TH Custom Effects Build Documentation · V4.2 · 2026

Pro-cessor

Splitter · Mixer · Filter — V4.2

A flexible signal routing tool for the pedalboard. Two switchable effect loops with 4th-order high-pass and low-pass filtering, blendable into a single output. Splitter, mixer, parallel effect blender or 24 dB tone control — all in one box.

4th-order Sallen-Key 2× effect loops 25k linear blend 1590B / 125B

01

Overview & Features

The Pro-cessor is a passive-look, active-design pedalboard utility built around four TL072 dual op-amps. Depending on how the patch is wired, the same circuit operates as a signal splitter, mixer, parallel effect blender, or a 24 dB/octave tone-control. Two independent send/return loops — channel A and channel B — are mixed by a single 25k BLEND pot at the output stage. Each channel has a three-position rotary selector that routes the signal either DIRECT, through a 4th-order high-pass filter, or through a 4th-order low-pass filter.

Two parallel loops with independent send / return jacks for any combination of effects, instruments or amps.

4th-order Sallen-Key filters on each channel — 24 dB/octave HP and LP, far steeper than typical 12 dB pedal filters.

25 kΩ linear blend pot mixes channel A and channel B at the output buffer.

Op-amp buffered throughout — input and output buffers, two filter stages per side, all inside four TL072 packages.

Unity gain with no inversion at the output — mix with the dry signal in series effects without phase cancellation.

Fits 1590B with 6 jacks, 2 rotaries, BLEND pot and battery clip; 125B for the on-on-on switch variant.

Typical Usage

Front-panel layout — central BLEND, IN at the bottom, OUT at the top, six jacks for two independent send/return loops, two channel selectors

Prototype build — 1590B enclosure

02

Circuit Theory

The Pro-cessor is built around four TL072 dual op-amps configured as unity-gain buffers and two cascaded Sallen-Key active filters per channel. The signal path is fully buffered from input to output — every send and return point sees a low source impedance, so external pedal load and cable capacitance do not interact with the filter response.

Pro-cessor V4.2 — full schematic, signal paths and switching

Stage-by-stage signal flow

Input buffer (IC1B). The IN jack feeds C1 (220n) coupling cap and R3 (1M) pull-down to virtual ground. IC1B is wired as a unity-gain follower to the input node: high input impedance, low output impedance. Its output drives the channel A and channel B paths in parallel.

Channel splitting. Both channel-A and channel-B sends pick up the buffered input signal directly — there is no series resistor between them, so the channels are isolated by IC1B's output impedance only and run in true parallel. R4 and R14 (1M) provide DC return paths at the send jacks.

Send / return loop. Each channel has SEND and RETURN jacks intended for an external effect. The RETURN signal is AC-coupled via C3/C4 (220n) to the filter selector. With nothing plugged into the loop, normalled jack switching routes the send signal directly back into the channel — the loop is transparent when not in use.

4th-order Sallen-Key filters. Each channel has a high-pass and a low-pass path, both built as two cascaded 2nd-order Sallen-Key stages for a 24 dB/octave slope. The HP path uses series capacitors with shunt resistors at the +IN node; the LP path uses series resistors with shunt capacitors. A rotary switch selects DIRECT, HP or LP output and feeds the channel mixer.

Channel mixer / blend (IC4B). The 25k linear BLEND pot is wired as a three-terminal mix control between channel-A and channel-B outputs. The wiper feeds IC4B configured as a unity-gain buffer driving the OUT jack. R6 (1M) at the output provides a DC return path.

Power supply. 9 V DC is reverse-polarity protected by D1 (1N4001), filtered by C9 (47 µF) and current-limited to the LED indicator D2 by R11 (3k9). A 33k/33k divider with C10 (22 µF) bypass generates a virtual-ground rail at 4.5 V (V_R). All op-amps reference V_R at their non-inverting inputs through 1M resistors, allowing single-supply operation.

03

Filter Analysis

Both the HP and LP paths are 4th-order Sallen-Key cascades (two 2nd-order stages in series). The 24 dB/octave slope is steep enough to do real frequency-band splitting between channel A and channel B — much more than the gentle 6 or 12 dB roll-off found in most pedal tone controls.

