TH Custom Effects Build Documentation · V1.5 · 2014

U-Boot

Clean Suboctave — V1.5

A clean analogue suboctave generator based on Merlin Blencowe's U-Boat circuit. The dry guitar signal is squared and divided by a CD4013 flip-flop to generate a precise one-octave-down square wave, smoothed and blended with the original signal at the output.

−1 octave (÷2) True bypass TL074 + LM833 + TL072 CD4013 divider

01

Overview & Features

Merlin Blencowe is famous for his deep knowledge of valve circuits, but every now and then he releases stompbox designs of his own. The U-Boat is one of those designs — an analogue suboctave with a different approach to the well-known OC-2. With a few tweaks added on top, it tracks more reliably for a wider range of pickups. Merlin's project page documents the original circuit; valvewizard.co.uk is the home of all his work.

This V1.5 board uses a true-bypass footswitch instead of the original FET switching, and adds a TRACK trimpot so you can find the sweet spot where the octave divider locks cleanly to your guitar's pickup output. V1.5 also addresses a handful of symptoms seen in a small number of earlier builds — slightly tighter tracking around the threshold, and improved decoupling on the comparator chain.

Clean −1 octave

A CD4013 D-flip-flop divides the squared input by 2 — one octave down, perfectly in tune with the original note as long as the comparator tracks reliably.

TRACK control

The TRACK pot adjusts the comparator threshold so you can tune the divider's locking range to your pickups (single-coil, humbucker, low-output vintage, hot-wound).

MIX control

Blends the suboctave signal with the dry guitar at the output, so you can dial in anything from subtle reinforcement to a pure octave-down voice.

Suboctave level trim

R22 (100 kΩ trimpot) sets the absolute level of the suboctave before the MIX blend — set once at build, then forget.

Populated U-Boot V1.5 — IC2 socketed for LM833, IC4 (TL072) at top-left, IC3 (CD4013) on top edge, R22 trimpot far right.

02

Circuit Theory

U-Boot V1.5 full schematic — Merlin Blencowe's clean suboctave circuit, TH Custom Effects 10/2014 layout.

Signal Flow

The signal enters at IN through R3 (100 Ω) and C4 (100 nF) into IC1A, configured as a unity-gain non-inverting buffer. R5 (1 MΩ) sets the input bias to VR (the 4.5 V virtual ground), and R4 (10 MΩ) provides a high-impedance pull-down to ground so the signal is well-defined when nothing is plugged in.

The buffered signal then enters IC1B, a 2nd-order Sallen-Key low-pass shaper with R6, R7, C5 and C6 setting the corner frequency, and R8/R9 setting a non-inverting gain of about ×5.5 (14.9 dB). The high gain combined with the Sallen-Key topology produces a peaking response near the corner that emphasises the strong harmonics of the guitar signal — this gives the comparator a cleaner, larger swing to lock onto.

IC1C is the comparator / squarer. R10 (10 kΩ) brings the signal to its non-inverting input, and R11 (10 kΩ) provides positive-feedback hysteresis from the output back to the same input. The output is a clean square wave at the input frequency — labelled SIG + on the schematic.

In parallel, IC1D together with D4, D5 and C15 forms a half-wave rectifier and threshold detector. R28, R29 and the TRACK pot (R30/R31 sit at the pot end-stops) set the comparator's tracking window. R14 (100 kΩ) and C8 (680 nF) form the envelope hold network with a time constant of about 68 ms — long enough to ride out one cycle of the lowest guitar fundamentals, short enough to release between notes.

IC2 (LM833) uses both halves: IC2A and IC2B, each with a clipping diode pair (D2/D3 and D6/D7) at the input. They condition the SIG+ and SIG− comparator outputs into clean logic-level edges that feed the CD4013 flip-flop's CLK and reset inputs. The LM833 is preferred here for its high slew-rate and clean edges; if a TL072 is substituted instead, R17 (10 MΩ) is fitted to compensate for the lower input bias current.

IC3 (CD4013) is a dual D-type flip-flop. With the D input tied to its own /Q output, each flip-flop divides its clock frequency by two — so one stage gives a perfect one-octave-down square wave. The two halves of the chip are wired so that the second flip-flop and the auxiliary switching keep the divider in a known state between notes, which is what makes this design track more reliably than a simple naked /2 divider.

