There has been much talk about the UcD amplifiers designed and patented by Bruno Putzeys. Philips has made an application note, that in details describes the circuit, and workings, and a demo board can be ordered. At the Class-D section of DIYAudio there has been several threads about the UcD patent and the Philips UcD Application note some of the more notably ones being by classd4sure, RX5, classd_fromru and johanps (probably forgot some, sorry).

Modules based on UcD are developed and sold by Hypex.

The UcD circuit is beautifully simple yet very powerful and in all a stroke of a genius. It uses as few as discrete 14 transistors, and a handful of passive components.

The circuit is basically a power comparator feeding the load through a 2. order passive filter. Feedback is taken after the filter, which causes the oscillation and thereby modulation of the signal.

The circuit can be divided into three blocks. The first stage is the comparator, which works from the PSU rails, eliminating the need for a lower voltage PSU for the input.

Second stage is the power stage. Here the MosFet drivers are made from discrete components, forming two identical drivers, driving high and low switch. The lower driver is feed from a 12V supply, but the upper driver supply is floating in order to provide the needed Vcc+12V to properly switch the upper Fet.

Third stage is a second order filter which demodulates the signal. Feedback is feed through a lead network back to the comparator.

Read more on Hypex Technology page for further information.

The basic circuit can be adapted to work as a pre-filter hysteresis design, with very few alterations. As I would like to investigate the difference in sound from these two quite different working methods, I have decided to build a small construction which can be configures for both. The intention is to build 4 amplifiers and try out the two designs in a stereo A/B test.