Module A-138 is a four channel mixer, which can be used with either control voltages or audio signals. Each of the four inputs has an attenuator, and there's a master attenuator, so that the mixer can be used at the end of the audio chain - ie. it can be used to interface directly with an external mixer, amplifier, etc.
The module can be supplied in two versions:
A-138 a: potentiometers with linear response, so especially suitable for control voltage mixing.
A-138 b: potentiometers with logarithmic response, so especially suitable for audio signal mixing.

For more detailed information please look at the English user's manual A138_man.pdf

Note: Since April 2004 improved versions of the modules are manufactured. For the revised versions control In1 works as a DC offset generator (about 0...+5V) provided that no patch cord is plugged into socket In1. If this feature is not required it can be deactivated by removing a jumper on the pc board.