ADDAC 105 (2019–)

A Eurorack module that sounds like a malfunctioning calculator choir filtered through a vintage fuzz pedal—and somehow, it’s musical.

Overview

Plug in the ADDAC 105 for the first time, and you might think something’s broken. The four square wave oscillators don’t lock into pitch like a traditional VCO—they wobble, drift, and interact in ways that feel more like circuit-bent chaos than precision synthesis. That’s the point. This isn’t a module for clean leads or stable drones; it’s a controlled demolition of orderly sound, a 4-voice cluster bomb of digital-ish analog square waves that thrive on unpredictability. Trigger it with a sequencer, and instead of a melody, you get a pulsing, twitching texture that evolves with every note—sometimes rhythmic, sometimes atonal, always alive. It’s the sound of a machine trying to compute something important and failing beautifully.

Built around the same T-Networks-inspired philosophy as the ADDAC 103 and 104, the 105 takes that concept into melodic (or anti-melodic) territory. Each of the four voices is a simple square wave oscillator with its own VCA, FM switch, and a slew/decay circuit on the trigger input. The trigger slew is key—it turns sharp pulses into gentle ramps, smearing the attack and giving each voice a soft, almost organic onset. That subtle glide keeps the square waves from sounding too brittle, rounding off the digital edges just enough to feel human. But don’t expect polyphonic harmony. Tuning these voices by hand is a patience-testing exercise. The pots aren’t the highest quality, and the oscillators aren’t temperature-compensated, so they’ll drift with heat and time. If you’re chasing stable chords or precise intervals, this isn’t your module. But if you want something that breathes, warbles, and refuses to sit still, the 105 delivers in spades.

All four voices feed into a shared multimode filter—lowpass, bandpass, and highpass—with resonance and CV control over cutoff. This filter is the secret sauce. It’s the same one found in the ADDAC 604 Dual Filter, and it’s got character: warm, slightly gritty, and capable of self-oscillation when pushed. Even with resonance dialed back, it adds a touch of vintage saturation that softens the square waves’ harshness. Crank it up, and the whole cluster can howl into feedback or collapse into resonant squeals. It’s not a surgical filter, but it’s musical—more like a piece of analog circuitry with opinions than a neutral processor. And because it’s shared, every voice passes through the same sonic lens, which helps unify the chaos into something cohesive. You’re not just hearing four separate oscillators; you’re hearing a single, evolving sound mass shaped by the filter’s response.

The module’s charm lies in its limitations. There are no CV inputs for oscillator frequency. No pitch modulation beyond the on/off FM switch per voice. No waveform selection. It’s deliberately stripped down, forcing you to work within its quirks. And that’s where the magic happens. Because you can’t precisely control pitch, you end up discovering sweet spots by accident—happy accidents where the detuning between voices creates beating patterns, or the FM switches add metallic overtones that weren’t there before. It’s a module that rewards experimentation over precision, noise over notes, texture over tune. It’s not the centerpiece of a melodic synth rig; it’s the wildcard, the wildcard you patch in when everything else is too clean, too predictable.

Specifications

Key Features

The Four-Voice Square Cluster

Four analog square wave oscillators, each with its own VCA and FM switch, form the core of the 105. These aren’t high-stability oscillators—they’re simple, raw, and intentionally imperfect. The lack of CV control over pitch is the module’s most controversial design choice, and it defines its character. Without precise tuning, the voices interact in unpredictable ways, creating complex phase relationships and beating patterns that evolve over time. The FM switch per voice is binary—on or off—but when engaged, it introduces a fixed amount of frequency modulation, adding metallic harmonics and instability. It’s not deep FM synthesis, but it’s enough to push the sound from orderly to chaotic. Because each voice responds slightly differently to temperature and power fluctuations, even identical settings yield slightly different results across voices, adding to the organic feel.

Trigger Slew and the Art of the Soft Attack

Instead of hard, instantaneous triggers, the 105 features a slew circuit on each trigger input. This means that when a gate or trigger is received, the oscillator doesn’t start instantly—it ramps up over a short, adjustable time. The result is a soft, rounded attack that transforms sharp pulses into gentle swells. This is critical to the module’s character: without it, the square waves would be too harsh, too digital. With it, they take on a breath-like quality, as if each voice is inhaling before it speaks. The decay time is fixed per voice, but the interaction between the slew and the VCA creates a pseudo-envelope effect. It’s not a full ADSR, but it’s expressive enough to shape the rhythm and texture of the sound. Patch in a Euclidean sequencer or random triggers, and the slew turns rigid patterns into fluid, evolving sequences.

