PSU design?

[Paul Eccles]

Tony wrote:
>Don't use large ground conductors in any psu. Use carefully thought out
>traces to give a star point grounding system. The amount of Guitar amps
>that use wodges of copper for the ground supply, and they hum like
>terrible.

and Haible wrote:
>One typical error is to make very broad GND connections
>and connect everything just anywhere. The currents must
>be *forced* into a certain path, or they will find the path with
>least resistance, not least ripple.

What is the theory behind this? I've always been quite enamoured with huge
GND tracks; in power supplies and in just about every other circuit. It was
my thinking that the broader the track, the lower the impedance and
therefore the lower the hum. What am I not considering here?

Paul.