ADDAC System ADDAC105 4 Voice Cluster ()
Four gritty square waves, a slick filter, and FM chaos in just 8 HP—this little Eurorack cluster punches way above its size.
Overview
The ADDAC105 4 Voice Cluster isn’t trying to be everything to everyone. It’s a focused, no-frills sound generator that packs four analog square wave oscillators, individual VCAs, FM routing, and a multimode filter into a mere 8 HP of Eurorack real estate. Built by ADDAC System, a modular synth boutique out of Lisbon, Portugal, this module feels like a love letter to minimalism—inspired, as the company notes, by the simplicity of their own ADDAC103 T-Networks. And it shows. There’s no flashy display, no menu diving, no digital trickery. Just knobs, inputs, and a layout that invites patching with your hands as much as your ears.
What makes the 105 stand out isn’t raw power—it’s density. Four voices, each with its own oscillator, VCA input, FM switch, and trigger slew, all summed into a single multimode filter with lowpass, bandpass, and highpass outputs. That alone makes it a compact powerhouse for percussive textures, metallic drones, or FM clangor. The fact that it can loop FM from voice to voice—Voice 1 modulating Voice 2, all the way around in a chain that wraps from Voice 4 back to Voice 1—opens up chaotic, self-modulating possibilities that feel almost accidental in the best way. It’s the kind of module that rewards random patching, where a misrouted cable might just give you your next favorite sound.
And despite its compact footprint, it doesn’t feel cramped. The front panel is clean, with each voice clearly laid out and color-coded for easy navigation. The [DECAY] knob doubles as a mute when turned fully counterclockwise—a clever, space-saving trick that also makes live performance intuitive. You can shape the envelope of each trigger simply by adjusting the slew, which is especially handy for creating snappy toms or softening sharp attacks without needing an external envelope generator.
Specifications
| Manufacturer | ADDAC System |
| Product type | Eurorack Module |
| Dimensions | 8 HP |
| Depth | 45 mm deep |
| Current Draw | 40 mA +12V, 40 mA -12V, 0 mA 5V |
| Module categories | Filter, Oscillator, Quad, Synth Voice, Drum |
| Original price | $165 |
Key Features
Four Square Waves, One Tight Package
Each of the four voices generates a raw, analog square wave with its own frequency control. These aren’t pristine sine waves—they’ve got edge, character, the kind of harmonic grit that cuts through a mix. The oscillators respond to trigger inputs, and each one includes a [DECAY] knob that applies slew to the incoming trigger. Turn it up, and the note fades in smoothly; crank it all the way down counterclockwise, and the voice mutes—no need for a separate mute bus. It’s a small detail, but one that saves space and speeds up workflow.
FM Looping: Chaos by Design
The FM routing is where things get interesting. Each voice has an [FM ON] switch that activates frequency modulation from the previous voice in the chain—Voice 2 gets modulated by Voice 1, Voice 3 by Voice 2, and so on, with Voice 1 looping back from Voice 4. This creates a closed FM network where each oscillator can modulate the next, forming feedback loops that generate bell-like tones, metallic resonance, or unpredictable digital-sounding artifacts. It’s not clean FM synthesis—it’s raw, unstable, and thrilling. Owners report “super nice FM sounds,” and it’s easy to see why: this isn’t Yamaha DX7 precision, but more like the gritty FM experiments of early digital synths filtered through analog chaos.
Shared Multimode Filter with Attenuverting Control
All four voices are summed and fed into a single multimode filter—a design choice that keeps the module compact but also forces cohesion. You can’t filter each voice independently, but that’s part of the charm. The filter offers [LOWPASS], [BANDPASS], and [HIGHPASS] outputs, giving you tonal flexibility without adding extra circuitry. The [CUTOFF] knob has an attenuverter on its CV input, meaning you can invert and scale incoming modulation—great for creating dynamic filter sweeps that dip down instead of rising up. Resonance is smooth and musical, and according to users, “one of the best filters by Addac inside.” It’s not a 24dB ladder or a state-variable monster, but it’s characterful—warm when subtle, shrieking when pushed.
External Input: Two Paths, One Jumper
On the back of the module, a physical jumper lets you route the [EXTERNAL IN] to one of two destinations: either into the filter input (so you can process outside audio), or to all four voice frequency controls at once. That second option is wild. It lets you apply a single CV source to modulate the pitch of all four oscillators simultaneously—great for tuning shifts, pitch bends, or experimental modulation. But there’s a catch: due to the circuit design and cost-saving measures, this CV input isn’t calibrated to 1V/octave. So don’t expect chromatic accuracy. Instead, think of it as a texture tool—something to warp the tuning in expressive, unpredictable ways. It’s not a flaw; it’s a feature disguised as a limitation.
VCA Inputs That Don’t Care
Each voice has its own VCA input, and the module doesn’t discriminate: it accepts Trigger, Gate, or CV signals. That flexibility means you can drive the amplitude with anything—envelopes, LFOs, sequencer steps, or even audio-rate signals for ring-mod-like effects. It’s a small thing, but it removes friction. You don’t need to match signal types or add attenuators just to get sound out. Plug in, turn up, and go.
Historical Context
The ADDAC105 4 Voice Cluster emerged as part of ADDAC System’s 100 Series, a line of compact, function-specific Eurorack modules designed for maximum utility in minimal space. It follows the ADDAC103 T-Networks, , and shares its philosophy of elegant simplicity. While the 103 is about control, the 105 is about sound generation—but both reflect the same design ethos: do one thing well, without bloat. Listed under “Modules - ADDAC100 Series” on the company’s website, the 105 fits into a family of small-format tools that prioritize hands-on usability and sonic character over feature stacking.
Collectibility & Value
The ADDAC105 has settled into the used market with modest but stable pricing. A unit listed on Reverb sold for $126 in used condition, while another was offered for €90 in November 2023, also described as “like new.” These prices suggest it’s not a rare grail, but neither is it a throwaway. At its original price of $165, it was positioned as an affordable yet capable voice module—especially for those building compact or budget-conscious systems. Sellers note that units often come with the ribbon cable and M3 screws included, which is a small but appreciated detail for new builders.
There’s no public data on common failures or maintenance issues. Given its analog-only design and lack of complex digital components, it’s likely robust over time—assuming standard care for Eurorack modules (clean power, no overvoltage, secure mounting). The jumper for external input routing is a mechanical component, so future servicing might require basic soldering if reconfiguration is needed, but it’s unlikely to fail under normal use.
Owners praise its versatility: “This little machine is perfect in little setups as well as in big ones.” That speaks to its scalability—it can be the heart of a desktop rig or a specialized texture generator in a larger system. Its strength lies in percussive and tuned drum sounds, thanks to the combination of square waves, individual VCAs, and the decay/slew function on triggers. It’s not a full synth voice with full envelope control, but it’s close enough to be dangerous.
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