Bucket-brigade flanger with Filter Matrix, 1978 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 Electro-Harmonix Deluxe Electric Mistress.

A SAD1024 delay chip holds a copy of the guitar about 1 to 8 ms behind. A slow triangle sweeps the delay, and the copy mixed with the dry signal cuts a row of notches that slide up and down. Color feeds the output back in for sharper peaks. Filter Matrix stops the sweep and leaves Range to set the delay by hand.

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. IC1 takes in the guitar and the feedback from Color. IC2a lifts the treble about 1.7 times at 3 kHz before the delay, and IC2b cuts it back after the mix, so the chip’s hiss comes down with it. The SAD1024’s two halves take the same signal on opposite clock phases, so together they sample twice each clock period. Q1’s steady current charges a 47 pF cap up to the voltage from IC4b, then the LM311 empties it, so the delay follows that voltage in a straight line: about 1 ms at 1.7 V and 7.4 ms at 9.6 V. The dry and wet signals mix in equal parts and cancel at a row of frequencies 1 / delay apart: at 4.2 ms the notches fall near 117, 356 and 594 Hz, about 28 dB deep. Color sends the output back into IC1 and lifts the peaks between the notches, up to about 13 dB. In Flange, IC4a and IC4c make a triangle, about 0.05 to 12 sweeps a second, and Range sets how far it reaches: the delay sweeps about 1 to 5 ms at half Range and 0.9 to 7.8 ms at full. In Filter Matrix, Range alone sets the delay.

Download.