The Multimode Filter as Unifier and Distorter

All four voices feed into a single multimode filter—lowpass, bandpass, and highpass—with resonance and CV control. This filter does more than shape frequency; it acts as a sonic glue, binding the four disparate voices into a single, cohesive output. The filter has a warm, slightly overdriven character, especially when resonance is increased. At high settings, it self-oscillates, producing a clean sine wave that can be used as a fifth sound source or as a modulation target. Because the filter is shared, any external audio signal patched into the pre-filter input is processed through the same circuit, making the 105 a useful filter module even when the oscillators are unused. The lack of individual filter routing is a limitation, but it reinforces the module’s philosophy: this is about collective sound, not individual voices.

Historical Context

The ADDAC 105 emerged in 2019, during a period when Eurorack was shifting from replication of classic analog synths toward more experimental, noise-oriented designs. While many manufacturers were focusing on high-fidelity VCOs, complex modulation, or digital wavetable engines, ADDAC—a Portuguese company with a taste for the idiosyncratic—leaned into the charm of imperfection. The 105 was a direct descendant of the ADDAC 103 T-Networks, a percussion module based on the same analog circuit principles found in vintage drum machines like the Roland TR-808. Where the 103 was designed for kicks and toms, the 105 repurposed that architecture for pitched, cluster-based synthesis. It arrived at a time when modular users were increasingly interested in texture, glitch, and generative music—artists like Holly Herndon, Oneohtrix Point Never, and Visible Cloaks were pushing the boundaries of what modular could do, and the 105 fit perfectly into that world. It wasn’t trying to be a Minimoog or a Prophet; it was trying to be a malfunctioning robot choir, and it succeeded.

Competitors like the Mutable Instruments Clouds (discontinued) or the MakeNoise Mimeophon offered complex granular or spectral processing, but the 105 stood out by being purely analog and refreshingly simple. It didn’t rely on digital algorithms to create texture—it used the inherent instability of analog circuits. In that sense, it was closer in spirit to the Buchla 200 series or the Serge Random*Source than to modern digital effects. It also shared DNA with DIY and circuit-bent aesthetics, appealing to users who valued unpredictability over reliability. While not a direct clone, it occupied a niche similar to the Intellijel Metropolis or the ALM Squid Salmple in its focus on rhythmic, atonal synthesis—but with a distinctly lo-fi, Portuguese twist.

Collectibility & Value

The ADDAC 105 is not a rare module, but it has developed a cult following among Eurorack users who appreciate its chaotic character. New units sell for around $249, and used prices typically range from $180 to $220 depending on condition. Because it’s still in production, there’s no urgency to buy, but its unique sound ensures steady demand. Condition is generally not a major concern—there are no delicate components or complex calibration procedures—but buyers should check for consistent oscillator tracking and smooth potentiometer operation. The tuning pots are the most common point of failure; gritty or scratchy pots can make fine-tuning nearly impossible and may require cleaning or replacement. The trigger slew circuit is passive and reliable, but if a voice fails to respond to triggers, it’s usually a cold solder joint or a faulty jack.

One of the most common complaints from owners is the lack of CV control over oscillator frequency. While this is by design, it limits the module’s integration with melodic sequencers. Some users have modded their units to add CV inputs, but ADDAC has not released an official VC version (as of 2026). If precise pitch control is essential, the 105 will frustrate. But for those who embrace its instability, it’s a source of endless inspiration. It’s also worth noting that the module draws 80mA on both +12V and -12V rails—modest by Eurorack standards, but users with tightly packed cases should account for it.

When buying used, test all four voices individually, check the FM switches for consistent operation, and verify that the filter responds smoothly to CV and resonance adjustments. The external audio input is a bonus—if the module is being used as a filter, ensure that the input accepts line-level signals without distortion. Overall, the 105 is a low-maintenance module with high character. It won’t replace a proper VCO or filter bank, but as a texture generator, a noise source, or a wildcard in a generative patch, it’s hard to beat.

eBay Listings

ADDAC System 105 4 Voice Cluster (Black) EURORACK - NEW - PE

$165

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