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 any collateral compromises. The term “XD” comes “Crossover Displacement”.
This technology is covered by a British patent and is proprietary of 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, 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, required to deal with the huge amount of heat generated by Class A operation.
Class AB amplification is far more efficient but can have a greater level of 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, allows us to spend less on heat sinks while delivering enough for the most demanding speaker setups. In fact, the 851A delivers a gigantic 120 watts per channel (into 8 ohms)!