Tape echo with three playback heads and a spring reverb, 1974 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 Roland RE-201 Space Echo.

A tape echo with a loop of tape running past one record head and three playback heads, and a spring reverb, after the Roland RE-201 Space Echo. Repeat Rate sets the tape speed, so how far apart the repeats come; Mode picks which heads play and whether the reverb is in; Intensity sends the repeats back to the record head.

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. The instrument comes in through Instrument and SW3. In Echo it goes to the mic amp (Q1, Q12 and Q2), whose output feeds the dry path, the reverb driver and, through R35, the record preamp (Q3). The record amp (Q4) lifts the treble with C35 and drives the record head through R48 and the bias trap. The tape carries what was recorded past three playback heads, 20.5, 45.5 and 73.5 mm on from the record head, so each repeat comes later by the time the tape takes to cover that distance: at half Repeat Rate the tape runs at 26 cm/s and the heads play 79, 175 and 283 ms behind. The heads Mode picks feed the playback amp (Q5, Q13 and Q6), whose C59 turns the heads’ rising treble back down. Its output goes to Echo Volume and, through Intensity, back into the record preamp, so each repeat is recorded again. The tape drops treble and saturates on each pass, and the speed wavers slightly with the capstan and motor. Echo Volume feeds Q7 and the Bass and Treble stage around Q8. The dry signal (through C6 and R10) and the reverb (IC1, the tank and Q10, then the OP-13 stage Q11 and R9) join it at Q9’s base, then the FL-7 twin-T notches out what is left of the bias, and SW4 sets the output level.

Download.

