Overview & Features
The Equinox II is Merlin Blencowe's brilliantly minimal take on a DIY reverb. Where commercial pedals reach for the Belton Digi-Log "brick" (which itself contains three PT2399 delay chips), Merlin asked the obvious follow-up: how few PT2399s can you actually get away with? The answer turns out to be two — passing the audio through a short and a long delay line in parallel, summing their outputs and feeding the sum back into both inputs. The result is a primitive but genuinely useful reverb tail with a recognisable analogue-digital character all of its own.
The original project lives at valvewizard.co.uk/equinox.html. This PCB lays out the circuit cleanly for through-hole construction in a single 16 mm pot board-mount format with all wiring brought to a single edge.
Two parallel PT2399s
Short (~80 ms) and long (~250 ms) delay lines summed and fed back into the input.
One-knob simplicity
A single MIX pot sets the wet/dry balance — Merlin's preferred minimalist control set.
Reverb tails
Buffered bypass keeps the wet signal trailing out naturally when the effect is switched off.
JFET input drive
A J201 stage drives both delay lines and provides the soft saturation character into the digital chain.
On-board 5 V regulator
78L05 supplies a clean digital rail for both PT2399 chips, isolated from the audio rail.
Single footswitch
Only an SPST or SPDT latching footswitch is needed for true buffered bypass with tails.
Built example
Circuit Theory
Below is the full Equinox II schematic. The signal path can be read left-to-right as five blocks: input network, op-amp buffer, JFET drive stage feeding two parallel PT2399 delay lines, a feedback summing network, and a wet/dry mix output stage.
Signal path, stage by stage
| Stage | Components | Function |
|---|---|---|
| Input network | R25, R24, C4, R4 | R25 (100 Ω) is series protection; R24 (10 MΩ) provides a high-impedance pulldown to reference; C4 (100 n) blocks DC; R4 (1 MΩ) sets the input bias to VR. |
| Buffer | IC1_A (TL072) | Unity-gain voltage follower. Its output is taken directly to the dry side of the MIX pot, to the input mixer of the output stage via R6, and to the source of Q1. |
| JFET drive | Q1 (J201), R5, C20 | J201 with input at the gate and output at the drain (R5 = 10 k load to +5 V). The summed feedback signal enters the gate; the drain output (via C20, 100 n) feeds both PT2399 delay lines in parallel. This stage adds the gentle non-linearity that makes the reverb sound musical rather than sterile. |
| Long delay | IC2 (PT2399), R15 | R15 = 22 kΩ sets a delay around 250 ms. The chip's two internal op-amp stages handle anti-alias filtering at the input and reconstruction filtering at the output. |
| Short delay | IC3 (PT2399), R12 | R12 = 2.2 kΩ sets a much shorter delay around 80 ms — close to the chip's practical lower limit. Same internal filter topology as IC2. |
| Feedback sum | R7, R9, R8, R20, R21, R16, R19, R18 | The two delay outputs are summed, mixed with the new input from Q1, and fed back into both delay inputs. This is what produces the reverb's continuous decay rather than two distinct echoes. |
| Mix & output | MIX, IC1_B, R23, C32, R13, C5 | The MIX pot blends the dry buffer signal with the wet feedback bus. IC1_B is a non-inverting summing stage with R23/C32 setting a gentle treble rolloff above ~16 kHz. R13 (100 Ω) is series output protection; C5 (10 µF) blocks DC; R17 (100 k) pulls the output to ground. |
| Tails network | D2, R26, C1 | 1N4148 + 1 MΩ + 2n2: when the footswitch grounds SW, the LED lights and Q1's gate is held; when released, the cap discharges slowly so the reverb tail keeps ringing as it decays naturally. |
| Power supply | D1, R1, C3, REG1, C2; R2/R3, C22 | 1N4001 reverse-polarity protection, then a 78L05 regulator for the +5 V digital rail (VC). R2/R3 (100 k each) form the analogue VR bias at half-supply, decoupled by C22 (10 µF). |
Delay & Filter Analysis
The two PT2399 chips run at different clock rates set by their respective REF resistors (pin 6). Inside each chip the audio is sampled, passed through the digital delay line, and reconstructed by two op-amp filter stages whose feedback caps set the smoothing rolloff.
Long delay — IC2
The long line provides the sense of room size. At 22 kΩ the PT2399 is running its clock relatively slowly, so the bandwidth is limited and a little grit is audible — this is part of the character.