The squared, divided suboctave signal is gated by Q1 (J112) — a JFET acting as a series switch controlled by D8, R20 and R21. The gate is held closed when no signal is detected, so the suboctave is silent during pauses (no buzzing or oscillation between notes). When Q1 is open, the divider's output passes through R22 (100 kΩ trimpot — the suboctave level set), C10 (10 µF) coupling cap, and into the summing amp.

IC4A (TL072) is an inverting summing amplifier. R23 and R24 (47 kΩ each) sum two signals in: the dry input from IC1A, and the suboctave from Q1's output via R22 and C10. R25 (47 kΩ) is the feedback resistor — gain is roughly ×1 per input — and C12 (2.2 nF) gives a high-frequency rolloff at about 1539 Hz, smoothing the square edges of the suboctave into something more usable. C11 (22 nF) shunts the inverting input to VR, removing any DC offset from the summing node.

The MIX pot (10 kΩ lin) sits between IC4A and IC4B, blending the summed signal back against the original dry. IC4B is the output buffer: R28 (100 kΩ) feedback, C15 (100 pF) HF rolloff at about 15.9 kHz (just outside the audio band), R26 (100 Ω) series, C14 (10 µF) DC-blocking output cap, and R27 (10 MΩ) pull-down to ground for pop-free switching.

Power Supply

D1 (1N4001) provides reverse-polarity protection on the +9 V rail. R1 and R2 (10 kΩ each) form the virtual-ground divider — the VR rail sits at half-supply, ~4.5 V, and is decoupled by C2 (22 µF). C1 (100 µF) reservoirs the +9 V rail and C3 (100 nF) provides high-frequency decoupling at the IC supply pins.

Note on tracking. Like every analogue octave-down circuit, U-Boot needs the input signal to have a well-defined fundamental for the divider to lock cleanly. It tracks single notes excellently — chords or heavily distorted input are not its territory. Place it before any drive pedals in the signal chain.

03

Filter & Stage Analysis

The U-Boot has three frequency-shaping points worth describing in detail: the IC1B input shaper, the envelope hold time, and the IC4A output smoothing filter. These are the only three places where component changes will meaningfully affect the way the pedal sounds — everything else is logic, switching or DC-blocking.

IC1B Sallen-Key Shaper

2nd-order active LP — feeds the comparator

R6, R7

10 kΩ × 2

C5

22 nF

C6

4.7 nF

Gain (R8/R9)

×5.55

f₀

1565

Hz

Slope

−40

dB / dec

Gain

14.9

dB

f₀ = 1 / (2π × √(R6 × R7 × C5 × C6)) ≈ 1565 Hz

The relatively high f₀ is intentional. The Sallen-Key topology combined with the high gain produces a resonant peak just below the corner, which strongly emphasises the harmonics of the guitar signal and gives the comparator a clean, large-amplitude waveform to trigger on.

IC4A Output Smoother

Inverting summing amp + LP feedback cap

R23, R24

47 kΩ × 2

R25 (fb)

47 kΩ

C12 (fb)

2.2 nF

C11 (in)

22 nF

f₀

1539

Hz

Slope

−20

dB / dec

Gain

≈1

×

f₀ = 1 / (2π × R25 × C12) ≈ 1539 Hz

Smooths the hard edges of the divided square wave into something resembling a triangle/sine — the harmonic content above ~1.5 kHz is gently rolled off, making the suboctave sit comfortably under the dry guitar instead of clashing with it.

Envelope & Tracking

R14 (100 kΩ) and C8 (680 nF) form the envelope hold network in IC1D's output:

τ = R14 × C8 = 100 kΩ × 680 nF = 68 ms

This is long enough to ride out one full cycle of even the lowest guitar fundamental (low E ≈ 82 Hz, period ≈ 12 ms — many cycles fit in the hold window) but short enough to release between phrased notes. The TRACK pot adjusts the comparator's threshold up or down, letting you fine-tune for your pickup's output level — turn it down for hot humbuckers, up for low-output single coils.

Output Buffer Rolloff

The IC4B feedback network (R28 = 100 kΩ, C15 = 100 pF) gives a final HF rolloff at 15.9 kHz — well above the audible range, but enough to clean up any switching noise from the CD4013 that might still be in the signal path.