High-pass · 4th order

≈ 275Hz

Stage 1 (IC2A): R20 820Ω · R19 3k9 · C14 560n · C13 330n
f₀ = 207 Hz · Q = 0.29
Stage 2 (IC2B): R8 2k7 · R7 3k9 · C15 180n · C2 100n
f₀ = 366 Hz · Q = 0.37
Slope: −24 dB/octave

Cuts everything below the lower-mids. Useful to send a guitar's body and fundamentals to one effect chain while letting the upper harmonics through.

Low-pass · 4th order

≈ 250Hz

Stage 1 (IC3A): R21 560Ω · R17 1k · C12 1µ · C11 330n
f₀ = 370 Hz · Q = 0.84
Stage 2 (IC3B): R10 1k8 · R9 3k3 · C17 680n · C16 220n
f₀ = 169 Hz · Q = 0.84
Slope: −24 dB/octave

Cuts everything above the lower-mids. Useful to feed only the guitar's body and fundamentals into an octaver or sub-octave effect, leaving the upper register clean.

Channel symmetry. Each channel (A and B) shares the same physical HP and LP filter blocks, selected by the rotary switches. The values above apply to both channels — any retuning is global.

Alternative HP Filter Values

The HP corner frequency is set by the four reactive components R19, R20, C13, C14 in stage 1 and R7, R8, C2, C15 in stage 2. To shift the corner up or down, replace all eight components according to the preset below. Keep both stages identical for a clean Butterworth-style response.

Ref	Value	Colour code
HP ≈ 100 Hz · stage 1 + stage 2
R20 / R8	5k1	Green · Brown · Black \| Brown · Brown
R19 / R7	5k1	Green · Brown · Black \| Brown · Brown
C14 / C15	330 n
C13 / C2	330 n
HP ≈ 200 Hz · stage 1 + stage 2
R20 / R8	820 Ω	Grey · Red · Black \| Black · Brown
R19 / R7	5k1	Green · Brown · Black \| Brown · Brown
C14 / C15	270 n
C13 / C2	560 n
HP ≈ 500 Hz · stage 1 + stage 2
R20 / R8	4k3	Yellow · Orange · Black \| Brown · Brown
R19 / R7	5k1	Green · Brown · Black \| Brown · Brown
C14 / C15	68 n
C13 / C2	68 n

Alternative LP Filter Values

The LP corner frequency is set by R17, R21, C11, C12 in stage 1 and R9, R10, C16, C17 in stage 2. As with HP, both stages should be retuned together. Note that the original LP design uses asymmetric resistor pairs and asymmetric capacitor pairs to achieve a specific Q — the simplified equal-component presets below trade Q precision for build simplicity.

Ref	Value	Colour code
LP ≈ 200 Hz · stage 1 + stage 2
R21 / R10	620 Ω	Blue · Red · Black \| Black · Brown
R17 / R9	10k	Brown · Black · Black \| Red · Brown
C12 / C17	680 n
C11 / C16	150 n
LP ≈ 500 Hz · stage 1 + stage 2
R21 / R10	4k3	Yellow · Orange · Black \| Brown · Brown
R17 / R9	5k1	Green · Brown · Black \| Brown · Brown
C12 / C17	68 n
C11 / C16	68 n
LP ≈ 1.2 kHz · stage 1 + stage 2
R21 / R10	1k6	Brown · Blue · Black \| Brown · Brown
R17 / R9	13k	Brown · Orange · Black \| Red · Brown
C12 / C17	47 n
C11 / C16	18 n

Feel free to substitute your own values. The PCB footprint accepts standard 6 mm metal-film resistors and 7.2 mm box-style film caps up to 1 µF; larger caps may not fit. The corner frequencies above are approximations — the cascaded Q interaction shifts the −3 dB point slightly from the geometric mean of the two stages.

04

Bill of Materials

All resistors are 1 % metal film, 6 mm body, ¼ W. All film capacitors are 7.2 mm box style. Electrolytic capacitors are radial, 5 mm or 6.3 mm body. The PCB accepts standard through-hole packages — verify each component fits before soldering.