Parts and what they do

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

R1, 120 kΩ
Input resistor to ground. It sets the input impedance near 90 kΩ with R2.
C1, 27 nF
Input coupling cap. With R2 it cuts below about 15 Hz.
R2, 390 kΩ
Input resistor of IC1. Against R3 the guitar comes through at a gain of 1, turned over.
IC1, LM741 / µA741
Input stage, a 741. The guitar goes into its − input at a gain of 1, and the feedback from Color into its + input at a gain of 2.
R3, 390 kΩ
Feedback resistor of IC1.
Color, 10 kΩ pot, linear taper
Color, 10 kΩ linear. Its wiper picks how much of the output goes back into IC1, so how sharp the peaks between the notches get.
C19, 33 µF electrolytic
Takes Color’s far end to ground for the signal only, so IC1’s + input keeps the output’s DC.
R5, 100 kΩ pot, linear taper
Feedback trim, 100 kΩ, used as a variable resistor in series with Color. It sets the most feedback Color can reach. At 0.84, set as the factory asks: with Color full, a 400 Hz note at 0.2 V peak to peak comes out at 0.7 V at the loudest point of Range.
R39, 68 kΩ
In the feedback line from the output. C3 across it lets more of the treble back.
C3, 33 nF
Across R39. Above about 70 Hz it passes the feedback around R39.
C2, 33 nF
Couples IC1 into IC2a. With R8 it cuts below about 48 Hz.
R8, 100 kΩ
Input resistor of IC2a.
R6, 8.2 kΩ
From the 15 V supply to the top of the bias trim.
R7, 10 kΩ pot, linear taper
Bias trim, 10 kΩ. Its wiper sets the DC on the delay chip’s input. At 0.85, about 7 V, assumed: the most clean swing on 15 V.
C4, 1 µF electrolytic
Filters the bias trim’s wiper.
R9, 100 kΩ
Feeds the bias to IC2a’s + input. IC2a passes it to its output, which is the delay chip’s input.
IC2a, RC4558
Drives the delay chip, a 4558. Its gain is 1 for low notes and rises to about 1.7 at 3 kHz (R11 and C6), so the delay chip’s hiss can be cut back the same amount after it. Its output is also the dry signal.
R10, 100 kΩ
Feedback resistor of IC2a. Against R8 the gain is 1.
C5, 56 pF
Across R10. Rolls the top end off above about 28 kHz.
R11, 240 kΩ
With C6, positive feedback to IC2a’s + input that lifts the treble.
C6, 300 pF
Sets where the treble lift starts, near 1.5 kHz.
IC3, SAD1024 (both halves in turn)
SAD1024. Its two 512-stage halves take the same signal on opposite clock phases, and their outputs add up to one line sampled twice each clock period. Modeled as one 1024-stage line at the LM311’s rate: a delay of 512 of its periods. Output resistance 300 Ω, assumed.
R12, 1 kΩ pot, linear taper
Balance trim, 1 kΩ, between the two halves’ outputs. The real one evens out the clock left on the two outputs. The model has one output, so both ends are on it.
R13, 10 kΩ pot, linear taper
Gain trim, 10 kΩ, used as a variable resistor: the load on the chip’s outputs. It sets the wet level. The factory sets it so the wet matches IC1’s output. At full the model comes within about 0.5 dB.
R14, 160 kΩ
Mixing resistor for the wet signal. Against R16 its gain is 0.63.
R15, 47 kΩ
With C7, holds IC2b’s + input at the wet signal’s DC, so the output sits there too.
C7, 4.7 µF electrolytic
Keeps the signal off IC2b’s + input.
C18, 100 nF
Couples the dry signal into the mixer.
R40, 160 kΩ
Mixing resistor for the dry signal, the same as the wet one, so the two cancel fully at each notch.
IC2b, RC4558
Output mixer and de-emphasis, a 4558. It adds the dry and wet signals and cuts the treble IC2a lifted.
R16, 100 kΩ
Feedback resistor of the mixer.
R17, 68 kΩ
With C8 beside R16, cuts the treble from about 1 kHz up.
C8, 1 nF
In series with R17.
C9, 180 pF
Across R16. Smooths what’s left of the chip’s steps above about 9 kHz.
C10, 4.7 µF electrolytic
Output coupling cap.
R18, 47 kΩ
Holds the output at 0 V so the jack doesn’t pop.
R19, 150 Ω
In series with the output jack.
R22, 3.9 kΩ
Top of the LFO’s divider. Its bottom, about 6.8 V, is the LFO’s center.
R23, 2.4 kΩ
Middle of the divider.
R24, 820 Ω
Bottom of the divider. Its top, about 1.7 V, is where Range starts.
IC4a, LM358
Comparator with hysteresis, a quarter of an LM324. Each time the triangle reaches a turn its output flips to the other rail, so it puts out a square wave.
R25, 30 kΩ
Positive feedback that holds the comparator where it last flipped.
R27, 10 kΩ
From the triangle to the comparator. Against R25 it sets the triangle’s height, about 4.5 V.
Rate, 1 MΩ pot, reverse audio taper
Rate, 1 MΩ reverse audio, used as a variable resistor. Its resistance falls as it turns up, so the triangle ramps faster.
R28, 3.9 kΩ
Sets the fastest sweep, with Rate at full.
C13, C14, 16.5 µF
Two 33 µF electrolytics back to back, drawn as one 16.5 µF. The integrator’s cap.
IC4c, LM358
Integrator, a quarter of an LM324. The square wave in makes a triangle out, about 4.6 to 9.1 V.
R29, 47 kΩ
With R30, scales the triangle down to about 1.7 to 3.3 V.
R30, 27 kΩ
Bottom of the triangle’s divider.
R31, 10 kΩ
In Filter Matrix, feeds IC4b’s + input the steady 1.7 V in place of the triangle.
IC4b, LM358
Range amplifier, a quarter of an LM324. Its output is the voltage the clock ramps up to, so it sets the delay.
Range, 100 kΩ pot, linear taper
Range, 100 kΩ linear, used as a variable resistor in IC4b’s feedback. Turned up, IC4b’s gain rises from 1 to about 5.5.
C15, 220 nF
Across Range. Keeps IC4b quiet.
R41, 22 kΩ
Against Range it sets IC4b’s gain. Its far end goes to 1.7 V in Flange and to ground in Filter Matrix.
R33, 33 kΩ
From IC4b to the clock’s control pin.
C16, 1 µF electrolytic
Smooths the control voltage (a 1 µF tantalum).
D4, 1N4001
Matches Q1’s base-emitter drop, so the clock drifts less with temperature.
R34, 62 kΩ
Sets Q1’s base about 0.7 V under the diode.
R35, 1.2 MΩ
Bottom of Q1’s base divider.
R36, 100 kΩ pot, linear taper
Clock trim, 100 kΩ, used as a variable resistor. It sets Q1’s current, so the whole delay range. At 0.87, set as the factory asks: 35 kHz on the 4013 with Range full up in Filter Matrix, a 7.4 ms delay.
R37, 3.9 kΩ
Sets Q1’s most current, with the trim at 0.
Q1, 2N5087
Current source, a 2N5087. Its steady current charges the clock’s 47 pF timing cap.
IC5, IC6, LM311 ramp (stand-in)
Stand-in for the LM311 clock and the 4013 that splits it into two phases. Q1’s current charges the 47 pF timing cap (C17) up to the control voltage, then the LM311 empties it through D5, so each ramp takes longer the higher the control voltage. The delay chip runs at the LM311’s rate.

About this version

Deluxe, 1979
Negative ground, 15 V from the pedal’s regulated supply (a 7815, not drawn). The audio and the delay chip work around the 7 V the bias trim sets. The LFO works around the 6.8 V from R22 and R23.

Sources

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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