The principles of class-G
Music has a large peak-to-mean level ratio. For most of the time the power output is a long way below
the peak levels, and this makes possible the improved efficiency of Class-G. Even rudimentary statistics
for this ratio for various genres of music are surprisingly hard to find, but it is widely accepted that the
range between 10dB for compressed rock and 30dB for classical material covers most circumstances.
If a signal spends most of its time at low power, then while this is true a low-power amplifier will be
much more efficient. For most of the time lower output levels are supplied from the lowest-voltage
rails, with a low-voltage drop between rail and output, and correspondingly low dissipation. The most
popular Class-G configurations have two or three pairs of supply rails, two being usual for hi-fi, while
three is more common in high-power PA amplifiers.
When the relatively rare high-power peaks do occur they must be handled by some mechanism that
can draw high power, causing high internal dissipation, but which only does so for brief periods. These
infrequent peaks above the transition level are supplied from the high-voltage pair of rails. Clearly the
switching between rails is the heart of the matter, and anyone who has ever done any circuit design will
immediately start thinking about how easy or otherwise it will be to make this happen cleanly with a
high-current 20k Hz signal.