Filters

LPF

A two-knob lowpass filter: cutoff, resonance. The simplest reduction of subtractive synthesis.

What is a LPF?

LPF is the simplest lowpass filter: two knobs, three jacks. Audio in, audio out, with a cutoff frequency that controls which highs survive and a resonance knob that adds a peak right at the cutoff for that classic synth squelch. At RES=1 the filter self-oscillates as a clean sine, doubling as an oscillator. This is the canonical "tone" reduction: every subtractive synth voice in history started here. The filter is 2-pole (12 dB/oct), gentle enough that you can still hear individual harmonics change as you sweep CUTOFF - perfect for the first filter lesson. When you outgrow it, the full Filter module adds simultaneous LP/HP/BP/Notch outputs, drive, and CV attenuverters.

In a patch

Patch a VCO into IN, the OUT into a VCA or straight into Output, then sweep CUTOFF to hear the canonical "open up" of subtractive synthesis. Patch an envelope (Basic AD or ADSR) into FREQ to get filter plucks: each note opens the filter briefly. Patch an LFO into FREQ for slow auto-wah motion. Crank RES to maximum (1.0) and the filter self-oscillates as a clean sine - at that point it doubles as a sine source whose pitch tracks the FREQ CV at 1V/oct. The classic subtractive bass: saw VCO -> LPF (cutoff ~300Hz, res ~0.2) -> VCA shaped by an envelope. Add a touch more RES for acid character, or modulate CUTOFF with the same envelope for the textbook "wow" sound.

Inputs

IN (audio) — Audio input. Standard ±5V Eurorack-level signal - patch any VCO, noise, sample, or processed audio here. The filter expects nominal full-scale audio (a VCO saw at default gain is exactly right). Below ~0.5V the resonance bump is harder to hear. The filter is linear (no internal soft-clip or saturation, unlike the full Filter module's drive stage), but resonant peaks at high RES can exceed input level - patch the OUT through an Attenuverter if you need to tame it for a downstream stage.
FREQ (cv) — Cutoff CV input. 1V/oct exponential: each volt doubles the cutoff frequency, so an envelope or LFO sweeps the filter in pitch terms - a +2V envelope opens the cutoff by two octaves. Bipolar AC modulation works as expected: an LFO's ±5V swing modulates the cutoff symmetrically around the knob setting. There is no attenuverter - patch through ibp.attenuverter or ibp.cv-mix if you need to scale the modulation. At RES=1 the filter self-oscillates and FREQ becomes the V/oct pitch input of a sine oscillator.

Outputs

OUT (audio) — Filtered audio output. ±5V Eurorack-level. With RES at 0 the filter is flat (no peak); with RES around 0.5 there is a clear peak at cutoff; past ~0.85 the filter rings audibly on transients; at RES=1 it self-oscillates as a clean sine. Patch into a VCA for envelope-shaped notes, or directly into Output for static filtering.

Controls

CUT — Cutoff frequency. Range 20Hz - 20kHz, exponential scale. The frequency above which the filter starts attenuating - at 12 dB/oct, an octave above cutoff is roughly half as loud, two octaves above is roughly a quarter. 200-600Hz for warm bass; 800-2000Hz for vocal/midrange tones; 4-12kHz to take the brittle edge off cymbals or buzzy synths. Also serves as pitch when the filter self-oscillates (RES=1).
RES — Resonance amount. Range 0 - 1, linear scale. Adds a feedback peak at the cutoff frequency. 0 is a clean flat lowpass (textbook starting point). 0.2-0.4 gives a subtle vocal coloration. 0.5-0.8 gives the classic acid squelch - pair with envelope on FREQ for plucky filter sweeps. Past ~0.85 the filter rings audibly on note transients; at exactly 1.0 it self-oscillates as a clean sine, usable as an oscillator (FREQ CV controls pitch at 1V/oct).

Inspired by

A 2-pole (12 dB/oct) state-variable lowpass - the gentlest, most musical "first filter" shape. Doepfer A-121 carries the same idea into Eurorack; the SEM made the sound famous. Our DSP is Andrew Simper's TPT discretization, stripped to the LP path: stable up to Nyquist, self-oscillates as a clean sine at max resonance.

Doepfer A-121
Oberheim SEM (LP path)
classic state-variable topology

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