Overview & Features
The Supreaux Deux is runoffgroove's JFET-based emulation of small Fender / Supro valve amplifiers — three JFET stages in cascade approximating the sound of a Champ-class or Supro 1×8 valve amp pushed into break-up. It runs three transistors with no source bypass caps (so the gain per stage is modest and the headroom is higher), and the JFETs run from an 18 V rail generated by an onboard charge-pump voltage doubler — which is what makes the difference between this and a typical 9 V JFET drive.
What's new in Revision 1.3
The V1.3 layout is a refinement of earlier revisions: smaller board footprint and three test points (TP1, TP2, TP3) added for easier biasing of the JFETs. The circuit itself is unchanged from the original runoffgroove design.
Features
- Three JFET stages — Q1 (2N5457), Q2 (J201), Q3 (2N5457) — emulating the cascaded triode stages of a small valve preamp
- Onboard 18 V charge-pump doubler using an ICL7660SCPA — gives the JFETs more headroom and a smoother break-up than 9 V designs
- Reverse-polarity protection (D1) on the +9 V input
- Three bias trimmers (R10, R11, R12) with dedicated test points (TP1, TP2, TP3) — set each JFET drain to ~12 V with a multimeter, no by-ear guesswork
- Optional bass-boost switch (SW1) — engages a low-frequency boost path; great for fattening up the response
- Three controls: GAIN, TONE, VOLUME — all 16 mm right-angle PCB-mount
- Fits a 1590B enclosure with the supplied drill template
Circuit Theory
Full schematic — runoffgroove's Supreaux Deux, TH Custom Effects layout V1.3 (04/2014). Original circuit and component values by runoffgroove.
Architecture
The signal path is three JFET stages in cascade: IN → Q1 (2N5457) → GAIN pot → Q2 (J201) → TONE → Q3 (2N5457) → VOLUME → OUT. Each stage is a common-source amplifier with no source bypass cap — so the gain per stage is determined by the JFET's transconductance and the source resistor, which gives a moderate, controllable amount of gain per stage and lets each stage clip softly when overdriven. The cascade of three such stages emulates the way a small valve preamp picks up complexity from each triode stage's slight non-linearity.
Stage 1 — Q1 input gain stage
Signal enters at IN through R1 (100 k) directly into Q1's gate. R2 (1 M) is the gate pulldown (sets input impedance). C1 (270 p) shunts RFI to ground above audio. Q1 is a common-source amplifier: drain via R10 (10 k bias trimmer) to VA (the 18 V rail), source via R3 (390 Ω) without a bypass cap. The drain output goes through R4 (56 k) and C3 (2 n2) to couple into the GAIN pot. TP1 is connected to Q1's drain — probe here with a multimeter to set the bias.
C2 (470 p) is a small HF-shunt cap at Q1's drain that softens the very top end before the next stage; with R4 (56 k) it sets an HF rolloff at about 6 kHz.
Bass boost network (C10, R14, SW1)
This is the Supreaux Deux's headline tweak. C10 (3 n3) and R14 (1 M) form an additional bass path that engages when SW1 (the bass-boost switch) is closed. The HP corner of this network is approximately 48 Hz, well below the low E of a standard-tuned guitar — so engaging the boost adds significant content in the deep bass / sub-bass region. With SW1 open, the bass-boost path is disconnected and the pedal has a more focused, mid-forward response. With SW1 closed, the bass blooms out for a fuller, fatter sound.
Stage 2 — Q2 (J201) middle stage
The GAIN pot (500 k log) drives Q2's gate through C4 (470 p). Q2 is a J201 — a different N-channel JFET from Q1/Q3 with a lower pinch-off voltage (VGS(off)) and a different transfer curve. Because of this, Q2 needs more bias-resistor adjustment range — hence the larger 50 k trimmer at R11 (versus 10 k for Q1 and Q3). R5 (1 k) source resistor, R6 (100 k) drain output series. TP2 is connected to Q2's drain — probe here to set the bias.
Tone stack — TONE pot
Single-knob treble cut. The TONE pot (500 k log) sits between Q2's output and ground via C5 (1 n5) — turning the pot down shunts more high-frequency content to ground (darker), turning it up reduces the shunting (brighter). The response is a single-pole low-pass with a corner that sweeps from ~210 Hz at full CW (dark) to ultrasonic at full CCW (bright).
