ADDAC System ADDAC601 VC Fixed Filterbank (20??–?)

Eight stiletto-sharp frequency bands, each with its own voice, its own envelope, and its own mind — this is spectral surgery with voltage-controlled precision.

Overview

The ADDAC601 VC Fixed Filterbank isn’t just a filter — it’s a full-body audio dissection table. You don’t shape sound so much as pull it apart and reanimate it, band by band, with surgical control. Built as a Eurorack module by Lisbon-based ADDAC System, it’s an analog fixed filterbank that gives you eight discrete frequency slices, each of which can be individually processed, gated, or tracked via CV. At its core, it’s a spectral microscope: you feed in a signal, and suddenly you’re staring at its beating heart across eight distinct physiological bands — from the sub-bass thump at 50 Hz all the way up to the crisp snap of 11 kHz.

It was priced at $499.00 when new, likely as a standalone module for modular synth enthusiasts who wanted more than just a sweeping low-pass — they wanted architecture. This isn’t a subtle coloration tool; it’s for making sounds that move, breathe, and stutter in rhythmic lockstep. Think vocoder-like articulation without the need for a carrier, or rhythmic gating that follows the natural dynamics of your source. The fact that it also doubles as an eight-channel envelope follower is no afterthought — it’s baked into the design, with each band outputting its own 0–10v CV signal based on amplitude. That’s eight independent envelope followers, ready to modulate other modules, lighting, or even motorized coffee grinders if you’re feeling theatrical.

What makes the ADDAC601 stand out in a sea of filter modules is its granularity. Most fixed filterbanks offer passive band splitting or basic level control. The ADDAC601 hands you a linear high-performance VCA for each band, each with its own attenuator knob and on/off switch. That means you’re not just analyzing the spectrum — you’re recomposing it in real time, live, with voltage. You can mute the even bands and sequence the odd ones. You can feed a drum loop in and use the envelope outs to trigger a bank of oscillators. You can blend two inputs — one at synth level, one at line level — and process them together, then route the wet/dry mix to a reverb, or send only the high-mid bands (2.2kHz and 5.2kHz) to a distortion stage.

It’s dense. It’s complex. And it rewards the tinkerer.

Specifications

Manufacturer	ADDAC System
Format	Eurorack
Width	20 hp
Depth	5.5 cm
Number of fixed frequencies/bands	8
Band distribution (frequencies)	50 Hz / 110Hz / 220Hz / 500Hz / 1.1kHz / 2.2kHz / 5.2kHz / 11 kHz
Control voltage inputs range	±5v
Control voltage outputs range	0 - 10v
Electrical compatibility	compatible with ±12v and ±15v power supplies
Max current	340 mA +12V, 250 mA -12V, 0 mA 5V
Bus board cable	8 × 2 idc (doepfer style) connector
Features 2 audio inputs	yes
Audio input 1 includes an attenuator knob	yes
Has a wet/dry mix knob	yes
Main outputs	1 wet output, 1 wet/dry output, 1 odd frequencies, 1 even frequencies

Key Features

Spectral Slicing with Full Voltage Control

Each of the eight bands isn’t just a filtered output — it’s a full processing node. The ADDAC601 assigns a linear high-performance VCA to every frequency band, with its own attenuator knob and on/off switch. That means you can manually set the level of each band, then dynamically modulate it via CV. The control voltage input range is ±5v, making it compatible with standard Eurorack modulation sources. Whether you're fading in the 110Hz band with an LFO or triggering the 5.2kHz band with a sequencer, you’re not just filtering — you’re orchestrating.

Eight Independent Envelope Followers

This is where the module transcends typical filterbank behavior. Each band includes an envelope follower with a dedicated CV output ranging from 0 to 10v. That’s eight separate envelope signals derived from your input audio, each corresponding to a specific frequency. You can use these to modulate filters, VCAs, panners, or anything else with a CV input. Feed in a vocal, and suddenly you’ve got eight control voltages that move with the syllables. Route them to oscillators, and you’ve built a partial vocoder or a dynamic harmonic generator. The envelope follower functionality isn’t an add-on — it’s integrated into every band’s signal path, making the ADDAC601 as much a modulation source as a processor.

Flexible Input and Output Routing

The module features two audio inputs: one designed for synth-level signals, and another for line-level, with an attenuator on input 1 to prevent clipping. These signals are mixed internally, so you can layer sources before spectral analysis. Outputs are equally versatile: you get a wet output (fully processed), a wet/dry mix output (blendable via front-panel knob), plus dedicated outputs for odd and even frequency bands. Need to isolate only the low-mid energy (220Hz and 500Hz)? Patch into the individual band outputs. Want to create a stereo effect by panning odd bands left and even right? The odd/even outputs make that trivial. Each band also has its own post-VCA audio output, so you can route individual slices to external effects or mixers.

Visual Feedback and Signal Monitoring

Every band includes a monitor LED that responds to signal level, giving you real-time visual feedback on which frequencies are active. This isn’t just cosmetic — when you’re working with complex, evolving sources like field recordings or full mixes, the LEDs help you see the spectral distribution at a glance. Combined with the on/off switches, you can mute bands experimentally and watch the LEDs to see how it affects the overall envelope output. It’s a rare module that lets you both hear and see the frequency domain simultaneously.

Collectibility & Value

The ADDAC601 is not a common module, but it’s not unobtainable. Current listings show a range of prices that suggest demand is steady but niche. Used units have appeared at €250.00 (listed on ModularGrid Marketplace in January 2026, including original box), while Reverb listings show prices between $426.64 and $602.67. The original retail price was $499.00, meaning some sellers are asking above MSRP, possibly due to scarcity or demand within experimental modular circles. However, no information is available about common failures, repair complexity, or long-term reliability. Given its active VCAs and envelope follower circuitry, it’s likely more complex than passive filterbanks, but no known failure points have been documented. Collectors should verify functionality, especially the envelope follower outputs and VCA response, before purchase.