Rides in the Storm - HFD

A compact high-pass filter with serious range and control

HFD is a discrete analog high-pass filter that packs a wide set of shaping and modulation options into just 4HP.

The filter offers switchable slopes at 12, 18, and 24 dB, moving from subtle low-end trimming to tight, surgical cuts. It stays musical across the range and responds well to both static settings and active modulation. When pushed, the circuit can self-oscillate, track 1V per octave, and function as a rough but usable sound source in its own right.

Cutoff modulation is a central part of the design. Four dedicated CV inputs cover different use cases and encourage layered control. You get classic 1V per octave tracking, exponential FM for expressive movement, linear FM for more precise modulation, and a dedicated envelope input. Each cutoff CV input includes its own attenuator, making it easy to balance complex modulation without extra utilities.

Resonance includes a CV input for dynamic control. A resonance boost and compensation circuit help keep the response stable across different settings, or allow the filter to push harder when driven. A built-in clipper sits after the filter and adds controlled overdrive, giving sharper edges and extra presence when needed.

Small, responsive, and easy to integrate, HFD works well as a tone shaper, a modulation target, or a compact oscillator. It pairs naturally with full voices like the SED-CSM, but stands on its own in any system that benefits from precise high-pass control.