Stage 3 — Q3 output stage
After the TONE control, signal goes through C6 (4 n7) into Q3's gate. Q3 is another 2N5457 — drain via R12 (10 k bias trimmer) to VA, source R8 (390 Ω). R9 (22 k) drain output series; C7 (1 n5) shunts HF to ground at the output node for stability (~4.8 kHz corner). TP3 is at Q3's drain — probe here to set the bias.
Output stage
Q3's output is AC-coupled by C8 (68 n) into the VOLUME pot (100 k log) and then to the OUT jack. The output HP corner (C8 / VOLUME) is at about 23 Hz — well below the low E, so the bass passes through cleanly regardless of VOLUME setting.
Power supply — the 18 V trick
9 V DC enters through D1 (1N4001) for reverse-polarity protection. C9 (100 µF) filters the +9 V rail. The ICL7660SCPA (IC2) is wired as a voltage doubler: its internal switched-capacitor circuit inverts the +9 V on pin 5, then D3 and D2 (1N5817 Schottky diodes) together with C11 (10 µF) reservoir rectify and stack the inverted output to produce ~18 V on the VA rail. R13 (100 Ω) is the rail series resistor, C12 (10 µF) is the VA decoupling cap. Schottky diodes are mandatory here — every 0.3 V drop costs headroom, and a standard silicon 1N4148 would cost twice that.
Filter & Tone Analysis
The Supreaux Deux has two user-controllable filters (TONE pot, BASS boost switch) and several fixed RC corners that shape the overall response. Working them out shows where the pedal lives in the frequency domain and what each control actually does.
Bass boost — SW1 closed
When the bass-boost switch is closed, the C10 / R14 path adds significant low-frequency content well below the guitar's fundamental range. Fattens the response; particularly useful at higher GAIN settings where it counteracts the natural HP tightening from cascading distortion stages.
Bass boost — SW1 open
With the switch open, the bass-boost path is disconnected and only the C3 inter-stage HP shapes the low end (~150 Hz approximate). Mid-forward response, less low-end thump — ideal for sitting in a busy mix or for higher-gain settings where bass mud would be a problem.
TONE control sweep
The TONE pot (500 k log) shunts HF to ground via C5 (1 n5). The pot's effective resistance sets the corner of a single-pole low-pass filter.
At TONE fully clockwise (pot wiper at high-resistance end): RTONE ≈ 500 kΩ, so the shunt-to-ground impedance is high and very little HF is removed → bright. The LP corner sits at f ≈ 1 / (2π · 500 k · 1 n5) ≈ 210 Hz, but at that low impedance the HF still passes mostly unattenuated through the signal path.
At TONE fully counter-clockwise: RTONE ≈ 0 Ω, so C5 effectively shorts HF to ground via the wiper → dark. The pedal loses high-frequency content above a few hundred Hz.
Other fixed filter corners
- Input RFI rolloff (C1 / R1): 1 / (2π · 100 k · 270 p) ≈ 5.9 kHz — softens the input transient, removes RF noise.
- Q1 drain HF shunt (C2 / R4): 1 / (2π · 56 k · 470 p) ≈ 6 kHz — tame the top end before the next stage.
- Q3 output HF shunt (C7 / R9): 1 / (2π · 22 k · 1 n5) ≈ 4.8 kHz — final HF rolloff for stability and to remove residual switching noise.
- Output HP (C8 / VOLUME 100 k): 1 / (2π · 100 k · 68 n) ≈ 23 Hz — well below audio, flat low end.
