The pre-amp has two inputs: an unbalanced input for line level signals, with a gain factor of 0 to 20, and a balanced input with a gain factor of 0 to 500, for insertion of low level signals, for instance from a microphone or electric guitar. The Envelope Follower reads the signal level of the input, and puts out a proportional voltage as an envelope at its own output. The comparator generates a gate signal whenever the input goes above an adjustable trigger threshold. Three LEDs help you keep track of overload, the envelope, and the gate signal.

Technical note:
The module was designed primarily to derive an envelope and gate signal from the incoming audio signal. The built-in preamp is not "high-end" and shows - depending upon the position of the gain control - a reduction of higher frequencies in the frequency response. It is possible to improve the frequency response by replacing IC1 (the upper of the two integrated circuits) by a more sophisticated circuit. For example the LME49740 can be used. It's no longer in production but available e.g. via Ebay.
Apart from this an incoming audio signal with sufficient level can be patched directly to the audio input of A-100 modules (e.g. filters), i.e. without the detour via the A-119. In this case the A-119 functions only as envelope and gate generator and the audio output of the A-119 remains unused.