ADDAC System 103 T-Networks (2019–2020)

Four voices of raw, squirming analog percussion that feel like digging up drum machine circuits from 1982 and stuffing them into a Eurorack slot.

Overview

Plug in the ADDAC System 103 T-Networks and you’re not so much programming drums as coaxing sounds out of four little analog temper tantrums—one kick, three toms, all built on the same deceptively simple twin-T architecture that powered the guts of vintage Roland, Korg, and Yamaha drum boxes. There’s no sample playback, no digital trickery—just resistors, capacitors, and op-amps wired into resonant networks that scream when hit with a gate. The result? A percussive module that doesn’t just emulate old drum machines; it *is* one, stripped down to its nervous system and reborn in black anodized aluminum. You don’t shape these sounds with envelopes or filters—you tweak decay and pitch with knobs that feel like tuning unstable oscillators, because that’s essentially what you’re doing. Each voice is a self-contained analog trigger-to-tone generator, and the moment you send a pulse into one, it erupts into a burst of resonant energy that collapses into silence like a collapsing lung. It’s not clean. It’s not predictable. But when you nail the right setting—say, a deep, wobbly kick that feels like it’s coming from a TR-808’s fever dream—you realize why anyone would bother building this in 2019.

And build it they did: ADDAC, the Portuguese modular outfit known for blending vintage topology with modern utility, didn’t just clone old circuits—they distilled them. The 103 gives you four of these 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—this is the key difference between the 103 and its successor, the 104—and 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 last bit is 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. The result is a kind of resonant, glitchy filtering effect—unstable, unpredictable, and oddly musical. It’s not what the module was designed for, but it’s exactly the kind of happy accident that makes Eurorack rewarding.

Despite its simplicity, the 103 isn’t just a one-trick analog kick generator. 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, while the bottom two dig into subby territory, capable of producing 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, but that’s not the point. The 103 is about organic, slightly unstable analog percussion—the kind that breathes, wobbles, and never plays the same way twice. It’s the sound of hardware that remembers when drum machines had to be tuned by hand.

Specifications

Key Features

Analog Twin-T Circuitry, Not Emulation

The heart of the 103 isn’t a DSP chip or a sample player—it’s 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 that, when overdriven with a sharp pulse, 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. That’s why the 103’s voices have such a natural, slightly uneven character. Each hit has subtle variations in pitch and decay, especially as the components warm up or respond to voltage fluctuations. It’s not a flaw—it’s the sound of analog unpredictability in its purest form.

Gate-to-Trigger Conversion Per Voice

Each of the four voice inputs includes a gate-to-trigger converter, meaning you don’t need to worry about normalizing your clock signals to sharp, short pulses. Whether you’re using a sequencer with sustained gate outputs or even an audio-rate LFO, the 103 will convert it into a usable trigger. This makes it incredibly flexible in a live patch, especially when paired with sequencers that output variable gate lengths. But it also opens up experimental uses: feeding an audio signal into a voice input doesn’t just trigger a drum—it excites the twin-T network into acting like a resonant filter. The result is a kind of self-oscillating, burst-mode filtering that can turn a drone into a stuttering, metallic texture. It’s not stable, and it’s definitely not a substitute for a proper filter, but in the right context—say, mangling a pad or creating glitchy transitions—it’s a secret weapon.

Summed Mix Output with Level Control

Sitting in the center of the panel is a simple but essential feature: a summed mix output with its own level knob. This means you can route all four voices through a single channel in your mixer or audio interface, which is a huge space-saver 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 particularly 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 right on the module and send it straight to your recorder or effects chain.

Historical Context

The ADDAC 103 didn’t appear out of nowhere—it arrived in 2019 at the peak of Eurorack’s obsession with 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 just sampled drum sounds in modular setups. But while many of those modules leaned on digital synthesis or complex algorithms, the 103 went the opposite direction: back to the roots of analog drum design. It was inspired by circuits found in machines like the Roland TR-808, where discrete twin-T networks generated toms and kicks without any microprocessors or digital logic. ADDAC didn’t just copy those designs—they simplified and standardized them for Eurorack use, making it easy to integrate vintage-style percussion into modern patches without needing 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—it was trying to be the most *authentic*. And for a brief window, it filled a niche no other module did: pure, unfiltered analog percussion generation with zero digital intervention.

Collectibility & Value

The ADDAC 103 is now discontinued, quietly replaced by the 104 VC T-Networks, which added voltage control over each voice’s pitch—a major upgrade that made the 103 feel limited by comparison. As a result, the 103 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, but 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 rather than modulate endlessly. It’s also slightly more compact than some competing modules, fitting into just 8 HP while delivering four distinct voices.

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 issues with the gate-to-trigger converters misfiring if the module is underpowered or exposed to voltage spikes. The most common complaint isn’t about reliability, though—it’s 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. It’s not a flaw, exactly, but it does require a learning curve. For buyers today, the advice is simple: if you want raw, hands-on analog percussion and don’t need CV, the 103 is a steal. But if you plan to modulate pitch or integrate with evolving sequences, the 104 is worth the upgrade. Either way, check that the module powers up cleanly, all four voices respond to triggers, and the summed output isn’t noisy or unbalanced—signs of a failing op-amp or cold solder joint.

eBay Listings

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

$149

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