Filters

HPF

A pure high-pass filter. One knob, audio in, CV in, audio out. Cuts the lows.

What is a HPF?

HPF is a single-mode high-pass filter: one knob, two inputs, one output. Frequencies below the cutoff knob are attenuated; frequencies above pass through. There is no resonance, no drive, no slope switch - the deliberate Basic-series reduction so beginners can hear the "cut lows, keep highs" effect without distractions. Sibling of the full Filter module, which offers simultaneous LP/HP/BP/Notch outputs plus resonance, drive, and CV attenuverters; reach for that one once you have the high-pass reflex internalized.

In a patch

Patch any audio source (saw, square, noise, sample) into IN. Sweep CUT from 20 Hz upward to hear lows progressively disappear; the timbre thins and brightens. Patch an LFO into FREQ for moving filter sweeps (at full ±5V the LFO covers ten octaves of cutoff - patch through an Attenuverter for a shallower musical range). Patch an envelope (Basic AD) into FREQ for cutoff that opens and closes per note. Layer HPF after the full Filter's LP output to carve a band-pass response with two independent corners. Use HPF on a kick or bass to remove sub-rumble before a delay or reverb so the tail does not muddy the mix.

Inputs

IN (audio) — Audio input. Any audio signal: a VCO, noise, an existing patch chain, an Audio-In live source. Nominal level ±5V (Eurorack audio). The HPF responds to the full audible range; signals well above the cutoff pass through nearly unchanged, signals well below are progressively attenuated at 12 dB per octave.
FREQ (cv) — Cutoff CV input, 1V/octave (each volt doubles the corner frequency). Adds to the CUT knob value: at +1V the corner is one octave higher than the knob position; at -1V it is one octave lower. Patch an LFO for sweeps, an envelope for per-note motion, or a sequencer CV for stepped cutoff. At ±5V LFO amplitude expect a 10-octave sweep around the knob - that is canonical 1V/Oct response, not a bug. Pass through an Attenuverter to scale down for subtler modulation.

Outputs

HP (audio) — High-pass output, ±5V audio. The same input minus its low-frequency content. With CUT at default 800 Hz a typical bass note is mostly silence; a hi-hat passes through nearly unchanged. Patch directly into Output, into a VCA for envelope-shaped volume, or into Reverb / Delay for ambient effects that do not muddy the low end.

Controls

CUT — Cutoff frequency. Range 20 Hz to 20 kHz, exponential scale. Below this corner, content is attenuated 12 dB/octave; above, it passes through. Sweet spots: 80-200 Hz removes sub-rumble (DC offset, room hum) without changing the perceived timbre much; 300-600 Hz thins out kick/bass body to reveal mids; 1.5-3 kHz creates "telephone vocal" or radio thinness; 5-10 kHz isolates the airy harmonics ("mosquito" character). Default 800 Hz is intentionally aggressive for the Modular Basics course - the lows of typical material are audibly cut at the knob's default position so the effect is obvious.

Inspired by

The HP-only sibling of the full Filter. Same TPT state-variable math (Andrew Simper) but with only the high-pass tap exposed and no resonance or drive - the simplest reduction of "cut the lows, hear the upper harmonics" for the Modular Basics course.

Doepfer A-121 (HP tap)
Oberheim SEM (state-variable topology)

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