Overview & Features
The Azabache is the Run Off Groove team's special take on a Fender-style overdrive — built not with op-amps and clipping diodes alone, but with a chain of JFETs that approximate the large-signal behaviour of a small Fender tube amp. The result is a warm, dynamic distortion that responds to picking attack the way a tube circuit does — quiet playing stays clean, hard playing breaks into harmonic-rich saturation, and rolling back the guitar volume cleans up the gain rather than just attenuating it.
This board is TH Custom Effects' compact PCB realisation of that circuit. The V1.2 layout is smaller than V1.1 and adds SMD alternate footprints for all four amplification transistors (Q1–Q4) — you can populate the board with through-hole TO-92 parts or SOT-23 surface-mount equivalents (BC548, BC547 family), whichever you have on hand. Four testpoints (TP1–TP4) make the trimmer biasing routine fast and accurate.
Front-panel controls
| Control | Type | Function |
|---|---|---|
| Gain | 100k log | Drive into the JFET cascade — clean at minimum, fully saturated at maximum. |
| Tone | 100k linear | First-stage tone control — sweeps from dark/wooly to bright/cutting. |
| Volume | 50k log | Output level after the recovery stage and tone-shaping network. |
| Scoop | SPST toggle | Mid-scoop on/off — when on, drops the midrange for that scooped Fender clean shimmer. |
| Bright | SPST toggle | Bright-cap shunt around the input source resistor — adds high-end sparkle when on. |
Tube-amp-like response
Three cascaded common-source JFET stages, each acting as a single triode-stage analogue.
Per-stage bias control
Four trimmers set drain bias for Q1–Q4 individually — JFETs vary, biasing makes them sing.
Soft-clip protection
Red-LED back-to-back diodes at the input clip extreme transients gently before they hit Q1.
Stage-to-stage clipping
Three pairs of 1N4148 silicon diodes between JFET stages provide hard-clipping at high gain.
Through-hole or SMD
V1.2 layout has both TO-92 and SOT-23 footprints for all four signal transistors.
Four testpoints
TP1–TP4 expose each transistor's drain so you can bias by multimeter rather than by ear.
Circuit Theory
The signal path is long but conceptually straightforward — input buffer, tone stack, three cascaded gain stages with per-stage clipping, recovery, and post-amp tone-shaping. Each block is small, the surprise is in the cumulative effect.
Signal flow
Power section
D1 (1N4001) provides reverse-polarity protection on the +9V input. R1 (68 Ω) is a small series resistor, and C1 (220 µF) is the bulk supply reservoir. Each amplification stage has its own dedicated +9V tap from this rail.
Input stage (Q1, 2N5457)
The input arrives through R3 (33 kΩ) — a series resistor that, together with R2 (1 MΩ to ground), sets the input impedance and gives a small protective resistance against capacitive loading. Q1 is a 2N5457 N-channel JFET in common-source configuration; its drain feeds TRIM1 (10 kΩ) which sits between drain and +9V to set the drain bias point. R4 (470 Ω) on the source establishes the gate-source bias by self-biasing — the BRIGHT switch optionally bridges R26 (1 MΩ) and lets C4 shunt audio frequencies around the source resistor, cancelling the source-degeneration high-frequency loss for extra brightness.
Two anti-parallel red LEDs (D2, D3) sit from the gate of Q1 to ground. Red LEDs have a forward voltage of roughly 1.8 V — far above the 0.6 V of a silicon diode — so they only conduct on extreme input transients (very loud signals or pops from cable connection). Under normal play they're invisible to the signal path; on a peak, they soft-clip the input and prevent ugly hard-edge transients from hitting the JFET gate.
Tone stack (TONE, R5, R6, R7, C5, C6, C7) and SCOOP switch
After Q1 the signal passes through C3 (220n DC-block) into a passive tone-stack network: a dual-path RC filter where R6/C6 forms a low-pass branch and R7/C5 a high-pass branch, blended by the TONE pot (100k linear). C7 (22n) and R8 (1 MΩ) couple to the SCOOP switch — when SCOOP is engaged it shunts the midrange branch to ground, producing the characteristic mid-scoop Fender clean tone.
Gain pot & cascaded JFET stages (Q2, Q3, Q4 — J201)
The GAIN pot (100k log) sets how much signal hits the first J201 stage. Q2, Q3, and Q4 are three identical J201 JFET common-source stages, each with:
— a 1.5 kΩ source resistor (R12, R15, R18) for self-biasing;
— a 25 kΩ trimmer (TRIM2, TRIM3, TRIM4) on the drain to set the bias point;
— a 220 nF DC-blocking cap on the input (C8, C10, C12) and output (C10, C12 again, plus C16);
— a 100 kΩ gate-leak resistor (R10, R14, R17) referencing each gate to its DC reference;
— a 1 MΩ pulldown to ground (R9, R13, R16) at each stage's input;
— a 150 pF compensation cap to ground (C9, C11, C13) — kills any ultrasonic instability;
— and a pair of anti-parallel 1N4148 diodes (D4/D5, D6/D7, D8/D9) at the gate
of each next stage to clip large signals symmetrically at ±0.6 V.
