ADDAC System ADDAC611 (2019–Present)

A tube howls inside your rack — raw, unpredictable, and gloriously out of control.

Overview

Turn it on and you hear it before you see it: a low, warm hum rising from the chassis like something alive waking up. The ADDAC611 isn’t just another Eurorack filter — it’s a temperamental beast with a vacuum tube at its core, salvaged from the 1970s and dropped into a 12HP slot like a live grenade with the pin pulled. This is not the kind of module you casually patch into a clean pad and expect refinement. It’s the sound of amplifiers pushed past redline, of feedback loops spiraling into self-destruction, of analog circuits sweating under the weight of their own distortion. And yet, for all its chaos, it’s astonishingly flexible — a three-channel mixer, a drive stage with attitude, a multimode filter with a personality disorder, and a feedback engine that can turn any signal into a snarling, phase-cancelling monster.

The ADDAC611 began life as the Gotharman Tubaz Filter, a boutique module from a fringe Danish designer who never cared much for textbook filter behavior. ADDAC System didn’t just clone it — they resurrected it, licensed the original circuit, and expanded it with voltage control, phase inversion switches, and a more robust front-panel layout. The result is a module that doesn’t so much filter sound as attack it, smear it, and reassemble it with missing pieces. It’s not a surgical tool; it’s a blunt instrument with a PhD in sonic mayhem. You don’t use it to clean up your mix — you use it to ruin everything in the most beautiful way possible.

Specifications

Manufacturer	ADDAC System
Production Years	2019–Present
Original Price	€335
HP	12
Depth	35 mm
Power Consumption (+12V)	90 mA
Power Consumption (-12V)	60 mA
Audio Inputs	3 (with individual gain controls)
CV Inputs	Drive, Cutoff, Resonance, Mix (all with attenuverters)
Filter Types	Low-pass, High-pass, Band-pass, Mix (band-pass + dry)
Filter Outputs	LP, HP, BP, Mix (each with level controls or trimmers)
Phase Inversion	Switchable for BP and Mix outputs
Feedback Path	Input 3 doubles as feedback from BP output when unpatched
Drive Stage	VC with attenuverter, up to x11 gain
Mix Output Gain	Adjustable via front-panel knob
Band-pass Output Gain	Adjustable via front-panel knob
LP/HP Output Level	Set via front-panel trimmers
Status LEDs	Drive, Mix, Filter

Key Features

A Genuine 1970s Tube in Your Rack

That glowing orb behind the circular cutout in the front panel? That’s a real vacuum tube, pulled from old stock, most likely built in the 1970s. ADDAC doesn’t cherry-pick audiophile-grade specimens — they use whatever old, imperfect tubes they can source, because imperfection is the point. Each tube has its own microphonic quirks, phase shifts, and harmonic distortions, meaning no two ADDAC611s sound exactly alike. Some might hiss more, others howl earlier, but all of them bring a warmth and unpredictability that solid-state filters can only dream of. The tube isn’t just a gimmick; it’s the heart of the distortion, the source of the grit, and the reason this module feels more like a vintage guitar amp than a synthesizer component.

Three-Channel Mixer with Feedback Tricks

Before your signal even hits the filter, it passes through three inputs, each with its own gain knob. This turns the ADDAC611 into a de facto three-channel mixer — useful if you’re stacking drum hits, layering oscillators, or blending external sources. But Input 3 has a secret: when left unpatched, it automatically routes the band-pass output back into the input, creating a feedback loop you can shape with the gain knob. Crank it and you get self-oscillation, howling resonance, and chaotic overtones that spiral out of control. Patch something in and you break the loop — it’s elegantly simple, and dangerously fun. Combine that with the phase inversion switches on the BP and Mix outputs, and you’ve got a playground for phase cancellation effects, letting you create band-reject or notch-like responses by mixing dry and inverted signals externally.

Drive, Distortion, and Sonic Abuse

The drive stage is where things get ugly — in the best way. With up to 11x amplification, it can push any signal into full-on saturation before it even reaches the filter. You can distort pre-tube (via input gain), in-tube (via drive), or post-tube (via output gain), giving you multiple flavors of destruction. At low levels, the filter behaves almost normally — dark low-pass tones, a usable band-pass, a high-pass that’s still a bit wild. But push it, and the low-pass starts bleeding harmonics, the band-pass turns into a “band-add” filter (as the original designer put it), and the high-pass gets downright rude. It’s not precise, it’s not clean, and it doesn’t pretend to be. But on drums? Unholy. On basslines? Terrifying. On vocals? Like running them through a blown Marshall stack in a thunderstorm.

Historical Context

The ADDAC611 exists because someone once decided filters didn’t have to be polite. Gotharman, a small Danish builder known for experimental designs, released the Tubaz Filter in 2014 as a no-compromise, high-gain tube filter that prioritized character over correctness. It gained a cult following but disappeared when production ceased. ADDAC System, no strangers to bold sonic statements, stepped in to license the design and bring it back — not as a museum piece, but as an evolved instrument. At a time when Eurorack was filling up with pristine digital filters and surgical VCAs, the ADDAC611 was a middle finger to cleanliness. It arrived in 2019, just as modular users were rediscovering the value of dirt, saturation, and analog unpredictability. Competitors like Metasonix had already proven there was a market for tube-based chaos, but the ADDAC611 brought something new: musicality within the madness, and enough CV control to make it a real performance module, not just a novelty.

Collectibility & Value

The ADDAC611 isn’t rare — it’s still in production — but it’s not common, either. You won’t find it gathering dust on every rack, but serious Eurorack users who crave analog filth know exactly what it is. New units sell for around €335, with custom-colored panels adding €70 and a 4–6 week wait. On the used market, prices hover between $250 and $350 depending on condition and whether it includes the original packaging. There are no known firmware updates or revisions, so all units are functionally identical aside from tube variance.

Ownership quirks? Yes. The tube has a finite lifespan — though it’s low-power, so it should last years under normal use. More pressing is the inrush current: some users report issues when powering up the module in tightly packed cases, especially on Mantis power systems. The solution? Give it its own power zone with sufficient headroom. Also, that background hiss from the tube is always present — not a flaw, but a feature. If you’re running quiet patches or clean mixes, you’ll want to gate the output or mute it when not in use. And those LP and HP output levels? Set with tiny front-panel trimmers — not the most user-friendly, but once dialed in, they stay put.

Buying advice: Test it if possible. Listen for microphonics (tap the panel and see if the tube picks it up), check that all CV inputs respond smoothly, and verify the phase inversion switches actually flip polarity. Avoid units with a dead or noisy tube — replacements are possible, but sourcing vintage tubes isn’t getting easier. This isn’t a module for beginners, and it’s not for those who value silence. But if you want something that sounds like nothing else in your rack, that can turn a simple sine wave into a howling beast, that feels more like a vintage amp than a synth module — the ADDAC611 earns its space.