ALM/Busy Circuits HPO (2018–Present)
A headphone output so unassuming it hides in plain sight—until you plug in and hear just how quiet and clear 2HP can be.
Overview
The ALM/Busy Circuits HPO (also known as ALM019 HPO) isn’t the star of your Eurorack build. It’s the quiet professional in the back who makes sure everyone else sounds good. Introduced in 2018 and still in production, this tiny headphone output module solves a problem every modular user eventually faces: how to monitor your patch without eating up precious rack space. At just 2HP wide and 38mm deep, it’s explicitly designed to be skiff-friendly—slim enough to tuck into the tightest setups without sacrificing function. It delivers a clean, low-noise stereo output via a 3.5mm jack, with a volume knob that feels responsive and precise. Owners report it’s “low noise, good sounding,” which might sound like faint praise until you’ve wrestled with hum, crosstalk, or weak output from cheaper solutions. This module doesn’t color your sound—it preserves it, cleanly, without fuss.
What makes the HPO stand out isn’t sonic character but practical brilliance. It accepts optional dual channel inputs, letting you route separate left and right signals for true stereo playback, or feed a single mono source and let the module double it across both ears—simple, but effective. It’s not packed with features: no mute button, no brightness toggle, no impedance switch. Just volume, stereo output, and a footprint so small you might forget it’s there—until you need it. And when you do, it’s rock solid. The design prioritizes reliability and minimalism, fitting the ALM/Busy Circuits ethos of functional, no-frills utility modules that do one thing well.
Specifications
| Manufacturer | ALM/Busy Circuits |
| Dimensions | 2 HP |
| Depth | 38 mm |
| Current Draw | 5 mA +12V, 5 mA -12V, 0 mA 5V |
| Inputs | Optional dual channel inputs |
| Output | 3.5mm stereo jack output |
| Features | Volume control |
| Supply | +12V 5mA / -12V 5mA |
Key Features
Skiff-Friendly Slim Profile
At 38mm deep and only 2HP wide, the HPO is built for tight spaces. “Skiff friendly” isn’t marketing fluff here—it’s a design mandate. In a world where even utility modules can hog real estate, this one slips into corners and gaps where others won’t fit. It’s a godsend for portable setups or packed cases where every millimeter counts. The compact construction doesn’t compromise build quality; it feels solid, with a clean layout and reliable jacks.
Flexible Input Options
The HPO supports optional dual channel inputs, allowing true stereo monitoring when needed. But it’s also smart enough to handle mono sources gracefully—simply patch one signal, and it doubles to both ears without phase issues or volume drop. This flexibility makes it useful across a wide range of systems, from minimalist mono rigs to expansive stereo setups.
Low-Noise, Transparent Audio Path
Described by users as “low noise, good sounding,” the HPO delivers a clean, uncolored signal. There’s no noticeable hiss, hum, or crosstalk under normal use. The volume control has a smooth taper, letting you dial in listening levels precisely, whether you’re tracking quietly at night or checking levels at higher volumes. It won’t add warmth or grit—its job is transparency, and it nails it.
Collectibility & Value
The HPO isn’t a grail module, but it’s a quietly essential one—and the market reflects that. Used modules in “Very Good” condition have sold for as low as $65, making it one of the more accessible ALM offerings. A few listings note the inclusion of extras like bus cable, screws, sticker, and anti-static bag—small touches, but appreciated. The most commonly mentioned cosmetic issue is “some slight rack rash” on used units, which is typical for modules handled frequently. With a ModularGrid user rating of Ø 4.60 (35 votes), it’s clearly a trusted workhorse. It doesn’t appreciate in value, but it holds its ground—because people keep needing headphone outs, and this one does the job without drama.
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