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understood and often catches out the unwary. To demonstrate this, Fig 2 shows THD plotted
against output level for Classes AB and B.
Fig 2: THD vs. level for Class B and Class AB. (0 dB is 30W into 8 Ohms)
So actually it would be much more desirable to have an amplifier that would give Class A
performance up to the transition level, with Class B after that, rather than AB. This would abolish
the AB gain changes that cause the extra distortion. This was the purpose of our Class XD
development program, could it be done?
The Class XD crossover displacement principle
We can see that in Class B it would be better if the crossover region were anywhere else rather
than where it is. If the crossover were displaced away from its zero-crossing position, then the
amplifier output would not traverse it until the output reaches a certain voltage level. Also below
this transition point the performance could be pure Class A; above it the performance normal Class
B.
