At the core of the 840A’s strength is a patent pending, Class XD (crossover displacement) technology. 

This unique design gives pure Class A operation at low levels, moving smoothly into an enhanced version of Class B at higher levels. This system should not be confused with Class AB, which inherently generates greater distortion at high levels than a pure Class B design. 

XD technology feeds a controlled current into the output stage in a new way so that the usual Class B crossover point no longer occurs at zero volume - the worst possible position in terms of distortion – but at a significant output level. 

The result is a smooth and linear transition between the two modes of operation, which differs significantly from the abrupt, distorted gain transition of a Class AB amplifier.