ADDAC 604 Dual Filter (2020–)
Two channels of raw, opinionated analog filtering in just 6hp—this little Eurorack brute doesn’t clean up your sound, it fights with it.
Overview
You know those filters that just sit quietly and do what they're told? The ADDAC 604 isn’t one of them. This is a module that leans in, gets in the mix, and makes it clear it’s got something to say—especially when you crank the resonance. It’s not trying to be transparent. It’s not trying to be polite. It’s a dual analog multimode filter with teeth, and it’s been turning heads in compact Eurorack builds since it quietly appeared in December 2020.
Born out of the development of the ADDAC105 4 Voice Cluster module, the 604 was originally meant to be an internal tool—a filter to follow the summing stage and shape the cluster’s dense harmonics. But the designers at ADDAC (Elevator Sound) liked what they heard so much that they decided to release it as a standalone module. And thank goodness they did. What you get is a full-featured, two-channel analog filter in a mere 6hp—a footprint so small it almost feels like cheating. Each channel gives you full multimode outputs (highpass, bandpass, lowpass), CV control with attenuator, and hands-on cutoff and resonance knobs. It’s dense with functionality, yet ruthlessly focused.
The 604 doesn’t pretend to be a luxury item. One reviewer noted the use of “wobbly trimmer pots” instead of more robust knobs—a cost-saving move that keeps the price in check but might give some builders pause. Still, that same reviewer praised its sonic character, calling it “great (stereo) sound in a small package.” And that’s the story of the 604: it trades polish for presence, and in doing so, delivers something far more interesting than a sterile signal processor.
Specifications
| Manufacturer | ADDAC (Elevator Sound) |
| Width | 6hp |
| Depth | 40mm |
| Power | 70mA +12V, 70mA -12V |
| Slope | 12dB per octave |
| Per channel controls | cutoff and resonance |
| Per channel inputs | cutoff CV input with attenuator, audio input |
| Per channel outputs | highpass, bandpass and lowpass |
| Features | Switchable stereo mode |
Key Features
Dual Analog Multimode Filtering in a Tiny Frame
At its core, the ADDAC 604 is a duplicated analog multimode filter circuit—two full channels of 12dB/octave filtering, each capable of highpass, bandpass, and lowpass outputs simultaneously. That alone is impressive at 6hp. But what makes it special is how it handles routing and control. The module includes a mode switch that toggles between Stereo and Dual Mono operation, fundamentally changing how the controls interact.
In Stereo Mode, the left channel takes command: the Left Cutoff knob controls both left and right channel cutoff frequencies. The Right Cutoff knob and its CV input are bypassed. This keeps your hands on one side while processing a stereo signal, which is ideal for master filtering or live tweaking. But here’s the clever bit: there’s an optional jumper on the back of the module. When installed, it reconnects the Right Cutoff CV input, letting you offset the right channel’s cutoff independently—even in Stereo Mode. It’s a subtle but powerful tweak for creating stereo width or phasing effects via CV.
Normalled Inputs and Independent Resonance
Both audio and CV inputs are normalled from the left to the right channel. Plug into the left, and your signal (and CV) automatically feed the right—perfect for quick stereo setups or using it as a single-channel filter with passive duplication. But if you want full independence, just patch into the right side and the normaling breaks. Resonance, meanwhile, is always independent per channel, no matter the mode. That means you can have a screaming, resonant lowpass on the left while the right stays clean and dull—or go full chaos with both cranked. Reviewers have noted the resonance is “very brutal,” in the best possible way: it doesn’t just peak, it bites.
A Filter With Character
This isn’t a neutral filter. One user put it perfectly: it “absolutely does not try to hide its own opinion on the signal.” It adds grit, warmth, and a slight saturation that keeps the output full-bodied—even before you push it into distortion. The lowpass has an “acid-y” crispness that cuts through a mix, while the highpass is described as “true” and “tight,” making it excellent for sculpting drum signals or adding bite to percussion. Another reviewer called it “great for grit in the signal,” especially when using the well-controllable CV response on the cutoff—ideal for aggressive, modulated drum processing or dynamic stereo sweeps.
Historical Context
The ADDAC 604 Dual Filter emerged from the development of the ADDAC105 4 Voice Cluster, where it was designed as a post-summing filter to tame (or enhance) the cluster’s rich, chaotic output. Instead of keeping it internal, ADDAC decided to release it as a standalone module—a smart move, given how well it performs on its own. It’s part of the ADDAC600 Series, a family of compact, function-focused Eurorack modules that prioritize sonic character and utility over flash. While no direct predecessors or competitors are cited in the documentation, its release in late 2020 placed it in a growing wave of boutique dual filters aimed at Eurorack users who want stereo processing without eating up half their case.
Collectibility & Value
As of the latest data, the ADDAC 604 lists for £195.00 (inc. UK VAT) directly from Elevator Sound, with an ex-VAT price of £162.50. That reflects its status as a current-production module rather than a vintage item, though its focused design and strong reception have made it a sought-after piece in modern Eurorack builds. A B-Stock listing on Thomann shows a price of $205, indicating steady demand and limited discounting. One listing was described as a “last display model,” suggesting low surplus inventory.
There is no confirmed original MSRP, and no data on common failures or maintenance issues—though the use of trimmer pots instead of solid knobs may warrant long-term attention. Given its recent release and ongoing availability, it’s not yet a “collector’s item” in the traditional sense, but its reputation for sonic character and efficient design makes it a high-utility module worth tracking. If you’re building a compact, character-driven system, the 604 isn’t just functional—it’s a statement.
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