Integrated Amplifiers

Cambridge Audio has ceased championing the ‘Class XD’ power amp topology that served its earlier models, including the 840W [HFN Aug ’08] and 851W [HFN Jun ’14], so very well. Nevertheless, with just one small reference to something called ‘Class XA’ in its 27-page promotional booklet, Cambridge’s novel XD, or Crossover Displacement, still looks to be alive and well, albeit with a tweak or two, in its new Edge A integrated and forthcoming Edge W power amps. Like its predecessors, the Edge A offers a marriage of low noise, impressive speaker load tolerance and supremely low distortion – right through to the highest frequencies – that the vast majority of ‘one size fits all’ solid-state amp brands might only dream about [see Lab Report]. And the low distortion, at least, is all down to Class XD, or XA, or whatever it’s called this season.

This is the Edge’s method of eliminating the crossover distortion suffered as music signals ‘cross’ from positive to negative-going (and vice-versa) and the pairs of power transistors momentarily switch off. Without, I might add, the cost and wasted energy of deploying a large standing current in the output stage (Class A operation). In practice, Cambridge’s elegant alternative doesn’t eliminate crossover distortion, but shifts it away from the zero-crossing point of the waveform – hence the name ‘Crossover Displacement’ or XD. By applying a negative bias current, the position of the crossover point is shifted without creating a DC offset. Indeed at low signal levels, where the effect is most deleterious, the crossover point is moved below the maximum negative swing of the audio waveform, yielding all the benefits of pure Class A. PM