Short delay — IC3
The short line fills in the early-reflection texture. At 2.2 kΩ the clock is much faster so the bandwidth is wider and the output is noticeably cleaner than the long line.
Output filter — IC1_B
The output op-amp has C32 (1 nF) in parallel with R23 (10 kΩ feedback). This sets a single-pole low-pass at:
f₀ = 1 / (2π × 10 kΩ × 1 nF) ≈ 15.9 kHz
Just above the audible band — it tames any high-frequency switching residue from the PT2399s without dulling the signal.
Bill of Materials
All resistors are 1 % metal film, ¼ W. Box film capacitors are non-polarised; check polarity carefully on all electrolytics. The colour bands shown below are calculated from the value — verify against the printed code on each part before populating.
| Ref | Qty | Value | Colour code | Notes |
|---|---|---|---|---|
| Resistors — metal film 1 % ¼ W | ||||
| R1, R13, R25 | 3 | 100 Ω | Brown · Black · Black | Black · Brown | Metal film ¼ W |
| R12 | 1 | 2.2 kΩ | Red · Red · Black | Brown · Brown | Sets short delay time on IC3 |
| R11, R16, R27 | 3 | 4.7 kΩ | Yellow · Violet · Black | Brown · Brown | R27 sets LED brightness — try other values for taste |
| R5, R6, R8, R9, R10, R18, R19, R20, R21, R22, R23 | 11 | 10 kΩ | Brown · Black · Black | Red · Brown | Feedback sum, op-amp gain setting, JFET load |
| R7, R14 | 2 | 15 kΩ | Brown · Green · Black | Red · Brown | Feedback path resistors |
| R15 | 1 | 22 kΩ | Red · Red · Black | Red · Brown | Sets long delay time on IC2 |
| R2, R3, R17 | 3 | 100 kΩ | Brown · Black · Black | Orange · Brown | VR bias divider and output pulldown |
| R4, R26 | 2 | 1 MΩ | Brown · Black · Black | Yellow · Brown | Input bias and tail-network discharge |
| R24 | 1 | 10 MΩ | Brown · Black · Black | Green · Brown | Input pulldown |
| Capacitors — film (box, 5 mm/7.2 mm pitch) | ||||
| C10, C32 | 2 | 1 nF | Box film — output filter cap (C32) and PT2399 feedback (C10) | |
| C1, C7, C27 | 3 | 2.2 nF | Box film — tail cap (C1), PT2399 internal LPF feedback (C7, C27) | |
| C9, C29 | 2 | 10 nF | Box film — PT2399 LPF2 feedback caps | |
| C6 | 1 | 15 nF | Box film — feedback path tone shaping | |
| C31 | 1 | 47 nF | Box film — dry path coupling at MIX pot | |
| C4, C11, C13, C14, C16, C17, C18, C19, C20, C21, C23, C26 | 12 | 100 nF | Box film — coupling and PT2399 supply decoupling | |
| C8, C28 | 2 | 1 µF | Box film, 7.2 mm pitch — PT2399 input coupling | |
| Capacitors — electrolytic (radial, 5 mm pitch) | ||||
| C12 | 1 | 10 µF | Tantalum or aluminium electrolytic — PT2399 reference decoupling | |
| C5, C22, C30, C33 | 4 | 10 µF | Polarised electrolytic — coupling and VR decoupling. Mind polarity. | |
| C2 | 1 | 22 µF | Polarised electrolytic — 78L05 output decoupling | |
| C3, C15, C24 | 3 | 47 µF | Polarised electrolytic — main supply rail and PT2399 supplies | |
| Semiconductors | ||||
| D1 | 1 | 1N4001 | Reverse-polarity protection on +9 V input. 1N4002–1N4007 1N5817 (Schottky, lower drop) | |
| D2 | 1 | 1N4148 | Tail network — small-signal silicon diode | |
| Q1 | 1 | J201 | N-channel JFET, TO-92. Pinout S-G-D from flat side. 2N5457 MPF102 (verify pinout) | |
| LED | 1 | 3 mm LED | Bypass indicator. Any colour — adjust R27 for brightness | |
| IC1 | 1 | TL072 | Dual op-amp, DIP-8. JRC4580 NE5532 OPA2134 | |
| IC2, IC3 | 2 | PT2399 | Digital delay IC, DIP-16. No pin-compatible substitute — Princeton Technology only. | |
| REG1 | 1 | 78L05 | +5 V positive linear regulator, TO-92. L78L05 MC78L05 | |
| Pots | ||||
| MIX | 1 | 10 kΩ log (A10K) | Board-mount, 16 mm Alpha-style PCB-pad pot. Mounted on the underside of the board. | |
Build Guide
There are a lot of parts on a relatively small board, so take your time. Working from the lowest-profile components upward keeps the board flat against the workbench and makes solder joints easier to inspect.
