Modulators
Reval
Dual discrete-core VCA with built-in overdrive - a colored amplifier that shapes and saturates signals in one stage.
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What is a Reval?
A discrete-core VCA is the loud, characterful cousin of a clean voltage-controlled amplifier. Built from matched transistor pairs and discrete feedback rather than an integrated VCA chip, it adds *coloration* to everything passing through - a faint even-order harmonic warmth even at unity gain. Push the gain past unity and the colour deepens into overdrive: harmonic richness, soft clipping, the kind of weight you cannot get from a clean amplifier turned up loud.
Discrete-core designs date back to the analog VCA modules in Buchla and the early Serge systems, where matching transistors and discrete topology was simply how amplifiers were built. Modern boutique builders - XAOC Devices, Doepfer, Mutable Instruments - revived the approach precisely because the *coloration* is desirable. A discrete-core VCA is to a clean integrated VCA what a tube preamp is to a transistor preamp: technically less linear, musically more interesting.
The signature trick of this kind of module is simultaneous expo + linear CV. Two CV inputs, both active at once, multiplied together like two VCAs in series. An envelope on the exponential input shapes amplitude naturally; an LFO on the linear input adds tremolo - but only while the envelope is open. That kind of interaction normally takes two VCAs and three cables; here it is one module, two cables, and the output sums both effects organically.
Our Reval
Ours is a dual discrete-core VCA in 6 HP modeled after XAOC Devices Tallin. Each channel has its own audio in, expo CV in (normalled to 8V), linear CV in (normalled to 8V), volume knob, and audio out. The two channels share a 3-position drive switch: SYM (symmetric / 3rd harmonic), LIN (clean), ASYM (asymmetric / 2nd harmonic).
The volume knob is not a simple gain - it scales CV sensitivity in the manner of the original. At center, 8V of CV produces unity gain. Below center, you need more CV to fully open the channel; above center, you are in boost territory up to +18dB. Below the lower limit, the channel is silent (-80dB). This mapping is the secret of how the same module behaves like a clean VCA, a colored amp, or an overdrive depending on knob position alone.
The drive functions are real waveshapers, not naive clipping. Symmetric mode uses a cubic soft clip (x - x^3/3) for that classic push-pull shape; asymmetric mode uses two different tanh curves on positive and negative half-cycles to mimic a single-ended tube hitting its supply rail. Even in linear mode we inject a 0.008 coefficient of even-order coloration to capture the discrete-core warmth that an integrated VCA chip would never produce. Output is soft-clipped at +/-9V (18Vpp) to prevent ugly hard clipping at the rail.
In a patch
Reval-class VCAs sit in two places. As a main voice VCA, with an envelope on the EXP input, set to ASYM drive at slight boost for warm tube-like notes - sounds great after a filter on saw waveforms. Or as a *colored preamp/distortion* on a send: route any signal through it with a high gain knob and a drive mode engaged, and the output is harmonically rich saturation, not a flat, brittle clip.
The two channels make a natural stereo pair. Run the same source through both, with slightly different drive modes or gain settings, and pan them left and right - the asymmetric harmonics differ slightly between channels and create a wider, more vivid stereo image than a single mono signal panned to both. Put a separate envelope on each channel's EXP input and you have an animated stereo voice.
For complex modulation routing, pair Reval with a function generator. One channel of the function gen on EXP for the note shape, the other channel cycling on LIN for self-modulating tremolo, and you get evolving voicings that change each note even though nothing in the patch is random.
Inputs
- IN A (audio) — Channel A signal input. DC-coupled, handles up to 20Vpp. Accepts audio, CV, or gates.
- IN B (audio) — Channel B signal input. Same specs as Channel A.
- EXP A (cv) — Channel A exponential CV input (0-10V). Normalled to 8V when unpatched. Amplitude drops rapidly near 0V. Optimized for 8V envelopes at knob center. Using 10V envelopes without adjusting the knob gives a compressed, thick sound.
- EXP B (cv) — Channel B exponential CV input. Same behavior as EXP A.
- LIN A (cv) — Channel A linear CV input (0-10V). Normalled to 8V when unpatched. Output scales proportionally to CV. Best paired with exponential envelopes for natural decay tails.
- LIN B (cv) — Channel B linear CV input. Same behavior as LIN A.
Outputs
- OUT A (audio) — Channel A output. Soft-clipped at 18Vpp to prevent harsh clipping from the gain stage.
- OUT B (audio) — Channel B output. Same soft-clip behavior as OUT A.
Controls
- Vol A — Channel A volume (0-1). Center (0.5) = 0dB unity with 8V CV. Left = -80dB silence. Right = +18dB boost. Also controls CV sensitivity - for 10V envelopes, turn slightly below center.
- Vol B — Channel B volume. Same range and behavior as Vol A.
- Drive — Overdrive mode (shared). SYM (0) = symmetric push-pull overdrive with 3rd harmonic emphasis - brighter, slightly aggressive. LIN (1) = clean linear, no distortion. ASYM (2) = asymmetric single-ended tube overdrive with 2nd harmonic emphasis - warmer, mellower.
Inspired by
A discrete-core dual VCA with overdrive, modeled after the XAOC Devices Tallin. Two channels share a 3-position drive switch (symmetric / linear / asymmetric); each channel has simultaneous exponential and linear CV inputs (both normalled to 8V), a knob spanning -80dB to +18dB through unity at center, and an output soft-clip stage. The discrete core adds subtle even-order coloration even in linear mode.
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