Parts and what they do

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

R34, 820 Ω
Drops the 17 V line for the mic amp.
C29, 100 µF electrolytic
Filters the mic amp’s rail.
R25, 15 kΩ
Feeds the mic amp’s bias divider from a quieter rail.
C23, 100 µF electrolytic
Filters the mic amp’s bias rail, about 12.5 V.
R72, 820 Ω
Drops the 17 V line for the playback amp, about 15 V.
C60, 100 µF electrolytic
Filters the playback amp’s rail.
R63, 15 kΩ
Feeds the playback amp’s bias divider.
C52, 100 µF electrolytic
Filters the playback bias rail, about 12.5 V.
R93, 8.2 kΩ
In series with the instrument jack. With the volume pot it sets the input’s load, about 16 kΩ (the spec).
C91, 1 nF
Across R93, a treble bypass: above about 19 kHz it starts to take R93 out.
VR3 Instrument, 10 kΩ pot, linear taper
Instrument volume, 10 kΩ linear, fed at the wiper: the signal comes in on the wiper and leaves from the top lug. Turned down, the wiper shorts the signal to ground. Turned up, it passes through R93 alone.
R3, 47 kΩ
With SW3 on Echo, the instrument goes through 47 kΩ into the mic amp’s input, along with the two mic jacks.
R1, 4.7 kΩ
From the Mic 1 volume’s wiper. With the jack empty and the pot at zero it’s 4.7 kΩ to ground, and with R2 it divides the instrument down by about 21 (26 dB) before the mic amp.
R2, 4.7 kΩ
From the Mic 2 volume’s wiper, 4.7 kΩ to ground with its jack empty and its pot at zero.
C22, 1 µF electrolytic
Input coupling cap.
R21, 1 kΩ
Base stopper.
R22, 82 kΩ
Top of Q1’s bias divider.
R23, 56 kΩ
Bottom of Q1’s bias divider.
R24, 47 kΩ
Brings the divider’s 5 V to Q1’s base.
Q1, 2SC1815
First stage of the mic amp. 2SC1000GR on the sheet, 2SC1815 values here (typical).
R26, 39 kΩ
Q1’s emitter resistor for DC: about 0.11 mA.
C25, 100 µF electrolytic
Couples Q1’s emitter to the feedback point.
R28, 270 Ω
Bottom of the feedback divider. With R33 it sets the mic amp’s gain near 445 (53 dB).
R33, 120 kΩ
Feedback from the output to Q1’s emitter.
C24, 100 pF
Collector to base, rolls off the top end above the audio band.
R27, 15 kΩ
Q1’s collector load.
Q12, 2SA1015-GR
Second stage, PNP. 2SA493GR on the sheet, 2SA1015 values here (typical).
R30, 180 Ω
Q12’s emitter resistor left in for the signal.
R29, 1.5 kΩ
Q12’s emitter resistor for DC.
C27, 100 µF electrolytic
Bypasses R29.
R31, 10 kΩ
Q12’s collector load.
C26, 47 pF
Q12’s collector to base (47 pF on these boards, 100 pF on earlier ones).
C28, 470 pF
Rolls off the top end at Q12’s collector.
Q2, 2SC1815
Emitter follower, the mic amp’s output.
R32, 5.6 kΩ
Q2’s emitter load.
C90, 10 µF electrolytic
Output coupling cap. From here the signal goes to the record preamp, the reverb driver and the dry output.
R35, 27 kΩ
From the mic amp to the record preamp. Intensity feeds the repeats into the same point.
C31, 1 µF electrolytic
Coupling cap.
R36, 1 kΩ
Base stopper.
R37, 470 kΩ
Base to ground, with R38 sets Q3’s bias.
R38, 1 MΩ
Collector to base: bias and shunt feedback.
C32, 100 pF
Collector to base, rolls off the top end.
R39, 10 kΩ
Q3’s collector load.
Q3, 2SC1815
Record preamp. 2SC1000GR on the sheet.
R40, 1 kΩ
Q3’s emitter resistor, unbypassed: gain about 10.
C33, 10 µF electrolytic
Coupling cap to the recording level trim.
VR11 Rec level, 20 kΩ pot, linear taper
Recording level trim (Recording Current Adj.), 20 kΩ. It sets how hard the tape is driven. The service notes set it for −12 dBm out of the echo with −50 dBm into a mic jack.
R41, 22 kΩ
Under the trim. The level meter’s feed comes off its top (not drawn).
R42, 27 kΩ
From the trim’s wiper to the record amp.
C34, 1 µF electrolytic
Coupling cap.
R43, 150 kΩ
Top of Q4’s bias divider.
R44, 22 kΩ
Bottom of Q4’s bias divider, about 2 V.
Q4, 2SC1815
Record amp. 2SC1000GR on the sheet.
R47, 1 kΩ
Q4’s emitter resistor.
C35, 68 nF
Across R47: the record treble lift. Above about 2.3 kHz it takes R47 out, so Q4’s gain climbs with frequency, ahead of the tape’s own treble loss.
R45, 4.7 kΩ
Q4’s collector load.
C36, 1 µF electrolytic
Coupling cap to the head.
R48, 6.8 kΩ
Sets the record head’s current from Q4’s voltage, so the head is fed from a high impedance.
L21, 14 mH
Trap coil, 14 mH (MC-128). With C37 it blocks the 60 kHz bias from running back into Q4.
C37, 470 pF
