Franc Zabkar wrote:

This month's Australian Silicon Chip magazine has an article on the
hum and RF noise produced by 2-pronged DVD players. The author(s)
tested quite a few of these players by connecting them to an old
3-pronged amp. They found that audible hum was introduced when earth
current flowed from a player's switchmode PSU to the amplifier's earth
via the analogue signal leads.

100% predictable. The usual cuplrit is the Y caps in the SMPS of the 2 wire
equipment which results is significant (but legal) leakage current of up to
around 2 mA. Of course, this won't happen if the amplifier is a 2-wire type too
since there's no earth for the current to flow to.

So, adding an earth to a Class II amplifier is likely to add 'hum'. Simple
really.

Graham