902 vca

moog902 · utilities

902 Voltage Controlled Amplifier (slice 3 of the moogafakkin System 55/35 clone — categorized under Ports -> moogafakkin). The classic moogafakkin DIFFERENTIAL VCA. A SIGNAL input is multiplied by a gain driven by a CONTROL SUM measured in volts: the manual GAIN pot ("fixed control voltage", 0..6 V) + the summing CONTROL INPUTS (cv, scaled by the CV-amount knob + sign) + a fixed-control-voltage bias input (fcv). Overall gain is x2 (+6 dB) at pot-max OR at CV = 6 V, and reaches its x3 ceiling near a control sum of ~7.5 V. The LIN / EXP RESPONSE switch picks the gain law: LINEAR rises linearly with the control voltage (6 V -> x2); EXPONENTIAL passes through the same x2 at 6 V then climbs faster, hitting x3 near ~7.5 V (the snappier VCA feel). Two complementary outputs form the differential pair: OUT (the amplified signal) and OUT- (audio_inv, its sample-accurate phase-inverted twin) — handy for stereo widening, sidechain feedback prevention, or mid/side work without a separate inverter. DSP is own-code: an amplifier gain law forked from the repo's own vca, re-implemented with the added exponential branch + the moogafakkin x2-at-6V / x3-ceiling scaling (not a port of any moogafakkin schematic or copyleft source - permissive only). Beige moogafakkin faceplate (the intrinsic always-on look shared by the moogafakkin module family).

A clean-room recreation of the Moog 902 Voltage Controlled Amplifier — the System 35/55 VCA that turns a control voltage into level. The signal you feed in is amplified by a gain that is the SUM, in volts, of the manual GAIN pot plus the two summing CONTROL INPUTS (CV scaled by the CV-depth knob, and FCV added straight). Overall gain reaches ×2 (+6 dB) when that control sum hits 6 V, and tops out at a ×3 ceiling whose voltage depends on which gain law is selected: 9 V in LINEAR, 7.5 V in EXPONENTIAL. A RESPONSE switch picks that law: LINEAR rises straight to ×2 at 6 V, EXPONENTIAL passes through the same ×2 at 6 V but climbs faster and hits the ceiling sooner. Be aware that the switch is also a LEVEL control — the two laws cross only at 0 V and at that 6 V anchor, so everywhere between them EXPONENTIAL is quieter for the same knob position (2.98 dB quieter at the factory setting, and up to 5.45 dB near the bottom of the pot). Like the hardware it has a true differential output pair — the normal output and a bit-exact phase-inverted twin. Patch an envelope or LFO into CV to shape dynamics or tremolo; leave everything unpatched and the GAIN pot is a static volume, sitting at exactly unity as shipped.

the faceplate

3 inputs · 2 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 input — the audio to be amplified by the VCA. audio signal
`cv`	cv	Summing CONTROL INPUT to gain (audio-rate). It is scaled by the CV-depth knob and added, in volts, to the control sum the worklet maps through the gain law, so an envelope here makes the VCA an amplitude shaper and an LFO makes tremolo. Bipolar CV with a negative CV-depth can duck the signal. control voltage (CV); modulates gain (summed directly (the destination DSP scales it))
`fcv`	cv	A second summing CONTROL INPUT to gain — a fixed-control-voltage bias added straight onto the control sum (no depth knob). Use it to offset the operating point, or to sum a second modulation source alongside CV. control voltage (CV); modulates gain (summed directly (the destination DSP scales it))

outputs

id	cable	what it does
`audio`	audio	The amplified signal — the input scaled by the gain the control sum produces. audio signal
`audio_inv`	audio	The differential − output: a phase-inverted twin of the main output (the same level, 180° out of phase). Handy for difference patches or driving a balanced pair. The inversion is BIT-EXACT, not merely sample-accurate — measured across 24064 samples of a rendered tone the worst \|OUT + OUT−\| is 0, so summing the two jacks cancels to digital silence rather than to a residue. audio signal

params

id	label	range	default	curve
`gain`	Gain	0..1	0.5	linear
`cvAmount`	CV	-1..1	1	linear
`mode`	Resp	0..1	0	discrete

controls

control	what it does
CV	Depth and SIGN of the CV control input: it scales how much the CV jack moves the gain. Full right (+1) is full positive depth, center is none, full left (−1) inverts the CV so a rising envelope ducks instead of opening. Defaults to +1. NOTE it does nothing at all until something is patched to the CV jack — with that jack empty, every position of this knob renders bit-identically, because it has nothing to scale.
Gain	The manual GAIN pot (the spec's fixed control voltage), mapped across 0..6 V of the control sum — at the top it alone gives ×2 (+6 dB). Its volts add to whatever CV/FCV contribute. Defaults to 0.5 (mid), which in the shipped LINEAR mode is 3 V and therefore EXACTLY unity gain: a bare 902 passes a patched signal at precisely the level it arrived. Fully closed is true silence, not merely a very small gain.
Resp	RESPONSE switch — the gain law. LINEAR rises straight to ×2 at a 6 V control sum and reaches the ×3 ceiling at 9 V; EXPONENTIAL passes through that same ×2 at 6 V then climbs faster, reaching ×3 at 7.5 V, and is the more musical curve for envelope-shaped amplitude. ⚠ It is ALSO a level control, which the two names do not suggest: the laws coincide only at 0 V and at the 6 V anchor, so at every setting between them EXPONENTIAL is the quieter of the two — 2.98 dB down at the factory pot setting, 5.45 dB down near the bottom of the dial — with no dial movement at all. Unity gain moves with it too, from pot 0.500 in LINEAR to pot 0.642 in EXPONENTIAL. Defaults to LINEAR.

source

moog902.ts on GitHub.