Janus HPF

Four-position high-pass filter · Vulpus Labs

Janus HPF is there to take the low-end mud out of a polysynth after the voices have been summed down to mono, and before the sum gets sent into an effects block for character.

It's simple on purpose: four distinct settings (one of which acts as a pure bypass), no modulation capability.

In a departure from the 80s polysynth the settings are taken from, there's a FREE button which enables the slider to sweep between them.

HPF

SELECTOR — four detents, the shelf at the bottom and the most filtering at the top. Each is labelled twice: a response curve to the right of the slot, a figure to the left. The curve tells you what kind of filter it is; the number tells you where.

All four positions pass their passband at unity., so switching between them is a tone change at constant level. You can audition all four against each other without having to adjust levels anywhere else.

FREE — takes the detents off. When set, the slider sweeps and the four positions are interpolated between:

• Bottom third, shelf up to FLAT — the shelf's 2.2 dB cut lifting back out to nothing, at a fixed shape.
• Middle third, FLAT up to 226 Hz — a high-pass fading in at a fixed corner. The depth moves, rather than the frequency.
• Top third, 226 Hz up to 720 Hz — the corner rising, at full depth. This is the lleg that sounds like a filter sweep.

Notes on the sound

No position boosts the bass. The bottom position is sometimes misdescribed as a bass lift; it is actually a 2.24 dB cut against the flat position beside it.

Position 0 is a shelf, not a filter. It flattens out below 90 Hz rather than carrying on down, so it takes a consistent couple of decibels off the whole bottom end instead of rolling it away. Used before a chorus it is a trim, not a tone control.

The two high-passes are first-order. a gentle 6 dB per octave, no resonance anywhere near the corner. This is a tone shaper, not a sweepable filter — for that, the ladder upstream is the module you want.

720 Hz is further up than it sounds like it should be. It is well into the body of most sounds rather than under them, and deliberately makes an small and bright to make room for a chorus or a pad underneath it.