Since 1991 we have experience with various Class-D amplifier concepts.
In 2003 we finished the a class D project LPC1, originally designed for making efficient high power audio till 1kW,
and later on optimized for higher quality, with almost inaudible distortion.
The LPC1 was meant as a demonstration-project in where many design-challenges, like Precision Audio, RF, Fast-Switching Power-electronics, and EMC are combined in one device.
The demands on speed and accuracy in parts of the LPC1 are similar to HDR-electronics.
In the LPC1, power is generated by modulating the duty-cycle ratio of a constant frequency square-wave signal from a switching inverter, with power-mosfets in a half- bridge. The duty-cycle ratio results in a low-frequency-component (after filtering) with the similar voltage levels in respect to supply-voltage as Class A or B, but with an efficiency of 90%.
From2012, with the availability of faster components, the LPC1 is upgraded to LPC2, in where the distortion is reduced to -100dB ( THD and IMD ), or less than 1mV errors per 100V audio.
A very simple DIY Class-D-amp for low power is constructed in 90’s. It is made with standard logic components, mosfets, and one opamp. The compact and EMC-proof design, made possible to use this amp in a portable FM-radio receiver, close to the antenna.