Let's face it; VCAs are pretty unglamorous. All they do is make it quieter, on command. And there's many ways to do it wrong; too much c/v transfer to audio, too noisy, not enough dynamic range. The Serge VCAs at first blush look pretty plain but are in fact very well done and versatile.
VCAs generally come in two different flavors: exponential response and linear response. You normally want exponential response (IF ANYONE SAYS LOGARITHMIC I'LL SPANK YOU WITH MY PHYSICS TEXTBOOK!) to gate audio into nicely enveloped notes. Sometimes, however, you want to vary the amplitude of a signal that then becomes a control voltage, and for that you want a linear response. Most of the Serge audio VCAs and mixers have exponential response, except for the top VCM module (Wave multiplier #1). The Active Processor is a DC-coupled crossfader (two complementary VCAs summed into one output). This has a linear response.
A funky feature of Serge envelope generators and transient generators is that they're capable of linear, log, and exponential curves. So if you have a linear response VCA you want to use to gate audio, just supply it with an exponential curved envelope! Likewise, if you want to vary vibrato depth through an exponential response VCA, supply it with a log envelope.
The Serge audio VCAs have a graduated response. Below .5 volts, they want to attenuate completely, keeping noise minimized. Turn the gain knob to 12 o'clock, you've got a standard VCA. Past 12 o'clock, the gain is sufficient that you will start to hear your signal without additional control voltage. The catalog says they won't overdrive past a gain factor of two if you pump lots of control voltage into the input.
Some words about panning and crossfading: If you simply have two VCAs connected together such that varying the control voltage brings the gain on one up and the other one down, you have a rudimentary panner, but the panner has a problem. At dead center (both VCAs equal), the loudness will be less than full right or full left. You need some flattening of the response curve to make this complete. The Serge VCAs provide this equalized left-center-right response curve, so panning sounds fairly natural.
Dual VCA (2VCA)
From the catalog:
The DUAL VCA (2VCA) employs two high-quality VCAs. This module is an excellent inter-patch VCA, featuring very low noise and exponential response. IT was designed as a small VCA function for use at various places within a system for internal VCA functions (as opposed to the VCAs for output mixing). Like the new Universal Audio Processor and the Cross-Fader this VCA has an audio taper that is an exponential curve with a 12 dB per volt sensitivity throughout the range except below about .5 volts. Below this threshold, the output of the VCA will decrease rapidly to completely attenuate the signal. This response is perfect for our envelope voltage range, and is responsible for exceptionally quiet operation. Control voltage rejection is very high, and the unit can't be overdriven beyond a gain factor of two.
Dual Mixer (MIX)
Simple, quiet, clean, to the point. 'Aux In' inputs let you chain mixers together, or allow another unity-gain input from another source.
This mixer is AC coupled. This module is available as the 2-inch pictured or a 3-inch configuration with optional 1/4-inch phone plug outputs.
Wizardry: As with the Dual Processor, you can use this module to realize greater than unity gain. Plug your incoming audio into one input, then multiple that banana plug into the next input. You can also get some subtle signal distortion this way.
Yes, you can play feedback tricks with this module too, to create a shoddy distorted oscillator.
Audio Mixer (AMX)
Functionally one-half of the dual mixer. Nice efficient skinny package if you're short on space.
Dual Channel Stereo Mixer (DCSM)
At first glance, it looks a lot like the UAP. But what we have here is a module that has two independent VCA's driving two channels. Each VCA has its own panner to the two channel output. Unlike the UAP, this module does not crossfade.
Universal Audio Processor (UAP)
Pro: Ideal small-system VCA, two VCAs really, cross-connected to provide panning, crossfading, or two independent VCAs Nice compact arrangement. This is an 'interface' module also in that its two output channels have both the banana jacks and 1/4" jacks (you can ask for minis or other jacks instead I think).
Wizardry: Be careful setting this one up! The Serge VCAs have very wide dynamic range. If you have your power amp cranked too high you may destroy your speakers and your eardrums. Since it's got the all-important 1/4" jacks, sometimes this module gets used for nothing more than interfacing. Turning the gain up past 12 noon gives you unity gain through a few dB's of gain, so you can treat it as a master volume/pan/interface in addition to being a regular VCA. The AM inputs are ac coupled and nice for tremolo or ring-modulator type sideband effects.