mixer with the S2 switch closed.
R25, 470 kΩ
In series with the 100 kΩ while S2 is open, so the band reaches the mixer at about a fifth of the level.
C8, 2.7 nF
From the supply to the collector. It rolls off the top of this band against the 100 kΩ load.
C10, 1 µF electrolytic
Into the bass stage, 1 µF, so the whole range reaches it. Its rail cap cuts the top.
R17, 200 kΩ
Collector to base, as on the treble stage.
R18, 33 kΩ
Base to ground.
Q5, 2N5088
The bass band’s amplifier, common emitter. Its gain is about 100 kΩ over 15 kΩ, under 7.
R16, 100 kΩ
Collector load.
R19, 15 kΩ
Emitter resistor, unbypassed.
C12, 100 nF
Output cap of this band.
R26, 100 kΩ
Into the mixer with the S1 switch closed.
R27, 470 kΩ
In series with the 100 kΩ while S1 is open, so the band reaches the mixer at about a fifth of the level.
C11, 10 nF
From the supply to the collector. It rolls off the top of this band against the 100 kΩ load.
R28, 47 kΩ
Top of the mixer’s reference divider.
R29, 47 kΩ
Bottom of the divider.
C13, 50 µF electrolytic
Filters the reference.
IC1, LM741 / µA741
The mixer, a 741, inverting. Each band’s gain is 1.5 MΩ over its input resistance: 15 with its switch on, about 2.6 off.
R30, 1.5 MΩ
The mixer’s feedback resistor.
C14, 25 nF
With Fuzz off, in series with R31 across the feedback. The gain falls above about 35 Hz toward 180 kΩ over the input resistance.
R31, 180 kΩ
With C14, across the feedback while Fuzz is off.
C15, 1 µF electrolytic
Output coupling cap.
Volume, 100 kΩ pot, audio taper
Volume, 100 kΩ. The taper isn’t on the drawing: audio assumed.

About this version

Stock
Negative ground on two 9 V batteries in series, 18 V. 2N5088 transistors and a 741, as the drawing lists them.

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.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

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