Bill of Materials
The BOM has been cross-checked against the V1.3 schematic. Three JFETs (two different types — see Q2 note), three bias trimmers with test points, eleven fixed resistors, twelve caps, and the charge-pump IC. Substitution alternatives are tagged in the Notes column.
| Ref | Qty | Value | Colour code | Notes |
|---|---|---|---|---|
| Resistors — metal film, ¼ W, 1% | ||||
| R1 | 1 | 100k | Brown · Black · Black | Orange · Brown | Q1 gate series input resistor. |
| R2 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Q1 gate pulldown / input impedance setter. |
| R3 | 1 | 390R | Orange · White · Black | Black · Brown | Q1 source resistor — unbypassed, sets gain and headroom. |
| R4 | 1 | 56k | Green · Blue · Black | Red · Brown | Q1 drain output series resistor. |
| R5 | 1 | 1k | Brown · Black · Black | Brown · Brown | Q2 (J201) source resistor — unbypassed. |
| R6 | 1 | 100k | Brown · Black · Black | Orange · Brown | Q2 drain output series resistor. |
| R7 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Q3 gate pulldown / bias return. |
| R8 | 1 | 390R | Orange · White · Black | Black · Brown | Q3 source resistor — unbypassed. |
| R9 | 1 | 22k | Red · Red · Black | Red · Brown | Q3 drain output series resistor. |
| R13 | 1 | 100R | Brown · Black · Black | Black · Brown | VA rail series resistor — final filtering after the charge pump. |
| R14 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Bass-boost HP path resistor — works with C10 to set the bass-boost corner. |
| Trimmers — JFET bias adjustment (6 mm PCB-mount) | ||||
| R10 | 1 | 10k | 6 mm trimmer — sets Q1 (2N5457) drain bias. Probe at TP1 with multimeter. Target: ~12 V at TP1. | |
| R11 | 1 | 50k | 6 mm trimmer — sets Q2 (J201) drain bias. Probe at TP2. Target: ~12 V at TP2. Note the higher value (50k) — J201 needs more bias-resistor range. | |
| R12 | 1 | 10k | 6 mm trimmer — sets Q3 (2N5457) drain bias. Probe at TP3. Target: ~12 V at TP3. | |
| Capacitors — ceramic | ||||
| C1 | 1 | 270p | Ceramic — Q1 gate input shunt to GND for RFI rejection (above audio). | |
| C2, C4 | 2 | 470p | Ceramic — Q1 drain HF shunt (C2), GAIN pot to Q2 gate coupling (C4). | |
| Capacitors — film (box-style) | ||||
| C3 | 1 | 2n2 | Box film — Q1 to GAIN pot coupling. | |
| C5, C7 | 2 | 1n5 | Box film — TONE pot shunt (C5), Q3 output HF rolloff (C7). | |
| C6 | 1 | 4n7 | Box film — TONE output to Q3 gate coupling. | |
| C8 | 1 | 68n | Box film — Q3 output to VOLUME pot coupling. | |
| C10 | 1 | 3n3 | Box film — bass boost HP path cap tuning. Works with R14 to set the bass-boost corner frequency. | |
| Capacitors — electrolytic (polarised) | ||||
| C9 | 1 | 100µF / 16 V | Polarised electrolytic — main +9 V supply rail filtering. Observe polarity. | |
| C11, C12 | 2 | 10µF / 25 V | Polarised electrolytic — charge pump reservoir (C11) and VA rail decoupling (C12). 25 V rating recommended — these caps see ~18 V on the doubled rail. Observe polarity. | |
| Potentiometers — 16 mm right-angle PCB-mount | ||||
| GAIN | 1 | 500k log | A taper (audio log) — drive amount. | |
| TONE | 1 | 500k log | A taper (audio log) — single-knob treble cut. | |
| VOLUME | 1 | 100k log | A taper (audio log) — output level. | |
| Diodes | ||||
| D1 | 1 | 1N4001 | Reverse polarity protection — observe polarity, band toward circuit. | |
| D2, D3 | 2 | 1N5817 | Schottky — charge pump voltage doubler rectifiers. Schottky required (low forward voltage drop) — every 0.3 V drop here costs headroom on the 18 V rail. Observe polarity. | |
| Transistors | ||||
| Q1, Q3 | 2 | 2N5457 | N-channel JFET, TO-92 — input (Q1) and output (Q3) stages. Pinout (flat side facing you, leads down): D – S – G. J201 MPF102 work but pinout may differ — verify. Sockets recommended. | |
| Q2 | 1 | J201 | N-channel JFET, TO-92 — middle stage. Different device from Q1/Q3 — J201 has different pinch-off characteristics, hence the larger 50 k bias trimmer (R11). Pinout (flat side, leads down): D – S – G. Socket recommended. | |
| Integrated circuits | ||||
| IC2 | 1 | ICL7660SCPA | Voltage inverter / charge pump, DIP-8. The "S" suffix is mandatory — non-S parts (plain ICL7660) oscillate in the audio band and whistle through the signal. MAX1044 works as an alternative. Use a socket. | |
| Switches | ||||
| SW1 | 1 | SPST | Bass boost switch (optional). When closed, grounds the B_B pad and engages the C10 / R14 bass-boost path. Any SPST mini-toggle works. | |
Build & Bias Setup
Moderate parts count, all through-hole. The bias procedure at the end is what gets this circuit sounding right — V1.3 added the three test points specifically to make it easier.
