Abstract: Improved audio reproduction apparatus permits upgrading a two channel high fidelity ("stereo") system to four channel ("quadraphonic") capability, while flexibly accommodating varying four channel source signal formats, e.g., four channel discrete, or encoded two channel SQ (e.g., tape, FM) or CD-4 (phonograph). The composite arrangement includes demodulation circuitry for the variously formated incoming signals, and plural variable gain amplifier arrays for final signal source selection which is readily effected by off-board minimal, DC switching.