ADDAC System ADDAC603 (2019–)

Three bandpass filters with distortion, feedback, and summing that turn any signal into a writhing, resonant beast.

Overview

It starts with a hum—low, resonant, almost metallic—that builds until it’s not a tone anymore but a texture, like sand shifting under tension. That’s the ADDAC603 in action: not just filtering sound, but transforming it into something alive, unpredictable, and often unhinged. This isn’t a polite equalizer or a surgical tone shaper. It’s a sonic alchemist, designed for those moments when you want your modular rig to stop behaving and start snarling. Built around three independent voltage-controlled bandpass filters, the ADDAC603 gives you surgical control over frequency, bandwidth, and resonance, but where it really diverges from the clinical norm is in its attitude. Each channel includes a switchable gain stage—either ×3 or ×100—feeding directly into the filter input, which means you can gently warm up a signal or absolutely obliterate it before it even hits the filter core. That distortion isn’t digital grit; it’s analog aggression, the kind that adds harmonics like splinters under the skin.

And then there’s the feedback. Not just resonance, but actual feedback control—voltage controllable, no less—allowing each filter to self-oscillate, scream, or hover on the edge of instability. Because the feedback is independent from bandwidth, you’re not just tweaking resonance; you’re dialing in how aggressively the filter feeds back into itself, opening up chaotic modulation possibilities when driven at audio rates. Pair that with the phase switch on each output and the ability to invert or attenuate CVs via built-in attenuverters, and you’ve got a module that doesn’t just process sound—it argues with it. The three filters can be fed from a single source (thanks to normalled inputs) or split across three different signals, letting you carve stereo fields, create moving spectral gaps, or build complex feedback loops across your system. There’s even a summed output with a dry/wet control, so you can blend the original signal back in for subtle tonal shaping or go full spectral mutation.

Positioned in ADDAC’s 600 series—dedicated to analog sound processors—the ADDAC603 sits at the intersection of utility and mayhem. It’s not the flagship in terms of raw polyphony or modulation density, but in terms of character, it’s unmatched in their lineup. Below it in complexity are simpler processors like the ADDAC601 Dual VCA or ADDAC602 Dual Mixer, while above it, you’d find modules like the ADDAC611 Gotharman’s VC Tube Filter, which trades surgical multi-band control for tube-driven warmth. The 603, though, is the Swiss Army knife of sonic destruction: precise when you need it, wild when you don’t. It’s the module you reach for when a patch feels too predictable, too clean, too *quiet*. And given how many Eurorack systems lean digital or quantized these days, the 603’s analog heart—built using CoolAudio’s modern reissue of the SSM2164 chipset—feels like a deliberate act of rebellion.

Specifications

Manufacturer	ADDAC System
Model	ADDAC603
Production Years	2019–
Original Price	€375
Format	Eurorack
HP	22
Depth	3.5 cm (35 mm)
Power Consumption +12V	250mA
Power Consumption -12V	250mA
Number of Filters	3 independent bandpass filters
Filter Type	Bandpass (6dB/oct or 12dB/oct selectable)
Frequency Control	Voltage-controlled with attenuverter
Bandwidth Control	Voltage-controlled with attenuverter
Feedback Control	Voltage-controlled resonance per filter
Input Gain	Switchable ×3 or ×100 per channel
Output Amplifier	Voltage-controlled per channel with attenuverter
Phase Switch	Per channel output
Mix Output	Summed output with dry/wet control and attenuverter
Inputs	Normalled: 1→2→3
Filter Core	SSM2164-based (CoolAudio)

Key Features

Three Bandpass Filters with Attitude

Most bandpass filters in Eurorack are afterthoughts—tacked onto multimode modules or used for narrow resonant peaks. The ADDAC603 treats them like lead instruments. Each of the three filters spans the full audio range, confirmed by users and verified in early communications with ADDAC, which means you’re not just shaping mids—you can isolate sub-bass thumps or slice through at 10kHz with surgical precision. The 6dB/12dB slope switch per filter lets you choose between a gentle sweep and a razor-edged cut, but the real magic lies in the voltage control. With attenuverters on both frequency and bandwidth, you can modulate not just where the filter sits, but how wide it opens—imagine an LFO widening the bandwidth while a sequencer steps the center frequency, creating a rhythmic, breathing effect. Because the filters are independent, you can stack them for a triple-resonance sweep or spread them across different sources to build a dynamic, evolving soundscape.

