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

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

Overview

This is not the module to reach for if you need a clean 20Hz to 20kHz sweep. If you want your modular rig to growl, shriek or collapse into harmonic rubble, the ADDAC 611 is the right one. It is less a filter and more a demolition crew with a vacuum tube for a foreman. Patch in a signal and crank the drive, and the module bites the sound, spits it back, and sometimes refuses to let go.

The 611 is not an original ADDAC design. It is a revival and expansion of the Tubaz Filter, a cult module from Danish builder Gotharman, discontinued after its 2014 run. ADDAC licensed the circuit outright and built around it, adding voltage control, feedback routing, phase inversion and multiple outputs. They kept the genuine 1970s vacuum tube, hand-sourced from old stock. It is not audiophile grade, and it comes with imperfections, microphonics and phase drift. No two units sound exactly alike, and that is by design.

At 12HP it is not a space hog, but it is dense. Three audio inputs, each with its own gain knob, let you blend and overdrive multiple sources before they hit the tube. Input 3 doubles as a feedback loop when unpatched, pulling the band-pass output back into the input path. That gives you self-oscillating howls, chaotic resonance, or smeared distortion like feedback through a blown speaker. The drive stage can push signals up to 11x amplification. Combined with the input gains and output trims, that gives you several points to add grit: pre-tube, in-tube or post-tube.

Specifications

Key Features

A Tube in the Middle of the Rack

The tube sits dead center, visible through a circular grille in the black front panel. It is the core of the circuit, a real thermionic valve from the 1970s, chosen for its quirks and not for sonic purity. These are old-stock tubes, often pulled from decommissioned gear. They bring microphonics, thermal drift and a slight hum that is always present. The hum is part of the texture. When you crank the resonance, the tube's phase shifts and nonlinearities turn the filter peak into something organic, almost vocal. It does not self-oscillate like a clean OTA filter. It howls, whines and sometimes sputters, closer to a guitar amp pushed to its limits than to a synth filter. Because each tube is unique, no two 611s behave the same, especially in resonance and feedback. One might scream at a focused pitch, and another might smear across a range of overtones.

Feedback, Phase, and Filter Cannibalism

The 611 lets you turn a signal against itself. Input 3, when unpatched, routes the band-pass output back into the input mix, which makes an internal feedback loop. Patch 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. The phase inversion switches for the BP and Mix outputs are the clever part. Flipping phase does nothing if you listen to the output solo. Patch the dry signal and the filtered output into an external mixer, though, and you get control over cancellation. Invert the BP output and mix it with the dry signal, and you have a band-reject filter. Do the same with the LP output, using an external phase inverter, and you can carve out lows by phase cancellation instead of filtering. The module has no native modes for this, so it is a workaround.

Gain Staging as a Creative Tool

The 611 does not assume you want clean filtering. The three input gains, the drive stage and the output trims give you five points to overdrive the signal. Keep it tame with low input levels, moderate drive and high output, and you get a dark, warm low-pass with a thick, tube-like sound. Or max out the inputs, crank the drive and send a kick drum through it, and the transient turns into a sustained burst of fuzz. It works well on drums, adding grit and glue at once, and it handles pads, vocals and external effects returns too. Users describe the high-pass output as "rude". It does not cleanly cut lows; it lets them bleed through with a snarl. The band-pass does not narrow to a precise peak. It keeps low-end weight even at high center frequencies, which makes it more of a "band add" than a band-pass, as Gotharman himself put it.

Historical Context

The ADDAC 611 exists because the original Gotharman Tubaz Filter was too odd to survive. Released in 2014, it was a personal project by a niche builder, designed to add "crunch, edge, and distortion" to a modular system, not to filter cleanly. When it went out of production, a small following was left without one. ADDAC saw value in keeping the circuit alive and expanded it with voltage control and phase switching. They positioned the 611 as a character module and not a utility module, in the line of tube-based filters from Metasonix or Industrial Music Electronics that put raw impact ahead of precision. It arrived in 2019, when Eurorack was full of clean, digital or highly stable analog filters.

It also fits a wider trend in modular synthesis. Early Eurorack focused on accurate copies of classic synth voices, and the 2010s saw more modules built around noise, instability and unpredictability. The 611 sits in that camp. It does not compete with Doepfer's A-106 or Make Noise's QPAS. It is closer in spirit to the Metasonix TM-70 or the Industrial Music Electronics Tiptop Audio Z4000, where the goal is transformation and not fidelity. By licensing the original design instead of reverse-engineering it, ADDAC gave Gotharman credit for the circuit.

Collectibility & Value

The ADDAC 611 is not rare, but it is not common either. It costs €335 new, and while it is still in production, not every modular dealer stocks it. Secondhand prices run between £280 and £320, depending on condition and whether it has the optional custom-colored panel. That panel comes in red, green, blue, white, gray, yellow or gold for an extra €70. There is no real markup for vintage status yet. Collectors of Eurorack gear with personality seek it out for its tube behavior and unit-to-unit variation.

Failures are rare but notable. The biggest concern is tube longevity and microphonics. The tube is low-power and designed for Eurorack use, but it is still a 50-year-old component. Some units are microphonic, so tapping the module introduces ringing or feedback. That can be useful in moderation and a problem if excessive. The tube cannot easily be swapped for a modern equivalent, and substitutions may change the sound significantly. Power draw is moderate at 90mA on +12V and 60mA on -12V, but the inrush current when cold can be high. That has caused issues in some busboards, particularly the Mantis. Users report success giving the 611 its own power zone with enough headroom. No circuit faults are known beyond standard electrolytic cap aging, though given the tube's age, a full recap is not unheard of in long-term ownership.

When buying used, listen for excessive hum or noise with no input patched. Some noise is expected from a tube module, but a loud hiss or buzz could mean a failing tube or a power issue. Test the phase switches and the feedback loop, and check that the trimmers for the LP and HP outputs are accessible and not stripped. Each unit sounds different, so hear it in action before buying if you can. Some units scream more musically than others.

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