Abstract: A quadraphonic decoder is described for providing connections between a stereo amplifier and four loudspeakers. Two signals each comprising in-phase and out-of-phase components are applied from the amplifier to the speakers by the decoder so that these four separable components are each reproduced by a different loudspeaker. Means are provided for trapping out-of-phase signal components off the speakers reproducing in-phase signal information. A different network is provided to allow the speakers reproducing out-of-phase signals to operate independently. Means are also provided for damping out all but the out-of-phase signal components on the speakers, so that the presence of such signal components may be detected.