Filter

VXN1’s filter section is a two-stage chain:

1. High-pass filter (HPF) — 1-pole, pre-VCF. Used to trim low-end ahead of the main filter; great for keeping pads from muddying low registers.
2. OTA-C ladder VCF — 4-pole transistor-ladder model (R3109 / IR3109-flavoured) with selectable LP / HP / BP / Notch outputs and a 12 / 24 dB/oct slope switch.

High-pass filter

Single knob: HPF Cutoff (20 Hz – 18 kHz, exp taper). Set it just above the fundamental of your lowest played note to remove rumble without thinning the body.

The HPF is a 1-pole (6 dB/oct) topology-preserving design with no resonance. It sits before the main filter, so the ladder receives an already-trimmed signal.

Main filter (VCF)

Cutoff (16.35 Hz – 16 kHz, exp taper centred at 800 Hz) controls the corner frequency. The taper sits a little above middle of the knob travel by default — this matches typical analog control voltages and gives even resolution around the most musically useful region.

Resonance (0–1, linear) increases feedback around the cutoff. The ladder will self-oscillate cleanly at the top of the range; settings around 0.5–0.7 give the characteristic emphasised cutoff peak without howl.

Drive (0.1–4, exp taper, default 1.0) saturates the input to the ladder. Below 1 the filter behaves cleaner / softer; above 1 the input clips into the per-stage tanh saturators (rational Padé(5,6) approximation, applied at each integrator input) and harmonic content blooms around the cutoff. Useful for adding bite without raising master volume.

Filter Mode selects which point on the ladder is tapped:

Filter Slope picks 12 dB/oct (2-pole) or 24 dB/oct (4-pole). 24 dB is the default — fatter, more “ladder-like.” 12 dB is brighter and lets more upper harmonics through, useful for leads where you want presence even at low cutoff.

Key Track

Key Track (0–1) is a continuous depth knob. At 1.0 the cutoff rises one octave per octave of key relative to C4 (the played note’s pitch tracks 1:1 into the filter). At 0 the cutoff stays fixed regardless of key. Intermediate values give proportional tracking — 0.5 is half-tracking, etc.

Set this to 1.0 for a filter that follows the keyboard fully; use lower values when you want bass notes to be darker than treble notes without going all the way to neutral.

Modulation

Filter modulation has four fixed depths with no source selector — every depth is live simultaneously. See Filter modulation:

• Cutoff LFO1 Dep — LFO 1 (per-voice) into cutoff.
• Cutoff LFO2 Dep — LFO 2 (global) into cutoff.
• Cutoff Env Dep — Env 1 into cutoff. Can be negative (envelope closes the filter).
• Vel→Cutoff — MIDI velocity into cutoff.

Plus from the Mod Wheel panel:

• Wheel→Cutoff — MIDI CC1 into cutoff.
• Wheel→Reso — CC1 into resonance.

Parameters