Ref	Qty	Value	Colour code	Notes
Resistors — ¼ W metal film, 1 %
R1, R2	2	33k	Orange · Orange · Black \| Red · Brown	Virtual-ground divider
R3, R4, R5, R6, R13, R14	6	1M	Brown · Black · Black \| Yellow · Brown	Pull-down / DC return
R7, R11, R19	3	3k9	Orange · White · Black \| Brown · Brown	HP filter VR · LED CLR
R8	1	2k7	Red · Violet · Black \| Brown · Brown	HP stage 2 feedback
R9	1	3k3	Orange · Orange · Black \| Brown · Brown	LP stage 2 series
R10	1	1k8	Brown · Grey · Black \| Brown · Brown	LP stage 2 series
R15	1	100R	Brown · Black · Black \| Black · Brown	Supply decoupling
R17	1	1k	Brown · Black · Black \| Brown · Brown	LP stage 1 series
R20	1	820R	Grey · Red · Black \| Black · Brown	HP stage 1 feedback
R21	1	560R	Green · Blue · Black \| Black · Brown	LP stage 1 series
Capacitors — film (box style, 7.2 mm pitch)
C1, C3, C16	3	220 n		Coupling / LP stage 2
C2	1	100 n		HP stage 2
C4	1	220 n		A return coupling
C11, C13	2	330 n		LP / HP filter
C12	1	1 µ		LP stage 1 feedback
C14	1	560 n		HP stage 1 input
C15	1	180 n		HP inter-stage
C17	1	680 n		LP stage 2 feedback
Capacitors — electrolytic (radial)
C5, C6, C7, C8	4	10 µF		Send/return coupling — observe polarity
C9	1	47 µF		Supply reservoir — observe polarity
C10	1	22 µF		V_R bypass — observe polarity
Semiconductors
IC1, IC2, IC3, IC4	4	TL072P		Dual op-amp, DIP-8. JRC4580 OPA2134 TLC2272 for low noise
D1	1	1N4001		Reverse-polarity protection — observe polarity
D2	1	LED 5 mm		Power indicator — superbright preferred, colour of choice
Pots & switches
BLEND	1	25k-B		Linear taper, 16 mm PCB-mount
SW_A, SW_B	2	Rotary 2P3T		Musikding rotary impulse switch — preferred PCB-mount type. on-on-on toggle alternative requires 125B enclosure and external wiring
Mechanical
Jack 1–6	6	NRJ6-HM-1		Neutrik 6.3 mm stereo jack, metal nut. Mouser link. NRJ4 plastic alternative — only metal nuts will fit side-by-side
V1	1	9 V battery clip		Optional — DC jack typically used instead
—	1	Enclosure		1590B for rotary version; 125B if using on-on-on toggles

Jacks. The original NRJ4 mono jacks with metal tips are no longer manufactured. Use the NRJ6 stereo version with the -HM (metal nut) suffix to ground the enclosure through the nuts. The plastic version works electrically but the plastic nuts are too wide to sit side-by-side on the PCB — substitute round metal nuts on the plastic shafts if you go that route. When ordering, make sure the order includes nuts.

Op-amp choice. TL072 is the standard fit; JRC4580D, NE5532, OPA2134 and TLC2272 are all pin-compatible DIP-8 dual op-amps and any can be substituted. The original prototype uses JRC4580D — the photographs on the next page reflect that.

05

Build Guide

Standard through-hole population order — lowest profile first, tallest last. The narrow PCB layout means component clearance is tight; verify each capacitor fits the footprint before soldering, and seat resistors flat against the board.

PCB silkscreen — top view, component reference

Populated prototype board with JRC4580D op-amps

Resistors

Populate all resistors first — they are the lowest-profile components and easiest to access before larger parts crowd the board. Double-check colour bands against the BOM as you go; the PCB has no value silkscreen on R-positions.

Diodes

Solder D1 (1N4001) in its marked orientation — the cathode band lines up with the silkscreen stripe. Solder D2 (LED) only after deciding final mounting: if you panel-mount the LED, fit it last with extension wires.

IC sockets

Fit DIP-8 sockets for IC1 through IC4. Do not insert the op-amps yet — they go in last to avoid heat damage and to allow easy substitution if you want to try different chips.

Film capacitors

Box-style film capacitors next, by ascending value (100 n, 180 n, 220 n, 330 n, 560 n, 680 n, 1 µ). Non-polarised — orientation does not matter. Check that the larger 1 µF cap (C12) and the 680 n cap (C17) clear adjacent components.

Electrolytic capacitors

Polarised — the long lead is positive, the silkscreen marks the positive pad. C5 through C8 (10 µF), C9 (47 µF) and C10 (22 µF) — verify each before soldering. Reversed electrolytics will fail when power is applied.

Jacks

Insert all six jacks at once and tack-solder one pin on each before doing the rest. This ensures co-planar seating against the enclosure face when the PCB is mounted.

