Any thoughts on the following?

Cambridge C70 pre and two A70 power amps. I've suddenly stated getting hum
from both power amps when:

* Preamp is plugged into mains (whether set to active or passive)
* If preamp is passive and not connected to the mains and I plug it into
Arcam Alpha 5 tuner, which I assume is earthed.
* I replace preamp with my own passive box with switch and volume pot.

Thing is this set up was fine for quite a while and I find it hard to
beleive that a loop has suddenly appeared affecting two amps.

Further information, this happens irrespective of whether it is plugged into
TV - but nowadays I have isolating transformers between TV and preamp. This
should be of no consequence to the discussion but I started getting hum on
my rear surround speakers that are connected to a media PC without isolating
transformers. These started humming at the same time. This latter one I
suspect is easy, Virgin have done something with the cable feed and I need
more isolating transformers.

Now, the latter issue seems to have occured at the same time as the first
one, but I cannot see how it is related due to my use of isolating
transformers between TV and Cambridge amps.

The only thing that seems logical is that the Cambridge power amps both
might have differential inputs to avoid earth loop problems, and that they
both failed simultaneously. But this does not seem credible.

Any thoughts on this?

Pete


--
http://www.petezilla.co.uk