Distortion as a Design Principle

The ×100 gain switch isn’t a gimmick—it’s a declaration of intent. Push a clean oscillator through that input gain and the filter stage starts clipping hard, generating rich, asymmetric distortion that feeds directly into the resonance path. This isn’t post-filter saturation; it’s pre-filter abuse, and it changes the character of the entire module. At ×3, you get a subtle warming, a bit of analog grit. At ×100, the signal clips aggressively, turning sine waves into square-ish pulses and noise into sizzling static. Because the gain is applied before the filter, the distortion interacts dynamically with the filter’s resonance and feedback, creating intermodulation effects that evolve with modulation. It’s a rare approach—most distortion comes after filtering—but here, it’s baked into the signal path like seasoning in a stew. And because the gain stage is analog, not digital, the clipping has a soft, organic character, even at extreme settings.

Feedback That Fights Back

Resonance is common. Feedback is rare. The ADDAC603 gives you both, and crucially, they’re decoupled. You can crank the feedback control to make a filter self-oscillate, but the bandwidth remains under independent control, so you’re not just boosting resonance—you’re shaping how the feedback behaves. This opens up wild possibilities: use audio-rate modulation on the feedback input to create FM-like effects, patch the output back into the input for unstable, chaotic oscillations, or modulate feedback with an envelope to make a filter “bark” at transients. The phase switch on each output adds another layer—flipping the polarity can create phase cancellations when mixing multiple filters, useful for creating hollow, metallic tones or comb-filter effects. And because the feedback is voltage controllable, you can automate how unhinged the filter gets over time, turning a stable drone into a howling vortex with a simple CV ramp.

Historical Context

The ADDAC603 arrived in 2019, a time when Eurorack was deep into its golden age of expansion—modules were getting more digital, more complex, and often, more sterile. ADDAC, a Portuguese company known for blending analog warmth with modern utility, responded with a series of modules that celebrated imperfection. The 603 wasn’t the first multi-band filter—precedents include the EMW Triple Bandpass VCF and various DIY designs—but it was one of the first to integrate distortion, feedback, and voltage control so thoroughly. It came from a lineage of experimental filter design, where bandpass filters were used not just for tone shaping but as sound generators in their own right. In that context, the 603 feels like a love letter to the underappreciated bandpass filter, elevating it from utility to centerpiece. Competitors at the time offered multimode filters or resonant low-pass designs, but few treated the bandpass as a primary voice. ADDAC did—and in doing so, created a module that appealed to drone artists, noise performers, and anyone tired of the same old filter sweeps.

The choice of the SSM2164 chipset—reissued by CoolAudio—was also significant. That chip, originally used in classic studio compressors and filters, is known for its smooth, musical character. By building the filters and VCAs around it, ADDAC ensured the 603 wouldn’t sound harsh or brittle, even when pushed to extremes. This was a deliberate contrast to the cold precision of digital filters or the brittle clipping of some analog designs. The 603 was meant to feel *alive*, and the SSM2164 helped deliver that. It also placed the module in a broader trend of Eurorack builders revisiting vintage topologies—not to clone them, but to recontextualize them. The 603 isn’t a recreation of a 1970s filter; it’s a modern interpretation with modern controls, built for a modular world that values both precision and unpredictability.

Collectibility & Value

The ADDAC603 has never been a budget module—€375 new put it in the upper mid-tier of Eurorack pricing—but it’s held its value well, especially given its niche appeal. On the used market, expect to pay between €250 and €320 depending on condition and whether it includes any custom panel options. ADDAC offered custom-colored front panels (red, green, blue, white, gray, yellow, gold) for an additional fee, and these can command a slight premium, though not enough to justify the upgrade unless you’re matching a specific build aesthetic. The module is built to last—aluminum panel, robust jacks, and a compact 35mm depth that fits in most skiffs—but there are a few ownership quirks to watch for. The ×100 gain stage, while glorious, can overload sensitive inputs if not patched carefully, and some users report that the feedback controls can become noisy over time if the pots aren’t cleaned. There are no known widespread failure points, no common capacitor issues, and the CoolAudio SSM2164 chips have proven reliable. Power draw is substantial—500mA total—but not excessive for a 22HP module with three VC filters and multiple VCAs.

Buying used? Check that all three filters respond evenly across the frequency range, that the gain switches engage cleanly, and that the attenuverters respond to both positive and negative CVs without dropout. Patch in a steady oscillator and sweep the feedback to ensure no crackling or dropouts. The module doesn’t require calibration out of the box, but if it sounds thin or lacks resonance, it may need a trim pot adjustment—documentation shows a single internal trim for overall filter response, accessible via the PCB. For collectors, the 603 isn’t a “holy grail” like a vintage Roland filter, but it’s increasingly seen as a modern classic among bandpass enthusiasts. It’s not rare—ADDAC didn’t limit production—but it’s not common either, and its specific character means it’s not easily replaced. If you’re building a drone, noise, or experimental rig, it’s worth the hunt. For more traditional synth players, it might feel excessive. But for those who want their filters to *do* something, not just sit there, the 603 remains a compelling, slightly dangerous choice.