The combination is a high-gain three-stage amplifier with multiple distinct clipping mechanisms active at different drive levels. At low gain only the JFETs themselves contribute mild even-order harmonics (the 'sound of a clean tube'). At medium gain the silicon diodes start asserting, hard-clipping the largest signal peaks. At full gain everything clips, the JFETs run flat-out, and the result is rich, compressed, harmonically dense overdrive.
Recovery stage (Q5, 2N5088)
After three stages of gain the signal is too hot to drive a passive tone control directly. Q5 is a 2N5088 NPN BJT in common-emitter configuration that absorbs the level, recovers a sensible output impedance, and provides the buffered drive for the post-amp tone-shaping filter. R19 (100k) is the collector load, R21 (39k) and C15 (470p) are an input low-pass network, and R22 (3k3) is the emitter resistor.
Post-amp tone-shaping & output
After Q5's collector, C16 (220n) couples to a multi-pole filter network: R23 (10k) gives a defined DC reference, R24/R25 (10k each) and C17 (68n) form a low-pass shelf, and C18 (22n) bridges around the resistive ladder for a high-frequency presence peak. This network is Run Off Groove's specific cabinet-sim approximation — a fixed EQ curve approximating a guitar speaker's loaded frequency response. The signal then arrives at the VOL pot (50k log) for final level control.
Bias Trim Points
JFETs are notoriously variable — two J201s out of the same bag can have pinch-off voltages differing by a volt or more. To make every build sound right, each transistor's drain voltage is set independently by a dedicated trimmer with a testpoint exposing the drain. The target voltages below put each stage in its sweet spot — symmetrical headroom between rail and pinch-off, where the harmonic content sounds best.
| Stage | Trimmer | Testpoint | Target voltage | Notes |
|---|---|---|---|---|
| Q1 (2N5457) | TRIM1 — 10 kΩ | TP1 | ~6.0 V | Input buffer, lowest gain stage |
| Q2 (J201) | TRIM2 — 25 kΩ | TP2 | ~5.0 V | First gain stage |
| Q3 (J201) | TRIM3 — 25 kΩ | TP3 | ~5.0 V | Second gain stage |
| Q4 (J201) | TRIM4 — 25 kΩ | TP4 | ~5.0 V | Third gain stage |
The biasing procedure itself is in section 06. Read this section first so you understand what the testpoints are showing — then run the procedure once your build is complete.
Bill of Materials
About 60 parts, mostly resistors and box-film caps. The BOM follows the schematic; values noted with footnotes are points where older revisions of this document differed.
| Ref | Qty | Value | Colour code (5-band, 1%) | Notes |
|---|---|---|---|---|
| Resistors — 1/4 W metal film | ||||
| R1 | 1 | 68 Ω | Blue · Grey · Black | Gold · Brown | Power-rail decoupling. |
| R4 | 1 | 470 Ω | Yellow · Violet · Black | Black · Brown | Q1 source self-bias. |
| R12, R15, R18 | 3 | 1.5 kΩ | Brown · Green · Black | Brown · Brown | J201 source resistors (Q2/Q3/Q4). |
| R11 | 1 | 2.2 kΩ | Red · Red · Black | Brown · Brown | Gain-pot ground end. |
| R22 | 1 | 3.3 kΩ | Orange · Orange · Black | Brown · Brown | Q5 emitter resistor. |
| R23, R24, R25 | 3 | 10 kΩ | Brown · Black · Black | Red · Brown | Post-amp filter network. |
| R6, R7 | 2 | 22 kΩ | Red · Red · Black | Red · Brown | Tone-stack series resistors. |
| R3 | 1 | 33 kΩ | Orange · Orange · Black | Red · Brown | Input series resistor. |
| R21 | 1 | 39 kΩ | Orange · White · Black | Red · Brown | Q5 base series resistor. |
| R5, R20 | 2 | 47 kΩ | Yellow · Violet · Black | Red · Brown | Tone branch / Q5 input. |
| R10, R14, R17, R19 | 4 | 100 kΩ | Brown · Black · Black | Orange · Brown | JFET stage gate-leak / Q5 collector load. |
| R2, R8, R9, R13, R16, R26 | 6 | 1 MΩ | Brown · Black · Black | Yellow · Brown | Pulldowns and bias references. |
| Trimmers — 6 mm vertical | ||||
