The same concerns most certainly do apply. The fact that the laptop or notebook runs on DC or that the output of the device is DC is irrelevant, it still runs off 120VAC. A laptop
power supply has 120VAC coming into it, which makes it just like any other AC powered device. With no
safety ground in place then unless the
power supply itself is designed to operate without a
safety ground, a loose
wire or arc inside the
power supply could make the supply itself 'hot' and anyone touching it then becomes the
ground path.
That's the thing with Dell
power supplies (and their laptops), they've got weird
ground designs. The mains input
ground is only terminated via a (if I recall correctly) 10k
resistor to the negative DC output
rail. This might not be the case on all of them, but on the few I have taken apart this has been the basic scheme.
The case is plastic on their
power supplies with no grounding cage, and typically they use only a 2-pin
connector to the laptop itself.
So normally, yes it isn't a good idea to lift the
ground on a grounded device, but in this instance it could be safely done in my opinion.
That said it's still not the ideal way to
tackle the issue. The problem is a simple
ground loop, only with a huge source of
RFI (the laptop) with close proximity to the loop.
That loop can be broken in two ways:
A: The method that has already been mentioned here, which is to
disconnect the laptop from earth
ground.
B: Break the
ground between the laptop and the sound board. A
direct box with a
ground lift switch is the easiest way to do this.
