> needed, something is fishy and using a ground lift potentially is
> dangerous to life. _But_ even professional gear sometimes provides a
> ground lift.
>
>
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Ground Lift is very often (read pretty much always) needed in PA systems! Hence why it is provided on every PA amplifier you can buy! BUT removing the ground from the mains input is not Ground Lift! As mentioned by others this is dangerous practice and should always be avoided as much as possible!! By that I mean you are welcome to risk your life in your own home if you really feel the need but it is never acceptable in any professional environment!

EG from the well-known amplifier manufacturer, Crown:
" The mixer, which drives the power amplifier, also receives its ground from the AC power cord. When the audio cable connects the mixer to the power amplifier the amplifier now sees a second ground from the mixer. If the mixer and power amplifier are both plugged into the same AC power strip then the mixer/amplifier interconnect cable shield can be cut to eliminate this problem. On most Crown amplifiers there is a "ground lift " switch on the back of the amplifier that performs this function and can be used to eliminate hum caused by ground loops"
http://crownaudio.com/kb/entry/19/

As you see the Ground-Lift switch doesn't lift the ground from the input and that term should not be used it this is what you are doing.

-

Now what has confused me is laptops where I get a ground hum. IE 50/60Hz (plus possibly harmonics) hum even when using nothing but headphone plugged into onboard audio output, which is eliminated (or vastly reduced) by running off batteries or by using a PSU with no earth pin. Never quite understood why that happens when there is no connected piece of equipment for the signal to loop around... But that's not really part of this discussion ;-)

Dale