ALM Busy Circuits MCFx2 (2020s)

Name: ALM Busy Circuits MCFx2
Brand: ALM Busy

A dual analog filter that doesn’t just shape sound—it generates it, warps it, and sometimes threatens to eat your patch cable for breakfast.

Overview

Plug in a square wave, twist the resonance past 12 o’clock, and the MCFx2 starts to sing on its own—clean, then snarling, then howling like a theremin trapped in a microwave. That’s the magic of a filter pushed into self-oscillation, and the MCFx2 doesn’t just flirt with it; it leans in hard. Born from ALM’s System Coupe but liberated into Eurorack format, this 14HP module packs two identical 3-pole analog state-variable filter cores, each capable of low-pass, high-pass, and switchable band-pass or notch outputs. It’s not a recreation of any one classic—no Moog ladder, no Oberheim SEM sweep—but it carries the DNA of Roland’s TB-303 in its DNA, with a raw, vocal character that cuts through dense patches like a knife. Yet it refuses to be pigeonholed as just a “303 filter clone.” It’s too flexible, too clever, too willing to misbehave in the best possible way.

Where the MCFx2 really flexes is in its routing options. The Link and Feed switches aren’t just conveniences—they’re game-changers. Hit Link, and the left filter’s cutoff knob controls both cores in parallel, letting you stack resonant peaks or split a signal into low-pass and high-pass outputs for stereo imaging. Flip Feed, and the first filter feeds directly into the second, creating a 6-pole cascade with a slope sharp enough to slice cheese. No patch cables needed. That kind of internal routing is rare in dual filters, and it turns the MCFx2 into a self-contained filtering powerhouse. Want to run two separate signals through different filter types at once? Done. Want to create feedback loops by routing a band-pass output back into the input? The module practically invites it. And with 1V/Oct tracking on each filter, you can use them as secondary oscillators—detuned, filtered, and modulated—effectively doubling your voice count in a minimalist setup.

It’s not all elegance, though. The MCFx2 is a product of its time—early 2020s Eurorack, when skiff-friendly depth and reverse power protection were table stakes. At 32mm deep and drawing a modest 85mA on both rails, it plays nice with compact cases. But it doesn’t skimp on control. Each filter gets dedicated cutoff and resonance knobs, CV inputs with attenuverters (crucial for inverting modulation sources), and manual resonance control that’s rare to see on dual filters. Most manufacturers would cut corners here, but ALM doesn’t. The band-pass/notch output is selected via a physical jumper on the back—a minor hassle, but one that keeps the front panel clean and avoids a fourth switch per filter. It’s a trade-off: you pick your flavor at install and stick with it, which encourages commitment, not indecision.

Specifications

Manufacturer	ALM Busy Circuits
Model	MCFx2
Production Years	2020s
Format	Eurorack
HP	14
Depth	32mm
Current Draw	+12V: 85mA, -12V: 85mA, 5V: 0mA
Filter Type	Dual analog 3-pole state-variable
Filter Modes per Core	Low-pass, High-pass, Band-pass or Notch (jumper selectable)
Resonance Control	Manual per filter, CV input with attenuator
Cutoff Control	Manual per filter, CV input with attenuverter
1V/Oct Input	Yes, per filter
Internal Routing	Link (cutoff sync), Feed (serial filter chaining)
Outputs per Filter	Low-pass, High-pass, Band-pass/Notch (switchable)
Power Connector	2x5 pin Eurorack power header
Reverse Power Protection	Yes
Manual Available	Yes (PDF)
Country of Origin	England

Key Features

Two Filters, One Brain

The MCFx2 doesn’t just give you two filters—it gives you a conversation between them. The Link switch synchronizes the cutoff frequency of both cores, which is invaluable for stereo processing or parallel filtering with matched resonance peaks. But it’s the Feed switch that turns the module into a beast. By internally routing the output of filter one into filter two, you create a 6-pole response without patching a single cable. This isn’t just about steeper slopes; it’s about interaction. The first filter can overdrive the input of the second, creating subtle distortion and intermodulation that a single 6-pole filter never achieves. It’s the difference between a clean surgical cut and a jagged, organic tear. And because each filter has its own resonance control, you can dial in feedback between stages—think of it as a modular version of a multi-stage resonant filter with cross-modulation.

