ADDAC 604 Dual Filter (2020–)

Two channels of raw, opinionated analog filtering in just 6hp. This little Eurorack module doesn't clean up your sound, it fights with it.

Overview

Most filters sit quietly and do what they're told. The ADDAC 604 doesn't. It gets in the mix, especially when you crank the resonance. It's not transparent and it's not polite. It's a dual analog multimode filter with teeth, and it has been turning up in compact Eurorack builds since it quietly appeared in December 2020.

The 604 came out of the development of the ADDAC105 4 Voice Cluster module. It was originally meant to be an internal tool, a filter to follow the summing stage and shape the cluster's dense harmonics. The designers at ADDAC (Elevator Sound) liked what they heard so much that they released it as a standalone module. What you get is a full-featured, two-channel analog filter in 6hp. Each channel has full multimode outputs (highpass, bandpass, lowpass), CV control with attenuator, and hands-on cutoff and resonance knobs.

The 604 doesn't pretend to be a luxury item. One reviewer noted the use of "wobbly trimmer pots" instead of sturdier knobs. That keeps the price in check, but it might give some builders pause. The same reviewer praised its sound, calling it "great (stereo) sound in a small package." It trades polish for presence.

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 the cutoff frequency of both channels, and the Right Cutoff knob and its CV input are bypassed. That keeps your hands on one side while processing a stereo signal, which suits master filtering or live tweaking. There is also an optional jumper on the back of the module. When installed, it reconnects the Right Cutoff CV input, so you can offset the right channel's cutoff independently, even in Stereo Mode. That is handy for 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. That works for quick stereo setups, or for using it as a single-channel filter with passive duplication. Patch into the right side and the normaling breaks. Resonance is always independent per channel, no matter the mode. So you can have a screaming, resonant lowpass on the left while the right stays clean and dull, or crank both. Reviewers have noted the resonance is "very brutal." It doesn't just peak, it bites.

A Filter With Character

This isn't a neutral filter. One user said 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. The highpass is described as "true" and "tight," which suits sculpting drum signals or adding bite to percussion. Another reviewer called it "great for grit in the signal," especially with the well-controllable CV response on the cutoff, 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. ADDAC decided to release it as a standalone module instead of keeping it internal. It's part of the ADDAC600 Series, a family of compact, function-focused Eurorack modules. No direct predecessors or competitors are cited in the documentation. Its release in late 2020 placed it among 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. It's a current-production module rather than a vintage item. 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.