Feel free to experiment with the IC1B caps if the divider isn't tracking your guitar reliably. Larger C5/C6 values lower f₀ and increase emphasis on the fundamental; smaller values raise it and pass more harmonics. The other shaping is cosmetic — IC4A's C12 controls only how "square" the suboctave sounds.

04

Bill of Materials

14 × 10 kΩ resistors are the most common value and the easiest to mix up — populate them all in one pass and double-check the colour bands before soldering. The LM833 for IC2 is highly recommended — using a TL072 in this slot is workable but requires R17 (10 MΩ) to be fitted as well, and tracking will not be quite as clean.

Ref	Qty	Value	Colour code	Notes
Resistors — Metal film, ¼ W, 1 % tolerance
R1, R2, R6, R7, R8, R10, R11, R12, R13, R15, R16, R18, R19, R20	14	10 kΩ	Brown · Black · Black \| Red · Brown	Metal film ¼ W
R3, R26	2	100 Ω	Brown · Black · Black \| Black · Brown	Metal film ¼ W
R4, R17, R27	3	10 MΩ	Brown · Black · Black \| Green · Brown	R17 — only fit if IC2 is not a LM833.
R5	1	1 MΩ	Brown · Black · Black \| Yellow · Brown	Input bias for IC1A
R9	1	2.2 kΩ	Red · Red · Black \| Brown · Brown	IC1B gain network
R21	1	4.7 kΩ	Yellow · Violet · Black \| Brown · Brown	Q1 source resistor
R23, R24, R25	3	47 kΩ	Yellow · Violet · Black \| Red · Brown	IC4A summing network
R14, R28, R29	3	100 kΩ	Brown · Black · Black \| Orange · Brown	Envelope / output stage
R30, R31	2	82 kΩ	Grey · Red · Black \| Red · Brown	TRACK pot end-stops
Capacitors
C1	1	100 µF		Polarised electrolytic — supply reservoir
C2	1	22 µF		Polarised electrolytic — VR decoupling
C3, C4	2	100 nF		Box film — supply / input coupling
C5, C11	2	22 nF		Box film
C6, C7	2	4.7 nF		Box film — IC1B Sallen-Key & IC1C feedback
C8	1	680 nF		Box film — envelope hold cap
C9	1	10 pF		Ceramic — IC2B compensation
C10, C13, C14	3	10 µF		Polarised electrolytic
C12	1	2.2 nF		Box film — IC4A feedback HF rolloff
C15	1	100 pF		Ceramic — IC4B HF rolloff
Pots & Trimmers
R22	1	100 kΩ lin		Trimpot 6 mm, ACP CV6 — sets suboctave level.
MIX, TRACK	2	10 kΩ lin		16 mm board-mount, B-taper (linear).
Diodes
D1	1	1N4001		Reverse polarity protection on +9 V supply.
D2 – D8	7	1N4148		Small-signal silicon. Watch the cathode (band) orientation.
Transistors
Q1	1	J112		N-channel JFET switch — gates the suboctave signal. 2N5485
Integrated Circuits
IC1	1	TL074		Quad JFET-input op-amp, DIP-14.
IC2	1	LM833		Dual low-noise op-amp, DIP-8 — highly recommended. TL072 works but R17 must then be fitted.
IC3	1	CD4013		Dual D-type flip-flop, DIP-14 — divides square wave by 2.
IC4	1	TL072		Dual JFET-input op-amp, DIP-8. NE5532 OPA2134
Switches & Sockets
SW1	1	2PDT		True-bypass footswitch (PCB-mounted or wired).
—	4	IC sockets		14-pin × 2 (IC1, IC3), 8-pin × 2 (IC2, IC4). Recommended.

IC2 substitution. If you cannot source an LM833, a TL072 will work — but you must also fit R17 (10 MΩ). The LM833 has lower input bias current and doesn't need the pull-down. Leave R17 unpopulated if you fit an LM833.

Q1 substitution. The J112 is the original choice. A 2N5485 will also work — same package (TO-92), same EBC pinout family, similar transconductance. Avoid 2N5457 / J201 here as their pinch-off voltages are different and the gating may not behave correctly.

