Analog delay with a compander and a 4096-stage bucket brigade, 1981 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 DM-2 Analog Delay.

A 4096-stage bucket brigade delays the signal by 20 to 300 ms. An NE570 compressor ahead of it and an expander after it keep the chip’s hiss down, and a share of the delayed signal goes back in for more repeats.

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. IC1a lifts the treble up to about 14 dB before the compressor, and IC1b cuts it the same amount at the output, so the dry signal comes through flat and the hiss added in between drops. IC2a halves the signal’s range in dB, the bucket brigade delays it, and IC2b doubles the range again. Two filters around the chip cut above about 3 kHz, so each repeat loses more top end, and the chip’s noise and clipping add up with every pass. Repeat Rate sets the clock from about 6.8 kHz (300 ms) to 102 kHz (20 ms), and most of that change comes in the last quarter of its turn. Echo sets how loud the repeats are. Intensity sends some of them back, and near full they run away. The footswitch, its flip-flop and the FET switch are left out, so the model is the pedal with the effect on. Each render needs a few delay times to settle, so it takes longer than most circuits.

Download.

Parts and what they do

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

C37, 100 µF electrolytic
Bulk cap across the supply.
R27, 22 Ω
With C38 it filters the supply for the bucket brigade’s side (VC, the solid arrows on the drawing).
C38, 220 µF electrolytic
Filter cap on VC.
R10, 10 kΩ
Top of the 4.5 V divider.
R33, 10 kΩ
Bottom of the 4.5 V divider. Both op-amps, Q1 and the switch work around it.
C26, 47 µF electrolytic
Filters the 4.5 V line.
R1, 10 kΩ
In series with the input.
C1, 47 nF
Input coupling cap. With R4 the corner is about 7 Hz.
R4, 470 kΩ
Biases Q1’s base from the 4.5 V line and sets the input impedance near 470 kΩ.
Q1, 2SC1815
Input emitter follower. 2SC732TM-GR on the drawing. 2SC1815 values.
R5, 10 kΩ
Q1’s emitter load.
C2, 1 µF electrolytic
Couples Q1 into IC1a.
R7, 47 kΩ
Input resistor of IC1a for the lows. With R8 the gain is 1 there.
C3, 6.8 nF
With R6 it adds a second input path for the treble. Its gain starts to rise above about 300 Hz.
R6, 10 kΩ
In series with C3. With R7 alongside, the treble gain tops out near 47 k / 8.2 k, about 5.7 times.
R9, 10 kΩ
Holds IC1a’s + input on the 4.5 V line.
IC1a, JRC4558D
Pre-emphasis stage, inverting. Flat at 1 for the lows, up to about 14 dB in the treble, so the treble goes through the delay louder than the hiss. uPC4558C on the drawing. JRC4558D values.
R8, 47 kΩ
IC1a’s feedback resistor.
C4, 100 pF
Rolls off IC1a’s gain above about 34 kHz.
C6, 10 µF electrolytic
Couples IC1a into the compressor’s input pin.
IC2 (1 to 8), NE570 (one half)
NE570 half wired as a compressor. The input goes in at pin 6, and the gain cell and rectifier listen to the output through C9. Every 2 dB change at the input is about 1 dB at the output, so quiet parts go into the delay louder.
C10, 220 nF
Compressor’s rectifier cap. With the 10 k inside the chip it averages the level over about 2.2 ms.
C9, 10 µF electrolytic
Feeds the compressor’s output to its rectifier and gain cell.
R12, 10 kΩ
DC feedback from the output to pin 5, with R11.
R11, 10 kΩ
DC feedback from the output, with R12.
C7, 10 µF electrolytic
Grounds the middle of R11 and R12 for audio, so they hold the DC only.
R13, 22 kΩ
Feeds the repeats from the Intensity wiper into the compressor’s summing node, next to the input through the 20 k inside the chip.
C8, 100 nF
Couples the Intensity wiper into R13.
R14, 10 kΩ
First resistor of the filter ahead of the bucket brigade.
C11, 6.8 nF
Filter cap to ground.
R15, 10 kΩ
Filter resistor.
C13, 82 nF
Filter cap from Q2’s emitter. With C14 and the 10 k resistors it makes a low-pass that cuts above about 2 kHz, so less of the treble aliases in the delay.
R16, 10 kΩ
Filter resistor into Q2’s base.
C14, 330 pF
Filter cap to ground at Q2’s base.
Q2, 2SC1815
Emitter follower of the input filter. 2SC732TM-GR on the drawing, 2SC1815 values.
R17, 10 kΩ
Q2’s emitter load.
C15, 1 µF electrolytic
Couples the filter into the bucket brigade.
VR1 Bias, 22 kΩ pot, linear taper
Bias trim. Sets the DC on the bucket brigade’s input. At half, as a factory-trimmed unit, about 4.49 V, the middle of the chip’s swing, so the wave clips evenly top and bottom.
C16, 1 µF electrolytic
Filters the bias.
R18, 100 kΩ
Carries the bias to the bucket brigade’s input.
IC4, MN3101
