ADDAC 611 (2019–)

A growling, howling, tube-driven filter that doesn’t filter so much as attack, smear, and resurrect your sound with valve-driven chaos.

Overview

Turn it on, and you hear it before you even patch a cable—a low, warm hum rising from the chassis like something alive powering up. The ADDAC 611 isn’t a filter in the traditional sense. It doesn’t politely carve frequencies; it chews them up, spits them out, and sometimes sets them on fire. This is a module built around a genuine 1970s vacuum tube, salvaged from old stock, wired into a circuit originally designed by Denmark’s cultish Gotharman for their now-discontinued Tubaz Filter. ADDAC didn’t just clone it—they licensed the design, then expanded it with extra CV control, phase inversion, and a feedback loop that turns the whole thing into a self-sustaining beast. It’s noisy, unpredictable, and gloriously imperfect. But if you’re after a filter that behaves, this isn’t it. If you want one that *feels*, that snarls when pushed and purrs when coddled, then the 611 might be the most character-packed 12HP you’ll ever install.

What makes the 611 so volatile is also what makes it special: every tube is different. ADDAC doesn’t cherry-pick pristine specimens—they use whatever old-stock tubes they can source, embracing the inconsistencies, microphonics, and harmonic bleed that come with decades-old glass. That means no two units sound exactly alike. One might howl with feedback at resonance like a dying siren; another might saturate smoothly, adding just enough grit to glue a drum bus. The circuit itself encourages abuse: three audio inputs with individual gain controls, a dedicated drive stage that can push pre-tube distortion, and a feedback path built into the third input. Patch nothing into Input 3, and it automatically routes the band-pass output back into the input, creating self-oscillation, feedback squeals, or just a thick, gnarled wash of resonance that feels more like a distortion effect than a filter sweep.

Despite its wild nature, the 611 isn’t without control. It offers full voltage control over cutoff, resonance, and drive, each with attenuverters so you can invert or scale incoming CV. The outputs are varied: separate high-pass, low-pass, band-pass, and a mix output that blends the band-pass with dry signal. There’s even a master gain for the mix and BP outputs, letting you tame or amplify the chaos. But here’s the catch: the HP and LP outputs don’t have front-panel level controls—only trimmer pots on the panel, meaning you’ll need a screwdriver to adjust their output levels. It’s a design quirk that feels slightly archaic in a modern Eurorack context, but it also hints at the module’s roots in boutique, hand-built gear where calibration was part of the ritual.

Specifications

Key Features

The Tube as a Character Generator

The heart of the 611 is its vacuum tube—not a simulation, not a transistor emulation, but an actual glass-encased, glowing valve from the 1970s. You can see it through the circular cutout in the front panel, a physical reminder that this module operates on a different set of rules than solid-state filters. Tubes don’t behave linearly. They distort, they phase-shift, they hum, and they react to heat. The 611 embraces all of it. When cold, the filter might sound thin, brittle, even unstable. After a few minutes of warming up, the tone thickens, the resonance blooms, and the drive stage starts to saturate in a way that feels organic, almost alive. This isn’t a flaw—it’s the point. The tube isn’t just adding warmth; it’s introducing a layer of unpredictability that digital filters can’t replicate. At high drive settings, the tube can produce rich, even-order harmonics that smear transients and glue sounds together. But push it too far, and it’ll scream, howl, or feed back like a cranked guitar amp in a metal solo. It’s not for the faint of heart, but for those who want their modular system to breathe, sweat, and occasionally misbehave, it’s essential.

