Class D is a technology that has been used for a few decades, but really only has taken hold in audio in the past 20 years or so. Spectron was an early example of a quality Class D amplifier used in hi-fi, with President John Ulrick building a low distortion commercial Class D amp way back in 1974. Spectron leverages a post-feedback filter; with connection points and specialized cables that allow post-speaker cable feedback (Hypex has this available on some of their latest products). Later on, OEM manufacturers began releasing modules and boards similar to what are used by DIY'ers today. Tripath and Bang & Olufsen had their respective Tripath and Icepower technologies used in this market, and supplied a number of major manufacturers. These include Bel Canto, Jeff Rowland, Motorola (in their very cool but ugly DCP-501 receiver), and Sonic Impact. Another player was Phillips, who paid an engineer named Bruno Putzeys to develop a technology, now known as Universal Class D, or UCD. Phillips retains the right to this technology, but Bruno's company, Hypex, licenses the rights to it from Phillips.
