On Sun, 10 Jan 2010 10:37:28 GMT, "Brian Gaff"
wrote:

The idea I think was to fast switch the output and use the duty cycle to
create the output, so small devices could be used. Trouble was that this
needed massive amounts of low pass filtering to get it to really work..
Ho hum.

It was Class D - used in just about every subwoofer you can buy today.
So it turned out ok in the end.

d