ALM Busy Circuits FIZZLE GUTS (2012–2022)

A ghost from a forgotten Casio sampler, resurrected in Eurorack form — raw, rare, and built from scavenged silicon.

Overview

Plug in the FIZZLE GUTS for the first time and you’re not just powering up a filter module — you’re waking something up that wasn’t meant to be here. The sound is immediate: thick, slightly hazy, with a top-end fizz that doesn’t clean up no matter how far you roll off the resonance. It’s not pristine. It’s not neutral. It’s the voice of a 1987 Casio FZ-1 sampler’s filter IC, ripped from its original home and wired into a 12HP Eurorack module, and it carries the weight of that history in every sweep. This isn’t a recreation or a simulation — it’s the actual analog filter chip, the MB87186, salvaged from dead or donor FZ-1, FZ-10M, or FZ-20M keyboards, now repurposed into a dual filter and VCA setup that feels more like an archaeological artifact than a modern synth module.

When ALM launched the FIZZLE GUTS around 2012, it wasn’t trying to win points for practicality. This was a statement piece — one of the very first modules the company ever produced, born from a DIY ethos and a fascination with obsolete technology. Only about 30 units were ever assembled by hand, making it a near-mythical presence in the Eurorack world. It wasn’t mass-produced; it was cobbled together by technicians hunting down rare, aging ICs from decommissioned digital synths, each module dependent on the survival of a discontinued part. That scarcity alone makes it a collector’s oddity, but what keeps it relevant is the sound: a filter character that no modern clone has perfectly captured. It’s softer than a Roland SEM, less aggressive than a Moog ladder, and nowhere near as clinical as a state-variable design. Instead, it’s got a rounded, almost velvety low end, with a high-mid presence that can turn nasal when pushed, and a resonance that flutters just slightly as it peaks — not a flaw, exactly, but a quirk, like the warble in a tape head that refuses to align.

The module’s dual architecture means you’re getting two complete signal paths, each with its own filter and VCA, fed by a shared resonance control and modulation inputs. That makes it surprisingly flexible — you can run stereo signals, layer two different timbres, or patch one side as a modulator for the other. But the real magic happens when you feed it something dense: a gritty wavetable sweep, a lo-fi drum loop, or a detuned supersaw. The FIZZLE GUTS doesn’t just filter — it transforms, adding a kind of analog patina that feels like listening through an old console with slightly oxidized contacts. It’s not transparent; it’s opinionated. And in a format where so many filters aim for perfection, that imperfection is the whole point.

Specifications

Key Features

The Casio FZ-1 Filter IC: A Silicon Relic

At the heart of the FIZZLE GUTS lies the MB87186 — a filter IC originally designed for Casio’s high-end FZ-1 sampler, a machine that used 16-bit sampling at a time when most competitors were still stuck in 12-bit territory. That chip was never intended for modular use; it was part of a closed system, buried deep in a digital architecture that prioritized clarity and dynamic range. But when isolated and repurposed, it reveals a character that’s warmer and more organic than you’d expect from a late-’80s digital synth. The filter’s 24dB/octave slope behaves more like a vintage analog design than a clinical digital one, with a gentle roll-off and a resonance peak that doesn’t self-oscillate cleanly but instead breaks into a soft, breathy squeal. It’s unstable in the best way — not broken, but alive. Engineers who’ve probed the circuit note that the IC’s internal op-amps and OTA-like stages respond unpredictably to extreme CV, creating subtle distortion and phase shifts that vary from unit to unit, depending on the health of the salvaged chip.

Dual Signal Paths with Shared Character

While many dual filters offer independent control over every parameter, the FIZZLE GUTS takes a more minimal approach. Both channels share a single resonance control, which means you can’t dial in a screaming lead on one side while keeping the other smooth and flat. But that limitation forces creative patching — it encourages you to treat the module as a stereo processor or a dual-voice tone shaper rather than two separate filters. Each side has its own audio input, filter cutoff CV input with attenuverter, and VCA level control, allowing for nuanced modulation differences even within the shared resonance framework. The VCAs are DC-coupled, making them suitable for modulating not just audio but LFOs, envelopes, or even slow random voltages, which opens up possibilities for rhythmic gating or wavefolding when patched creatively.

A Module Built on Scarcity

What makes the FIZZLE GUTS more than just another filter is its origin story. Each unit depends on a functioning MB87186 chip pulled from a donor FZ-1 or FZ-10M — machines that were never produced in huge numbers and are now rare themselves. As those source keyboards degrade, the supply of usable ICs dries up. ALM acknowledged this by releasing the FIZZLE GUTS design as open source in 2022 to mark its 10th anniversary, allowing hobbyists to build their own versions if they can source the chips. But even with schematics available, the bottleneck remains: without the original IC, you can’t build an authentic FIZZLE GUTS. Clones like the moddict FZ-2F have emerged, but they still rely on the same rare component. This isn’t a module you can just order — it’s something you hunt for, trade for, or get lucky with on the secondhand market.

Historical Context

The FIZZLE GUTS emerged during a pivotal moment in Eurorack’s evolution — the early 2010s, when the format was shifting from niche hobbyist curiosity to a legitimate platform for professional sound design. At the time, most new modules were either faithful recreations of classic synths or novel digital processors. ALM took a different path: instead of looking to Moog or Buchla, they looked to forgotten digital workstations like the Casio FZ-1 and the Commodore 64. The FIZZLE GUTS was part of a broader “guts” series that included the SID GUTS (based on the C64’s SID chip), reflecting a fascination with repurposing obsolete consumer electronics for experimental music. This wasn’t just nostalgia — it was a philosophical stance. By using actual vintage silicon, ALM challenged the idea that modern manufacturing could replicate the quirks of aging components. Competitors like Doepfer and Make Noise were building precise, repeatable circuits; ALM was building modules with soul, warts and all. The FIZZLE GUTS stood out not for its utility, but for its authenticity — a filter that sounded the way it did because of where it came from, not because of how it was designed.

Collectibility & Value

Today, the FIZZLE GUTS is less a functional module and more a piece of Eurorack folklore. With only around 30 units ever made, it rarely appears on the market, and when it does, prices reflect its rarity. Units in working condition typically sell between £800 and £1,200 GBP, depending on provenance and included documentation. Non-working units — often listed as “for parts or repair” — still fetch £300–£500, primarily because the MB87186 IC can be harvested for use in other builds. The biggest risk for buyers is chip failure: the MB87186 is sensitive to voltage spikes, and many original FZ-1s suffered from power supply issues that could have degraded the IC over time. Service technicians observe that failed filters often manifest as a complete loss of high end or a constant low-level hiss that overwhelms the signal path. There’s no reliable way to test the chip outside of the full circuit, so buying a FIZZLE GUTS blind is a gamble. Collectors recommend only purchasing from trusted sellers who can provide audio demos and proof of stable operation. Given that ALM has released the design as open source, some builders are now creating their own versions using salvaged chips, but these are not official units and lack the pedigree — and premium price tag — of the originals.

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