ADDAC 611 Gotharman's VC Tube Filter (2019–)

A tube howls inside a Eurorack slot, hungry for feedback, distortion, and the kind of resonance that sounds like a rusty hinge screaming in a storm.

Overview

It’s not the first thing you’d reach for if you need a clean sweep from 20Hz to 20kHz. But if you want your modular rig to growl, shriek, or collapse into a pile of harmonic rubble—then the ADDAC 611 is less a filter and more a controlled demolition crew with a vacuum tube for a foreman. This isn’t subtle shaping; it’s sonic street fighting. The moment you patch in a signal and crank the drive, you’re not filtering so much as summoning a presence—something alive, unpredictable, and a little dangerous. It doesn’t just color sound; it bites it, spits it back, and sometimes refuses to let go. And that’s exactly the point.

The 611 isn’t an original ADDAC design—it’s a resurrection and expansion of the Tubaz Filter, a cult-classic module from Danish fringe builder Gotharman, discontinued after its 2014 run. ADDAC didn’t just clone it; they licensed the circuit outright and built around it, adding voltage control, feedback routing, phase inversion, and multiple outputs. But they kept the soul: a genuine 1970s vacuum tube, hand-sourced from old stock, not audiophile-grade but full of character—imperfections, microphonics, phase drift, and all. No two units sound exactly alike, and that’s by design. These aren’t precision instruments; they’re hand-fed, tube-warmed beasts with individual temperaments.

At 12HP, it’s not a space hog, but what’s packed in is dense. Three audio inputs, each with its own gain knob, let you blend and overdrive multiple sources before they even hit the tube. Input 3 doubles as a feedback loop when unpatched, pulling the band-pass output back into the input path—perfect for self-oscillating howls, chaotic resonance, or that gnarly, smeared distortion that feels like feedback through a blown speaker. The drive stage can push signals up to 11x amplification, and combined with the input gains and output trims, you’ve got multiple points to introduce grit—pre-tube, in-tube, or post-tube. It’s not just about distortion; it’s about choosing where the filth enters the signal chain.

Specifications

Manufacturer	ADDAC System
Production Years	2019–
Original Price	€335 / $352
Format	Eurorack
Width	12HP
Depth	3.5cm
Power Consumption	+12V: 90mA, -12V: 60mA
Audio Inputs	3 (with individual gain controls)
CV Inputs	Drive (with attenuverter), Cutoff (with attenuverter), Resonance (with attenuverter), Mix (with attenuverter)
Filter Outputs	Low-Pass (LP), High-Pass (HP), Band-Pass (BP), Mix (BP + dry)
Output Level Controls	LP and HP: front-panel trimmers; BP and Mix: front-panel trim pots
Phase Inversion	Switchable for BP and Mix outputs
Feedback Path	Input 3 acts as feedback from BP output when unpatched
Visual Indicators	3 LEDs: post-Drive, post-Mix, post-Filter
Tube Type	Vintage 1970s vacuum tube (specific type varies per unit)
Amplification Range	Input gain up to 2x, Drive up to 11x, Output up to 2x

Key Features

A Tube in the Middle of the Rack

There’s no hiding it—the tube sits dead center, visible through a circular grille in the black front panel. It’s not a gimmick; it’s the core of the circuit, a real thermionic valve from the 1970s, not selected for sonic purity but for character. These are old-stock tubes, often pulled from decommissioned gear, and they bring all the quirks: microphonics, thermal drift, and a slight hum that’s always present. But that hum isn’t a flaw—it’s part of the texture. When you crank the resonance, the tube’s inherent phase shifts and nonlinearities turn the filter peak into something organic, almost vocal. It doesn’t self-oscillate like a clean OTA filter; it howls, whines, and sometimes sputters, more like a guitar amp pushed to its limits than a synthesizer filter. And because each tube is unique, no two 611s behave the same—especially in resonance and feedback behavior. One might scream in a focused pitch, another might smear across a range of overtones. It’s not consistency you’re buying; it’s individuality.

Feedback, Phase, and Filter Cannibalism

The 611 doesn’t just process audio—it lets you turn it against itself. Input 3, when unpatched, routes the band-pass output back into the input mix, creating an internal feedback loop. Patch in a little CV to the drive or cutoff, and you can modulate that loop into self-oscillation, chaotic noise, or sustained drones that evolve over time. But the real magic lies in the phase inversion switches for the BP and Mix outputs. At first glance, flipping phase does nothing if you’re listening to the output solo. But patch the dry signal and the filtered output into an external mixer, and suddenly you’ve got surgical control over cancellation. Invert the BP output, mix it with the dry signal, and you’ve created a band-reject filter. Do the same with the LP output (using an external phase inverter), and you can carve out lows with phase cancellation instead of filtering. It’s a clever workaround for a module that doesn’t offer those modes natively—and a nod to the kind of patching creativity Eurorack thrives on.

