Free Modular ALU - 4HP Eurorack Arithmetic & Logic Utility

The Free Modular ALU is one of those utility modules that looks simple until you start patching it. Feed it a few gates, envelopes, LFOs or sequencer voltages and it gives you eight different processed outputs simultaneously - without menus, modes or sacrificing half the rack to do it.

ALU stands for Arithmetic Logic Unit, borrowing the name from the part of a CPU that performs mathematical and logical operations. The module contains two completely independent sections: one for gates and logic, and one for analog CV mathematics.

Logic section

The upper section takes two gate inputs, A and B, and simultaneously provides:

• OR - high when A, B or both are high
• AND - high only when both inputs are high
• NOR - inverted OR
• XOR - high when exactly one input is high

This makes it particularly useful for combining clocks and rhythmic patterns, deriving additional triggers, creating conditional events or turning two relatively simple gate sequences into something considerably less predictable.

Arithmetic section

The lower section takes two analog signals, again A and B, and simultaneously provides:

• A+B - sum of both input voltages
• A-B - difference between A and B
• MAX - whichever of the two voltages is currently higher
• |A-B| - the absolute difference between A and B

Patch in LFOs, envelopes, sequencer CV or other modulation sources and the ALU can derive several new modulation shapes from signals you already have. It is a particularly compact way of making a patch interact with itself instead of simply adding yet another LFO.

Specifications

• Width: 4HP
• Depth: approximately 35 mm
• 12 x 3.5 mm jacks
• 2 gate inputs + 4 logic outputs
• 2 analog inputs + 4 arithmetic outputs
• Logic functions: OR, AND, NOR, XOR
• Arithmetic functions: A+B, A-B, MAX, |A-B|
• No +5V rail required
• Typical current draw: less than 10 mA on +12V and less than 10 mA on -12V
• Maximum current draw: 20 mA per ±12V rail

About this build

This is a privately hand-built implementation of the open-source Free Modular ALU design, not a commercial production run or factory-built module.

I ordered the PCBs for my own DIY builds, and PCB manufacturing inevitably comes with minimum quantities. Rather than leaving the remaining board sets in a drawer indefinitely, I assembled a small number of them into complete modules.

This particular module has been assembled by hand, visually inspected and functionally tested. It is supplied fully assembled and ready to install - this is not a kit or partially completed DIY project.

Included

• Free Modular ALU module
• Approx. 30 cm Eurorack power cable
• Mounting screws

The original ALU is an open-source Free Modular design. Full project documentation, including the manual, schematics, BOM, assembly information and design files, is available from the Free Modular project website:

freemodular.org/modules/Logic