PCB layout
Population order
Resistors
Populate all 27 resistors first. They are the lowest-profile components and the most numerous. Verify the colour bands against the BOM table — a quick multimeter check on the value before bending leads is good practice.
Diodes — D1 and D2
1N4001 (D1) is the larger black-bodied power diode at the input — check the cathode stripe matches the silkscreen. 1N4148 (D2) is the small glass diode in the tail network — also polarised.
IC sockets
Fit a DIP-8 socket for IC1 (TL072) and DIP-16 sockets for IC2 and IC3 (the two PT2399s). Mind the notch orientation against the silkscreen. Do not insert the ICs yet.
JFET — Q1
J201 in TO-92. Match the flat side of the package to the silkscreen outline. Pinout is S-G-D reading from left to right with the flat facing you. If you are substituting (2N5457, MPF102), verify the pinout from the datasheet — it can vary.
Voltage regulator — REG1
78L05 in TO-92. Pinout differs from the J201 — check the silkscreen and the 78L05 datasheet (typically OUT-GND-IN reading flat-side-up, but verify your part).
Film capacitors
All box-film caps in ascending value order: 1 nF, 2.2 nF, 10 nF, 15 nF, 47 nF, then the twelve 100 nF supply-decoupling caps, finally the two 1 µF caps (C8, C28). Box film is non-polarised — orientation does not matter electrically, but a consistent direction looks tidier.
Electrolytic capacitors
All polarised — long leg is positive. Match against the silkscreen + marking. Order: 10 µF (×4), 22 µF (C2), 47 µF (×3). C12 (10 µF) is shown as tantalum in the original BOM but a standard aluminium electrolytic at the same value works fine.
LED
The LED is needed for bypass to function correctly — do not omit it. Long leg is the anode and goes to the +5 V side via R27. Mount on the component side facing out so it is visible through the enclosure.
MIX pot — board-mount on the underside
The 16 mm board-mount pot mounts on the back of the PCB, not the component side. Place a strip of double-sided tape between the pot body and the PCB copper to prevent shorts. Solder the centre wiper pin first, gently pull it back about 1 mm so the pot seats flat, let it cool, then solder the two outer pins. This avoids the wide pot pads bridging to nearby traces.
Final assembly
Clip off the small mounting bracket on the pot body before fitting into the enclosure. Insert IC1, IC2 and IC3 into their sockets last, mind the notch orientation, and double-check no pins are bent under.
Wiring & Bypass
The Equinox II uses buffered bypass — the input signal always passes through IC1_A regardless of the footswitch state. This is required to give the reverb its tail behaviour: when you switch off, the dry signal is cut but the wet signal continues to ring as the feedback loop empties.
Only a single-pole latching footswitch (SPST or SPDT) is required. Wire it to short the SW pad to ground when engaged.
Setup & Usage
There is no calibration. Power up, plug in, and turn the MIX knob.
The single control
MIX — sets the wet/dry balance. Fully counter-clockwise is dry only; fully clockwise is mostly wet. The reverb feedback amount and the delay times themselves are fixed by the resistor network — there is no decay or time control by design. Merlin's one-knob philosophy keeps the pedal honest about what it does well.
Reverb tail in bypass
Engage the effect, play a chord, then disengage with the footswitch. The dry signal drops out immediately while the wet tail continues for roughly a second before fading to silence. This is intentional and is the reason the bypass is buffered rather than true bypass.
Position in your signal chain
Reverb is conventionally placed near the end of the chain — after distortion, modulation and EQ, but before any noise gate. Equinox II's buffered bypass means it can sit anywhere without loading the previous pedal's output.
Listening notes
The two delay times (≈80 ms and ≈250 ms) and the JFET drive give the Equinox II a short, slightly grainy room sound rather than a long ambient wash. It excels on rhythm guitar where you want a sense of space without smearing the attack. For lead lines it adds a sympathetic decay rather than a dramatic tail. As Merlin himself puts it — this is not only a proof of concept; it is a cheap, good-sounding reverb.
Disclaimer & Licence
PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.
Original Equinox II design © Merlin Blencowe / ValveWizard (valvewizard.co.uk/equinox.html). PCB layout and this build documentation © TH Custom Effects 2015–2026. Build documentation V1.1.