ALM Busy Circuits AB2 (2020–2023)

A tiny dual comparator that clicks and clacks and cuts your signal path into clean timing and logic.

Overview

It doesn't look like much. Two LEDs, two knobs, a handful of jacks, and a faceplate the size of a postage stamp. It doesn't make sound on its own, and it won't win any beauty contests. But the AB2 is the kind of utility that goes unnoticed because it works exactly as it should, all the time. Think of a well worn screwdriver in an old toolbox: unassuming, essential, and built to solve real problems.

The AB2 is a dual comparator. It constantly compares voltages and puts out clean, sharp gate signals when a threshold is crossed. Turn the threshold knob and you set a voltage tripwire. Patch a wobbly LFO into it and you get a crisp square wave at exactly the point you choose. Feed it a messy envelope with a soft rise and it slices that into a precise gate the moment it hits your set level. It also creates new signals from chaos. You can trigger a drum hit only when a random voltage climbs above 3V. You can convert an audio-rate sine wave into a steady clock. It turns analog slop into digital certainty.

What sets the AB2 apart from other comparators, even within ALM's own lineup, is simplicity. There are no menus, no shift functions, no hidden modes. Just threshold, input, output, and a comparator LED that blinks clearly. Each side works completely independently, so you get two separate logic tasks in just 4HP. In a crowded rack, that kind of efficiency counts. It's the kind of module you install early and never unplug, like a power supply or a buffered mult. You don't notice it until it's gone, and then your patch falls apart.

It isn't magic, though. The AB2 doesn't add slew, it doesn't debounce, and it doesn't care if your input signal is noisy. Feed it a jittery, unstable CV and it responds with jittery, unstable gates. That's not a flaw, it's fidelity, but it means you need to think about what you're feeding it. Pair it with a clean source or add a filter upstream and it's rock-solid. Let it run wild with dirty randomness and it becomes a glitch generator.

Specifications

Key Features

Dual Independent Comparators with Instant Feedback

Each side of the AB2 is a fully isolated comparator with its own threshold knob, signal input, gate output, and LED. The threshold knob sets a voltage level between -5V and +5V, and when the input signal crosses that level, the output fires a 5V gate. The LED lights when the input is above threshold. There's no hysteresis control or response curve adjustment, which keeps the design clean and the behavior predictable. This isn't a module for nuanced analog logic. It's for when you need a binary decision, fast. Turn the knob, patch the cable, get the gate.

High-Speed Response for Audio-Rate Processing

With a signal bandwidth up to 100kHz, the AB2 isn't just for slow control voltages. It handles audio signals too. That makes it useful for wavefolding, zero-crossing detection, or converting sine or triangle waves into square waves with precise duty cycle control. Patch in a smooth LFO and tweak the threshold to generate rhythmic pulses at any point in the waveform. Feed it a chaotic noise source and use it to extract rhythmic triggers from the peaks. It's not a VCO or a waveshaper, but in the right patch it can act like both. It won't lag or distort fast signals, which matters when you use it to clock other modules or generate timing references.

Minimal Footprint, Maximum Utility

The AB2 is 4HP wide and 38mm deep, so it's skiff-friendly and easy on rack space. Utility modules often sprawl across 6HP or more. You can tuck this one into a gap between oscillators and filters, or mount it on the edge of a case where bulkier modules won't fit. The jacks are full-sized and spaced for standard patch cables, and the knobs are ALM's usual sturdy type, with no wobble. It's built like a tank, even if it doesn't look like one. It draws only 30mA on each rail, so it won't tax your power supply, even in dense systems.

Historical Context

The AB2 arrived in 2020, in the middle of Eurorack's utility boom. By then the format had matured beyond oscillators and filters, and patchers were building complex, self-contained systems that needed logic, clock management, and signal conditioning. ALM, already known for modules like Pamela's Workout and the Squid Salmple, had carved a niche in smart, playful utilities that didn't sacrifice functionality for whimsy. The AB2 fit that approach: no frills, no gimmicks, just a tool that solved a common problem in the most direct way.

It wasn't the first comparator in Eurorack. Modules like the Intellijel Dual ADSR or the Mutable Instruments Maths could perform similar functions. But the AB2 was among the first to make the comparator its sole purpose. That focus gave it an edge in usability and reliability. Other modules buried comparator functions behind modes or menus, and the AB2 put them front and center. It also arrived when more users were exploring digital logic, bit reduction, and clock manipulation in their patches, which made it a natural fit for experimental and glitch-oriented workflows.

Compared to contemporaries like the WMD UGM-3 or ALM's own Pamela's New Workout, the AB2 doesn't do sequencing or clock division. It's not trying to. It's a specialist, not a generalist, and in its narrow lane it excels.

Collectibility & Value

The AB2 is not a rare module, but it's not always easy to find. Production ran from 2020 to 2023, and while it was never discontinued due to failure, ALM periodically rotates out older modules to make room for new designs. As of 2026, used units typically sell between £80 and £120, depending on condition and seller. It has no moving parts beyond the knobs and no onboard firmware, so failure rates are extremely low. The most common issue reported by technicians is damaged jacks from over-tightening or cable strain, but even that is rare.

The AB2 is a passive utility with no complex circuitry, so it doesn't require recalibration or maintenance. There are no trimmers to adjust, no batteries to replace, and no firmware to update. When inspecting a used unit, check that the LEDs respond cleanly to input signals and that the knobs turn smoothly without crackling. Also verify that the gate output is consistently 5V when active. Some early units had minor voltage regulation quirks, but these were resolved in later production runs.

For collectors, the AB2 isn't a showpiece. It won't turn heads at a modular meetup. For working musicians and patch designers, it's quietly indispensable. It ends up in 90% of ALM-based systems, not because it's flashy, but because it works. If you're building a compact or travel-friendly rig, or running low on logic modules, it's a smart buy. Don't expect to resell it for a profit.

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