Feedback and Phase Inversion as Creative Tools

Most filters give you cutoff, resonance, and maybe CV control. The 611 goes further by giving you tools to manipulate phase and feedback—features that open up entire signal routing possibilities. The phase inversion switches for the mix and band-pass outputs might seem like minor additions, but they’re powerful when used creatively. Flip the phase on the band-pass output, mix it with a dry signal externally, and you’ve created a band-reject (notch) filter through phase cancellation. Do the same with the low-pass, and you can craft a high-pass response. These aren’t precise surgical tools—phase cancellation with a tube circuit is inherently messy, with frequencies bleeding through and cancellation points shifting with temperature and tube variance. But that’s the charm. You’re not building a clean EQ; you’re sculpting sound with controlled chaos. The feedback loop, activated when Input 3 is unpatched, turns the filter into a self-oscillating monster. Even without external input, you can modulate the cutoff and resonance to create evolving drones, metallic shrieks, or rhythmic pulses that feel more like a sound source than a processor.

Gain Staging as a Sonic Weapon

The 611 doesn’t just filter—it amplifies, distorts, and reshapes at multiple stages. You can overdrive the signal pre-tube (via input gain), within the tube stage (via the drive control), or post-tube (via the output gain). This layered approach means you can dial in anything from subtle saturation to full-on destruction. At low levels, the filter behaves almost normally—dark, warm, with a slightly soft low-pass character. But crank the inputs or the drive, and the filter starts to break up, adding grit, compression, and harmonic complexity. It’s particularly effective on drums: feeding a snare or kick through the 611 with moderate drive can add body and glue, while maxing it out turns transients into roaring bursts of noise. Synth leads gain a snarling edge, basslines get thicker and more aggressive. The module even functions as a three-channel mixer thanks to the input attenuators, making it a multitool for both processing and blending. But beware: the gain structure is aggressive. It’s easy to overload the input or output stages, especially with hot modular signals. A gate or attenuator downstream is often necessary to keep the noise floor in check.

Historical Context

The ADDAC 611 exists because the original Gotharman Tubaz Filter was too wild, too niche, and too limited to survive in the mainstream Eurorack market. Gotharman, a small Danish builder known for idiosyncratic, hand-wired modules, created the Tubaz as a personal tool for adding “crunch, edge, and distortion” to his system. It wasn’t designed to be a precise filter—it was designed to be a character generator. When production ceased, the design risked vanishing into obscurity. ADDAC, known for their thoughtful, well-engineered modules with a vintage soul, stepped in and officially licensed the circuit. They didn’t just replicate it—they enhanced it with CV control, phase inversion, and better integration into modern systems. The 611 arrived in 2019, a time when Eurorack was saturated with clean, digital, or overly precise modules. The 611 was a counterpoint: a reminder that analog doesn’t have to be pristine to be powerful. It joined a lineage of tube-based Eurorack modules like those from Metasonix and Industrial Music Electronics, but with a more accessible form factor and deeper modulation options. It wasn’t the first tube filter, but it was one of the first to make tube chaos practical for everyday patching.

Collectibility & Value

The ADDAC 611 isn’t a rare module, but it’s not common either. Built in limited runs with genuine vintage tubes, it carries a premium price—€335 new from ADDAC, with used units typically trading between €280 and €350 depending on condition and custom panel options. The value holds well because demand remains steady among modular users seeking organic, unpredictable textures. However, there are real ownership considerations. The tube, while low-power, does create heat and has a finite lifespan. Though failures are rare, a dead tube means sending the module in for replacement or repair—there’s no user-swappable socket. The inrush current at power-on can also be a concern in tightly packed cases, particularly with power supplies like the Mantis, where users have reported instability when the 611 shares a power zone. Giving it its own dedicated power rail is recommended. Noise is another factor: even with no input, there’s a noticeable hiss and hum from the tube circuit. This isn’t a module for pristine, clean systems—it’s for those who welcome noise as part of the character. When buying used, check for microphonics (tap the module lightly—if the tube howls, it’s overly sensitive), and verify that all CV inputs respond correctly, especially the attenuverters. Custom panels add cost and wait time (4–6 weeks), but they’re a popular way to personalize a module that’s already unique by design. Overall, the 611 isn’t for everyone, but for those who want a filter that feels more like a collaborator than a tool, it’s a standout piece worth the investment.

eBay Listings

ADDAC ADDAC611 Gotharman's VC Tube Filter EURORACK - NEW - P

$459

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