Across the trap coil: the trap tunes near 62 kHz.
VR4 Repeat Rate, 1 kΩ pot, linear taper
Repeat Rate, 1 kΩ. It sets the motor’s speed: up is faster, so the repeats come sooner. The motor circuit on the power supply board is stood in for: the tape’s speed follows this wiper’s voltage in a straight line, from 12 cm/s at 0 V to 40 cm/s at 17 V, the service notes’ range.
Tape, Tape loop (RE-201 transport, typical)
The tape loop, record head and three playback heads. The record head magnetizes the tape with the current through it, the motor moves the tape past the heads, and each playback head puts out a voltage set by the rate of change of what it reads. Every figure is typical or estimated.
C51, 1 µF electrolytic
Input coupling cap from the heads.
R60, 1 kΩ
Base stopper.
R61, 82 kΩ
Top of Q5’s bias divider.
R62, 56 kΩ
Bottom of Q5’s bias divider.
R64, 47 kΩ
Brings the divider’s 5 V to Q5’s base.
C54, 47 µF electrolytic
Bootstraps R64 from the emitter, so the bias network hardly loads the heads.
Q5, 2SC1815
First stage of the playback amp. 2SC1000GR on the sheet.
R65, 39 kΩ
Q5’s emitter resistor for DC.
C55, 22 µF electrolytic
Couples Q5’s emitter to the feedback point.
R67, 270 Ω
Bottom of the feedback network.
C59, 6.8 nF
Feedback through a cap: the gain falls 6 dB an octave as the frequency rises, about 87 at 1 kHz. That undoes the heads’ rise with frequency (playback EQ).
C53, 100 pF
Collector to base, rolls off the top end.
R66, 15 kΩ
Q5’s collector load.
Q13, 2SA1015-GR
Second stage, PNP. 2SA493GR on the sheet.
R69, 180 Ω
Q13’s emitter resistor left in for the signal.
R68, 1.5 kΩ
Q13’s emitter resistor for DC.
C58, 100 µF electrolytic
Bypasses R68.
R70, 10 kΩ
Q13’s collector load.
C57, 470 pF
Rolls off the top end at Q13’s collector.
Q6, 2SC1815
Emitter follower, the playback amp’s output.
R71, 5.6 kΩ
Q6’s emitter load.
C56, 100 nF
Output coupling cap to Echo Volume and Intensity. With the pots’ load it cuts below about 80 Hz.
R73, 10 kΩ
In series with the Intensity trim, into Intensity’s wiper.
VR12 Intensity adj, 50 kΩ pot, linear taper
Intensity trim, 50 kΩ as a variable resistor. It sets how much Intensity can send back, so where on the knob the repeats run away.
C61, 2.2 nF
With R73 and the trim, a low-pass on the repeats going back to the tape: each pass loses more treble.
VR5 Intensity, 20 kΩ pot, linear taper
Intensity, 20 kΩ linear, fed at the wiper: the playback signal comes in on the wiper and leaves from the top lug into the record preamp. Up, more of each repeat is recorded again.
VR6 Echo Volume, 20 kΩ pot, linear taper
Echo Volume, 20 kΩ linear: how much of the playback reaches the output.
R74, 1 kΩ
From Echo Volume’s wiper.
C62, 470 nF electrolytic
Coupling cap into Q7.
C63, 4.7 nF
From Q7’s base to the rail, a low-pass far above the audio band.
R75, 47 kΩ
Top of Q7’s bias divider.
R76, 100 kΩ
Bottom of Q7’s bias divider, about 11 V.
Q7, 2SC1815
Emitter follower, drives the tone network.
R77, 39 kΩ
Q7’s emitter load.
C64, 1 µF electrolytic
Coupling cap into the tone network.
R79, 1 MΩ
Holds the tone network’s input at ground for DC.
R78, 10 kΩ
Input side of the bass control.
VR7 Bass, 100 kΩ pot, linear taper
Bass, 100 kΩ linear, in Q8’s feedback (a Baxandall): about ±10 dB at 100 Hz, the service notes’ figure.
C65, 39 nF
Across the input half of the bass pot.
C66, 39 nF
Across the feedback half of the bass pot.
R82, 10 kΩ
Feedback side of the bass control.
R80, 10 kΩ
From the bass wiper to Q8.
VR8 Treble, 100 kΩ pot, linear taper
Treble, 100 kΩ linear, across the input and the feedback: about ±10 dB at 5 kHz.
C67, 3.9 nF
Passes only the treble from the treble wiper.
R81, 4.7 kΩ
In series with C67.
C68, 4.7 µF electrolytic
Coupling cap into Q8.
R84, 68 kΩ
Top of Q8’s bias divider.
R83, 27 kΩ
Bottom of Q8’s bias divider, about 4.5 V.
Q8, 2SC1815
Tone amp, inverting, with the tone network from collector to base.
R87, 2.2 kΩ
Q8’s emitter resistor.
C70, 47 µF electrolytic
Bypasses R87.
R85, 3.9 kΩ
Q8’s collector load.
C69, 4.7 µF electrolytic
Couples Q8’s collector to the tone network’s feedback side.
R86, 22 kΩ
The echo into the output buffer, DC-coupled: Q8’s collector also biases Q9.
R9, 27 kΩ
