Analog delay with clock-tracking filters, 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 Analog Delay (M-118).

Six 512-stage bucket brigades in three SAD1024s delay the signal by about 15 to 230 ms. An NE571 compressor ahead of them and an expander after keep the hiss down. The filters on both sides run through CMOS gates that close for a short pulse on every clock cycle, so as the delay gets longer and the clock slows, the filters close down with it.

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. IC10b buffers the guitar and feeds the dry side of Mix and the compressor, IC8b. Its output passes a diode limiter, then two two-pole low-pass filters, then the six delay halves, then two more low-pass filters and the expander, IC8a. Each filter resistor charges its cap only while its CD4016 gate is closed, about 0.6 µs on every cycle of the clock oscillator. At the shortest delay the gates are closed most of the time and the filters run near 20 kHz. At the longest the gates are closed about 7 % of the time, so the resistors act about 15 times larger and the filters close down near 1.4 kHz, keeping the clock out of the audio band all the way. Regen sends the expander’s output back to the compressor. Mix sets dry against wet into IC10a, which inverts.

Download.

Parts and what they do

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

C37, 15 µF electrolytic
Filter cap on the 12 V line after the 340L12 regulator. The mains transformer, rectifier and regulator are drawn as the supply.
R43, 470 kΩ
Top of the 6 V divider (Vref).
R44, 470 kΩ
Bottom of the 6 V divider.
C38, 4.7 µF electrolytic
Filters the 6 V line.
R1, 1 kΩ
In series with the input.
C1, 10 nF
Input coupling cap. With R2 the corner is about 34 Hz, so the lowest notes come through slightly thinner.
R2, 470 kΩ
Biases IC10b from the 6 V line and sets the input impedance near 470 kΩ.
IC10b, MC1458
Input buffer, a follower. CA1458 on the drawing (the bench’s MC1458, the same part).
C2, 150 nF
Couples the buffer into the compressor. 0.1 µF on the factory sheet.
R3, 47 kΩ
Input resistor into the compressor’s summing node.
C3, 4.7 nF
Across R3. It lifts the treble into the compressor above about 720 Hz.
IC8b, NE571 (one half)
NE571 half wired as a compressor. The input comes into the summing node through R3. The gain cell and rectifier read the output, so the louder the output, the less gain. 20 dB of range in comes out as about 10 dB.
R6, 36 kΩ
Half of the compressor’s DC feedback. With R7 and the chip’s 30 k it sets the output near 6.1 V.
R7, 36 kΩ
Other half of the DC feedback.
C6, 1 µF electrolytic
Grounds the middle of R6 and R7 for audio, so they set only the DC.
C7, 1 µF electrolytic
Feeds the compressor’s output to its rectifier and gain cell.
C8, 470 nF
The compressor’s rectifier cap. With the chip’s 10 k it sets how fast the gain follows the level, about 5 ms.
R8, 10 MΩ
10 MΩ from the 12 V line into the rectifier cap: a small standing current, so the compressor’s gain stays bounded when there’s no signal.
R5, 33 kΩ
Feeds the repeats into the compressor’s summing node, beside the input.
C5, 150 nF
Couples the Regen wiper back into the compressor. 0.1 µF on the factory sheet.
R9, 2 kΩ
Feeds the limiter diodes and the first filter.
C9, 15 µF electrolytic
Couples the limiter diodes to the signal line.
D1, 1N914 / 1N4148
With D2, clips the signal at about 0.6 V peak before the delay chips, so a hard pick can’t drive them past their range.
D2, 1N914 / 1N4148
The other half of the limiter, for the negative peaks.
TP1, 15 kΩ
Trim, 20 kΩ, set to 15 kΩ. It sets the first filter’s corner. Drawn as a fixed resistor at its setting.
