Amplifier Class XD

The Amplifier Class-XD, patented by Cambridge Audio, is based on Crossover Displacement technology. As a new output stage technology, it eliminates crossover distortion up to a certain power level without collateral compromises. The term "XD" comes from "Crossover Displacement". A British patent covers this technology and is proprietary to Cambridge Audio. It first appeared in 2006 in the Azur 840A integrated amplifier and consists of a class-B output stage combined with a class-A output stage, working in parallel. This displaces crossover distortion away from the zero-crossing point. The combination operates as class-A at low levels and Class B at higher levels but without gain steps associated with class-AB. 

Benefits of Class XD

Class-A amplification sounds wonderful thanks to its extremely low distortion, but it's unfortunately very inefficient. It's easy to find big class-A monoblock amps the size of coffee tables! That means costly heat sinks are required to deal with the huge heat generated by class-A operations. Class-AB amplification is far more efficient but can have more significant distortion. This is due to the fragility of the audio signal as it crosses transistors. Class-XD displaces the controlled current's distortion away from the zero-crossing, where the transfer functions of the transistors match better, at a significant output level where distortion is far less audible. The combined sonic benefits of class-A amplification, with the efficiency and power of class-AB, allow us to spend less on heat sinks while delivering enough for the most demanding speaker setups. The 851A delivers 120 watts per channel (into 8 ohms)!

Source Texts

• Class XD Amplifier