Audio Engineering

(Barry) #1
Audio Amplifi er Performance 323

be specifi ed as a mixture of Class-A and -C, which says nothing about the basic principle
of operation, which is error correction.


10.4.11 Variations on Class-B


The solid-state Class-B three-stage amplifi er has proved both successful and fl exible,
so many attempts have been made to improve it further, usually by trying to combine
the effi ciency of Class-B with the linearity of Class-A. It would be impossible to give
a comprehensive list of the changes and improvements attempted, so I give only those
that have been either commercially successful or particularly thought provoking to the
amplifi er-design community.


10.4.12 Error-Correcting Amplifi ers


This refers to error-cancellation strategies rather than the conventional use of NFB.
This is a complex fi eld, for there are at least three different forms of error correction, of
which the best known is error feedforward as exemplifi ed by the ground-breaking Quad


405.^15 Other versions include error feedback and other even more confusingly named
techniques, some of which turn out on analysis to be conventional NFB in disguise. For a
highly ingenious treatment of the feedforward method, see Giovanni Stochino.^16


10.4.13 Nonswitching Amplifi ers


Most of the distortion in Class-B is crossover distortion and results from gain changes in
the output stage as the power devices turn on and off. Several researchers have attempted
to avoid this by ensuring that each device is clamped to pass a certain minimum current
at all times.^17 This approach has certainly been exploited commercially, but few technical
details have been published. It is not intuitively obvious (to me, anyway) that stopping the
diminishing device current in its tracks will give less crossover distortion.


10.4.14 Current-Drive Amplifi ers


Almost all power amplifi ers aspire to be voltage sources of zero output impedance. This
minimizes frequency response variations caused by the peaks and dips of the impedance
curve and gives a universal amplifi er that can drive any loudspeaker directly.


The opposite approach is an amplifi er with a suffi ciently high output impedance
to act as a constant-current source. This eliminates some problems, such as rising

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