R10, 2 kΩ
Rest of the first filter’s first resistor.
IC5B, CD4016B
CD4016 gate in series with R9, TP1 and R10. It closes for about 0.6 µs on every clock oscillator cycle, so those 19 kΩ charge C11 only that share of the time.
C11, 1 nF
Sallen-Key feedback cap to IC6b’s output.
IC5A, CD4016B
CD4016 gate in series with R11.
R11, 10 kΩ
Second filter resistor.
C10, 300 pF
Filter cap to ground. With the gates closed all the time the filter would turn down near 22 kHz.
IC6b, RC4558
Follower closing the first filter.
TP2, 14.6 kΩ
Trim, 20 kΩ, set to 14.6 kΩ. It sets the second filter’s corner. Drawn as a fixed resistor at its setting.
R12, 4.7 kΩ
Rest of the second filter’s first resistor.
IC5D, CD4016B
CD4016 gate in series with TP2 and R12.
C13, 750 pF
Sallen-Key feedback cap to IC6a’s output.
IC5C, CD4016B
CD4016 gate in series with R13.
R13, 10 kΩ
Second filter resistor.
C12, 470 pF
Filter cap to ground.
IC6a, RC4558
Follower closing the second filter.
R14, 10 kΩ
With R15 a divider into the delay chips, and with C14 one more pole near 26 kHz.
R15, 6.8 kΩ
Bottom of the divider: the signal into the chips is 0.4 of the filter’s output.
C14, 1.5 nF
Fixed pole across R15.
C15, 150 nF
Couples the filtered signal into the first delay half.
R30, 56 kΩ
Top of the bias divider.
TP4 Bias, 20 kΩ pot, linear taper
Bias trim, 20 kΩ, set to 15.05 kΩ (read here from the R31 end). It sets the DC on every delay chip’s input, 5.1 V here. MXR’s note: it also sets how much the repeats feed back at the longest delays.
R31, 30 kΩ
Bottom of the bias divider.
C22, 15 µF electrolytic
Keeps the signal off the bias line.
R16, 100 kΩ
Bias into IC7a’s input.
IC7a, SAD1024 (one 512-stage half)
SAD1024 half A, 512 stages. Its two outputs are joined, as on the board. Each half here passes 1.4 times its input, assumed: as drawn the path to the expander loses about 18 dB more than the compander can make up, and the repeats would come back near 38 dB down.
R17, 4.7 kΩ
Load on IC7a’s outputs.
C16, 150 nF
Couples IC7a into IC7b.
R18, 100 kΩ
Bias into IC7b’s input.
IC7b, SAD1024 (one 512-stage half)
SAD1024 half B, 512 stages.
R19, 4.7 kΩ
Load on IC7b’s outputs.
R20, 47 kΩ
In series between the chips. With R21’s 100 kΩ it drops the level about 3.3 dB between the chips.
C18, 150 nF
Couples IC7 into IC4.
R21, 100 kΩ
Bias into IC4a’s input.
IC4a, SAD1024 (one 512-stage half)
SAD1024 half A, 512 stages.
R22, 4.7 kΩ
Load on IC4a’s outputs.
C19, 150 nF
Couples IC4a into IC4b.
R23, 100 kΩ
Bias into IC4b’s input.
IC4b, SAD1024 (one 512-stage half)
SAD1024 half B, 512 stages.
R24, 4.7 kΩ
Load on IC4b’s outputs.
R25, 47 kΩ
In series between the chips, as R20.
C20, 150 nF
Couples IC4 into IC2.
R26, 100 kΩ
Bias into IC2a’s input.
IC2a, SAD1024 (one 512-stage half)
SAD1024 half A, 512 stages.
R27, 4.7 kΩ
Load on IC2a’s outputs.
C21, 150 nF
Couples IC2a into IC2b.
R28, 100 kΩ
Bias into IC2b’s input.
IC2b, SAD1024 (one 512-stage half)
SAD1024 half B, the last 512 stages. Its two outputs are one pin here. On the board R29, 1 kΩ, sits between them with the balance trim across it.
TP3 Balance, 2 kΩ pot, linear taper
Balance trim, 2 kΩ, across the last half’s two outputs. Its wiper is the delayed signal. Set so the clock’s two phases cancel. Here both ends are on the one output pin.
R32, 10 kΩ
Load on the last half’s outputs, through the balance trim.
R33, 10 kΩ
First resistor of the third filter.
IC1D, CD4016B
CD4016 gate in series with R33.
C24, 1 nF
Sallen-Key feedback cap to IC3a’s output.
IC1C, CD4016B
CD4016 gate in series with R34.
R34, 10 kΩ
Second filter resistor.
C23, 300 pF
