ADDAC 105 (2019–)

Name: ADDAC System 105
Brand: ADDAC System

A Eurorack module that sounds like a malfunctioning calculator choir run 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 sound 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 4-voice cluster of digital-ish analog square waves that thrive on unpredictability. Trigger it with a sequencer and you don't get a melody. You get a pulsing, twitching texture that changes with every note, sometimes rhythmic, sometimes atonal.

The 105 is built around the same T-Networks-inspired philosophy as the ADDAC 103 and 104, but it 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 turns sharp pulses into gentle ramps. That smears the attack and gives each voice a soft, almost organic onset, and it keeps the square waves from sounding too brittle. Don't expect polyphonic harmony. Tuning these voices by hand takes patience. The pots aren't the highest quality, and the oscillators aren't temperature-compensated, so they drift with heat and time. If you're chasing stable chords or precise intervals, this isn't your module. If you want something that warbles and refuses to sit still, the 105 does that.

All four voices feed into a shared multimode filter - lowpass, bandpass, and highpass - with resonance and CV control over cutoff. It's the same filter found in the ADDAC 604 Dual Filter. It sounds warm and slightly gritty, and it self-oscillates when pushed. Even with resonance dialed back, it adds a touch of vintage saturation that softens the harshness of the square waves. Crank it up and the whole cluster can howl into feedback or collapse into resonant squeals. It's not a surgical filter. Because it's shared, every voice passes through the same circuit, which pulls the chaos together into one evolving sound mass.

The module's appeal comes from its limitations. There are no CV inputs for oscillator frequency. There is no pitch modulation beyond the on/off FM switch per voice. There is no waveform selection. It's deliberately stripped down, so you work within its quirks. Because you can't control pitch precisely, you find sweet spots by accident, where the detuning between voices creates beating patterns or the FM switches add metallic overtones. It rewards experimentation over precision. It's not the centerpiece of a melodic synth rig. It's the wildcard you patch in when everything else is too clean.

Specifications

Manufacturer	ADDAC System
Production Years	2019–
Original Price	$249
Format	Eurorack
HP	10
Depth	40mm
Current Draw +12V	80mA
Current Draw -12V	80mA
Oscillator Type	4x Square Wave
FM Control	Switch per voice
Trigger Input	4x with slew/decay
VCA	Per voice
Filter Type	Multimode (Lowpass, Bandpass, Highpass)
Filter Resonance	Adjustable with self-oscillation
Filter CV Input	Yes, with attenuverter
External Audio Input	Yes, pre-filter
Outputs	Mixed output, individual voice outputs
Weight	0.3 kg

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. They 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 the sound. 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 adds a fixed amount of frequency modulation, which brings in metallic harmonics and instability. It's not deep FM synthesis, but it's enough to push the sound from orderly to chaotic. Each voice responds slightly differently to temperature and power fluctuations, so even identical settings give slightly different results across voices.

Trigger Slew and the Art of the Soft Attack

Instead of hard, instantaneous triggers, the 105 has a slew circuit on each trigger input. When a gate or trigger arrives, the oscillator doesn't start instantly. It ramps up over a short, adjustable time. The result is a soft, rounded attack that turns sharp pulses into gentle swells. Without it, the square waves would be too harsh. 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 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. It does more than shape frequency. It binds the four voices into a single output. The filter has a warm, slightly overdriven sound, 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 runs through the same circuit, so the 105 works as a filter module even when the oscillators are unused. The lack of individual filter routing is a limitation.

Historical Context

The ADDAC 105 appeared in 2019, when Eurorack was shifting from replicating classic analog synths toward more experimental, noise-oriented designs. While many manufacturers were focused on high-fidelity VCOs, complex modulation, or digital wavetable engines, ADDAC, a Portuguese company with a taste for the idiosyncratic, leaned into 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 when modular users were increasingly interested in texture, glitch, and generative music, and artists like Holly Herndon, Oneohtrix Point Never, and Visible Cloaks were pushing what modular could do. It wasn't trying to be a Minimoog or a Prophet.

Competitors like the Mutable Instruments Clouds (discontinued) or the MakeNoise Mimeophon offered complex granular or spectral processing. The 105 stood out by being purely analog and 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, and appealed 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 a cult following among Eurorack users who like its chaotic character. New units sell for around $249, and used prices typically range from $180 to $220 depending on condition. It's still in production, so there's no urgency to buy. Condition is generally not a major concern, since 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 need 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. It's by design, but it limits 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. The module draws 80mA on both +12V and -12V rails. That's 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, make sure the input accepts line-level signals without distortion. The 105 is a low-maintenance module. It won't replace a proper VCO or filter bank.