05

Build Guide

There are a lot of parts on a relatively small board — take your time and everything will work out well. Use IC sockets so you can swap chips later if you want to experiment.

PCB silkscreen — component placement reference.

Populated reference build for visual comparison.

Resistors

Populate all 30 resistors first — they sit lowest on the board. Verify each colour code before soldering. The 14 × 10 kΩ resistors share a colour pattern (Brown · Black · Black · Red · Brown), so don't mix them with the 100 kΩ or 82 kΩ values during placement.

Diodes

Fit D1 (1N4001) for reverse polarity protection on the +9 V rail — watch the cathode band orientation. Then the seven 1N4148s (D2 – D8) in the comparator and JFET-gate networks.

IC sockets

Solder all four IC sockets now: 14-pin for IC1 (TL074) and IC3 (CD4013), 8-pin for IC2 (LM833) and IC4 (TL072). Match the notch with the silkscreen orientation.

Transistor & trimpot

Insert Q1 (J112) — match the flat side to the silkscreen. Solder R22 — the 6 mm trimpot. Set it to mid-position for now; you'll dial it in during Setup.

Ceramic and film capacitors

Fit the small ceramics first (C9 = 10 pF, C15 = 100 pF), then the box film caps in ascending value: C12 (2.2 nF), C6/C7 (4.7 nF), C5/C11 (22 nF), C3/C4 (100 nF), C8 (680 nF). Box film caps have no orientation.

Electrolytic capacitors

Fit C2 (22 µF), C10/C13/C14 (10 µF) and finally the tall C1 (100 µF). All electrolytics are polarised — the long leg is positive, and the silkscreen marks the positive pad. Double-check before soldering.

Pots

The MIX and TRACK pots mount on the other side of the board (component side facing down). Use a piece of double-sided tape between the pot body and the PCB to prevent the wide pot tabs from shorting against the board through-holes. Solder the centre pin first, then pull it back about 1 mm and let it harden — this aligns the pot horizontally before you commit the outer pins.

Insert the ICs

With all soldering complete, insert IC1 (TL074), IC2 (LM833), IC3 (CD4013) and IC4 (TL072) into their sockets. Match the notch to the silkscreen.

Wire the footswitch and jacks

SW1 is a standard 2PDT true-bypass footswitch. Wire input jack tip → switch input → IN pad on PCB; OUT pad → switch output → output jack tip. The middle pole switches the LED if you fit one.

Pot mounting — note the small bracket has been clipped off before fitting. Don't forget this step before you mount the board in the enclosure.

Clip the pot bracket. The small mounting bracket on the side of each board-mount pot must be clipped off before you mount the assembly into the enclosure — otherwise it will foul on the pot mounting hole.

06

Setup & Trimming

Once built, the U-Boot needs two settings dialled in: the suboctave level (R22 trimpot) and the tracking threshold (TRACK pot, normally on the front panel).

R22 — Suboctave level

R22 (100 kΩ trimpot, on-board) sets the absolute level of the suboctave signal before it is summed with the dry. Plug in your guitar, set MIX fully clockwise (suboctave only), and play a clean note in the middle of the guitar's range (around the 12th fret of the A string). Adjust R22 until the suboctave is roughly the same level as the dry signal would be. Once set, you rarely need to revisit this.

TRACK — Tracking threshold

The TRACK pot adjusts where the comparator triggers. Start with TRACK at noon. Play a clean sustained note on the lowest string. If the suboctave wobbles, drops out or octaves up unexpectedly, sweep TRACK slowly until you find the cleanest lock. Single-coils typically want TRACK between noon and 2 o'clock; humbuckers between 9 and noon.

Best signal chain placement. Place U-Boot first in the chain after your guitar (before tuner, drives, fuzzes). The cleaner the input fundamental, the better the divider locks. Compressors before U-Boot can help with sustain; drives after U-Boot give classic Boss OC-2-style fuzz-bass tones.

Tracking limits. Like every analogue octave-down design, U-Boot tracks monophonic signals only. Chords will produce intermodulation artefacts (this is by design). For best results, play single notes with clear attack and let them ring.

Final word

Thanks to Merlin there's a clean alternative to the well-known OC-2 circuit that most octave-down pedals are built around. With the small additions on the V1.5 board it tracks even better for some pickups. Enjoy the build.

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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.