904b vcf

moog904b · filters · schema v1

904B Voltage Controlled High Pass Filter (moogafakkin System 55/35 clone — categorized under Ports -> moogafakkin). The high-pass companion to the 904A LPF: a 24 dB/oct transistor-LADDER HIGH-pass, built (like the hardware) by SUBTRACTING the ladder's low-passed signal from the input (input - lp4 -> the complementary 4-pole high-pass). Cutoff is set by the FIXED CONTROL VOLTAGE (Cutoff) pot, shifted by a two-position RANGE switch (LOW = the full 4 Hz–20 kHz span / HIGH = +1.5 octaves), and swept by the summing 1V/oct CONTROL INPUT (cutoff_cv — each volt = one octave, summed per-sample). Unlike the 904A there is NO regeneration / resonance knob (the hardware 904B has no resonance pot), so the ladder runs with zero feedback. DSP is own-code, CLEAN-ROOM: it CONSUMES the same shared transistor-ladder core the 904A uses (TPT/Zavalishin zero-delay-feedback ladder via its hpDerive high-pass tap) — NOT a port of the LGPLv3 Huovilainen code, the CC-BY-SA musicdsp model, or any moogafakkin schematic (permissive only). Beige moogafakkin faceplate (the intrinsic always-on look shared by the moogafakkin module family).

A clean-room recreation of the Moog 904B Voltage Controlled High Pass Filter — the high-pass companion to the 904A. It is a 24 dB/octave transistor-ladder HIGH-pass, built by subtracting the ladder's low-passed output from the input (input − four-pole-lowpass = the complementary four-pole highpass), so everything BELOW the cutoff is rolled off at a steep slope and the highs pass through. Use it to thin out a sound, remove rumble, or — paired with a 904A — bracket a band. Unlike the 904A there is NO resonance/regeneration knob (the hardware 904B has none): just a CUTOFF pot, a two-position RANGE switch, and a summing 1 V/octave control input for sweeping the corner with an envelope or LFO. It consumes the same shared ladder core as the 904A.

the faceplate

2 inputs · 1 outputs · 2 params

about this module

Hover a control, or open the patch panel and hover a jack.

inputs

id	cable	what it does
`audio`	audio	The signal to be filtered — the audio fed into the ladder. audio signal
`cutoff_cv`	cv	Summing 1 V/octave CONTROL INPUT to the cutoff (audio-rate). It adds exponentially onto the CUTOFF knob inside the worklet, so an envelope or LFO patched here sweeps the high-pass corner — opening up the lows as it falls, thinning the sound as it rises. control voltage (CV); modulates cutoff (summed directly (the destination DSP scales it))

outputs

id	cable	what it does
`audio`	audio	The 24 dB/octave high-passed output — the input with everything below the cutoff rolled off at the four-pole slope. audio signal

params

id	label	range	default	curve
`cutoff`	Cutoff	4..20000Hz	1000	log
`range`	Range	1..2	1	discrete

controls

control	what it does
Cutoff	The FIXED CONTROL VOLTAGE pot — the high-pass corner frequency, 4 Hz to 20 kHz on a log taper. Content below it is attenuated at the four-pole slope; raise it to thin the sound and strip bass, lower it to let more low end through. CV adds on top of this. Defaults to 1 kHz.
Range	RANGE switch — LOW (the full 4 Hz–20 kHz span) or HIGH (the same sweep shifted up about 1.5 octaves), so you can place the CUTOFF pot's travel where you need it. Defaults to LOW.

source

moog904b.ts on GitHub.