ADDAC System ADDAC601 VC Fixed Filterbank (20??–?)
Eight sharp frequency bands, each with its own VCA and its own envelope follower, all under voltage control.
Overview
The ADDAC601 VC Fixed Filterbank is a Eurorack module from Lisbon-based ADDAC System. It's an analog fixed filterbank with eight discrete frequency bands, and each band can be processed, gated, or tracked via CV. You feed in a signal and get eight slices of it, running from sub-bass at 50 Hz up to 11 kHz.
It was priced at $499.00 when new, likely as a standalone module for modular players who wanted more than a sweeping low-pass. It isn't a subtle coloration tool. It's for sounds that move and stutter in rhythm. Think vocoder-like articulation without the need for a carrier, or rhythmic gating that follows the natural dynamics of your source. It also doubles as an eight-channel envelope follower, built into the design, with each band outputting its own 0-10v CV signal based on amplitude. That's eight independent envelope followers to modulate other modules, lighting, or even motorized coffee grinders.
Most fixed filterbanks offer passive band splitting or basic level control. The ADDAC601 gives you a linear high-performance VCA for each band, each with its own attenuator knob and on/off switch. So you can recompose the spectrum in real time, 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 and complex, and it rewards tinkering.
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 gets a linear high-performance VCA, with its own attenuator knob and on/off switch. You set the level of each band by hand, then modulate it via CV. The control voltage input range is ±5v, which suits standard Eurorack modulation sources. You can fade in the 110Hz band with an LFO or trigger the 5.2kHz band with a sequencer.
Eight Independent Envelope Followers
Each band also 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 tied to a specific frequency. Use them to modulate filters, VCAs, panners, or anything else with a CV input. Feed in a vocal and you get 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 is integrated into every band's signal path, so the ADDAC601 is as much a modulation source as a processor.
Flexible Input and Output Routing
The module has two audio inputs: one for synth-level signals and one for line-level, with an attenuator on input 1 to prevent clipping. The signals are mixed internally, so you can layer sources before the filterbank. Outputs include a wet output (fully processed), a wet/dry mix output (blendable via front-panel knob), and dedicated outputs for odd and even frequency bands. To isolate only the low-mid energy (220Hz and 500Hz), patch into the individual band outputs. Panning odd bands left and even right is easy with the odd/even outputs. Each band also has its own post-VCA audio output, so individual slices can go to external effects or mixers.
Visual Feedback and Signal Monitoring
Every band has a monitor LED that responds to signal level, so you can see which frequencies are active. With complex, evolving sources like field recordings or full mixes, the LEDs show the spectral distribution at a glance. Combined with the on/off switches, you can mute bands and watch the LEDs to see how it affects the overall envelope output.
Collectibility & Value
The ADDAC601 is not a common module, but it's not unobtainable. Current listings 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, so some sellers are asking above MSRP, possibly due to scarcity or demand within experimental modular circles. 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.
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