Cambridge Audio Azur 840A White Paper page 14

White paper
also reduced at output powers less than the maximum. Similarly, there is a canonical sequence of
efficiency in Crossover Displacers, though the differences are smaller.
Fig 8: Left is a resistive Class A amplifier giving 12.5% efficiency, while centre shows constant-
current Class A giving 25%. On the right is push-pull Class A, which achieves 50%; the current
source may be controlled by either voltage or current conditions in the output stage.
The push-pull displacement approach even has another benefit; it reduces distortion when
operating above transition in the Class B mode. This is because the push-pull system acts to
reduce the current swings in the output devices, as the displacement current varies in the correct
sense for this. This is equivalent to a decrease in output stage loading; this is the exact inverse of
what occurs with resistive displacement, which increases output loading. Lighter loading is known
to make the current crossover between the output devices more gradual, and so reduces the size
of the gain-wobble that causes crossover distortion. [Ref 2] In addition the crossover region is
spread over more of the output voltage range, so the distortion harmonics generated are lower-
order and receive more linearization from a negative feedback factor that falls with frequency. In
push-pull displacement operation, the accuracy of the current variation does not have to be high to
get the full reduction of the distortion, because of the low output impedance of the main amplifier,
which maintains control of the output voltage. The global feedback around this amplifier is effective