Le doc's analogy is good. Now think of this increasing and decreasing (of speed or voltage) in the very short term by turning the road and hill into music with a very stong pulse. At every beat of the thumpdrum (or bass guitar, or...), with an unregulated system, the voltage sags downward and then back upward. That pulsing puts ('modulates') that ryhthm on the amp's voltage supply, which puts some unpredictable portion of that pulsing signal on every other portion of the amplifier, thereby making the entire system sound not as good. In the best systems, EVERY voltage-gain stage in EVERY channel has its own regulator thereby making not only that gain stage sound better but also making EVERY gain stage sound better.
Poweramps often have regulators on their voltage-gain and driver stages but not on the output stage. The output stage is only infrequently regulated because regulating such a high-current supply is expensive. Regulating voltage-gain stages is relatively inexpensive.
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