The LC-31 has been designed to mix together two line level signals to one signal. It has two independent signal ways (A and B) so that for example the main outputs (Left and Right) of two mixers may be connected to the input of one amplifier. For galvanic isolation of the two signal sources, each input signal is routed via a transformer. In addition, the signal ground of each input can be decoupled by means of a switch from the ground of the output and the ground of the other input. Thus, a ground loop which may cause an interfering hum noise can be interrupted. It occurs e. g. when the two signal sources have contact in the rack both via the signal ground and via the earthed conductor of the power supply or a conductive connection of the housings.