Optimum sound quality
Class XD (Crossover Displacement), a patented Cambridge Audio technology, is a unique amplifier circuit design created to offer the optimum sound quality currently available from existing amplifier configurations, whilst retaining the efficiency and high power levels required for a truly dynamic performance. It was conceived by our London-based engineering team to combine the best of both worlds from Class A and Class AB amplifier operation.
Class A amplification sounds wonderful thanks to its extremely low distortion, but unfortunately it’s horrendously inefficient. We’ve all seen big Class A mono-block amps the size of coffee tables – that’s all costly heat sink, required to deal with the huge amount of heat generated by Class A operation.
Class AB amplification on the other hand is far more efficient but can often have a greater level of distortion due to how fragile the audio signal is when it crosses through the zero crossing point from transistor to transistor. By eliminating this distortion, Class XD, delivers the sonic benefits of pure Class A amplification, combined with the efficiency of Class AB. In turn, this allows us to spend more of the budget on the quality of amplification, power supplies, etc rather than on heat sinks!
Distinct from Class AB, XD moves the controlled current’s crossover point away from either side of zero volume (the worst possible position in terms of distortion) and instead displaces it to a different point where the transfer functions of the transistors are better matched, at a significant output level where distortion is far less audible.
The benefits of this approach give the sonic refinements of Class A operation and the uncompromising efficiency and power of Class AB operation. In fact, the 851A delivers a gigantic 120 watts per channel (into 8 ohms) - enough for the most demanding speaker set-up.