A linear voltage-controlled amplifier (VCA) uses a simple mathematical relationship between control voltage input and signal level output – for example, 50% of nominal control voltage in would result in the output signal being at 50% of the level of the input signal. This, however, is not how our ears perceive loudness; a sound must be amplified by 10x in order to be perceived as twice as loud. This makes a linear VCA desirable for scaling control voltages, but perhaps less so for scaling audio signals. If you connect an envelope generator with an exponential output to a linear VCA, then you will get the desired aural result. Confusing? That’s why it’s great when an envelope generator or VCA has a switch or control to vary it between linear and exponential response.
A linear mixer is similar to a linear VCA: “half” on the input level control equals the output having half the voltage swing as the input. Again, this is fine for altering control voltages, but not for mixing audio signals; in that case you want a mixer with exponential controls.
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