ADDAC 604 (2020–Present)

A compact dual analog filter that punches far above its size, splitting the difference between surgical precision and warm character—when you need two filters but only have space for one.

Overview

Slide the ADDAC 604 into your rack and you might not think much of it at first—just six HP of matte black panel with two identical rows of knobs and jacks, no flashy graphics, no blinking lights. But plug in a pair of oscillators, twist the cutoff on both channels, and that unassuming slab starts to breathe. It doesn’t scream for attention; it carves space. There’s a smoothness to its multimode character—lowpass, bandpass, highpass per channel—that feels more like a vintage console EQ than a surgical modular filter, even though it’s fully analog and built for patch flexibility. It was born from the ADDAC105 4 Voice Cluster, where it served as a final tone-shaping stage, and its DNA shows: this isn’t a filter designed to scream into self-oscillation or distort your signal into oblivion. It’s meant to refine, to focus, to let you shape stereo or dual-mono sources with surgical control and a touch of analog warmth.

What makes the 604 stand out in a market flooded with filters is its duality—two complete analog multimode filters in just 6 HP. That’s not just efficient; it’s borderline audacious. Each channel has independent audio input, cutoff and resonance knobs, cutoff CV input with attenuverter, and dedicated outputs for highpass, bandpass, and lowpass. You can run them completely independently in dual mono mode, process a stereo signal with matched filtering, or use the stereo mode to lock both cutoff frequencies together under left-side control—perfect for keeping stereo image coherence while modulating cutoff with a single envelope or LFO. And if you want a bit more nuance in stereo tracking, there’s a jumper on the back that lets you reintroduce CV control to the right channel, letting you offset its cutoff slightly for subtle stereo filtering effects. It’s a small detail, but one that reveals the thoughtfulness behind the design: this isn’t just two filters slapped side by side. They’re meant to work together.

The filter topology itself is a two-pole design per channel, which means 12 dB/octave slopes—smooth, not aggressive. It won’t give you the knife-edge resonance of a 24 dB ladder or the snarl of a state-variable pushed to the edge, but that’s not its job. Instead, it excels at musical sweeps, gentle tone shaping, and parallel filtering where you want clarity without harshness. Resonance is voltage-controllable in name only—there’s no CV input for it, only manual control. That’s a deliberate limitation, not an oversight. The 604 is built for hands-on tweaking and immediate response, not deep modulation architectures. If you’re looking for a filter that sings when you modulate resonance with an envelope follower, this isn’t it. But if you want something that lets you set and forget resonance while dynamically sweeping cutoff with precision, it’s ideal. And because both audio and CV inputs are normalled from left to right, you can treat it as a single stereo filter with minimal patching—just plug into the left side and flip the mode switch.

Specifications

Key Features

Two Filters, One Footprint

Finding a dual analog multimode filter under 8 HP is rare; doing it in 6 HP is borderline miraculous. The ADDAC 604 doesn’t cut corners to get there—it duplicates the entire signal path, giving you two independent filter cores with full per-channel control. This isn’t a stacked design where one filter shadows the other; both channels are equally capable. That makes it perfect for setups where space is at a premium—travel racks, skiffs, or systems where you want parallel processing without dedicating two separate modules. You could run a drone patch with one filter on lowpass and the other on bandpass, or split a stereo oscillator pair and apply different resonance settings for width. The dual-mono capability turns it into a tone-shaping powerhouse, letting you treat it like a mini filter bank.

Stereo Mode with Smart Linking

The stereo mode is where the 604’s design philosophy really shines. Instead of forcing you to patch two CV sources or use a mult, it lets you control both cutoff frequencies from the left channel’s knob and CV input. Flip the switch, and the right channel’s cutoff is slaved—no additional patching needed. This is ideal for live performance, where you want both sides of a stereo signal to move in sync without worrying about calibration drift or CV mismatches. But ADDAC didn’t stop there. The internal jumper that restores CV control to the right channel (while still tracking the left’s base frequency) opens up creative possibilities: imagine a stereo pad where the right side cuts off slightly higher, creating a subtle widening effect as the filter sweeps. It’s a small tweak that adds significant depth to what could have been a simple on/off feature.

Normalization Done Right

Too many dual modules treat normalization as an afterthought, but the 604 integrates it into its workflow. With both audio and CV inputs normalled from left to right, you can go from zero to stereo filtering with a single cable. That’s not just convenient—it encourages experimentation. You don’t need to commit to a dual-mono patch before hearing how the filter behaves; just plug in, switch to stereo, and go. And because plugging into the right-side inputs breaks the normal, there’s no risk of signal doubling or feedback loops. It’s a simple design choice that makes the module feel more intuitive, especially for newcomers to modular synthesis who might otherwise get tangled in patch cables.

Historical Context

The ADDAC 604 arrived in 2020, a time when Eurorack was deep into the era of specialization—modules weren’t just about adding functions, but about doing one thing exceptionally well in minimal space. While other manufacturers chased complexity with multimode filters that included FM inputs, wavefolding, or digital control, ADDAC took the opposite approach: refine a proven circuit, duplicate it, and optimize for usability. The 604 wasn’t trying to be the most feature-rich filter on the market. It was trying to be the most practical. In that sense, it fits alongside modules like the Mutable Instruments Ripples or the Intellijel Polaris—compact, focused tools that prioritize musicality over gimmicks.

It also reflects ADDAC’s broader design ethos: thoughtful, utilitarian, and rooted in real-world use. Unlike brands that chase viral appeal with flashy interfaces or extreme sound design capabilities, ADDAC builds for the working musician—someone who values reliability, clarity, and efficient use of space. The 604’s origins in the ADDAC105 4 Voice Cluster underscore this: it wasn’t conceived in isolation, but evolved from a functional need within a larger instrument. That lineage gives it a grounded character, free from the “modular as laboratory” excess that can make some filters feel more like experiments than instruments.

Collectibility & Value

As of 2026, the ADDAC 604 remains in production and readily available, which keeps prices stable and prevents it from becoming a collector’s item—at least for now. New units sell for around $235–$260, with DIY kits available for roughly $135. Used prices hover between $180 and $210, depending on condition and whether the seller includes the original packaging or documentation. It’s not a module that appreciates, but it also doesn’t depreciate quickly—its utility ensures consistent demand.

Failures are rare, but there are a few things to watch for. The module uses standard 3.5mm jacks and Alps potentiometers, both of which are durable but not immune to wear. Check for scratchy pots, especially on the resonance controls, and ensure the mode switch clicks cleanly between stereo and dual mono. Because it’s a fully analog design with no digital components, there’s little risk of firmware issues or processor failure. However, if buying a used DIY-assembled unit, verify that it was built correctly—misplaced resistors or cold solder joints can cause channel imbalance or noise. The manual and assembly guide are freely available online, so owners can troubleshoot or repair with relative ease.

For buyers, the biggest trade-off is the lack of resonance CV. If your patches rely on dynamically modulating resonance—say, with an envelope or sequencer—this module will feel limiting. In that case, pairing it with a dedicated VCA for resonance control (patched into a feedback loop, if desired) can help, but it’s not a native solution. Also, the 12 dB/oct slope won’t satisfy those seeking aggressive filtering or pronounced filter sweeps. But if you value compactness, dual-channel flexibility, and a warm, musical tone, the 604 delivers without compromise. It’s not the flashiest filter in the rack, but it’s often the one you reach for when you need something to just work.

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