Quad digital power amplifier in a lightweight 1U rack-mountable format. An energy-efficient switch-mode power section and class-D amplifiication circuit delivers accurate power-on-demand to four independent speaker outputs. Proprietary cool guide technology provides stable temperature management for dependable operation. Each pair of output channels can be separately parallel switched or bridged as a full or part stereo amplifier configuration. For channel A, there is a switchable 120Hz low-pass filter for use with subwoofers and a channels C and D have a switchable stereo pair of 120Hz high pass filters to effectively focus power in full-range cabs operating as mid-tops in conjunction with sub-woofers. For permanent or mobile installations, the D-series offers an invaluable and flexible problem solver for audio designers and sound engineers.