A-119 is mono: "The pre-amp has two inputs: an unbalanced input for line level signals, with a gain factor of from 0 to 20, and a balanced input with a gain factor of from 0 to 500, for insertion of low level signals, for instance from a microphone or electric guitar"
A-139-2 is a headphone amplifier. You need to bring down the level going into the computer.