Mad Box V3.2 — Build Documentation

Overview & Features

The Mad Box is a multi-purpose pedalboard utility. It buffers an input signal and presents it on two independent send jacks, accepts two return signals from external effects, and blends them at the output through a single 25kΩ pot. Whether you use it as a splitter, a mixer, a parallel effect blender, or a clean-blend front-end is purely a matter of how you wire it into your rig.

This circuit is essentially a stripped-down Pro-cessor without the active filters. The four op-amp gates that remain are all configured as unity-gain voltage followers — there is no signal shaping, no gain, no rotary switching. The only user control is the BLEND pot.

Buffered I/O

All five signal nodes (input, two sends, two returns) are op-amp buffered for low output impedance and high input impedance.

Single BLEND pot

25kΩ linear pot mixes the two buffered return signals into the output. End-to-end: 100% A or 100% B; centred: equal mix.

4 modes from one box

Splitter, mixer, parallel effect blend, clean blend — selected entirely by external wiring, no switches inside the pedal.

Reverse-polarity safe

1N4001 series diode protects the circuit from a wrongly-polarised power adapter.

Four typical usage modes — Splitter, Mixer, Clean Blend, Effect Blend

About the diagram: The DIRECT/LP/HP toggle switches drawn on the boxes above are leftover labels from the Pro-cessor diagram — the Mad Box does not include filter switches. Read those toggle markers as decorative; the only physical control on the Mad Box enclosure is the BLEND pot.

Circuit Theory

The signal chain consists of four unity-gain op-amp buffers arranged in two pairs. IC1 (TL072) handles input buffering and provides the two SEND signals. IC4 (TL072) handles return buffering and feeds the BLEND pot. Both halves share a single 9V supply with a virtual-ground reference.

Mad Box V3.2 schematic — input buffer pair on the left (IC1), return buffer pair on the right (IC4), BLEND pot, and power supply at the top

Input buffer pair (IC1A & IC1B)

The guitar signal enters through C1 (220nF) which DC-blocks the input. R3 (1MΩ) pulls the cap junction to virtual ground, setting the bias point and presenting a 1MΩ input impedance to the source. Both op-amp +IN pins of IC1 share this same node — a single input drives two parallel followers in lock-step. Each output is taken through a 10µF electrolytic coupling cap (C5, C6) to the SEND jacks. R4 and R14 (1MΩ each) pull the SEND outputs down so they are well-defined when no cable is plugged in.

Why two parallel followers instead of one? Splitting the load between two op-amp gates keeps each output driver lightly loaded, gives independent short-circuit current to each SEND jack, and prevents one effect's input impedance from interacting with the other. The cost is one extra op-amp gate — well worth it for the isolation.

Return buffer pair (IC4A & IC4B)

A_RET and B_RET each go through their own DC-blocking cap (C4, C3 — both 220nF) into a 1MΩ pull-up to virtual ground (R5, R13). Each signal then drives one half of IC4 as a unity-gain follower. IC4A (B-side) and IC4B (A-side) outputs go through 10µF coupling caps (C7, C8) to the BLEND pot.

BLEND pot and output

The BLEND pot is a 25kΩ linear (B-taper) wired as a passive mixer between the two buffered return signals. Pin 1 receives the B-side return; pin 3 receives the A-side return; the wiper (pin 2) is the output. R6 (1MΩ) pulls the output to ground so there is no DC offset present at the OUT jack when the wiper is moved. Note that the output is not buffered after the pot — the source impedance at the wiper varies between 0Ω (at the extremes) and ~6.25kΩ (at the centre, worst case), which is low enough to drive any standard pedal or amplifier input without trouble.

Power supply

D1 (1N4001) provides reverse-polarity protection — a wrongly-wired DC adapter forward-biases D1 to ground and the rail collapses safely. R15 (100Ω) limits inrush current and forms a simple RC filter with C9 (47µF) for ripple smoothing. R1 and R2 (33kΩ each) form the virtual-ground divider, generating VR ≈ 4.5V at the junction. C10 (22µF) decouples this VR rail to AC ground so it acts as a clean reference for all four op-amp +IN bias networks.

VR = V_supply × R2 / (R1 + R2) = 9V × 33k / (33k + 33k) = 4.50 V

No filters, no calculations to make: Every R/C network in this circuit is either a DC-blocking high-pass (corner well below 1Hz) or a power-supply decoupling node. There is no signal-shaping filter and no gain stage — the entire pedal is a clean buffer + passive blend. That's why this is "Pro-cessor without the filters."

Bill of Materials

IC substitution note: The schematic specifies TL072P. For lower noise on quiet signal paths (acoustic, single-coil clean tones), the Burr Brown OPA2134 is a recommended drop-in pin-compatible alternative. The JRC4580D and TLC2272 are also pin-compatible and work well here.

