ADDAC 603 VC Triple Bandpass Filter (2019–)

Three bandpass filters that work as a clean parametric EQ or as a howling harmonic storm.

Overview

Feed the ADDAC 603 a drone or a screaming sawtooth and switch the input gains to x100, and it does a lot more than filter. It pushes the signal through three parallel bandpass filters, and each one can be precise or unhinged. Max out the feedback, crank the bandwidth, and the module stops sounding like a filter. It growls, shrieks, or blooms into vowel-like formants. One LFO modulating all three frequency controls can turn a static pad into a breathing, pulsing mass.

The 603 came out of the Eurorack boom in utility and experimental modules. It draws a direct line from the Resonator in the Korg PS-3300, a filter bank from 1977 that could mimic human vocal formants and orchestral swells. The 603 rebuilds that idea with full voltage control, aggressive gain staging, and a layout that invites patching chaos. Many triple bandpass modules aim for neutrality. This one doesn't. Each filter uses a modern reproduction of the SSM2164 VCA chip, known for its clean yet slightly colored response, so the filters add subtle saturation even before you hit the x100 gain switch.

What sets this module apart isn’t just the triple filter architecture—it’s the gain structure. The input gain per channel can be switched between x3 and x100, and that second option is where things get dangerous. Suddenly, quiet noise sources become snarling feedback loops, and simple waveforms distort into complex, harmonically rich textures before they even hit the filters. Combine that with the feedback (resonance) control on each filter, and you’ve got a system that can oscillate into self-sustained tones, though not quite self-oscillate cleanly like a traditional low-pass filter. It’s a deliberate limitation: these filters are meant to enhance and warp, not replace your oscillators. The result is a module that’s equally at home as a dynamic EQ on a mix bus or as the centerpiece of a noise performance, where audio-rate modulation and feedback patches create sounds that feel more like tectonic shifts than synth effects.

Specifications

Manufacturer	ADDAC System
Production Years	2019–
Original Price	$396 / £385
Format	Eurorack
Width	22HP
Depth	40mm
Current Draw +12V	250mA
Current Draw -12V	250mA
Number of Filters	3
Filter Type	Bandpass (6dB or 12dB slope)
Frequency Range	Approx. 10Hz – 16kHz
Bandwidth Control	Voltage controlled with attenuverter
Frequency Control	Voltage controlled with attenuverter
Input Gain	Switchable x3 or x100 per channel
Resonance	Feedback control per filter (non-self-oscillating)
Output	Individual outputs per filter, mix output, dry/mix output
Dry/Wet Control	Dedicated knob and CV input
Phase Inversion	Switchable per filter
Input Normaling	Input 1 normalled to 2, 2 normalled to 3
Output VCA	Per filter with CV input and attenuverter

Key Features

A Three-Headed Filter with Attitude

Each of the three filters is identical, with its own frequency and bandwidth controls. Both are voltage-controllable through CV inputs with dedicated attenuverters. You can sweep each band independently with an LFO, envelope, or sequencer, or lock them together for synchronized sweeps. A 6dB/12dB slope switch on each filter changes the tone. The 6dB mode is broader and almost shelving in nature. The 12dB setting gives the classic narrow, resonant peak that formant synthesis depends on. Bandwidth defines the sound most. At its tightest it makes a sharp notch that can isolate a single harmonic. At its widest it lets nearly the full spectrum through and turns the filter into a resonant boost rather than a cut. That makes the 603 useful for enhancing frequencies as well as carving them out, especially with the input gain stages in play.

Gain as an Instrument

Most filter modules assume you'll feed them line-level or Eurorack-level signals and leave distortion to dedicated effects. The 603 ignores that assumption. Each input gain switches from a modest x3 to x100, which pushes even quiet signals into clipping. This is analog overdrive built into the front end, and it interacts dynamically with the filter response. Push a noise source through all three channels with high gain and wide bandwidth and you get a roaring wall of sound that CV modulation can shape into rhythmic pulses or chaotic sweeps. The filters themselves don't distort much. The input stage does, and that distortion feeds the filters, which emphasize certain harmonics and build a feedback loop of complexity. The line between processor and sound generator gets blurry, and the 603 ends up feeling more like a synth voice than a passive filter.

Flexible Mixing and Phase Manipulation

Besides the individual filter outputs, the 603 has a summed mix output and a dry/mix output with its own level control and CV input. This lets you blend the processed signal with the original in real time, by hand or by modulation, without an external mixer. The dry/mix control suits rhythmic effects, such as a sequencer modulating the mix level to make a "wah" that pulses in and out. Each filter also has a phase inversion switch. Flipping the phase on one or two filters changes more than polarity. It alters how the bands interact when mixed, which gives comb filtering, cancellations, or unexpected resonances. Patch the mix output into a scope and you can watch the waveform change as phase relationships shift. It's a sound design tool, and a simple patch can turn unpredictable with it.

Historical Context

The triple bandpass filter isn't a new idea. It dates to the late 1970s, most famously in the Korg PS-3300, a polyphonic analog monster that used three parallel bandpass filters to make vocal-like formants and dynamic timbral shifts. The idea trickled down into modules from Doepfer and Analogue Systems in the early Eurorack days, often with minimal voltage control or gain staging. The ADDAC 603 arrives decades later, in a modular market saturated with filters, and it stands out by going to extremes. It isn't trying to be the most musical or the most accurate. It's trying to be the most expressive. While many Eurorack manufacturers were focused on digital hybrids or complex sequencers, ADDAC doubled down on analog character and hands-on control.

The 603 was released in 2019, during renewed interest in filter banks and dynamic EQs, driven by artists working in drone, noise, and ambient genres where texture matters more than pitch. Competitors like the Cwejman RES-4 offered a fourth filter and more precise tracking, but at a much higher price and with less aggressive gain options. The ADDAC 603 found its niche by being practical and wild at once. It's compact at 22HP, logically laid out, and deeply patchable, yet it can make sounds that feel uncontrolled and alive. The PS-3300's Resonator was fixed in its architecture. The 603 invites you to run signals at extreme levels, modulate everything, and treat filtering as a performance tool rather than a static setting.

Collectibility & Value

The ADDAC 603 isn't a rare bird. ADDAC has kept steady production going since 2019, and it's widely available from dealers like Thomann, Perfect Circuit, and Reverb. That keeps prices stable, with new units typically selling for around $390 - $420 and used ones in mint condition going for $300 - $350. There's little secondary market speculation. People buy this module to use, not to flip. Its value comes from versatility and build quality, not scarcity.

There are a few quirks to watch for. The module draws a hefty 250mA on both the +12V and -12V rails, which can surprise you on a smaller power supply. Some users have reported noise issues when it runs from low-current or poorly regulated supplies, so a solid power setup is recommended. The switches and pots are standard quality, nothing exotic. They're reliable, though heavy stage use might eventually lead to crackling. No widespread failure modes have been reported, and there are no known capacitor or component issues that would call for preventative servicing. It contains no microcontrollers or firmware, so there's no risk of bricking or update problems.

For buyers, reliability isn't the issue. Knowing what you're getting into is. This isn't a 1V/oct module, and the filters aren't musically tuned, so you can't play them like a keyboard. If you want a precise, trackable filter bank for melodic work, the Analogue Systems RS-360 or Cwejman RES-4 might be better choices. If you want a module that can turn a simple patch into a living, breathing sound sculpture, the 603 delivers. It's especially worth considering for drones, noise, or experimental textures, where control matters less than character. Patch boldly, and push it into the red.