On Thu, 27 Oct 2011 09:11:17 -0700, DaveC wrote:

Or, as is not uncommonly done these days in portable digital players,
establish a floating ground for the purpose of providing and signal for the
(floating) load, so that the difference between the output and the floating
ground can become negative.

Which raises the question I was pondering recently: is it OK to connect 2
computers' sound cards' grounds together?

No. You will certainly create a ground loop which will result in mains
hum.

d