904c voltage controlled filter coupler

moog904c · filters · schema v1

904C Voltage Controlled Filter Coupler (moogafakkin System 55/35 clone — categorized under Ports -> moogafakkin). Chains an internal moogafakkin-ladder low-pass + high-pass into a CV-controlled BAND-PASS / band-reject: CUTOFF, WIDTH (LP/HP spread) and MODE (BP to BR) knobs plus a 1V/oct cutoff CV input. DSP composes two ladder instances (own-code clean-room ladder). Beige moogafakkin faceplate (the intrinsic always-on look shared by the moogafakkin module family).

A clean-room recreation of the Moog 904C Voltage Controlled Filter Coupler — a single module that couples a 904A-style low-pass and a 904B-style high-pass around ONE shared cutoff, so the pair tracks together as a voltage-controlled BAND-PASS: the low-pass keeps the lows under the corner, the high-pass keeps the highs over it, and you hear the band between them. WIDTH spreads the two corners apart (a wide, gentle band) or pulls them together (a narrow, vocal peak), and MODE crossfades the whole thing from band-PASS to its complement, a band-REJECT notch that scoops out the band instead. A single CV input sweeps the band centre. It is built on the same transistor-ladder core as the 904A and 904B, so the band has the Moog ladder's character.

the faceplate

2 inputs · 1 outputs · 3 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 band-passed (or band-rejected at MODE = 1). audio signal
`cutoff_cv`	cv	CV → the band CENTRE (cutoff). Wired through the worklet's cutoff AudioParam with a log scaling, so a bipolar ±1 CV sweeps the band across its full octave span — patch an envelope or LFO here to make the band-pass move (a wah, a vowel sweep). control voltage (CV); modulates cutoff (multiplicative ≈ octaves — ±1 CV spans the param’s log range)

outputs

id	cable	what it does
`audio`	audio	The band-passed output (or, at MODE = 1, the band-rejected / notched output). audio signal

params

id	label	range	default	curve
`cutoff`	Cutoff	20..20000Hz	800	log
`width`	Width	0..1	0.5	linear
`mode`	Mode	0..1	0	linear

controls

control	what it does
Cutoff	The band CENTRE frequency, 20 Hz to 20 kHz on a log taper — where the low-pass and high-pass meet, i.e. the centre of the pass band (or the notch). CV adds on top of this. Defaults to 800 Hz.
Mode	Crossfade from band-PASS (0) to band-REJECT / notch (1). At 0 you hear only the band; at 1 the band is subtracted from the input so it is scooped OUT; in between is a partial blend. Defaults to 0 (band-pass).
Width	How far the low-pass and high-pass corners are spread around the centre. Low = the corners pull together for a narrow, resonant, vocal band; high = a wide, gentle band that passes more of the spectrum. Defaults to 0.5.

source

moog904c.ts on GitHub.