ADDAC System ADDAC603 (2019–)

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

Overview

The ADDAC603 is built around three independent voltage-controlled bandpass filters. You get control over frequency, bandwidth, and resonance. It is not a polite equalizer or a surgical tone shaper, and it is designed for the moments when you want your modular rig to stop behaving. Each channel has a switchable gain stage, either ×3 or ×100, feeding directly into the filter input. You can warm a signal up gently or obliterate it before it reaches the filter core. The distortion is analog, not digital grit, and it adds harmonics.

Then there's the feedback. The module has actual feedback control, and it is voltage controllable. Each filter can self-oscillate, scream, or sit on the edge of instability. The feedback is independent from bandwidth, so you set how hard the filter feeds back into itself separately, and that opens up chaotic modulation when driven at audio rates. Each output has a phase switch, and built-in attenuverters invert or attenuate the CVs. The three filters can be fed from a single source thanks to normalled inputs, or split across three different signals for stereo fields, moving spectral gaps, or feedback loops across your system. There is also a summed output with a dry/wet control, so you can blend the original signal back in for subtle shaping or go for full spectral mutation.

The ADDAC603 sits in ADDAC's 600 series, which is dedicated to analog sound processors. It is not the flagship for raw polyphony or modulation density. Below it in complexity are simpler processors like the ADDAC601 Dual VCA and the ADDAC602 Dual Mixer. Above it you'd find modules like the ADDAC611 Gotharman's VC Tube Filter, which trades multi-band control for tube-driven warmth. The 603 is the one to reach for when a patch feels too predictable or too clean. Its analog core is built using CoolAudio's modern reissue of the SSM2164 chipset.

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 as lead instruments. Each of the three filters spans the full audio range, confirmed by users and verified in early communications with ADDAC. You can isolate sub-bass thumps or slice through at 10kHz. A 6dB/12dB slope switch on each filter lets you pick between a gentle sweep and a sharper cut. Attenuverters sit on both frequency and bandwidth, so you can modulate where the filter sits and how wide it opens. An LFO can widen the bandwidth while a sequencer steps the center frequency. Because the filters are independent, you can stack them for a triple-resonance sweep or spread them across different sources.

Distortion as a Design Principle

The ×100 gain switch is not a gimmick. Push a clean oscillator through that input gain and the filter stage starts clipping hard, producing asymmetric distortion that feeds directly into the resonance path. This is pre-filter abuse, not post-filter saturation, and it changes how the whole module behaves. At ×3, you get a subtle warming and 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 with the filter's resonance and feedback, creating intermodulation effects that change with modulation. Most distortion comes after filtering, so this is a rare approach. The gain stage is analog, and the clipping stays soft and organic even at extreme settings.

Feedback That Fights Back

Resonance is common. Feedback is rare. The ADDAC603 gives you both, and they are decoupled. You can crank the feedback control to make a filter self-oscillate, while the bandwidth stays under its own control. That means you shape how the feedback behaves, not only how much resonance there is. Audio-rate modulation on the feedback input gives FM-like effects. Patching the output back into the input gives unstable, chaotic oscillations. Modulating feedback with an envelope makes a filter "bark" at transients. The phase switch on each output flips polarity, which can create phase cancellations when mixing multiple filters, useful for hollow, metallic tones or comb-filter effects. Because the feedback is voltage controllable, a simple CV ramp can turn a stable drone into a howling vortex.

Historical Context

The ADDAC603 arrived in 2019, when Eurorack modules were getting more digital and more complex. ADDAC is a Portuguese company known for blending analog warmth with modern utility. 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 this thoroughly. It came from a lineage of experimental filter design, where bandpass filters were used for tone shaping and also as sound generators in their own right. Competitors at the time offered multimode filters or resonant low-pass designs, and few treated the bandpass as a primary voice. ADDAC did, and the module 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. The chip was originally used in classic studio compressors and filters, and it is known for a smooth, musical sound. By building the filters and VCAs around it, ADDAC kept the 603 from sounding harsh or brittle, even when pushed to extremes. That was a deliberate contrast to the cold precision of digital filters and the brittle clipping of some analog designs. It also fits a broader trend of Eurorack builders revisiting vintage topologies, not to clone them but to recontextualize them. The 603 is not a recreation of a 1970s filter. It is a modern interpretation with modern controls.

Collectibility & Value

The ADDAC603 has never been a budget module. At €375 new it sat in the upper mid-tier of Eurorack pricing, but it has held its value well 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. These can command a slight premium, though not enough to justify the upgrade unless you're matching a specific build. The module has an aluminum panel, sturdy jacks, and a compact 35mm depth that fits in most skiffs. There are a few quirks to watch for. The ×100 gain stage can overload sensitive inputs if not patched carefully, and some users report that the feedback controls can get noisy over time if the pots aren't cleaned. There are no known widespread failure points and no common capacitor issues, and the CoolAudio SSM2164 chips have proven reliable. Power draw is substantial at 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 check for crackling or dropouts. The module doesn't require calibration out of the box. 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 is increasingly seen as a modern classic among bandpass enthusiasts. It isn't rare, since ADDAC didn't limit production, but it isn't common either, and its specific sound means it's not easily replaced. For traditional synth players it might feel excessive.