Doepfer A-131

Modules A-130 and A-131 provide voltage-controlled amplification. For audio signals, you would normally use the exponential VCA (A-131), and for control voltages, the linear VCA (A-130). It doesn't always have to be that way, though.
The amount of amplification the VCAs provide is determined by the voltage at the CV input, and the position of the gain control, which sets the overall gain in the system.
The VCA has two audio inputs, each with an attenuator. They are amplified by an amount determined by the combination of the gain and the two CV controls.

The latest versions of A-130 and 131 are equipped with the high-end VCA circuits SSM2164/V2164 (A-130) and THAT2159/V2159/SA2159 (A-131). Even they are able to process slowly varying control voltages as they have DC coupled signal in/outputs. These are manufactured since March 2013. Unfortunately it's not possible to upgrade older modules because the new circuitry is totally different.