Analog delay on three SAD1024s, 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 Memory Man (EH1309B, three knobs).

Three SAD1024 chips in series, six 512-stage bucket brigades on one clock, delay the signal by about 15 to 280 ms. No compander: filters before and after the chips keep the clock out, and Feedback sends a share of the repeats back in.

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 sets the input gain: 1 with Boost off (rising to 2 in the treble) and about 3.7 with it on. It feeds both Blend and the delay. IC2B, R15 and C10 cut the treble ahead of the chips, so less of it aliases at the slow clock. Six 512-stage bucket brigades in three SAD1024s run in series on one clock from a CD4047, so the delay is 1536 clock periods, about 15 to 280 ms. IC2A makes up the level halfway along. IC1A, with R22 and C12 ahead of it, smooths the chips’ steps. The longer the delay, the lower the clock and the darker the repeats: they reach about 2.5 kHz at the shortest Delay, 2 kHz at half and 1 kHz full up. Feedback sends the echo back into the input filter, cut above about 2.2 kHz by R16 and C11, so each repeat is darker than the last. There’s no compander, so nothing keeps the hiss down but the filters.

Download.

Parts and what they do

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

C30, 100 µF electrolytic
Filter cap on the 15 V line after Q1, the series regulator. The regulator, transformer and rectifier are drawn as the supply.
R1, 1 MΩ
Holds the input jack at 0 V.
C1, 100 nF
Input coupling cap.
R2, 470 kΩ
Top of IC1B’s bias divider.
R3, 390 kΩ
Bottom of IC1B’s bias divider. With R2 it holds the input near 6.8 V and sets the input impedance near 210 kΩ.
IC1B, RC4558
Input stage, non-inverting. Its gain is set by R4 against the Boost switch’s networks. Its output is the dry signal and feeds the delay. RC4558, typical values.
R4, 220 kΩ
IC1B’s feedback resistor. A trimmer on the traced unit, 220 k on Electro-Harmonix’s EH-7500 drawing.
C2, 39 pF
Rolls off IC1B above about 18 kHz.
R7, 82 kΩ
With Boost on, it sets IC1B’s gain for the lows: 1 + 220 k / 82 k, about 3.7.
C5, 220 nF
Keeps R7 out at DC.
R8, 10 MΩ
Keeps C5 charged with Boost off, so the switch doesn’t pop.
R5, 62 kΩ
With C3 and Boost on, it lifts the treble further, toward about 7 times.
C3, 2.2 nF
Treble cap for Boost on, grounded by the switch.
R6, 220 kΩ
With C4 and Boost off, it lifts the treble up to twice.
C4, 620 pF
Treble cap for Boost off, grounded by the switch.
C6, 33 µF electrolytic
Couples the dry signal to Blend.
Blend, 10 kΩ pot, linear taper
Blend, 10 kΩ, between the dry signal and the echo. Turned up, more echo and less dry.
R9, 10 kΩ
In series with the output.
C7, 15 nF
With R10 and R9 it cuts the treble at the output, toward about 9 dB down at the top.
R10, 5.6 kΩ
In series with C7.
R11, 75 kΩ
Feeds the dry signal into the filter ahead of the delay.
R12, 75 kΩ
Feeds the repeats from the Feedback wiper into the same node.
C9, 1.8 nF
Filter cap from IC2B’s output.
R13, 36 kΩ
Filter resistor into IC2B.
C8, 1.8 nF
Filter cap to ground.
IC2B, RC4558
Two-pole low-pass ahead of the delay chips, gain about 2.3, with a peak near 2 kHz before the cut. It cuts the treble that would alias at the slow clock. RC4558.
R14, 55 kΩ
IC2B’s feedback resistor.
R17, 43 kΩ
With R14 it sets IC2B’s gain, 1 + 55 k / 43 k.
R15, 55 kΩ
With C10, a third pole of filtering.
C10, 1.8 nF
Filter cap to ground, a pole near 1.6 kHz with R15.
C13, 100 nF
Couples the filter into the first delay chip.
R25, 8.2 kΩ
Top of IC4’s bias trim.
IC4 Bias, 10 kΩ pot, linear taper
Bias trim for both halves of IC4, 10 kΩ from R25 to ground. At 8.5, assumed for a factory-set unit, it puts 7.0 V on both inputs, the middle of the chip’s swing, so the wave clips evenly top and bottom.
R23, 100 kΩ
Carries the bias to IC4’s pin 15.
R24, 100 kΩ
Carries the bias to IC4’s pin 2.
IC4 (B), SAD1024 (one 512-stage half)
SAD1024 half B (pin 15 in, pins 11 and 12 out), 512 stages, the first of six in series. All six run on one clock, 3072 stages, so the delay is 1536 clock periods. Output resistance 300 Ω, assumed, so with its 2.4 k load it passes about 0.9 of its input.
R26, 2.4 kΩ
Load on IC4’s half B outputs.
C18, 1 µF electrolytic
Couples half B into half A.
IC4 (A), SAD1024 (one 512-stage half)
SAD1024 half A (pin 2 in, pin 6 out), 512 stages, on the same clock.
R27, 3.3 kΩ
Load on IC4’s half A output.
C19, 1 µF electrolytic
Couples IC4 into IC5.
R29, 8.2 kΩ
Top of IC5’s bias trim.
