The F_VCAx2 module is equipped with two complete VCA modules
The VCA module has three inputs. The left one is used for AM modulation fro which the level can be set using the green knob. The middle one is used for an ADSR's envelope signal to 'open' the VCA based on the envelope... To use it - push the button underneath this input. The right input is where your audio signal enters the amplifier. The level is set with the black knob. The GAIN knob sets the overall output level of the signal when it exits the bottom port. The DRIVE knob overdrives the signal to add some distortion and through that extra frequency information in the signal.