Gain Staging as a Creative Tool

The 611 doesn’t assume you want clean filtering. It assumes you want to destroy things. The three input gains, the drive stage, and the output trims give you five points to overdrive the signal. You can keep it relatively tame—low input levels, moderate drive, high output—and get a dark, warm low-pass with a thick, tube-like character. Or you can max out the inputs, crank the drive, and send a kick drum through it to turn a transient into a sustained burst of fuzz. It excels on drums, adding grit and glue simultaneously, but it’s equally at home mangling pads, vocals, or even external effects returns. The high-pass output, in particular, is described by users as “rude”—it doesn’t cleanly cut lows so much as let them bleed through with a snarl. The band-pass doesn’t narrow to a precise peak; it retains low-end weight even at high center frequencies, making it more of a “band add” than a band-pass, as Gotharman himself put it. It’s not a surgical tool; it’s a blunt instrument with attitude.

Historical Context

The ADDAC 611 exists because the original Gotharman Tubaz Filter was too weird to survive. Released in 2014, it was never meant to be a mainstream module. It was a personal project by a niche builder, designed to add “crunch, edge, and distortion” to a modular system, not to provide clean filtering. When it disappeared from production, a small but passionate following was left without access. ADDAC, known for their thoughtful, expressive Eurorack designs, saw value in preserving the circuit—not just cloning it, but expanding it with modern features like voltage control and phase switching. In doing so, they positioned the 611 not as a utility module, but as a character module, joining a lineage of tube-based filters like those from Metasonix or Industrial Music Electronics that prioritize raw sonic impact over precision. It arrived in 2019, a time when Eurorack was saturated with clean, digital, or highly stable analog filters. The 611 was a deliberate counterpoint—a reminder that not every filter needs to be transparent, predictable, or quiet.

It also reflects a broader trend in modular synthesis: the embrace of imperfection. Where early Eurorack focused on replicating classic synth voices with accuracy, the 2010s saw a rise in modules that celebrated noise, instability, and unpredictability. The 611 fits squarely in that camp. It’s not competing with Doepfer’s A-106 or Make Noise’s QPAS; it’s doing something entirely different. It’s closer in spirit to the Metasonix TM-70 or the Industrial Music Electronics Tiptop Audio Z4000, where the goal is not fidelity but transformation. And by licensing the original design rather than reverse-engineering it, ADDAC gave credit where it was due, ensuring that Gotharman’s vision wasn’t lost to obscurity.

Collectibility & Value

The ADDAC 611 isn’t a rare module, but it’s not common either. Priced at €335 new, it’s a premium product, and while it’s still in production, it’s not something you’ll find at every modular dealer. On the secondhand market, prices hover between £280 and £320, depending on condition and whether it includes the optional custom-colored panel (available in red, green, blue, white, gray, yellow, or gold for an extra €70). There’s no significant markup for vintage status—yet—but collectors of character-driven Eurorack gear often seek it out for its unique tube behavior and limited per-unit consistency.

Failures are rare but notable. The biggest concern is tube longevity and microphonics. While the tube is low-power and designed for Eurorack use, it’s still a 50-year-old component. Some units exhibit microphonic behavior—tapping the module introduces ringing or feedback—which can be desirable in moderation but problematic if excessive. The tube can’t be easily replaced with a modern equivalent; substitutions may alter the sound significantly. Power draw is moderate (90mA on +12V, 60mA on -12V), but the inrush current when cold can be high, which has caused issues in some busboards, particularly the Mantis. Users report success when giving the 611 its own power zone with sufficient headroom. There are no known circuit faults beyond standard electrolytic cap aging, but given the tube’s age, a full recap isn’t unheard of in long-term ownership.

When buying used, listen for excessive hum or noise with no input patched. Some noise is expected—this is a tube module—but a loud hiss or buzz could indicate a failing tube or power issue. Test the phase switches and feedback loop to ensure they’re functioning, and check that the trimmers for LP and HP outputs are accessible and not stripped. Because each unit is sonically unique, if possible, hear it in action before purchasing. Some units scream more musically than others.