ADDAC System ADDAC611 (2019–Present)
A tube howls inside your rack - raw, unpredictable, and out of control.
Overview
Turn it on and you hear a low, warm hum rising from the chassis before you see anything. The ADDAC611 is a Eurorack filter with a vacuum tube at its core, salvaged from the 1970s and fitted into a 12HP slot. It isn't a module you casually patch into a clean pad and expect refinement. It sounds like amplifiers pushed past redline and feedback loops spiraling into self-destruction. It's also flexible: a three-channel mixer, a drive stage, a multimode filter, and a feedback engine that can turn any signal into a snarling, phase-cancelling mess.
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 licensed the original circuit and expanded it with voltage control, phase inversion switches, and a sturdier front-panel layout. The module doesn't so much filter sound as attack it, smear it, and reassemble it with missing pieces. It's a blunt instrument, not a surgical tool. You don't use it to clean up a mix.
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
The glowing orb behind the circular cutout in the front panel is 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, so no two ADDAC611s sound exactly alike. Some might hiss more, others howl earlier. The tube is the source of the distortion and the grit, and it makes the module feel more like a vintage guitar amp than a synthesizer component.
Three-Channel Mixer with Feedback Tricks
Before the signal hits the filter, it passes through three inputs, each with its own gain knob. That makes the ADDAC611 a de facto three-channel mixer, useful for stacking drum hits, layering oscillators, or blending external sources. 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. Patch something in and you break the loop. The phase inversion switches on the BP and Mix outputs add 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. With up to 11x amplification, it can push any signal into full saturation before it reaches the filter. You can distort pre-tube (via input gain), in-tube (via drive), or post-tube (via output gain). 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. 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 and it's not clean. On drums, unholy. On basslines, terrifying. On vocals, like running them through a blown Marshall stack in a thunderstorm.
Historical Context
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 stepped in to license the design and bring it back as an evolved instrument. It arrived in 2019, when Eurorack was filling up with pristine digital filters and surgical VCAs, and modular users were rediscovering dirt, saturation, and analog unpredictability. Competitors like Metasonix had already proven there was a market for tube-based chaos. The ADDAC611 added musicality and enough CV control to work as a performance module.
Collectibility & Value
The ADDAC611 isn't rare, since it's still in production, but it's not common either. 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.
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 fix is to give it its own power zone with sufficient headroom. The background hiss from the tube is always present. If you're running quiet patches or clean mixes, gate the output or mute it when not in use. The LP and HP output levels are set with tiny front-panel trimmers, which aren't very 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.
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