White paper
Cambridge Audio prides itself on employing the finest staff and we are lucky enough to have
Douglas Self, the renowned writer, designer and amplifier researcher working in our engineering
team. When Doug joined us he already had the seed of an idea to develop a new amplifier topology
and it wasn’t long before he’d pitched us his ‘brave new idea’ and what was to become the Class
XD™ development programme began. The 840A is the first ever Class XD product and the result of
over two years development work.
This new patent-pending technology is only a part of the 840A but is significant enough to warrant
separate treatment here, this white paper aims to outline Class XD technology in simple terms.
Please bear in mind this paper is by its nature technical in style and does not cover the many other
small circuits and tweaks developed as part of the 840A’s extensive measurement and listening
test programme.
For a more in depth technical description of Class XD, please refer to our UK Patent application
number GB 0505024.0.
Background: The state-of-the-art
The great divide in solid-state amplifier technology has always been between the efficient
but ultimately compromised Class B approach and the beautifully linear but dishearteningly
inefficient Class A.
The basic difference between the two methods being that in Class A the output transistors are
modulated by the audio signal to turn more or less ‘on’ but never actually turn off, in Class B the
output transistors actually at some point turn off as the output is passed from one transistor to
another. It is at the point at which the output moving from one transistor to another (the crossover
point) that a small amount of distortion is created. This ‘crossover distortion’ is inevitable and
although it can be minimized, it can be shown that it cannot be completely eliminated. Class A of