IC5 Bias, 10 kΩ pot, linear taper
Bias trim for IC5: pin 15 through R28, and IC2A’s + input through R30, so IC2A’s output holds pin 2 at the same DC. At 8.5, assumed, 7.0 V.
R28, 100 kΩ
Carries the bias to IC5’s pin 15.
IC5 (B), SAD1024 (one 512-stage half)
SAD1024 half B (pin 15 in, pins 11 and 12 out), 512 stages, on the same clock.
R31, 2.4 kΩ
Load on IC5’s half B outputs.
C21, 1 µF electrolytic
Couples half B into IC2A.
R33, 56 kΩ
IC2A’s input resistor, with the Gain trim.
100k Gain, 100 kΩ pot, linear taper
Gain trim, 100 kΩ as a variable resistor in series with R33. IC2A’s gain is 100 k / (56 k + the trim): about 1.5 at 0.1, assumed, set with the 5k Gain trim so the first repeat comes back at the dry level.
R30, 100 kΩ
Carries the bias to IC2A’s + input.
C20, 1 µF electrolytic
Filters the bias on IC2A’s + input.
IC2A, RC4558
Make-up amp between the two halves of IC5, inverting. It wins back what the first three halves cut, and its output sits at the bias, so it feeds IC5’s pin 2 directly. RC4558.
R32, 100 kΩ
IC2A’s feedback resistor.
IC5 (A), SAD1024 (one 512-stage half)
SAD1024 half A (pin 2 in, pin 6 out), 512 stages, on the same clock.
R34, 2.4 kΩ
Load on IC5’s half A output.
C23, 1 µF electrolytic
Couples IC5 into IC6.
R37, 8.2 kΩ
Top of IC6’s bias trim.
IC6 Bias, 10 kΩ pot, linear taper
Bias trim for both halves of IC6. At 8.5, assumed, 7.0 V on both inputs.
R36, 100 kΩ
Carries the bias to IC6’s pin 15.
R35, 100 kΩ
Carries the bias to IC6’s pin 2.
IC6 (B), SAD1024 (one 512-stage half)
SAD1024 half B (pin 15 in, pins 11 and 12 out), 512 stages, on the same clock.
R38, 3.3 kΩ
Load on IC6’s half B outputs.
C22, 1 µF electrolytic
Couples half B into half A.
IC6 (A), SAD1024 (one 512-stage half)
SAD1024 half A (pin 2 in), 512 stages, the last of the six. Its two outputs, pins 5 and 6, go to the ends of the Balance trim. Here they are one pin. Clock feedthrough isn’t modeled.
Balance, 1 kΩ pot, linear taper
Balance trim, 1 kΩ, across IC6’s two outputs (pins 5 and 6). It balances them so the clock cancels. Here both ends are on the one output pin, so at half it’s 250 Ω in series. The clock it cancels isn’t modeled.
5k Gain, 5 kΩ pot, linear taper
Gain trim, 5 kΩ as a variable resistor from the delayed signal to ground. It loads IC6’s outputs, so it sets the echo level too. At 0.8 (4 kΩ), assumed, with the 100k Gain trim the first repeat comes back at the dry level.
R20, 51 kΩ
Carries the delayed signal’s DC to IC1A’s + input, so its output sits at the same DC.
C17, 4.7 µF electrolytic
Keeps the audio off IC1A’s + input.
R22, 27 kΩ
First resistor of the filter after the delay chips.
C12, 5.6 nF
Filter cap to ground.
R21, 27 kΩ
IC1A’s input resistor. With R18 the gain is 82 k / 27 k, about 3.
C16, 6.8 nF
Filter cap to ground.
R18, 82 kΩ
Feedback from IC1A’s output.
R19, 18 kΩ
Into IC1A’s − input.
C15, 330 pF
Feedback cap of the multiple-feedback low-pass.
IC1A, RC4558
Output filter, a multiple-feedback low-pass turned over, gain about 3. With R22 and C12 ahead of it, it smooths the chips’ steps. Its output is the echo, to Blend and to Feedback. RC4558.
C14, 4.7 µF electrolytic
Couples the echo to Blend.
C27, 3.3 µF
Couples the echo to Feedback. Mounted under the board on the traced unit, not on Electro-Harmonix’s EH-7500 drawing.
R16, 330 Ω
In series with C27.
C11, 220 nF
With R16 it cuts the treble going back in above about 2.2 kHz, so each repeat is darker.
Feedback, 10 kΩ pot, linear taper
Feedback, 10 kΩ. It sets how much of the echo goes back into the delay, so how many repeats.
IC3, CD4047 astable
CD4047 wired as an oscillator. Its Q and Q-bar outputs are the two clock phases for all three SAD1024s. It runs at 1 / (4.4 R C): C26, 390 pF, drawn as this part’s cx, and R the 5.6 k R40 plus Delay. About 102 kHz with Delay at 0, 5.5 kHz full up. The 4.4 is the data sheet’s typical figure.
R40, 5.6 kΩ
In series with Delay. It sets the fastest clock, with Delay at 0.
Delay, 100 kΩ pot, linear taper
Delay, 100 kΩ as a variable resistor in series with R40. Turned up, its resistance rises and the clock slows, so the delay gets longer: about 15 ms at 0, 147 ms at half and 279 ms full up. Linear assumed.

About this version

As traced
Negative ground, on a 15 V supply. The op-amps work around their own bias dividers, and each delay chip’s inputs are biased by a trim.

Sources

  • Szabó Máté, traced 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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