Four-stage OTA phaser, 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 Electro-Harmonix Small Stone (Issue J).

Four transconductance amps each turn the phase as the pitch goes up, and the wet signal mixed with the dry cancels at two notches. The LFO drives the four bias pins straight, so the sweep follows their diode curve.

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. Each stage is a CA3094, an OTA and a Darlington follower. The signal reaches the OTA’s minus input through 27 kΩ (30 kΩ in the first stage) and its output through 6.8 nF, and the follower’s output comes back through 27 kΩ. The OTA’s current into the cap makes the stage pass every pitch at the same level and only turn its phase, 90° at a frequency set by its bias current. Wet and dry meet at the mixer, and where the four stages have turned the phase 180° and 540° they cancel: the notches. The LFO is a fifth CA3094 ramping 33 µF up and down, with two PNPs as a latch that turns it round, and its Darlington drives the four bias pins straight. The pins are diodes, so the LFO line’s 50 mV swing (0.625 to 0.677 V) swings the bias current from about 18 µA to 140 µA a stage, and the notches from 115 Hz and 670 Hz up to 860 Hz and 5 kHz, 34 to 48 dB deep. Rate runs from about one sweep every 5 s to about 22 a second. With Color on the notches go about 14 dB deeper, the peaks between them rise about 4 dB, and the LFO runs from about one sweep every 11 s to 16 a second. The CA3094s are the CA3080’s typicals with 2N5088s for the Darlingtons, not measured.

Download.