Self-Oscillation as a Feature, Not a Bug

Many filters will self-oscillate if you push resonance far enough, but few embrace it like the MCFx2. With 1V/Oct tracking, each filter core becomes a fully voltage-controlled oscillator, capable of tracking a keyboard or sequencer. The tone isn’t pristine—it’s buzzy, rich in harmonics, with a slight instability that feels alive. It won’t replace your main VCO, but it’s perfect for generating sub-basses, metallic percussion, or detuned drones that sit just outside the harmonic center. Pair it with a slow LFO on cutoff, and you’ve got a pseudo-wavetable oscillator with character. The resonance CV input, complete with attenuator, lets you modulate how hard the filter sings, from a faint hum to a full-on shriek. It’s a sound design playground, especially when you start feeding external audio into a self-oscillating filter—ring modulation without a ring modulator.

Outputs That Do the Work

Three outputs per filter isn’t overkill—it’s freedom. You can take the same input signal and split it into low-pass, high-pass, and band-pass versions simultaneously, feeding different parts of your system. Need a sub-bass, a crisp hi-hat, and a mid-focused lead from one oscillator? The MCFx2 can do that in a single module. The band-pass/notch duality is handled via a jumper on the PCB, which means you choose your behavior at install time. It’s a minor inconvenience—flipping it requires removing the module—but it keeps the front panel clean and avoids a fourth knob or switch. Most users pick band-pass for its musicality, but notch has its niche in subtractive effects and phasey textures. And because all outputs are active at once, you can patch creatively: send the high-pass to a reverb, the low-pass to a distortion, and the band-pass to a delay, all from one source.

Historical Context

The MCFx2 arrived in the early 2020s, a time when Eurorack was shifting from boutique curiosity to mainstream synth tool. Modules weren’t just expected to sound good—they had to be smart, efficient, and flexible. ALM, known for its System Coupe standalone system, had already proven the MCF filter’s worth in that context. The MCFx2 wasn’t a reinvention; it was a liberation. By extracting the filter core and doubling it, ALM gave modular users access to a proven topology without requiring a full system. At a time when many manufacturers were chasing digital complexity or FPGA wizardry, the MCFx2 stood out for its analog purity and mechanical cleverness. It competed with filters like the Doepfer MS-20-style modules and Mutable Instruments’ Brains, but it carved its own niche: not a clone, not a Swiss Army knife, but a focused, high-character tool for those who wanted resonance with attitude. It arrived alongside a wave of “intelligent analog” modules—devices that didn’t just process signals but interacted with them—and it held its own by being both simple and deep.

Collectibility & Value

The MCFx2 isn’t a vintage relic—it’s a recent module, and that affects its collectibility. It won’t appreciate like a 1970s ARP, but it’s already earned a reputation as a modern classic among Eurorack enthusiasts. New units sell for around $325 USD, with European prices in the £220–£250 range. Used units trade for 20–30% less, depending on condition and market demand. It’s not a rare module, but it’s not oversaturated either—ALM’s production is steady but not massive, and the MCFx2 sells quickly when restocked. There are no known manufacturing defects or widespread failure points. The PCB is well laid out, the jacks are sturdy, and the reverse power protection means it’s unlikely to fry your case. The only real maintenance concern is the jumper for band-pass/notch selection—if you change your mind, you’ll need to pull the module and flip it. No firmware, no hidden menus, no digital components to fail. It’s refreshingly simple. For buyers, the advice is straightforward: check that both filters self-oscillate cleanly, that the Link and Feed switches function as expected, and that the CV inputs respond linearly. A dead filter core is rare but possible—ALM’s support is responsive, and repairs are usually straightforward. Given its utility and build quality, the MCFx2 is a keeper. It’s not flashy, but it’s the kind of module you buy once and keep forever.