Ref	Qty	Value	Colour code	Notes
Resistors — Metal film, ¼ W, 1%
R1, R2	2	33k	Orange · Orange · Black \| Red · Brown	Virtual ground divider — match for best supply rejection
R3, R4, R5, R6, R13, R14	6	1M	Brown · Black · Black \| Yellow · Brown	Input/return bias and output pull-down to VR or GND
R15	1	100R	Brown · Black · Black \| Black · Brown	Power-rail current limiter / RC filter with C9
Capacitors — Film (input/return DC-block)
C1, C3, C4	3	220n		Input and return AC-coupling — film type, polyester or polypropylene box
Capacitors — Electrolytic (signal coupling and supply)
C5, C6, C7, C8	4	10µF		Op-amp output coupling caps — 16V or 25V rated, observe polarity
C9	1	47µF		Main +9V supply bulk decoupling — 16V or 25V rated
C10	1	22µF		VR (virtual ground) decoupling cap
Semiconductors
D1	1	1N4001		Reverse polarity protection — observe cathode band orientation
IC1, IC4	2	TL072P		Dual JFET-input op-amp OPA2134JRC4580DTLC2272
Pots
BLEND	1	25k-B		B-taper = linear (European convention); 16mm PCB-mount
Hardware (not on PCB)
IC sockets	2	DIP-8		Strongly recommended — allows op-amp swap for tone experiments
Jacks	5	¼″ mono		IN, OUT, A_SEND, A_RET, B_SEND, B_RET — wire to PCB pads
DC jack	1	2.1mm		Centre-negative standard, wire to V1 pads

Build Guide

PCB silkscreen — component placement reference

Populated PCB — note IC sockets and electrolytic polarity

Populate the board in order from lowest to highest profile. This keeps everything sitting flat against the board while you flip and solder.

Resistors

Mount R1, R2, R3, R4, R5, R6, R13, R14, R15 first. Resistors have no polarity — you can fit them either way around. Bend the leads close to the body, drop them in, and flush-solder from the back. Trim leads to about 1mm.

Diode D1 (1N4001)

Polarity matters. The black band on the diode body marks the cathode (the bar end of the diode symbol). Match the band to the silkscreen marking on the PCB.

IC sockets (DIP-8)

Insert the two DIP-8 sockets, but do not put the ICs in yet. The notch on the socket should match the notch printed on the PCB silkscreen — getting this right now means the chips drop straight in later. Solder corner pins first to fix the socket flat against the board, then the rest.

Film capacitors

C1, C3, C4 (all 220nF film box caps). No polarity — fit either way around.

Electrolytic capacitors

C5, C6, C7, C8 (10µF), C9 (47µF), C10 (22µF). Polarity matters — the long lead is positive, and the negative side is marked with a stripe on the body. The PCB silkscreen marks the positive pad with a +. Get this wrong and the cap can be damaged or fail.

BLEND pot (mount last, from the back)

The 25kΩ linear pot is a tall component and is mounted from the back side of the PCB. This sets the overall enclosure height. Insert from the back, push fully home, and solder from the component side.

External wiring & insert ICs

Wire the off-board components: 5 audio jacks (IN, OUT, A_SEND, A_RET, B_SEND, B_RET — six total — depending on whether you split the SEND/RETURN pads or run separate jacks per side, see the silkscreen labels) and the 2.1mm DC jack. Then insert IC1 and IC4 (TL072P) into the sockets — notch matches notch.

First power-up check: Before plugging in a guitar, apply 9V and measure pin 4 (V−) and pin 8 (V+) of each IC socket. Pin 4 should read 0V (GND); pin 8 should read close to +9V. Then measure the +IN pins (3 and 5) — they should all sit at about 4.5V (the VR rail). If anything is significantly off, power down and re-check polarity of D1, the electrolytics, and resistor placement.

Usage Modes

The Mad Box has no internal mode switching — the four operating modes are achieved entirely by how you wire it into your signal chain. The BLEND pot is the only control that interacts with the signal in real time; it sets the mix ratio between channel A and channel B at the output.

Splitter (1 → 2)

Guitar into IN. A_SEND and B_SEND drive two amps or two parallel pedal chains. A_RET and B_RET are unused. The OUT jack is also unused in this configuration — the SENDS are your two outputs.

IN: guitar · A_SEND/B_SEND: → amps · BLEND: not used

Mixer (2 → 1)

Two guitars (or any two line-level sources) into A_RET and B_RET. OUT goes to the amp. BLEND sets the ratio between the two inputs. A_SEND, B_SEND, and IN are unused.

A_RET/B_RET: ← sources · OUT: → amp · BLEND: A↔B mix

Effect blend (parallel loops)

Guitar into IN. A_SEND → effect 1 → A_RET. B_SEND → effect 2 → B_RET. OUT to amp. Both effects run in parallel; BLEND mixes the wet signals from each loop.

A & B both as effect loops · BLEND: effect-A ↔ effect-B

Clean blend (one wet, one dry)

Guitar into IN. A_SEND → effect → A_RET. B_SEND directly into B_RET (a short patch cable). OUT to amp. The B-side carries the dry buffered signal; the A-side carries the effect. BLEND adjusts the wet/dry mix.

A: effect loop · B: SEND→RET patched · BLEND: wet ↔ dry

Tip: Because every signal node in the Mad Box is op-amp buffered, you can drive long cable runs from the SEND jacks without losing high-frequency content, and you can feed the OUT jack into a high-impedance amp input or a low-impedance line input without trouble. The pedal is essentially transparent — it does not colour the tone.

—

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.