Mixer/Splitter V4.2 — Build Documentation

Overview & features

The Mixer/Splitter V4.2 is a stripped-down version of the Pro-cessor — same routing core, same six jacks, same blend control, but with the high-pass and low-pass filter section removed. It comes in handy when filtering is not needed but splitting or mixing signals is the only job at hand.

Two TL072 dual op-amps handle everything: IC1 buffers the input and fans it out to the two parallel send loops; IC4 buffers each return and feeds the BLEND pot for the final summing stage. Every stage runs at unity gain — there is no amplification anywhere in the signal path, only buffering and mixing.

Splitter

One guitar in, two amplifiers out — with independent send levels and full isolation between channels.

Mixer

Two instruments into one amp — blend ratio set by the front-panel pot.

Two parallel loops

Run two effect chains in parallel and blend them with a single knob — wet/wet or wet/dry.

Clean blend

One loop holds an effect, the other stays direct — classic clean-blend topology for parallel drive or compression.

Key specs

Two TL072 dual op-amps. 25 kΩ linear (B taper) blend pot. 1 MΩ input impedance, low output impedance from the buffered output stage. 9 V single-supply with reverse-polarity protection. Six Neutrik ¼″ jacks (IN, OUT, A_SEND, A_RETURN, B_SEND, B_RETURN) with normalled tip-to-return contacts so unused loops pass straight through. Fits a 1590B enclosure.

Four configuration examples: splitter, mixer, clean blend, parallel effect blend.

Circuit theory

Full schematic — Mixer/Splitter V4.2 (TH Custom Effects, 10/2014)

Signal architecture

The circuit is built around two TL072 dual op-amps, giving four op-amp stages in total. All four run as unity-gain voltage followers — output tied directly to the inverting input, signal arriving at the non-inverting input through a coupling capacitor. There is no resistor ratio in any feedback path, so there is no amplification anywhere in the signal chain.

Virtual ground (VR rail)

R1 and R2 (both 33 kΩ) form a divider from +9 V to GND, creating a +4.5 V virtual ground reference (the VR rail). C10 (22 µF) decouples this rail. All op-amp non-inverting inputs are biased to VR through 1 MΩ pull-up resistors, allowing the TL072 to handle AC audio signals on a single 9 V rail without a split supply.

Input fan-out (IC1A and IC1B)

The guitar signal enters through the IN jack via C1 (220 nF coupling cap). R3 (1 MΩ) pulls the input down to VR to prevent floating and reduce pop noise. The signal then arrives simultaneously at both IC1A and IC1B non-inverting inputs — this is the splitter node. Each op-amp half buffers its own copy of the signal independently, providing low output impedance to drive whatever effects are patched into the send loops.

Send buffers and loop normalling

IC1A drives B_SEND through C6 (10 µF) and IC1B drives A_SEND through C5 (10 µF). R14 and R4 (1 MΩ each) sit on the send-side of each coupling cap and pull the tip down to GND, keeping it quiet when no plug is inserted. The Neutrik switching jacks (NRJ6-HM-1) provide a normally-closed contact that ties the SEND tip directly to the RETURN tip when no cable is connected — so a loop with no effect plugged in still passes signal through, undisturbed.

Return buffers (IC4A and IC4B)

Each return signal is AC-coupled into IC4 — A_RETURN through C4 (220 nF), B_RETURN through C3 (220 nF) — with R5 and R13 (1 MΩ each) providing a path to VR for the bias current at the non-inverting inputs. IC4A buffers the B-loop return and IC4B buffers the A-loop return. Both stages are unity-gain followers.

Blend and output stage

IC4A's output drives one end of the BLEND pot through C7 (10 µF); IC4B's output drives the other end through C8 (10 µF). The BLEND wiper is the mix point — a 25 kΩ linear pot smoothly crossfades between the two return channels. R6 (1 MΩ) at the wiper provides a DC path to GND and a defined output impedance. The wiper feeds the OUT jack tip directly.

Why no output buffer? The 25 kΩ pot's wiper has output impedance under 12.5 kΩ at any rotation — low enough to drive a guitar amp's input directly. Adding another buffer would change nothing audibly and would only add component count. The earlier filtered Pro-cessor needs IC4 as an output buffer because its filter chain ends in higher impedance; this mix-only build does not.

Power supply

D1 (1N4001) provides reverse-polarity protection in series with the 9 V input. R15 (100 Ω) limits any inrush and forms a small RC filter with C9 (47 µF / 100 µF) for supply-side decoupling. R11 (3k9) is the LED current-limit resistor — adjust if the indicator is too bright or too dim, depending on the LED you fit.

Bill of materials

Ref	Qty	Value	Colour code / detail	Notes
Resistors — metal film, ¼ W, 1 % tolerance
R1, R2	2	33 kΩ	Orange · Orange · Black \| Red · Brown	VR divider
R3, R4, R5, R6, R13, R14	6	1 MΩ	Brown · Black · Black \| Yellow · Brown	Pull-downs and bias
R11	1	3.9 kΩ	Orange · White · Black \| Brown · Brown	LED current limit — check with your LED firstTune
R15	1	100 Ω	Brown · Black · Black \| Black · Brown	Supply RC filter
Capacitors — film (box) and polarised electrolytic
C1, C3, C4	3	220 nF		Box film, signal coupling
C5, C6, C7, C8	4	10 µF		Polarised electrolytic, inter-stage coupling
C9	1	47 µF / 100 µF		Polarised electrolytic, supply decouplingEither fits
C10	1	22 µF		Polarised electrolytic, VR decoupling
Semiconductors
IC1, IC4	2	TL072		Dual op-amp, DIP-8 — use OPA2134 or TLC2272 for extra-low noisePin-compatible
D1	1	1N4001		Reverse-polarity protection, observe stripe
D2	1	LED 5 mm		Indicator, colour of choice (superbright recommended)
Pots, jacks & hardware
BLEND	1	25 kΩ B		B taper = linear (European convention)
Jack ×6	6	NRJ6-HM-1		Neutrik switching ¼″ — IN, OUT, A_SEND, A_RETURN, B_SEND, B_RETURNSee note
IC sockets	2	DIP-8		Recommended — allows op-amp swaps without desoldering
Battery clip	1	9 V		Or DC jack of choice
Enclosure	1	1590B		Powder-coated or raw aluminium

