ADDAC System ADDAC604 (Years Not Confirmed)
A compact dual multimode filter that punches way above its 6 HP weight, born from a voice cluster and built for stereo flexibility
Overview
You know that moment when a manufacturer builds something for an internal project, only to realize they’ve accidentally made a standalone gem? That’s the ADDAC604 in a nutshell. It started life as a new multimode filter developed for the ADDAC105 4 Voice Cluster module—specifically intended to sit after the summing mix stage and shape the final stereo output. But the results were so promising that ADDAC System decided to spin it off into its own dedicated module. And thank goodness they did, because what emerged is a dual filter that manages to be both ruthlessly practical and sonically rich, all in a footprint so small it barely registers on a crowded Eurorack case.
The ADDAC604 isn’t trying to be flashy. It doesn’t modulate its own personality with LFOs or offer digital wizardry. Instead, it delivers two identical multimode filter circuits, each capable of highpass, bandpass, and lowpass outputs—perfect for parallel processing, stereo imaging, or dual-mono patching across two separate voices. At just 6 HP wide and 40 mm deep, it’s a space-saving hero, but don’t mistake compactness for compromise. This thing is built for real work, with normalled audio and CV inputs that let you daisy-chain signals efficiently. It’s the kind of module you install and forget about—until you tweak it and realize how much it’s been doing for your sound all along.
Owners report it’s immediately useful: no cryptic menus, no hidden functions, just two filters that respond predictably and musically. One reviewer called it “a very good sounding multi mode dual filter with a very tight footprint,” and that about sums it up. It’s not a boutique filter with self-oscillation quirks or analog drift for character—it’s clean, focused, and flexible. And at a time when Eurorack modules keep creeping wider and deeper, the ADDAC604 feels like a quiet rebellion against bloat.
Specifications
| Manufacturer | ADDAC System |
| Dimensions | 6 HP |
| Depth | 40 mm |
| Current Draw | 70 mA +12V, 70 mA -12V, ? mA 5V |
| Price | $202 |
| Filter outputs per channel | highpass, bandpass, and lowpass |
| Modes | dual mono or stereo |
| Product type | Eurorack Module, Dual Filter |
Key Features
Dual Identical Multimode Filters
At its core, the ADDAC604 houses two complete multimode filter circuits—each offering highpass, bandpass, and lowpass outputs. These aren’t approximations or shared resources; they’re full, independent filter paths. That means you can route two completely different signals through their own filter chains, or process a stereo pair with matched or divergent settings. The fact that both filters are identical ensures consistency, whether you're using them in dual mono or stereo mode. It’s rare to find this level of completeness in such a narrow module, and it speaks to ADDAC’s design philosophy: give users real tools, not gimmicks.
Stereo vs. Dual Mono Flexibility
The mode switch is where the ADDAC604 reveals its true utility. Flip it to Dual Mono, and both filter channels operate independently—perfect for processing separate voices, drum layers, or effects returns. Switch to Stereo Mode, and the Left Cutoff knob and CV input take control of both channels, effectively locking the cutoff frequencies together. This is ideal for applying unified filter movement to a stereo signal, like a master tone sweep across a mixed pad or loop. But here’s the clever bit: in Stereo Mode, the Right Cutoff knob and CV input are bypassed by default. That means you can’t independently tweak the right channel’s cutoff unless you intervene.
Jumper for Offset Control
For those who want more nuance, there’s a jumper on the back of the module that reconnects the Right CV input to the right filter’s cutoff. This doesn’t restore full independent control—since the Left Cutoff still dominates—but it allows you to apply an offset via CV, letting the right channel respond slightly higher or lower than the left. It’s a subtle but powerful option for creating dynamic stereo width or phasing effects without needing external modulation sources for both sides. It’s the kind of thoughtful detail that suggests ADDAC didn’t just duplicate a circuit and call it a day—they considered how people actually patch.
Normalled Inputs for Efficient Patching
Both audio and CV inputs are normalled from Left to Right. That means if you only plug into the Left side, the signal automatically feeds the Right channel as well. This is incredibly useful in stereo mode, where you’re often sending the same source to both filters. It cuts down on cable clutter and makes the module faster to deploy in a mix. But the normaling doesn’t force your hand—you can always break the chain by plugging into the Right inputs, restoring full independence. It’s a small thing, but in a format where every cable matters, these kinds of optimizations add up.
Independent Resonance Control
Even in Stereo Mode, resonance remains independent for each channel. That’s a big deal. While the cutoff frequency is linked (unless you use the jumper trick), you can still shape the character of each side differently. Want a bright, ringing peak on the left and a softer, warmer tone on the right? Go ahead. This separation keeps the stereo image lively and prevents the sound from collapsing into uniformity. It’s one of those design choices that feels obvious in hindsight but is surprisingly rare—many stereo filters link resonance by default, which limits expressive potential.
Collectibility & Value
The ADDAC604 doesn’t scream “collector’s item,” but it’s quietly appreciated by those who value utility and efficiency. On ModularGrid, it holds an average rating of Ø 4.20 from 10 votes—solid, if not ecstatic. Reviewers praise its immediacy and usefulness. One noted, “I’m really impressed by these recent releases from ADDAC. They’ve been simple, straight forward, immediately useful and affordable.” Another called it “a very good sounding multi mode dual filter with a very tight footprint,” while a retailer described it as offering “immense amounts of sonic possibilities in the realm of filtering with flexible control, routing, and timbre.”
Pricing reflects its practical nature. The listed price is $202, though current new prices vary—synthCube lists it at $193.00, with a kit version available for $124.00. Used market listings show prices at $223.50, $235, and $241.62, suggesting modest appreciation or seller variance. There’s no data on common failures, maintenance, or repair issues, but the lack of reported problems in forums hints at solid build quality. One user did point out a limitation: “No CV for Resonance though.” That’s true—while resonance is adjustable per channel, it’s manual only. If you need automated resonance sweeps, you’ll need to pair it with a separate CV-controlled attenuator or processor.
It’s not a rare grail, nor is it overproduced. It occupies that sweet spot of being available, functional, and sonically competent—perfect for builders who want to add dual filtering without sacrificing real estate. It won’t dominate a build, but it will earn its place.
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