BLEND pot and rotary switches — backside

The BLEND pot and the two rotary switches mount from the back of the PCB so the shafts protrude through the enclosure top. Insert from the underside and solder from the component side — the photo on the next page shows the correct orientation.

Insert ICs into sockets

Last step — insert the four TL072 (or JRC4580D / OPA2134 / TLC2272) op-amps into their sockets. Pin 1 is marked by the dot or notch on the IC; the socket has a notch on one end — line them up. Bend the leads slightly inward if they splay.

Backside mounting detail

BLEND pot and rotary switches mounted from the underside of the PCB

06

Switching & Wiring

Each channel selector picks one of three signal paths: DIRECT (the buffered loop output, no filtering), HP (4th-order high-pass), or LP (4th-order low-pass). The default build uses a PCB-mount 2P3T rotary switch — no external wiring required.

Rotary switch positions

Position	Pad routed to channel mixer	Effect
1 — DIRECT	X_DIRECT	Buffered loop output, full bandwidth
2 — LP	LP	4th-order low-pass at ≈ 250 Hz
3 — HP	HP	4th-order high-pass at ≈ 275 Hz

Alternative: on-on-on toggle wiring

If you use an on-on-on toggle switch in place of the rotary, external wiring is needed and a 125B enclosure is recommended for the extra room. First link pins 3 and 5 together. Then connect:

Toggle pin	Connect to PCB pad
2	X_FILTER (filter input)
1	X_DIRECT (direct output)
4	LP (low-pass output)
6	HP (high-pass output)

On-on-on toggle wiring requires a 125B enclosure. The 1590B variant is too small to accommodate the additional external wiring loom comfortably — switch leads will collide with the rotary footprints. If you intend to use toggles, plan for the larger box from the start.

07

Drilling

The Pro-cessor mounts in a 1590B with all six jacks along the back face and the controls (BLEND, two rotaries, LED) on the top. A drill template at 600 DPI is available at diy.thcustom.com/drill-templates.

Backside jack measurements (1590B)

Backside jack hole spacing — all dimensions in mm, measured from left edge

1590B jack holes must be slotted to the bottom edge. Without slotting, the populated PCB will not fit into the enclosure — the jack body sits below the PCB surface and needs to drop in from the open side. The photograph above shows the correct slotted geometry.

For 125B or other enclosures, the 21 mm top-edge offset still applies — measure from the top of the enclosure face down to the jack centreline, then space the jacks at the indicated 17 / 16 / 16 / 16 / 16 / 16 mm intervals from the left edge.

08

Usage Modes

The Pro-cessor is the same circuit in every patch — the routing and BLEND setting determine what it does. Six common configurations are shown below; many more are possible by mixing wet and dry, filtered and unfiltered, mono and stereo signal paths.

Splitter

One guitar in, two amps out. Patch GUITAR → IN, AMP I → SND_A, AMP II → SND_B. RET_A and RET_B are unused. BLEND becomes a balance control between the two amps.

Mixer

Two instruments in, one amp out. GUITAR I → RET_A, GUITAR II → RET_B, OUT → AMP. BLEND mixes the two sources at the output stage.

Splitter + Filter

BASS → SND_A → external bass amp; TREBLE → SND_B → external treble amp. Switch A to LP, switch B to HP. The Pro-cessor does the active crossover internally — 24 dB/oct on each side.

Clean blend

One guitar, parallel effect chain. GUITAR → IN, OUT → AMP. Patch effect send/return into either channel. Set the unused channel to DIRECT and use BLEND to mix wet and dry.

24 dB tone control

GUITAR → IN, OUT → AMP, no external loops. Set channel A to HP and channel B to LP — BLEND becomes a brutal tilt-EQ between the two halves of the spectrum.

Effect blend

Two parallel external effects. EFFECT 1 in the channel-A loop, EFFECT 2 in the channel-B loop. BLEND mixes their wet outputs at the OUT jack.

Six routing presets — splitter, mixer, splitter & filter, clean blend, 24 dB tone control, effect blend

Acoustic + octaver tip. Set channel B to LP, route B's send through an octaver, and return it. Set channel A to DIRECT. BLEND now mixes a clean acoustic guitar with sub-octave on the body frequencies only — the upper register stays dry. This was the patch the Pro-cessor was originally designed around.

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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.