About the jacks: The original BOM specified NRJ4 — these are no longer available with metal tip terminals. The replacement is the stereo NRJ6 series; choose NRJ6-HM-1 if you want the jack body grounded to the enclosure metalwork. Plastic-shaft variants need the enclosure grounded with a separate wire. Plastic nuts are too wide to sit next to each other on the 1590B side wall — order metal nuts (sometimes sold separately) or specify them with the jacks.

Op-amp substitution: The TL072 is the standard, low-cost choice. For lower noise floor try the OPA2134 or TLC2272 — both pin-compatible duals with better specs. JRC4580 also works. All are unity-gain stable, which is required here.

Build guide

PCB layout (top view) — jacks along the long edge, BLEND pot mounted from the backside.

Populated prototype — note the resistor stripes, IC socket, and BLEND pot mounted on the underside.

Build in profile order — flattest components first, tallest last. The BLEND pot mounts on the back of the PCB so the shaft pokes through to the front of the enclosure; install it after everything else.

Resistors

Populate all twelve resistors first — they sit lowest and are easiest to solder before taller parts get in the way. Watch R3, R4, R5, R6, R13, R14 (all 1 MΩ) and R1, R2 (both 33 kΩ) — same colour-band order, easy to mix up across the board.

Diode D1

Install D1 (1N4001) — observe the stripe (cathode) orientation against the silkscreen mark. This is the reverse-polarity protection diode; getting it backwards means no power reaches the circuit at all.

IC sockets

Solder both DIP-8 sockets for IC1 and IC4. Match the notch on the socket to the notch on the silkscreen. Do not insert the ICs yet — leave the sockets empty until soldering is complete.

Film capacitors (C1, C3, C4)

The three 220 nF box film caps are non-polarised — orientation does not matter. Seat them flush against the PCB.

Electrolytic capacitors

C5–C8 (10 µF), C9 (47 µF or 100 µF), C10 (22 µF). All polarised — match the long leg (positive) to the + mark on the silkscreen. Getting any of these reversed will at best sound bad and at worst pop the cap.

Jacks

The six Neutrik jacks need their middle contact row clipped off, plus any plastic alignment pins, before they fit the PCB. Insert all six before soldering any — that way they self-align flat against the board edge. Solder one pin on each jack first to set the position, then complete the rest.

BLEND pot (backside)

Mount the 25 kΩ B-taper pot on the underside of the PCB so the shaft passes through to the enclosure top. This is the last component installed because it sets the overall vertical height of the assembly.

LED & battery clip

Solder D2 (LED) — long leg = anode = side marked + on the silkscreen. Connect the battery clip or DC jack of your choice. Adjust R11 (3k9) up or down if the LED is too bright or dim — for a typical superbright LED, 3k9 sits in the middle of the comfortable range.

Insert ICs

Once all soldering is complete, insert IC1 and IC4 (TL072 or your chosen alternative) into the sockets, matching the notch on each chip to the notch on the socket.

First power-up

Inspect for solder bridges and missed joints under raking light. Plug a guitar into IN, an amp into OUT, and confirm sound passes through with all loops empty. Then test each loop individually with an effect plugged in.

Diode and electrolytic polarity: Three places to watch — D1's stripe, the LED's long leg, and every electrolytic cap's + marking. Reverse-mounting any of these is the most common source of a dead-on-arrival build.

Usage modes

The same six jacks support four distinct setups depending on what you plug in where. Loops that have nothing connected stay normalled — the SEND tip ties to the RETURN tip internally, so signal still passes.

Splitter

One guitar into IN, two amps into A_SEND and B_SEND. The OUT jack is unused. The BLEND pot has no effect — both sends always pass full signal.

IN: guitar · SEND: amp 1, amp 2

Mixer

Two guitars into A_RETURN and B_RETURN. One amp into OUT. BLEND sets the mix ratio. The IN and SEND jacks are unused.

RETURN: guitar 1, guitar 2 · OUT: amp

Clean blend

Guitar into IN, amp into OUT. One loop runs an effect (e.g. dirt or compressor) on its SEND/RETURN; the other loop is empty and stays normalled — clean. BLEND mixes wet and dry.

IN: guitar · loop A: effect · loop B: empty · OUT: amp

Parallel effect blend

Guitar into IN, amp into OUT. Both loops run different effects. BLEND crossfades between the two parallel chains.

IN: guitar · loop A: effect 1 · loop B: effect 2 · OUT: amp

Loop normalling explained: Each Neutrik switching jack contains an internal contact that closes when no plug is inserted, tying the SEND tip directly to the RETURN tip on the same loop. This means an empty loop is invisible to the signal — full bypass with no degradation. Plug an effect in and the contact opens; signal then runs through the cable instead.

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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.