ADDAC 611 (2019–)

A tube-driven filter that growls and howls. It doesn't so much filter as attack, smear and resurrect your sound.

Overview

Turn it on and you hear it before you patch a cable - a low, warm hum from the chassis. The ADDAC 611 isn't a filter in the traditional sense. It doesn't politely carve frequencies. It chews them up and spits them out. The module is built around a genuine 1970s vacuum tube, salvaged from old stock, wired into a circuit originally designed by Denmark's Gotharman for their now-discontinued Tubaz Filter. ADDAC licensed the design, then expanded it with extra CV control, phase inversion, and a feedback loop that lets it sustain itself. It's noisy, unpredictable and imperfect. If you want a filter that behaves, this isn't it. If you want one that snarls when pushed and purrs when coddled, it fits in 12HP.

Every tube is different, and that's what makes the 611 volatile. ADDAC doesn't cherry-pick pristine specimens. They use whatever old-stock tubes they can source, with the inconsistencies, microphonics and harmonic bleed that come with decades-old glass. 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 invites 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. That gives you self-oscillation, feedback squeals, or a thick, gnarled wash of resonance that feels more like a distortion effect than a filter sweep.

Despite its wild nature, the 611 has plenty of 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 separate high-pass, low-pass and band-pass, plus a mix output that blends the band-pass with dry signal. A master gain covers the mix and BP outputs. Here's the catch: the HP and LP outputs have no front-panel level controls, only trimmer pots on the panel, so you need a screwdriver to adjust their levels. It feels a bit archaic in a modern Eurorack context, and it points back to boutique, hand-built gear where calibration was part of the routine.

Specifications

Key Features

The Tube as a Character Generator

The vacuum tube is the core of the 611. It's not a simulation or 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. Tubes don't behave linearly. They distort, phase-shift, hum, and react to heat. 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. That's not a flaw. At high drive settings, the tube can produce rich, even-order harmonics that smear transients and glue sounds together. Push it too far and it will scream, howl, or feed back like a cranked guitar amp.

Feedback and Phase Inversion as Creative Tools

Most filters give you cutoff, resonance and maybe CV control. The 611 adds tools to manipulate phase and feedback, which open up more signal routing. The phase inversion switches for the mix and band-pass outputs look 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 get 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 tools. Phase cancellation with a tube circuit is inherently messy, with frequencies bleeding through and cancellation points shifting with temperature and tube variance. 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 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). That lets you 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. Crank the inputs or the drive and it starts to break up, adding grit, compression and harmonic complexity. It's particularly effective on drums. A snare or kick through the 611 with moderate drive gains body and glue, while maxing it out turns transients into roaring bursts of noise. Synth leads gain a snarling edge, and basslines get thicker and more aggressive. Thanks to the input attenuators, it also works as a three-channel mixer. The gain structure is aggressive, though. 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. ADDAC officially licensed the circuit and enhanced it with CV control, phase inversion, and better integration into modern systems. The 611 arrived in 2019, when Eurorack was saturated with clean, digital, or overly precise modules. 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 rare, but it's not common either. It's built in limited runs with genuine vintage tubes and 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. There are real ownership considerations. The tube, while low-power, creates heat and has a finite lifespan. Failures are rare, but a dead tube means sending the module in for replacement or repair, since there's no user-swappable socket. The inrush current at power-on can 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. 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).

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