Erica Synths Resonant Filterbank

Erica’s take on the filterbank is highly programmable and characteristically boisterous.

The prolific Erica Synths strike again. This time they’re tackling the hallowed grounds of the filterbank, a product that is designed to grab your sound by the scruff, rough it up a bit, and send it home with its tail between its legs. Excuse my mixed metaphors.

This desktop unit is an evolution of two earlier Eurorack modules, the Graphic Resonant Filterbank and its Expander, both of which are still available to purchase at the time of this review. Essentially, this is an analogue, stereo, 10‑band, fixed‑frequency band‑pass filterbank that can be configured in several ways, modulated using LFOs and envelope followers, controlled via MIDI CC, and even used as a standalone drone instrument thanks to the self‑oscillating filter design.

At 258 x 176 x 70mm, the Resonant Filterbank is a medium‑sized desktop unit with a solid metal construction. Ten centre‑indented sliders handle the individual filters’ boost/cut amounts during normal operation (in some modes, they adopt other functions). Along the top of the unit is an OLED display, six fixed‑purpose knobs (for input, output, resonance, spread, dry/wet and a data encoder), plus various buttons for mode switching and navigation.

Filterbank Mode

The 10 stereo band‑pass filters operate at fixed frequencies, ranging from 29Hz to 11kHz. The machine can run in one of four modes. The first is Filterbank mode. Here the 10 sliders control the boost and cut of each stereo band‑pass filter with stereo offset courtesy of the Spread control. If you prefer, you can switch the Spread function via the global settings to let you dial in completely independent filter settings for each stereo channel.

The filter resonance is the bad boy here. You can turn resonance on or off per band, and the main resonance knob will increase feedback for any band that is opted in. The resonance control is bipolar, so at 12 o’clock there is no feedback; clockwise increases feedback, and anti‑clockwise applies polarity‑inverted feedback. In addition to individual band feedback (just to be clear, feedback and resonance are the same thing here), there is a main feedback loop that takes the combined signal from all feedback‑enabled channels and feeds it back into the main input. The combination of these feedback types (and both can coexist happily) produces a wide variety of tones, from subtle overdrive to howling distortion.

The 10 sliders offer plenty of hands‑on control, but it’s when you get into the programming that the Resonant Filterbank really comes to life.

Filter Mode

The second operating mode is Filter. In this mode, the 10 band‑pass filters can work in unison to emulate multi‑mode filter behaviour (LP/HP/BP/notch). It does this by automating the 10 bands. For example, in low‑pass mode, it will reduce the band‑pass filter gain for the top band, then the next band down, and so on, as you close the filter. In this mode, the sliders take on different roles: cutoff, slope, filter type, etc.

The feedback system works here too, causing all sorts of sonic mayhem. It rarely sounds like traditional multi‑mode filter resonance, and is much more of a distortion tool. As such, Filter mode is not particularly convincing as a classic filter. You cannot get...