The A-101-9 is a universal vactrol module, for putting other modules into vactrol control. It contains two identical sub-units. Each unit has two vactrol outputs available that are controlled manually (Man control) and by an external control voltage (CV In) with attenuator (CV). A vactrol is a combination of a light depending resistor (LDR) and a light emitting diode (LED) both put into a small light-proof case.
Increasing the LED brightness causes a decrease of the LDR resistance. The LED at the front panel displays the brightness of the LEDs of the vactrol pair in question. The resistance of the two vactrols changes simultaneously. This allows to control two parameters at the same time or to control a parameter that requires two resistors at the same time (e.g. certain filter circuits). The LDR terminals of each vactrol are connected to the jack sockets labelled VAC1 and VAC2. In contrast to other A-100 sockets the VCA1/2 sockets are not grounded, i.e. the two terminals of the vactrol are connected to the two terminals of the socket in question.
The A-101-9 module is intended only for users who are familiar with electronics as some soldering is required to install the vactrol connections on the module or device that has to be controlled by the A-101-9