Component placement reference — top view. Trimmers (R10, R11, R12) are visible at the top corners; test points (TP1, TP2, TP3) are clearly labelled next to each trimmer. The three pots (GAIN, TONE, VOLUME) mount on the back side.
Build order
Diodes and resistors first
Lowest profile, do these first. Fit D1 (1N4001), D2 and D3 (1N5817 Schottky) — all polarity-sensitive, band toward the silkscreen mark. Then all 11 resistors — the colour-band column in the BOM makes verification quick.
Transistor sockets — recommended
Three JFETs, one IC — fit DIP-style sockets or 3-pin SIP sockets for Q1, Q2, Q3 and a DIP-8 socket for IC2. JFET parameters vary significantly between individual devices (especially VGS(off)), so sockets let you swap and find a set that biases well. Don't insert the chips yet.
Q1, Q3 = 2N5457: pinout D – S – G (flat side facing you, leads down). Q2 = J201: pinout D – S – G in TO-92. IC2 = ICL7660SCPA: match the socket notch to the silkscreen.
Trimmers (R10, R11, R12)
Fit the three 6 mm trimmers. R10 = 10 k, R11 = 50 k, R12 = 10 k. Use the silkscreen footprint to confirm orientation — the trimmer's adjustment screw should face up so you can probe TP1/TP2/TP3 with one hand and tune with the other. Set all three trimmers to mid-rotation as a safe starting point before powering up.
Ceramic and box film capacitors
Non-polarised, no orientation. Populate ascending: 270 p (C1), 470 p (C2, C4), 1 n5 (C5, C7), 2 n2 (C3), 3 n3 (C10), 4 n7 (C6), 68 n (C8). Push them flat against the board.
Electrolytic capacitors
Polarised — long lead is positive (+), the can has a stripe marking the negative side. Match the (+) on the silkscreen. C9 (100 µF) is the big main filter cap. C11 and C12 (10 µF each) sit in the charge-pump network — note the 25 V rating recommended since these caps can see ~18 V on the doubled rail. Reversing any electrolytic risks a small explosion when powered.
Off-board pads and bass-boost switch (SW1)
Solder pads / pin headers for the off-board connections: IN, OUT, +9V, GND, and the two B_B pads for the optional bass-boost switch. The bass boost is optional — leave the B_B pads unconnected (or short them with a wire) if you don't want the switch.
Pots — back-side mounting
The three pots (GAIN, TONE, VOLUME) mount on the opposite side of the board to all other components. Use double-sided tape on the metal pot bodies to prevent shorts to any pin tips that protrude. Tack-solder the centre pin of each pot first, pull the pot back ~1 mm so it sits flat, let the solder set, then solder the rest. Don't forget to clip off the small anti-rotation bracket on each pot before final assembly.
Insert ICs and transistors
With all soldering done and double-checked for shorts, insert IC2 (ICL7660SCPA) and the three JFETs in their sockets. Match each notch / flat side to the silkscreen. Don't force anything.
