Frap Tools CUNSA: Quadruple Filter Alchemy in Eurorack

Frap Tools introduce CUNSA as their so-called “sound seasoning tool,” but that’s underselling it. At its core, CUNSA is a quadruple mono—or double stereo—pingable multimode resonant filter, but it also moonlights as a saturator, mixer, and even an oscillator. The design isn’t just about stacking four filters; each filter can be coaxed into wildly different sonic roles, making the module a veritable Swiss Army knife for the modular enthusiast.

Each of CUNSA’s four analog filters offers lowpass, bandpass, and highpass outputs, with the lowpass filter switchable between 12 dB and 24 dB per octave slopes. Combined outputs and semi-normalled inputs hint at a module that wants to be patched in creative, interconnected ways. This opening segment sets the stage for a deep dive into how a single filter behaves before the video explores the more complex multi-filter territory.

The video quickly moves to the bread and butter of filter operation: cutoff frequency, resonance (Q), and the intriguingly named Character parameter. Cutoff and input VCA are straightforward, but the Q knob—akin to resonance or emphasis—lets you dial in anything from subtle contouring to pronounced peaks at the cutoff frequency. Notably, CUNSA’s lowpass filter retains its bass presence even at high Q, sidestepping the dreaded thinness that plagues some designs. Adjusting input level further shapes the tone, from clear and open to tightly focused.

The Character knob is where things get spicy. This control determines how the filter circuit behaves when pushed—at its lowest setting, you get a soft-clipping safety circuit modeled after Frap Tools’ CGM mixer, while cranking it up removes these protections, inviting more aggressive saturation. The interplay between Q and Character yields a spectrum of filter behaviors, from clean resonance to wild, distorted screams. Oscilloscope views reveal how Character warps waveforms, especially when the filter is overloaded, and how these changes differ between 12 dB and 24 dB slopes.

CUNSA isn’t content to just filter—its high Q settings let each filter self-oscillate, turning them into sine wave oscillators that track volt per octave quite respectably. The tutorial showcases how different filter outputs (lowpass, bandpass, highpass) yield distinct oscillator timbres, and how the fine-tune knob allows precise pitch control. There’s even a bonus tip: pairing CUNSA with Frap Tools’ Brenso enables true zero FM patches, effectively creating a pair of complex oscillators from what started as a filter module.

The video also explores CUNSA’s ping circuit, a custom-designed, nonlinear integrator that generates organic, falling envelopes from triggers or CV sources. This pinging is DC-coupled and sensitive to amplitude and duration, making it more versatile than typical filter pings. The combo mode shines here, linking cutoff and VCA for natural, decaying envelopes—ideal for percussive or plucked sounds. The distinction between filter pinging and filter ringing is clarified, underscoring CUNSA’s flexibility in both classic and experimental patching.

The tutorial wraps up by spotlighting CUNSA’s routing options and semi-normalled connections, which are central to its creative workflow. Any signal patched into an input is automatically routed to all filters to its right, making parallel processing and stereo setups a breeze. The mixed outputs allow for various combinations—filters 1+2, 3+4, or all four—while routing switches let users select which filter mode is sent to the mix.

This flexibility means CUNSA can serve as four independent filters, two stereo pairs, or a single, complex filter bank. The dry output sums pre-filter VCA signals for parallel processing, and its behavior is shaped by combo mode and modulation sources. The video closes with patch examples that combine filtering, pinging, and mixing, demonstrating how CUNSA integrates into larger modular systems and rewards adventurous patching.