| TRIM1 | 1 | 10 kΩ | — | Q1 (2N5457) drain bias. |
| TRIM2, TRIM3, TRIM4 | 3 | 25 kΩ | — | Q2/Q3/Q4 (J201) drain bias. |
| Potentiometers — 16 mm right-angle PCB-mount | ||||
| GAIN | 1 | 100 kΩ log | — | Audio (logarithmic) taper. |
| TONE | 1 | 100 kΩ lin | — | Linear taper for tone-stack symmetry. |
| VOL | 1 | 50 kΩ log | — | Audio (logarithmic) taper. |
| Capacitors — ceramic (multilayer) | ||||
| C2, C9, C11, C13 | 4 | 150 pF | — | Compensation caps at each stage gate. |
| C15 | 1 | 470 pF | — | Q5 input low-pass. |
| Capacitors — box film (5 mm pitch) | ||||
| C14 | 1 | 1 nF | — | Q5 input network. |
| C5, C6 | 2 | 10 nF | — | Tone-stack capacitors. |
| C7, C18 | 2 | 22 nF | — | SCOOP coupling / output presence. |
| C17 | 1 | 68 nF | — | Post-amp shelf cap. |
| C3, C4, C8, C10, C12, C16 | 6 | 220 nF | — | Inter-stage DC-blocks. |
| Capacitors — polarised electrolytic | ||||
| C1 | 1 | 220 µF | — | Bulk +9V decoupling. 16 V or higher rated. |
| Diodes | ||||
| D1 | 1 | 1N4001 | — | Reverse-polarity protection. |
| D2, D3 | 2 | LED 3 mm red | — | Input soft-clip diodes — anti-parallel from Q1 gate to ground. |
| D4, D5, D6, D7, D8, D9 | 6 | 1N4148 | — | Inter-stage clipping diodes — three pairs, anti-parallel. 1N914 equivalent |
| Transistors | ||||
| Q1 | 1 | 2N5457 | — | Input JFET. TO-92 through-hole. SOT-23 BC548 footprint also available |
| Q2, Q3, Q4 | 3 | J201 | — | Cascaded gain-stage JFETs. TO-92 through-hole. SOT-23 alternates Q2A/Q3A/Q4A on V1.2 board for SMD parts. |
| Q5 | 1 | 2N5088 | — | Recovery stage NPN BJT. TO-92. |
| Switches | ||||
| SCOOP | 1 | SPST | — | Mid-scoop on/off toggle (PCB-mount or panel-mount slide). |
| BRIGHT | 1 | SPST | — | Bright cap on/off toggle. |
| Hardware | ||||
| — | 5 | TO-92 sockets | — | Strongly recommended for Q1–Q5 — JFETs benefit from being swappable for batch-matching. |
| — | — | Hookup wire | — | Stranded 22–24 AWG for the perimeter pads (IN, OUT, GND, BATT, +9V, footswitch wiring). |
Build Guide
Build by part height. Diodes and resistors first, then sockets and trimmers, then capacitors, then the right-angle pots, finally the LEDs and electrolytic. Plan ahead — once the pots are in, it's hard to reach earlier components for rework.
Diodes (D1, D4–D9)
Solder the seven small signal diodes first — they're the lowest profile. Watch the cathode bands. D1 is the 1N4001 power-protection diode (one orientation matters most here). D4–D9 are six 1N4148s in three anti-parallel pairs — installed in opposing orientations within each pair. Match the silkscreen marking carefully.
Resistors
26 resistors total. Mount them lying flat with leads bent close to the body. Tip: sort them into piles by value first, then install groups so you don't lose track. See section 04 for colour codes.
Transistor sockets (Q1–Q5)
Install five TO-92 sockets. JFETs vary part-to-part, so sockets let you A/B test pulls from your stash without resoldering. Skip this step if you're populating the SMD footprints for Q1–Q4 — Q5 is always TO-92, so always socket Q5 at minimum.
Trimmers (TRIM1–TRIM4)
Four 6 mm vertical trimmers. TRIM1 is 10 kΩ; TRIM2/3/4 are each 25 kΩ. Orientation is fixed by the part's three pins matching the silkscreen pads.
Ceramic capacitors
Five small ceramic caps — C2, C9, C11, C13 (each 150 pF) and C15 (470 pF). Not polarised, so orientation doesn't matter. They're physically small — easy to mistake for each other, so check each one with a meter or use a magnifier on the markings before installing.
Box-film capacitors
Twelve box-film caps spanning four values: C14 (1 n), C5/C6 (10 n), C7/C18 (22 n), C17 (68 n), and C3/C4/C8/C10/C12/C16 (220 n). All 5 mm pitch. Not polarised. Take time to install the right value in each spot — mixing 10 n and 100 n gets you a wildly wrong tone-stack response.