Bias setup — multimeter required
With three JFETs in the signal path and individual device variation between samples, biasing by ear is essentially impossible — there are too many trimmers interacting. The V1.3 board provides three dedicated test points (TP1, TP2, TP3) specifically for setting bias with a multimeter. The procedure is simple:
| Step | Action |
|---|---|
| 1 | Power up with 9 V DC. No audio connected. All three front-panel pots at noon for now (the bias-setting doesn't depend on them). |
| 2 | Set multimeter to DC volts (20 V range or higher). Black probe to GND, red probe to TP1 (next to R10). |
| 3 | Adjust R10 until TP1 reads ~12 V. This sets Q1's drain bias to about 2/3 of the VA rail. |
| 4 | Move probe to TP2. Adjust R11 until TP2 reads ~12 V. (R11 is 50 k because the J201 needs more range — don't be surprised if R11 sits at a different mechanical position than R10/R12.) |
| 5 | Move probe to TP3. Adjust R12 until TP3 reads ~12 V. |
| 6 | Re-check TP1 once TP3 is set — there's usually no interaction between the stages, but verify each test point still reads close to 12 V. |
| 7 | Done. The pedal is now in its working range. You can fine-tune by ear from here. |
Wiring & Enclosure
Standard external-bypass wiring with a 3PDT footswitch. Board pads: IN, OUT, +9V, GND, plus the two B_B pads for the optional bass-boost toggle.
| Pad | Connect to |
|---|---|
| IN | Effect input — middle pole of 3PDT bypass footswitch |
| OUT | Effect output — middle pole of 3PDT bypass footswitch |
| +9V | Power jack centre-positive (DC jack tip), routed through the 3PDT for status LED switching |
| GND | Sleeves of input / output / power jacks; footswitch ground tab if present |
| B_B (2 pads) | The two terminals of the SPST bass-boost toggle (SW1). Leave unconnected if you don't want the bass boost — the pedal works without the switch, just with bass-boost permanently disabled. |
1590B enclosure — drill template
This board fits a 1590B enclosure. The drill template below shows all the hole positions to scale — measurements in millimetres. Always print and verify scale before drilling.
Drill template — 1590B enclosure, measurements in mm. Three pots across the top (32 mm from top edge, 17 mm spacing between centres). Audio jacks on left and right sides. Power jack and footswitch on the bottom. Verify scale on your printout before drilling.
Usage
Controls
GAIN — drive amount into the Q2 / Q3 stages. Audio log taper. Low settings = clean preamp character (the 18 V headroom keeps it clean further than a 9 V design would). Higher settings = JFET clipping into mild-to-moderate distortion. Even at full the pedal stays in "valve amp pushed hard" territory — it's not a high-gain fuzz.
TONE — single-knob treble cut. Audio log taper. Fully CW = bright, fully CCW = dark. Centre detent gives a balanced response.
VOLUME — output level. Audio log taper. The pedal has plenty of swing — VOLUME can drive a clean amp's input cleanly or push it into break-up depending on the GAIN setting.
Bass boost (SW1) — two-position toggle. Off = tight / mid-forward voicing. On = fat / full voicing with significant low-end content added below ~150 Hz.
Suggested starting points
Clean Fender-style chime — GAIN at 9–10 o'clock, TONE at 1–2 o'clock, VOLUME to taste, bass boost off. The 18 V headroom keeps this completely clean; the JFETs add subtle harmonic colour that closely emulates a valve preamp at modest levels.
Edge-of-breakup — GAIN at noon, TONE at noon, VOLUME up to taste, bass boost off or on (try both). The classic small-amp-pushed-hard sound — articulate, dynamic, responsive to pick attack.
Crunchy rhythm — GAIN at 2–3 o'clock, TONE at 11–12 o'clock, VOLUME to taste, bass boost on for fatness. Full Fender / Supro crunch territory — the LEDs nominally aren't clipping anything here (this is pure JFET soft saturation), so it stays musical at high settings.
Lead voice — GAIN max, TONE at 1 o'clock, bass boost on, VOLUME up. Maximum saturation that the cascade can produce — still surprisingly clean compared to silicon-clipping drives, perfect for cutting solos.
Final note
Once you've biased it correctly, you have all the Fender / Supro-flavoured tones available — clean chime, edge-of-breakup, full crunch — in a single small box. Have fun.
Disclaimer & Licence
PCBs based on runoffgroove circuits purchased from TH Custom Effects are intended for DIY / non-commercial use only. Any commercial use whatsoever is forbidden. Please contact runoffgroove for further information.
Supreaux Deux circuit design © runoffgroove.com. PCB layout and build documentation © TH Custom Effects 2014–2026. Build documentation V1.3.