MN3101 clock chip. Its timing cap is C39, 100 pF, between pins 5 and 7’s side of the network (drawn as this part’s cx). The current out of pin 6 through VR3 and R47 and through R48 and Repeat Rate sets the clock. kf 3.0 and a 15 k series term are fitted to the published 20 to 300 ms. R49 22 k (pin 7) is left out.
VR3 Clock, 1 MΩ pot, linear taper
Clock trim, wired as a variable resistor. It sets the slowest clock, with Repeat Rate at 0. Set at 0.8, assumed for a factory-trimmed unit: about 6.8 kHz, 300 ms.
R47, 100 kΩ
In series with VR3.
R48, 18 kΩ
In series with Repeat Rate. With the knob full up it sets the fastest clock, about 102 kHz, 20 ms.
Repeat Rate, 1 MΩ pot, linear taper
Delay time, as a 1 MΩ variable resistor beside VR3. Turned up, its resistance drops and the clock runs faster, so the delay gets shorter: about 300 ms at 0, 211 ms at half, 73 ms at 0.9 and 20 ms full up. Lugs as the pedal is built.
IC3, MN3005
MN3005 bucket brigade, 4096 stages. Each clock cycle moves the samples one stage on, so the delay is 2048 clock periods. Its two outputs are drawn as one pin. Swing set to 1.6 V for the 9 V supply. Clock feedthrough isn’t modeled.
R19, 100 kΩ
Load for OUT1, from VC.
R20, 100 kΩ
Load for OUT2, from VC. With the two outputs as one pin, R19 and R20 are in parallel.
VR2 Cancel, 10 kΩ pot, linear taper
Cancel trim, across OUT1 and OUT2. It balances the two so the clock cancels. Here both ends are on the one output pin, so at half it’s 2.5 kΩ in series. The clock it cancels isn’t modeled.
R21, 10 kΩ
First resistor of the filter after the bucket brigade.
C17, 2.2 nF
Filter cap to ground. It smooths the steps of the held output.
R22, 10 kΩ
Filter resistor.
C19, 33 nF
Filter cap from Q3’s emitter.
R23, 10 kΩ
Filter resistor into Q3’s base.
C18, 1 nF
Filter cap to ground at Q3’s base.
Q3, 2SC1815
Emitter follower of the first output filter. 2SC732TM-GR on the drawing, 2SC1815 values.
R25, 10 kΩ
Q3’s emitter load.
R24, 10 kΩ
Second filter’s first resistor.
C20, 39 nF
Filter cap from Q4’s emitter. Against C21’s 330 pF it makes a peak near 2 kHz before the cut, so the two output filters together are about 5 dB up at 2 kHz and 14 dB down at 5 kHz.
R26, 10 kΩ
Filter resistor into Q4’s base.
C21, 330 pF
Filter cap to ground at Q4’s base.
Q4, 2SC1815
Emitter follower of the second output filter. 2SC732TM-GR on the drawing, 2SC1815 values.
R28, 10 kΩ
Q4’s emitter load.
C22, 10 µF electrolytic
Couples the filter into the expander.
IC2 (9 to 16), NE570 (one half)
NE570 half wired as an expander. The rectifier and gain cell take the delayed signal, and pins 10 and 11 are joined. Every 1 dB at its input is about 2 dB at its output, which undoes the compressor and pushes the delay line’s hiss down when the signal is quiet.
C12, 220 nF
Expander’s rectifier cap, 2.2 ms with the 10 k inside the chip, the same as the compressor’s so the two track.
C23, 1 µF electrolytic
Couples the expander to Q5. Q5, the effect switch, is drawn as a wire: it’s on with the effect on.
R31, 1 MΩ
Holds Q5’s source side on the 4.5 V line.
R30, 1 MΩ
Holds Q5’s drain side on the 4.5 V line. With Q5 on, R30 and R31 are in parallel.
C27, 1 µF electrolytic
Couples the delayed signal to the tops of Intensity and Echo.
Intensity, 50 kΩ pot, linear taper
How much of the delayed signal goes back into the compressor, so how many repeats. Linear, as drawn. Near full the repeats grow instead of dying.
Echo, 50 kΩ pot, linear taper
How loud the repeats are in the output. The dry signal doesn’t change.
C31, 1 µF electrolytic
Couples the Echo wiper into the mixer.
R37, 47 kΩ
Mixing resistor for the repeats.
R32, 47 kΩ
Mixing resistor for the dry signal, from IC1a’s output.
IC1b, JRC4558D
Mixer, inverting, with de-emphasis. Gain 1 for the lows, cutting toward −14 dB in the treble, which undoes IC1a’s lift. JRC4558D values.
R35, 47 kΩ
Mixer feedback resistor. With R32 and R37 the gain is 1 for the lows.
C28, 100 pF
Rolls off the mixer above about 34 kHz.
R36, 10 kΩ
In series with C29, beside R35. It brings the treble gain down to about 0.18.
C29, 6.8 nF
De-emphasis cap, the same value as C3 so the cut matches IC1a’s lift.
R34, 470 Ω
In series with the output.
C30, 1 µF electrolytic
Output coupling cap.
R3, 100 kΩ
Holds the output at 0 V so the jack doesn’t pop.

About this version

As drawn
Negative ground. The op-amps and the switch work around a 4.5 V line from R10 and R33. The bucket brigade and its bias trim run from the supply through R27.

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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© 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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