Slightly Off Topic: Amp hum (double insulation?)

[brett]

Hi.
I'm working on a small, simple amp, and have it going well EXCEPT for a really annoying hum.  It's quite loud.  

I *think* that it might be related to the use of a double-insulated wall-wart for a power supply - thereby preventing a true ground.

I have done the usual design things that would reduce hum, such as used different pcb traces for the pre-amp and power amp earths (Class A 4W).  The filtering on the main supply line is 2 x 2200uF caps, which should be enough for an amp that only draws 0.8A at 30V.  I increased the caps from 2 x 1000uF with no significant loss of noise.  The pre-amp (a jfet stage followed by a TL071) is on a regulated, filtered (100uF) supply.

Any ideas?  Thanks.