Bucket-brigade flanger, 1977 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 MXR Flanger (M-117).

A SAD1024 delay chip holds a copy of the guitar 0.5 to 10 ms behind, and a slow triangle sweeps that delay through a discrete VCO. Mixed with the dry signal, the copy cuts a row of notches that slide up and down, and Regen feeds the copy back in for sharper peaks.

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. IC1B lifts the treble 21 dB before the delay, and IC1A cuts it back down after the mix, so the delay chip’s hiss comes down with it. IC2A feeds the guitar into the SAD1024, whose two 512-stage halves run in series on one clock, so the delay is 512 clock periods. Two low-pass filters near 12.7 and 7.9 kHz after the chip smooth its steps. Mixed with the dry signal, the delayed copy cancels it at a row of frequencies spaced 1 / delay apart: at 2.6 ms the notches fall near 210, 570 and 940 Hz, about 11 dB deep. Regen sends the wet signal back into the delay and lifts the peaks between the notches. Q1 and Q2 make an exponential converter, so each volt at the Width wiper multiplies the clock by about 2.3. With Width at 0, Manual fully down holds the delay near 10.6 ms, and by half turn it reaches the shortest, 0.51 ms. With the knobs at their default settings the sweep runs 0.93 to 3.6 ms about 0.7 times a second. Width full and Manual down run it 0.51 to 8.6 ms. Speed runs the sweep about 0.07 to 7.8 times a second. The pedal’s VCO is a ramp oscillator and a 4013 divider. The model stands it in with a clock run by Q1’s current at the rate the real ramp sets, held under 1 MHz. R30 is 10 kΩ in the model, not the drawing’s 15 kΩ, which leaves the LFO stuck at the bottom. The drawing has no footswitch, so the model is the pedal with the effect on. The chip’s clock ripple at its outputs isn’t modeled.

Download.

