ADDAC System 103 T-Networks (2019–2020)

Four voices of raw analog percussion, built like the drum machine circuits of 1982 and fitted into a Eurorack slot.

Overview

The ADDAC System 103 T-Networks has four analog voices: one kick and three toms. All of them use the same twin-T architecture found in vintage Roland, Korg, and Yamaha drum boxes. There's no sample playback and nothing digital. It's resistors, capacitors, and op-amps wired into resonant networks that ring when a gate hits them. You don't shape these sounds with envelopes or filters. You set decay and pitch with knobs that behave like tuning unstable oscillators, because that's essentially what they are. Each voice is a self-contained analog trigger-to-tone generator. Send it a pulse and it bursts into resonance, then dies away to silence. It's not clean and it's not predictable. But a deep, wobbly kick in the neighborhood of a TR-808 is in there, which is why anyone would bother building this in 2019.

ADDAC is a Portuguese modular outfit known for blending vintage topology with modern utility. The 103 gives you four twin-T voices, each with a frequency knob and an L/M/H range switch to jump between tonal zones. There's no CV control over pitch - that's the key difference between the 103 and its successor, the 104. There's also no attenuation on the trigger inputs, just a gate-to-trigger converter that lets you fire each voice from any pulse source, even audio-rate signals. That's where things get weird. Patch an audio oscillator into a voice input and you're not triggering a drum, you're feeding a signal into a narrow bandpass filter that only responds when excited. You get a resonant, glitchy filtering effect that is unstable, unpredictable, and oddly musical. It's not what the module was designed for.

The 103 isn't limited to kicks. With careful tweaking it can produce short, wooden claves, hollow toms that sound like they're played with rubber mallets, snappy rimshots, and even metallic clangs if you push the frequency high enough. The top two voices are brighter by design, better suited for rack toms or high-pitched snaps. The bottom two dig into subby territory and can produce surprisingly weighty kicks, especially when layered or slightly detuned against each other. There's no noise source onboard, so you won't get snares with body or cymbals.

Specifications

Key Features

Analog Twin-T Circuitry, Not Emulation

The core of the 103 is four discrete twin-T networks, each built from passive components and op-amps to recreate the behavior of vintage drum machine tone generators. A twin-T circuit, named for the schematic shape of its resistor-capacitor network, acts as a narrow band-reject filter. When it's overdriven with a sharp pulse, it rings at a specific frequency before decaying. That ring is your drum sound. Unlike a VCO with a traditional envelope, this isn't a controlled process. It's a physical resonance, more like striking a tuning fork than playing a synth note. So each hit has subtle variations in pitch and decay, especially as the components warm up or respond to voltage fluctuations.

Gate-to-Trigger Conversion Per Voice

Each of the four voice inputs includes a gate-to-trigger converter, so you don't need to shape your clock signals into sharp, short pulses. A sequencer with sustained gate outputs or even an audio-rate LFO will work, and the 103 converts it into a usable trigger. That helps in a live patch, especially with sequencers that output variable gate lengths. It also opens up experimental uses. Feeding an audio signal into a voice input excites the twin-T network into acting like a resonant filter. The result is a self-oscillating, burst-mode filtering that can turn a drone into a stuttering, metallic texture. It's not stable, and it's not a substitute for a proper filter, but it can work for mangling a pad or creating glitchy transitions.

Summed Mix Output with Level Control

Sitting in the center of the panel is a summed mix output with its own level knob. You can route all four voices through a single channel on your mixer or audio interface, which saves space in a crowded rack. The mix is passive, so there's no buffering or coloration, just a clean sum of the four outputs. The level control is useful because the individual voices can vary in output amplitude depending on frequency and decay settings. Without it, you'd need an external mixer or attenuator to balance the levels, especially when layering kicks with higher-pitched toms. As it stands, you can dial in a balanced mix on the module and send it straight to your recorder or effects chain.

Historical Context

The ADDAC 103 arrived in 2019, at the peak of Eurorack's interest in analog percussion. Modules like the 4ms RCD, Noise Engineering's Basimilus Iteritas, and Qu-Bit's Nebulae had already proven there was demand for more than sampled drum sounds in modular setups. Many of those modules leaned on digital synthesis or complex algorithms. The 103 went back to analog drum design. It was inspired by circuits in machines like the Roland TR-808, where discrete twin-T networks generated toms and kicks without any microprocessors or digital logic. ADDAC simplified and standardized those designs for Eurorack, so vintage-style percussion fits into modern patches without a full drum machine clone. The 103 was part of a broader trend of "circuit archaeology" in modular, where designers dug into old service manuals and reverse-engineered forgotten topologies. It wasn't trying to be the most advanced drum module. For a brief window it filled a niche no other module did: pure analog percussion generation with zero digital intervention.

Collectibility & Value

The ADDAC 103 is now discontinued. It was replaced by the 104 VC T-Networks, which added voltage control over each voice's pitch. That made the 103 feel limited by comparison, and it trades at a discount, typically between $80 and $110 in good condition, depending on seller location and whether it's still in original packaging. Units listed as "new old stock" occasionally pop up around $130. That's near the original $149 MSRP and hard to justify when the 104 offers CV control for only $20 more on the used market. Still, the 103 has its fans, especially among players who prefer hands-on, non-CV-based percussion that forces them to commit to settings. It's also compact for what it does, fitting four distinct voices into just 8 HP.

Mechanically, the 103 is solid. There are no moving parts beyond the knobs and switches, and the PCB build quality is consistent with ADDAC's reputation for reliability. Failures are rare, but owners report occasional misfiring of the gate-to-trigger converters if the module is underpowered or exposed to voltage spikes. The most common complaint is about the L/M/H switches. Some users note that the frequency ranges don't always behave intuitively. Switching from M to H doesn't always produce a clear pitch jump, and the overlap between voices can make it tricky to dial in distinct sounds without careful knob adjustment. If you want raw, hands-on analog percussion and don't need CV, the 103 is a steal. If you plan to modulate pitch or integrate with evolving sequences, the 104 is worth the upgrade. When buying, check that the module powers up cleanly, all four voices respond to triggers, and the summed output isn't noisy or unbalanced. Those are signs of a failing op-amp or cold solder joint.

eBay Listings

ADDAC System 103 T-Networks (Black) Modular EURORACK - NEW -

$159

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