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.

technical details

Width: 8HP 

Connections: 
Audio input 1 
Audio input 2 
Audio input 3 
Audio input 4 
Audio output