ADDAC System 104 VC T-Networks (2020–Present)

A pocket-sized Eurorack powerhouse that resurrects the thump and crack of vintage analog drum machines—one T-network at a time.

Overview

Plug in the ADDAC System 104 for the first time and you’re not greeted with lush pads or sweeping arpeggios—you get a sharp, resonant thwack, like a snare hit through a blown speaker in a 1982 basement rehearsal space. That’s the sound of a Twin T-network doing its thing: a deceptively simple analog circuit that’s been the secret sauce behind the percussive guts of classic drum machines like the Roland TR-808. The 104 isn’t trying to be a full drum module with dedicated kick, snare, and hi-hat voices. Instead, it’s a minimalist, no-frills playground of four identical analog T-networks, each capable of producing richly resonant, pitch-definite percussive tones that live somewhere between a tom, a tuned kick, and a metallic thud. It’s the kind of module that makes you forget about melodic content for an hour while you obsess over tuning four slightly detuned toms into a hypnotic, tribal roll.

At just 8HP wide, the 104 is a skiff-builder’s dream—compact, efficient, and dense with character. Each of the four voices features a frequency knob with a three-position range switch (Low, Mid, High), letting you dial in anything from subby thumps to glassy pings. The top two voices are subtly tuned to favor higher frequencies, while the bottom two are optimized for deeper tones, a small but thoughtful design choice that reflects real-world use. You won’t find envelopes or VCAs here; the 104 is purely a sound source, meant to be paired with triggers and VCAs elsewhere in your system. But that simplicity is its strength. It forces you to think like a drummer, to patch deliberately, to treat each voice as a physical instrument with its own tuning and response.

What elevates the 104 beyond a mere retro clone is voltage control—specifically, CV inputs with attenuverters for each voice’s frequency. This isn’t just a novelty; it transforms the module from a static drum machine voice into a dynamic, modulatable instrument. Patch in an LFO with positive and negative offset, and a single tom voice can morph between high and low pitches on every hit, like a mechanical heartbeat. Run a sequencer through the CV inputs and you’ve got melodic percussion that dances around a scale. The attenuverters mean you can invert the CV response, so rising control voltages lower the pitch instead of raising it—perfect for creating disorienting, descending booms. It’s this blend of vintage circuitry and modern modular flexibility that makes the 104 feel both nostalgic and forward-thinking.

And then there’s the mix output. Nestled in the center of the panel is a master volume knob feeding a summed output, alongside four individual outs at the bottom. This dual-output design is quietly brilliant: you can send the full mix to a recorder or effects chain while still routing individual voices to different processors—say, slap reverb on voice 3, tight delay on voice 1. It’s a small detail, but one that speaks to ADDAC’s understanding of how modular users actually work. Whether you’re building a live rig or crafting intricate studio patches, the 104 doesn’t get in your way. It just makes noise—good, gnarly, analog noise.

Specifications

Key Features

The Twin T-Network Circuit: Analog Resonance, Simplified

At the heart of the 104 lies the Twin T-network—a passive filter topology made from two T-shaped arrangements of resistors and capacitors. When hit with a sharp pulse, the network resonates at a specific frequency determined by those components, then decays naturally. Unlike a traditional VCO or resonant filter, it doesn’t sustain; it thumps. The brilliance of the design is its simplicity: no op-amps, no complex feedback loops, just RC networks doing what they do best—ringing like a bell when struck. ADDAC doesn’t simulate this behavior; they implement it in analog hardware, preserving the slight imperfections and nonlinearities that give these circuits their character. The result is a sound that feels physical, almost acoustic in its decay, even though it’s entirely electronic. It’s not a perfect sine wave—it’s got grit, a little overshoot, a hint of distortion at the attack—exactly what makes vintage drum machines feel alive.

Voltage Control with Attenuverters: Precision Modulation

While the original T-network circuits in machines like the 808 were fixed-pitch, the 104 embraces the modular ethos with full CV control over each voice’s frequency. But it’s not just a CV input slapped on—the attenuverters are the real magic. They let you scale and invert the incoming control voltage, so you can fine-tune how much a sequencer or LFO affects the pitch. Want a subtle pitch wobble on every third hit? Dial in a small positive CV. Need a dramatic drop from high to low on a tom roll? Invert the CV and crank the attenuverter. This level of control turns the 104 into more than a drum module—it becomes a percussive synth voice, capable of rhythmic melodies and evolving textures. And because the CV response is exponential, the pitch changes feel musical, not linear or robotic.