The OP-13 stage’s output (the reverb, or the instrument on Normal) into the mix at Q9’s base, which a shielded wire carries from the OP-13 board.
Q9, 2SC1815
Output buffer, an emitter follower.
R88, 4.7 kΩ
Q9’s emitter load.
C72, 10 µF electrolytic
Output coupling cap (R89 1.5 kΩ after it was taken out on these boards).
C300, 2.2 nF
FL-7, twin-T notch near 72 kHz: keeps the bias oscillator’s leak off the output. Capacitor arm.
C301, 2.2 nF
Twin-T, capacitor arm.
R301, 470 Ω
Twin-T, the capacitor arm’s leg to ground.
R300, 1 kΩ
Twin-T, resistor arm. The audio passes through R300 and R302.
R302, 1 kΩ
Twin-T, resistor arm.
C302, 2.2 nF
Twin-T, the resistor arm’s leg to ground (with C303).
C303, 2.2 nF
Twin-T, the resistor arm’s leg to ground (with C302).
R92, 3.3 kΩ
Output divider, H to M.
R91, 1.5 kΩ
Output divider, M to L.
R90, 560 Ω
Output divider, L to ground.
R50, 330 kΩ
From the mic amp’s output to the reverb driver, with R51 about 24 dB down.
C40, 2.2 nF
Into IC1’s input (added on these boards). With R52 it cuts the lows going to the springs below about 600 Hz.
R51, 22 kΩ
Bottom of the reverb driver’s input divider.
R52, 100 kΩ
Biases IC1’s input from its own 5 V pin.
IC1 bias, upper, 24 kΩ
Not on the sheet: the TA7200P’s own bias for pin 9 (5 V), stood in for by a divider.
IC1 bias, lower, 10 kΩ
Not on the sheet: the lower half of the stand-in for the chip’s bias.
C41, 47 µF electrolytic
Filters IC1’s bias pin.
IC1, NE5532
Reverb driver, a TA7200P power amp chip. The bench’s op-amp with NE5532 values stands in, on the 17 V line. Its output current isn’t limited.
R54, 100 kΩ
Feedback (added on these boards). With R53, a gain of about 120 (42 dB).
R53, 820 Ω
Bottom of the driver’s feedback.
C42, 4.7 µF electrolytic
Keeps the driver’s DC gain at one.
C47, 47 µF electrolytic
Output coupling cap to the tank.
R59, 10 Ω
In series with the tank’s input coil.
Tank in R, 800 mΩ
Resistance of the tank’s input coil. A measured long tank stands in for the Z-3F tank. The current through this coil drives the springs, which play from the measured tank’s impulse response.
Tank in L, 1.3 mH
Inductance of the tank’s input coil, about 8 Ω at 1 kHz. Typical of a low-impedance tank.
Tank out L, 360 mH
The tank’s output coil, about 2250 Ω at 1 kHz, measured on the stand-in tank. The springs come back in series with it.
Tank out R, 181 Ω
Resistance of the tank’s output coil.
C48, 470 nF electrolytic
Coupling cap from the tank.
R56, 390 kΩ
Collector to base: bias for Q10.
R55, 120 kΩ
Base to ground.
Q10, 2SC1815
Reverb recovery. 2SC1000GR on the sheet.
R57, 1 kΩ
Q10’s emitter resistor.
C49, 47 µF electrolytic
Bypasses R57: full gain.
R58, 10 kΩ
Q10’s collector load.
C50, 4.7 µF electrolytic
Coupling cap from the recovery.
VR10 Reverb adj, 50 kΩ pot, linear taper
Reverb level trim, 50 kΩ. Set at half, assumed.
C92, 4.7 nF
On the trim’s wiper, softens the reverb’s top end.
VR9 Reverb Volume, 50 kΩ pot, linear taper
Reverb Volume, 50 kΩ linear, from the OP-13 stage’s input to ground, fed at the wiper through R11. Turned down, the wiper shorts the reverb to ground. Turned up, it reaches the stage through less of the track.
R11, 100 kΩ
From SW1a into Reverb Volume’s wiper.
C1, 1 µF electrolytic
Coupling cap into Q11.
R6, 47 kΩ
Q11’s base to ground.
R5, 220 kΩ
Collector to base: bias and shunt feedback.
C2, 47 pF
Collector to base, rolls off the top end.
Q11, 2SC1815
The OP-13 stage. It amplifies the reverb, and on Normal the instrument as well.
R7, 560 Ω
Q11’s emitter resistor.
C4, 10 µF electrolytic
Bypasses R7.
R8, 27 kΩ
Q11’s collector load.
C3, 470 nF electrolytic
Couples Q11’s collector to R9.
R10, 15 kΩ
The dry signal into the mix at Q9’s base. With R9 and R86 on the other side it sets the dry level, about 8 dB under the mic amp’s output.
R4, 100 kΩ
From the From PA jack’s tip into the mic amp’s input. With nothing plugged in the tip is open and R4 does nothing.
C6, 330 nF electrolytic
Couples the mic amp’s output to R10. It comes through the From PA jack’s switch, closed with nothing plugged in.

About this version

As drawn, 1978
Boards OP-13, OP-14B and FL-7, as on units from serial 587250. The supply is drawn as a 17 V source.

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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