Filter cap to ground.
IC3a, RC4558
Follower closing the third filter.
R35, 10 kΩ
First resistor of the fourth filter.
IC1A, CD4016B
CD4016 gate in series with R35.
C26, 750 pF
Sallen-Key feedback cap to IC3b’s output.
IC1B, CD4016B
CD4016 gate in series with R36.
R36, 10 kΩ
Second filter resistor.
C25, 470 pF
Filter cap to ground.
IC3b, RC4558
Follower closing the fourth filter.
R37, 1 kΩ
With C27, a fixed pole near 10 kHz that catches what the gated filters leave of the clock.
C27, 15 nF
Fixed filter cap.
TP5, 1.46 kΩ
Trim, 20 kΩ, set to 1.46 kΩ. With the expander’s input it sets the level the expander sees, and so how much the repeats feed back. Drawn as a fixed resistor at its setting.
C28, 1 µF electrolytic
Couples the filtered delay into the expander.
IC8a, NE571 (one half)
NE571 half wired as an expander, the compressor’s mirror. The gain cell takes the delayed signal and the rectifier its level, so 10 dB of range in comes out as 20 dB and the hiss between notes drops.
C29, 470 nF electrolytic
The expander’s rectifier cap, the same as the compressor’s so the two track.
R38, 100 kΩ
The expander’s feedback resistor, in place of the chip’s own 20 k: it sets the expander’s gain and, with the chip’s 30 k, its output near 7.8 V.
C30, 2.2 nF
Across R38. It rolls the expander off above about 720 Hz, the mirror of C3 at the compressor.
C31, 1 µF electrolytic
Output coupling cap of the expander.
Regen, 50 kΩ pot, linear taper
How much of the delayed signal goes back into the compressor, so how many repeats. 50 kΩ linear.
C32, 1 µF electrolytic
Couples the delayed signal to Mix.
Mix, 50 kΩ pot, linear taper
Mix, 50 kΩ linear, between the dry signal and the delayed one. Up is more delay.
R39, 100 kΩ
Input resistor of the output stage. With R40 the gain is 1.
C33, 150 nF
Couples the mix into IC10a.
IC10a, MC1458
Output stage, inverting, gain 1. CA1458 on the drawing.
R40, 100 kΩ
IC10a’s feedback resistor.
C34, 47 pF
Across R40. It rolls the output off above about 34 kHz.
C35, 4.7 µF electrolytic
Output coupling cap.
R41, 100 Ω
In series with the output.
R42, 47 kΩ
Holds the output at 0 V so the jack doesn’t pop.
Clock, VCO and divider (stand-in)
Stands in for the clock: the CD4069 oscillator (IC12e, IC12d, C40 30 pF, this part’s cx, R49, and Q2 refilling it), the four 4013 halves dividing it by 16 for the delay chips, and the one-shot (D5, C41 30 pF, R52 and TP7) that closes the CD4016 gates for about 0.6 µs on each oscillator cycle. Q1’s current runs it: the delay is 1536 / clock.
Q1, 2N3904
Current sink for the oscillator. Its base sits on TP6, so its emitter current is that voltage, minus about 0.6 V, over R46, R45 and Delay. Type unreadable on both drawings. A 2N3904 here, typical.
R46, 1 kΩ
Q1’s emitter resistor.
C39, 1 µF electrolytic
Smooths the Delay pot’s end of the emitter network, so turning it doesn’t crackle.
R45, 2.4 kΩ
Sets the shortest delay with Delay at 0.
Delay, 50 kΩ pot, linear taper
Delay, 50 kΩ linear as a variable resistor. Turned up, Q1’s current falls, the clock slows and the delay gets longer: from about 15 ms to 230 ms.
R47, 100 kΩ
Top of Q1’s base divider.
TP6 Range, 20 kΩ pot, linear taper
Delay range trim, 20 kΩ, set to 16.65 kΩ (read here from the R47 end): Q1’s base at 1.7 V. It moves the whole delay range up or down.
R48, 16 kΩ
Bottom of Q1’s base divider.

About this version

As drawn
Negative ground, on a 12 V regulated supply. The op-amps work around a 6 V line from R43 and R44. The delay chips’ inputs sit on the bias trim’s 5.1 V.

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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