Janus VCF

Sixteen-voice OTA ladder filter, seven responses · Vulpus Labs

Each polyphonic voice goes through four one-pole filter stages in series with a feedback loop around all four, and a saturator inside every stage. It can be driven into overdrive, or into self-resonance.

IN and OUT

IN — poly audio, ±5 V nominal. Drive it harder and it will saturate more.

OUT — poly, one filtered voice per lane. Polyphony follows the patch.

FILTER

Five controls, left to right in the order the signal meets them.

DRIVE

0 to +24 dB of gain into the ladder.. Zero is unity gain. Drive does not affect the feedback level: self-oscillating resonance tends to disappear at higher drive levels.

FREQ

Cutoff, twelve octaves from 5 Hz.. At the top, in LFO mode, it's essentially a pass-through (but an envelope can duck it).

RES

Nothing to self-oscillation. Rests at the bottom.

The peak arrives late and suddenly. Nothing much happens for the first half of the travel — the peak is still below unity at 0.5 — and the last quarter is where the filter starts to whistle.

The bass goes as the peak comes. The two columns above are one fact: every decibel of peak is paid for out of the passband.

Where self-oscillation starts depends on the cutoff. High cutoffs sing sooner: at 4 kHz the filter sustains at RES 0.75, while at 440 Hz it needs the slider at the very top. The oscillation is a clean sine with odd harmonics only, and its level rises with frequency — about 0.44 V at 440 Hz, 0.67 V at 1 kHz, 1.43 V at 4 kHz.

It tracks. The ladder's poles are detuned to cancel the loop's own phase lag, so the self-oscillation lands on the frequency FREQ is set to rather than drifting flat as it rises. With KYBD at the top the singing filter is a 1 V/oct oscillator.

MODE

Four detents, counting up from the bottom: LP, BP, HP, NOTCH.

All four come out of the same ladder, mixed from taps it is already computing, so they share its character: the same resonance, the same saturation, the same passband loss.

The notch is a true null, and it does not sit where the scale says. The null lands on the ladder's detuned pole, which is about 4% sharp of the printed cutoff — so in NOTCH, tune by ear rather than by the numbers beside the slot.

Use notch with moderate resonance. Past about RES 0.5 the filter's own ring sits at the resonant frequency, which is exactly where the null is, and fills it in. There is no null left to hear near self-oscillation.

DEPTH

Two positions: 24 at the bottom (the default) and 12 above it.

12 is not a two-pole filter. The feedback is taken from the fourth stage in both settings, so the resonance behaviour and the self-oscillation are identical — what changes is where the output is tapped, which halves the slope of the skirt.The result is a ladder with a shallower response: the resonant character stays put while the filtering gets less severe.

It does nothing at all in NOTCH. A ladder cannot form a steeper notch with a comparably clean null, so both positions give the same response.

MODULATION

Four amount sliders, each sitting over the input it attenuates. Every amount is unipolar, nothing to full, but they have different ranges.

All four are summed in octaves before anything happens: two sources that each add an octave add two octaves, wherever FREQ is set.

ENV — envelope, ±14 octaves at full

At full amount a 5 V envelope reaches both ends of the sweep from wherever FREQ is parked. Most patches want it well below half.

The jack is poly, so each voice gets its own envelope: a chord's notes open and close independently rather than as one gesture.

LFO — ±3.5 octaves at full

Mono and poly jacks, summed. The mono jack allows one LFO to modulate every voice at once. The poly jack can be fed from Janus LFO, enabling per-voice delay, or from a fan output from Janus KEY, enabling a spread of cutoffs across unison stacks of voices.

KYBD — key follow

At the top of the travel the filter tracks the keyboard exactly, one octave per volt. Feed it the same 1 V/oct signal the oscillator is playing. Partial settings track partially, which keeps the top of the keyboard from getting shrill without making the bottom dull.

BEND — ±3.5 octaves at full, mono

Plug pitch bend or mod wheel into this.

Notes on the sound

Resonance and drive are separate, both add harmonics. RES adds them by ringing, DRIVE by saturating. A high-resonance sweep at unity drive is the classic sound; adding drive underneath it thickens the body rather than the peak.

A quiet patch will sound cleaner than a loud one at the same settings. The saturator is level-dependent, so the filter's grit follows what you feed it. If a patch loses its character when you pull the mixer down, the fix is DRIVE, not RES.

The 12 dB setting keeps the resonance. Using it to tame a too-aggressive filter sweep will not tame the peak — reach for RES for that.

Self-oscillation is a tone, not a fault. With RES at the top, nothing patched into IN, and KYBD at the top, the module is a sine oscillator that plays the keyboard. Feed it noise and back the resonance off slightly for a whistle.