Parts and what they do

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

R43, 15 kΩ
Top of the Vbias divider.
R44, 10 kΩ
Bottom of the Vbias divider: 3.6 V.
C14, 33 µF electrolytic
Vbias filter cap. Every OTA’s plus input sits on Vbias, so it has to be quiet.
R1, 100 kΩ
Input resistor. With Color on, the feedback comes in at its far end, so the guitar’s source resistance doesn’t change how much feedback gets in.
R26, 270 kΩ
From the input node to the Color switch. With Color off it loads the input to ground.
C1, 6.8 nF
Input cap. Against R2 it sets a corner near 50 Hz.
R2, 470 kΩ
Biases Q1 from Vbias.
Q1, 2N5088
Input buffer, first half.
R3, 10 kΩ
Q1’s collector load, which drives Q2’s base.
R4, 4.7 kΩ
Q1’s emitter resistor.
Q2, 2N5087
Input buffer, second half, a PNP. R5 from its collector back to Q1’s emitter sets the pair’s gain at about 2.
R5, 4.7 kΩ
Feedback in the input buffer. With R4 it sets the gain, 1 plus 4.7 k over 4.7 k.
C2, 10 µF electrolytic
Couples the buffer to the first stage.
R6, 30 kΩ
The dry signal into the mixer.
R7, 30 kΩ
Into the minus input. With the 27 kΩ feedback resistor and the 1 kΩ to Vbias it makes a divider, so the OTA sees about a thirtieth of the signal and stays clean.
R8, 1 kΩ
From the minus input to Vbias, the bottom of the divider.
C3, 6.8 nF
From the stage’s input to the OTA’s output. The OTA’s current into it sets the frequency where this stage turns the phase 90°. More bias current, a higher frequency.
U1, CA3094
A CA3094’s OTA, its plus input on Vbias. Its output current follows the bias current from the LFO line, so with the cap it acts as a resistor the LFO varies.
Q1a, 2N5088
The CA3094’s Darlington follower, first half. It buffers the OTA’s output so the 27 kΩ and 10 kΩ don’t load the cap.
Q1b, 2N5088
The Darlington’s second half. Its emitter is the stage’s output.
R10, 10 kΩ
The follower’s emitter load.
R9, 27 kΩ
Feedback from the output to the minus input, about equal to the input resistor, so every pitch comes out near the same level and only the phase moves.
R11, 27 kΩ
Into the minus input, as in the first stage.
R12, 1 kΩ
Bottom of this stage’s divider.
C4, 6.8 nF
This stage’s 6.8 nF. All four are the same, so the stages turn together.
U2, CA3094
This stage’s OTA, on the same LFO line as the others.
Q2a, 2N5088
This stage’s Darlington, first half.
Q2b, 2N5088
This stage’s Darlington, second half.
R14, 10 kΩ
Emitter load.
R13, 27 kΩ
Feedback, equal to the input resistor.
R15, 27 kΩ
Into the minus input, as in the first stage.
R16, 1 kΩ
Bottom of this stage’s divider.
C5, 6.8 nF
This stage’s 6.8 nF. All four are the same, so the stages turn together.
U3, CA3094
This stage’s OTA, on the same LFO line as the others.
Q3a, 2N5088
This stage’s Darlington, first half.
Q3b, 2N5088
This stage’s Darlington, second half.
R18, 10 kΩ
Emitter load.
R17, 27 kΩ
Feedback, equal to the input resistor.
R19, 27 kΩ
Into the minus input, as in the first stage.
R20, 1 kΩ
Bottom of this stage’s divider.
C6, 6.8 nF
This stage’s 6.8 nF. All four are the same, so the stages turn together.
U4, CA3094
This stage’s OTA, on the same LFO line as the others.
Q4a, 2N5088
This stage’s Darlington, first half.
Q4b, 2N5088
This stage’s Darlington, second half.
R22, 10 kΩ
Emitter load.
R21, 27 kΩ
Feedback, equal to the input resistor.
R27, 27 kΩ
The wet signal into the mixer, slightly stronger than the dry. Where the four stages have turned it 180°, and 540°, the two cancel: the notches.
C8, 100 nF
Output cap. The mixer has no buffer, so the pedal’s output impedance is the two mixing resistors, about 14 kΩ.
470k, 470 kΩ
Output pulldown, so the jack sits at 0 V.
R23, 3.3 kΩ
Carries the feedback from the fourth stage’s output.
R24, 4.7 kΩ
With R23 it divides the feedback down to about 60 %.
R25, 27 kΩ
Sets how much feedback reaches the input, against R1.
C7, 100 nF
Blocks DC in the feedback path.
R32, 4.7 kΩ
Feeds the two PNPs’ emitters.
R33, 22 kΩ
With Color off, in parallel with R34. It changes where the LFO turns round.
R34, 15 kΩ
From the emitters to Q4’s collector.
Q4, 2N5087
Half of the LFO’s switch. Q5’s collector drives its base, and its collector feeds back to Q5’s base through R35, so the pair flips one way or the other and holds.
Q5, 2N5087
The other half of the switch. Its base follows the plus input node, its collector drives Q4’s base and the minus input.
R35, 1 kΩ
From Q4’s collector to the plus input node.
R36, 1.8 kΩ
From the plus input node to C10.
C10, 100 nF
A small cap on the switch node.
R37, 27 kΩ
From the PNPs’ bases to ground, and the LFO OTA’s minus input.
U5, CA3094
The LFO’s OTA. Its output current charges and discharges C11, so C11 ramps up and down. The Rate pot sets its bias current, so how fast it ramps.
C11, 33 µF electrolytic
The LFO’s timing cap. Its ramp is the sweep.
Q6, 2N5088
The LFO CA3094’s Darlington, first half.
R40, 2 kΩ
The Darlington driver’s collector resistor, from the Sunstone’s discrete version.
Q7, 2N5088
The Darlington’s second half. Its emitter drives the four bias pins. Its collector current comes out of the switch node, so the sweep turns the switch round.
R41, 47 kΩ
Across the output transistor’s base and emitter, from the Sunstone.
R42, 100 Ω
From the LFO to the four bias pins.
Rate, 1 MΩ pot, reverse audio taper
Rate, 1 MΩ as a variable resistor into the LFO OTA’s bias pin. Turned up, less resistance, more bias current, a faster ramp. Reverse audio, as the Sunstone’s 1MC.
R38, 27 kΩ
Top of the Rate divider.
R39, 7.5 kΩ
Bottom of the Rate divider, about 2 V for the Rate pot.

About this version

Issue J, CA3094
Issue J as drawn: five CA3094s, each an OTA and a Darlington. The OTA is the CA3080’s typicals and the Darlington two 2N5088s, typical of the parts, not measured.

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, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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