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 is a focused, no-frills sound generator. It packs four analog square wave oscillators, individual VCAs, FM routing, and a multimode filter into 8 HP of Eurorack space. ADDAC System, a modular synth boutique out of Lisbon, Portugal, built it, and the company notes it was inspired by the simplicity of their own ADDAC103 T-Networks. There's no display, no menu diving, no digital trickery. Just knobs, inputs, and a layout meant for patching by hand.
The 105 is about 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 makes it a compact box for percussive textures, metallic drones, or FM clangor. It can also loop FM from voice to voice, with Voice 1 modulating Voice 2, all the way around in a chain that wraps from Voice 4 back to Voice 1. That opens up chaotic, self-modulating results that feel almost accidental. Random patching pays off here, and a misrouted cable might give you your next favorite sound.
Despite the small footprint, it doesn't feel cramped. The front panel is clean, with each voice clearly laid out and color-coded. The [DECAY] knob doubles as a mute when turned fully counterclockwise, which saves space and makes live performance easy. You can shape the envelope of each trigger by adjusting the slew. That's handy for snappy toms or for softening sharp attacks without 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 are not pristine sine waves. They have edge and harmonic grit that cuts through a mix. The oscillators respond to trigger inputs, and each one has 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, so there's no need for a separate mute bus.
FM Looping: Chaos by Design
The FM routing is where things get interesting. Each voice has an [FM ON] switch that turns on 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 makes a closed FM network where each oscillator can modulate the next, and the feedback loops generate bell-like tones, metallic resonance, or unpredictable digital-sounding artifacts. It's not clean FM synthesis. It's raw and unstable. Owners report "super nice FM sounds," but this isn't Yamaha DX7 precision. It's closer to 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. That keeps the module compact, but it also means you can't filter each voice independently. The filter offers [LOWPASS], [BANDPASS], and [HIGHPASS] outputs without any extra circuitry. The [CUTOFF] knob has an attenuverter on its CV input, so you can invert and scale incoming modulation. That's useful for 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 warm when subtle and shrieking when pushed.
External Input: Two Paths, One Jumper
On the back of the module, a physical jumper routes the [EXTERNAL IN] to one of two destinations: the filter input, so you can process outside audio, or all four voice frequency controls at once. The second option applies a single CV source to the pitch of all four oscillators simultaneously, which works for tuning shifts, pitch bends, or experimental modulation. There's a catch. Because of the circuit design and cost-saving measures, this CV input isn't calibrated to 1V/octave, so don't expect chromatic accuracy. Treat it as a texture tool for warping the tuning in unpredictable ways.
VCA Inputs That Don’t Care
Each voice has its own VCA input, and it accepts Trigger, Gate, or CV signals. You can drive the amplitude with envelopes, LFOs, sequencer steps, or even audio-rate signals for ring-mod-like effects. You don't need to match signal types or add attenuators just to get sound out.
Historical Context
The ADDAC105 4 Voice Cluster is part of ADDAC System's 100 Series, a line of compact, function-specific Eurorack modules. It follows the ADDAC103 T-Networks, , and shares its philosophy of simplicity. The 103 is about control and the 105 is about sound generation. Both aim to do one thing well, without bloat. The company's website lists it under "Modules - ADDAC100 Series."
Collectibility & Value
The ADDAC105 has settled into the used market at modest but stable prices. 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." At its original price of $165, it was positioned as an affordable voice module for compact or budget-conscious systems. Sellers note that units often come with the ribbon cable and M3 screws included.
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 to hold up 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 reconfiguring it might require basic soldering, 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." It can anchor a desktop rig or act as a specialized texture generator in a larger system. Its strength is 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.
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