Parts and what they do

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

R1, 30 kΩ
Top of the 7.5 V bias divider.
R2, 30 kΩ
Bottom of the bias divider.
C1, 2.2 µF electrolytic
Filters the 7.5 V bias rail.
C2, 100 nF
Input coupling cap.
R3, 1 kΩ
In series with the input.
R4, 470 kΩ
Biases IC1B’s + input at 7.5 V and sets the input impedance near 470 kΩ.
IC1B, NE5532
Input stage and pre-emphasis. It passes low notes at a gain of 1 and lifts the treble 21 dB, so the delay chip’s hiss can come down the same amount after it. NE5532, as marked on the drawing.
R5, 10 kΩ
Feedback resistor. Against R6 it sets the treble gain at 11.
R6, 1 kΩ
With C3 it sets where the treble lift levels off, near 4.8 kHz.
C3, 33 nF
Blocks DC from the feedback leg, so the gain is 1 for low notes and starts rising near 440 Hz.
R8, 150 kΩ
Mixing resistor for the dry signal. Against R9 its gain is 0.67.
R11, 82 kΩ
Mixing resistor for the wet signal. Against R9 its gain is 1.2, which makes up some of what the delay side loses.
R7, 30 kΩ
Biases IC1A’s + input at 7.5 V.
IC1A, NE5532
Output mixer and de-emphasis. It adds the dry and wet signals and cuts the treble by the 21 dB the input stage added. NE5532, as marked on the drawing.
R9, 100 kΩ
Feedback resistor of the mixer.
R10, 10 kΩ
With C4 beside R9 it cuts the treble from about 440 Hz to 4.8 kHz, the input stage’s curve turned over.
C4, 3.3 nF
In series with R10.
C5, 2.2 µF electrolytic
Output coupling cap.
R12, 10 kΩ
Holds the output at 0 V so the jack doesn’t pop.
R13, 1 kΩ
In series with the output jack.
R14, 30 kΩ
Biases IC2A’s + input at 7.5 V.
R15, 150 kΩ
Feeds the pre-emphasized guitar into the delay side. Against R17 its gain is 0.67.
IC2A, RC4558
Mixes the guitar with the Regen signal and drives the delay chip. The diodes in its feedback hold its swing near a volt, so the chip doesn’t clip hard. A 4558, as the drawing’s note says.
R17, 100 kΩ
Feedback resistor of the delay side’s mixer.
C8, 100 pF
Rolls the top end off above about 16 kHz.
R18, 27 kΩ
In series with the diodes. Once they conduct, it pulls the stage’s gain down to about a fifth of what R17 sets.
D1, D2, 1N914 / 1N4148, one each way
Two 1N4148s back to back. Past about half a volt across the feedback they start to conduct and round the peaks off.
C7, 100 nF
Couples the wet signal into Regen.
Regen, 50 kΩ pot, linear taper
Regen, 50 kΩ linear. It sets how much of the delayed signal goes back into the delay, so how deep the comb’s peaks get.
C6, 15 nF
With R16 it keeps the lows out of the Regen path (below about 90 Hz).
R16, 120 kΩ
Regen mixing resistor. Against R17 the delayed signal comes back at up to 0.83.
R19, 1 kΩ
With C9 a low-pass near 16 kHz ahead of the delay chip.
C9, 10 nF
Low-pass cap ahead of the chip.
C10, 100 nF
Couples the signal into the delay chip.
R20, 100 kΩ
Sets the DC on the first half’s input from the bias trim.
TR1, 5 kΩ pot, linear taper
Bias trim, 5 kΩ. Its wiper sets the DC on both halves’ inputs. At half turn it sits at 7.5 V, assumed: the factory set it for the cleanest delay.
R21, 100 kΩ
Sets the DC on the second half’s input from the same trim.
IC3A, SAD1024 (one 512-stage half)
First half of the SAD1024, 512 stages. Its two outputs are joined on the drawing and drawn as one pin. Both halves run on one clock, so the whole chip is 1024 stages: a delay of 512 clock periods.
R22, 2.4 kΩ
From the first half’s outputs toward the second half.
R23, 10 kΩ
Load on the first half’s outputs, to ground.
C11, 100 nF
Couples the first half into the second.
IC3B, SAD1024 (one 512-stage half)
Second half of the SAD1024, 512 stages, on the same clock. Its two outputs are joined and drawn as one pin.
TR2, 22 kΩ pot, linear taper
Trim, 22 kΩ, a load on the chip’s outputs. Its wiper sets the wet level. At full turn, assumed: that brings the wet closest to the dry at the mixer, for the deepest notches.
C12, 100 nF
Couples the delayed signal into the filters.
R24, 30 kΩ
Input resistor of the first low-pass.
C13, 6.8 nF
First low-pass cap, to ground.
R25, 30 kΩ
Feedback resistor of the first low-pass. Against R24 the gain is 1.
R26, 5.1 kΩ
Into IC2B’s − input.
C14, 150 pF
Second low-pass cap. With C13 it sets the corner near 12.7 kHz.
R27, 30 kΩ
Biases IC2B’s + input at 7.5 V.
IC2B, RC4558
Two-pole low-pass near 12.7 kHz, turned over. It smooths the chip’s steps. A 4558.
R28, 7.5 kΩ
First resistor of the second low-pass.
R29, 2.4 kΩ
Second resistor of the second low-pass.
C15, 3.3 nF
Cap to ground on IC4A’s + input.
C16, 6.8 nF
Feedback cap from the output. With C15 it sets a two-pole low-pass near 7.9 kHz.
IC4A, RC4558
Follower of the second low-pass. Its output is the wet signal, to the mixer and to Regen. A 4558.
R30, 10 kΩ
Top of the LFO’s 7.5 V divider. 10 kΩ, not the drawing’s 15 kΩ: with 15 kΩ the reference is 6 V and the triangle can’t reach its lower turn, so the LFO stops.
R31, 10 kΩ
Bottom of the LFO’s divider.
IC6A, RC4558
Comparator with hysteresis. Its output flips to the other rail each time the triangle reaches a turn, so it puts out a square wave. A 4558.
R32, 100 kΩ
Positive feedback that holds the comparator where it last flipped.
R33, 82 kΩ
From the triangle to the comparator. Against R32 it sets the triangle’s height at about 0.8 of the square’s.
C17, 10 nF
From the comparator’s output to its − input.
R34, 100 kΩ
Holds the comparator’s − input at the LFO’s 7.5 V.
Speed, 500 kΩ pot, reverse audio taper
Speed, 500 kΩ marked Log, used as a variable resistor. Its resistance falls along an audio curve as it turns up, so the triangle ramps faster: about 0.07 to 7.8 sweeps a second.
R35, 4.7 kΩ
Sets the fastest sweep, with Speed at full.
C18, C19, 7.5 µF
Two 15 µF electrolytics back to back, drawn as one 7.5 µF. The integrator’s cap: a larger one slows every Speed setting.
IC6B, RC4558
Integrator. The square wave in makes a triangle out, about 1.7 to 13.2 V. A 4558.
Manual, 50 kΩ pot, linear taper
Manual, 50 kΩ. It sets the delay the sweep moves around. Turned up, the clock runs faster and the delay gets shorter.
R36, 100 kΩ
From Manual’s wiper to the follower.
R37, 470 kΩ
From the supply. With R36 it keeps the follower above about 2.6 V with Manual fully down.
IC4B, RC4558
Follower for Manual’s voltage, about 2.6 to 13.8 V. A 4558.
Width, 50 kΩ pot, linear taper
Width, 50 kΩ linear, between Manual’s voltage and the triangle. Its wiper picks a mix of the two, so it sets how far the sweep reaches. At 0 the delay holds where Manual sets it.
R38, 1 MΩ
Turns Width’s voltage into a current into the control node.
C20, 4.7 µF electrolytic
Smooths the control node. Against R39 and Q2 it rounds the triangle’s corners and narrows the sweep at the faster Speed settings.
R39, 22 kΩ
Between Q1’s base and Q2. The control current through it lifts Q1’s base above Q2’s, and each 0.6 µA multiplies Q1’s current by about 1.7.
Q2, BC550B
A transistor wired as a diode. Its voltage is the start of Q1’s exponential curve. BC548 on the drawing, modeled as a BC550B.
R40, 470 kΩ
Feeds Q2 a standing current from TR3.
TR3, 500 kΩ pot, linear taper
Clock trim, 500 kΩ. It sets Q2’s standing current, so the slowest clock. At 0.3, assumed: the longest delay near 10 ms.
Q1, BC550B
The VCO’s current source. Its collector current empties the 62 pF timing cap each cycle, so it sets the clock. BC548 on the drawing, modeled as a BC550B. The BC548 above it on the pedal, a cascode, is not drawn.
VCO, VCO and divider (stand-in)
Stand-in for the VCO and 4013 divider, its timing cap the pedal’s 62 pF. The clock runs at Q1’s current over about 1 nF, as the real ramp does, and stops at 1 MHz (0.51 ms), the top a builder measured. The BC557 that refills the cap, the cascode, the 4013 and their resistors are not drawn.

About this version

As drawn
Negative ground, 15 V from a single supply. The audio works around the 7.5 V R1 and R2 set. The LFO has its own 7.5 V from R30 and R31.

Sources

  • Stellan Lehrberg, redrawn schematic

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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