Compact Design with Full Output Flexibility

In an ecosystem where every HP counts, the 104’s 8HP footprint is a triumph. It packs four fully independent voices, four CV paths, four trigger inputs, and dual output options into a space smaller than many dual VCAs. Yet it never feels cramped. The layout is clean, the knobs are spaced well, and the jacks are full-size. The mix output with its own volume knob is a rare inclusion at this size—most modules force you to use an external mixer. Here, you can have both individual routing and a ready-to-go mix, making the 104 ideal for small cases or traveling setups. The individual outputs also open up creative routing: pan voices spatially, send different ones to different effects, or use them as modulation sources via envelope followers. It’s a module that gives you maximum utility without demanding maximum space.

Historical Context

The ADDAC 104 didn’t emerge in a vacuum. It arrived in 2020, a time when Eurorack was bursting with digital oscillators, complex sequencers, and FPGA-based modules that could emulate entire studios. Amid that high-tech noise, there was a quiet counter-movement—a return to analog simplicity, to circuits with names like “T-network,” “ladder filter,” and “vactrol.” The 104 taps directly into that nostalgia, drawing inspiration from the analog drum machines of the late ’70s and early ’80s, particularly the Roland TR-808, whose deep toms and snappy snares were built around variations of the Twin T design. Unlike modern drum modules that use samples or digital synthesis, the 104 goes back to the source, offering the same kind of raw, resonant tones that defined early electro and hip-hop.

It also follows in the footsteps of its sibling, the ADDAC 103 T-Networks, which offered similar circuitry but without voltage control. The 104 is essentially the evolved version—same soul, smarter brain. At a time when many manufacturers were chasing complexity, ADDAC doubled down on minimalism, proving that a single analog circuit, well-executed and thoughtfully expanded, could still feel fresh. Competitors like Malekko’s Varigate 8+ or Intellijel’s Bifold Perc offered more features—envelopes, noise sources, multiple trigger modes—but the 104 carved its niche by doing one thing exceptionally well: analog resonance with CV control. It’s not a drum machine replacement, but a specialist tool for modular users who want authentic, tweakable percussive textures without bloating their racks.

Collectibility & Value

The ADDAC 104 isn’t a rare unicorn—production has been steady since 2020, and it’s available both pre-built and as a DIY kit. That accessibility keeps prices grounded. On the secondhand market, a used 104 typically sells for $120–$150, depending on condition and whether it’s boxed with accessories. New units still retail around $172, making it a relatively affordable entry into analog drum synthesis for modular users. The DIY kit option, priced lower, appeals to builders and tinkerers, though the surface-mount components may deter beginners.

Failures are rare, but not unheard of. Service technicians observe that the most common issue is damaged power cables or reverse polarity insertion, which can fry the module’s power regulation—always double-check your cable orientation. The jacks are standard 3.5mm types, robust but not indestructible; repeated plugging and unplugging in tight cases can loosen them over time. There are no user-serviceable fuses, so protection relies on the case’s power supply. Owners report that the knobs are secure and the PCB is well-laid out, with no known cold solder joints or design flaws. Because it’s a passive resonant circuit with minimal active components, there’s little to wear out—no electrolytic capacitors to dry up, no transistors to drift. It’s about as low-maintenance as a Eurorack module can get.

When buying, check that all four voices respond evenly to triggers and that the CV inputs modulate pitch smoothly without jumps or dead zones. Test the mix output’s volume knob for crackling, and ensure the attenuverters respond symmetrically in both positive and negative ranges. Since the module relies on external triggers and VCAs, it’s worth testing it in a live system if possible—its true character only emerges in context. For those seeking vintage drum machine tones without the bulk or expense of a full TR-808, the 104 is a compelling alternative. It won’t replace a sample-based drum module for realism, but it delivers something more valuable: analog unpredictability, warmth, and the kind of hands-on immediacy that makes modular synthesis feel alive.

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