SCOOP and BRIGHT switches
Two SPST switches. Polarity doesn't matter for SPST, but mount them at the height you need for your enclosure. PCB-mount slides sit flat; through-hole mini-toggles need their threaded shaft pointing through the enclosure top.
Right-angle pots — from the back
The three 16 mm pots (GAIN, TONE, VOL) mount on the back of the board with their shafts pointing forward. Apply double-sided tape to the pot bodies before seating them — the metal pot case sits very close to PCB traces and can short them. Alternatively, a thin square of electrical tape between pot and PCB does the job.
Solder tip: tack the centre pin first, push or pull the pot to align it horizontally, let the solder cool, then solder the remaining pins.
LEDs (D2, D3) and electrolytic (C1)
The two red LEDs (D2, D3) at the input clip-protection are not indicator LEDs — they sit flat against the board. Long lead = anode = goes into the square pad. Last, install C1 (220 µF) — long lead is +; the silkscreen marks the polarity.
Wiring
Solder hookup wires to the perimeter pads — IN, OUT, GND, +9V, BATTERY, and the two footswitch pads. Clip off the small mechanical bracket on the PCB if your enclosure layout calls for it (mentioned in the original build doc).
Insert transistors and bias
With power off, insert Q1 (2N5457), Q2/Q3/Q4 (each J201), and Q5 (2N5088). Match the flat side of each TO-92 to the flat side on the silkscreen. Then proceed to section 06 for the biasing procedure.
Biasing Procedure
Four transistors with adjustable drain bias means there's no “adjust by ear” route — too many interactive variables. Use a multimeter on the testpoints. Once each stage is in its working window, the circuit will sound right; you can then fine-tune by ear if you want a particular tonal flavour.
Equipment
One digital multimeter set to DC volts (20 V range). A small flathead screwdriver for the trimmers. Power supply (9 V regulated). No signal needs to be present.
Procedure
Apply power, no signal connected
Connect 9 V to the +9V and GND pads. The board should draw a few milliamps of quiescent current. If you see any smoke, hear any sizzle, or measure more than 50 mA, power down immediately and check D1 orientation, the polarity of C1, and the placement of all transistors.
Connect meter ground to circuit ground
Black multimeter probe to GND pad on the board, or to the negative terminal of the battery clip. Leave it there for the rest of the procedure.
Bias Q1 — TP1 to ~6.0 V
Touch the red probe to TP1. Adjust TRIM1 until the meter reads as close to 6.0 V as you can get. Range: anywhere between 5 V and 7 V is acceptable; 6.0 V is the sweet spot.
Bias Q2 — TP2 to ~5.0 V
Move red probe to TP2. Adjust TRIM2 to read close to 5.0 V. Range: 4.5 V to 5.5 V.
Bias Q3 — TP3 to ~5.0 V
TP3 with TRIM3 — same target as Q2. The earlier stages affect later stages slightly, so once you've biased Q4 in the next step, come back and re-check Q2 and Q3 — small re-adjustments may be needed.
Bias Q4 — TP4 to ~5.0 V
TP4 with TRIM4. Same target.
Re-check all four points
Sweep TP1 → TP2 → TP3 → TP4 once more, nudging trimmers as needed. Once all four are within their target ranges, biasing is done.
Fine-tune by ear (optional)
With the pedal in your signal chain, play through it at high gain. If a stage sounds gated or dry, lower its trimmer voltage by 0.5 V; if it sounds soggy or compressed, raise it. Listen, adjust, listen again. Stay within the ranges above — going outside them will either pinch the JFET off or saturate it permanently.
Enclosure & Drilling
The board fits a standard 1590B enclosure. The drilling template below gives the hole positions in millimetres. Always print the template at 100 % and verify measurements before drilling — printer scaling can throw the dimensions off by enough to make pots not line up with the board.
Layout suggestions
Top edge of the enclosure: GAIN (left), TONE (centre), VOL (right) at 17 mm spacing. Bottom edge: footswitch in the centre and the input/output jacks on the long sides. The two toggle switches (SCOOP and BRIGHT) typically go on the top edge above their respective controls — but their exact placement is up to you and depends on what feels natural under your hand on a stage.
Disclaimer & Licence
The Azabache circuit is the work of Run Off Groove and remains their intellectual property. The PCB layout and this build documentation are by TH Custom Effects.
PCBs based on Run Off Groove circuits purchased from TH Custom Effects are intended for DIY and non-commercial use only. Any commercial use whatsoever is forbidden — please contact Run Off Groove directly for licensing information. Redistribution of PCBs and artwork from this document is also not permitted.
© TH Custom Effects 2014–2026. Build documentation V1.2.