Power supply musings

[macdonald at evenfall.com]

I have some basic questions about bipolar power supplies.  Most designs
seem to use a center tapped transformer with the center tap acting as
ground (the mains ground wire is connected to the center tap I believe)
and the other two outputs going off to the rest of the supply
circuitry.   To me this means your supply ground is then a "real"
ground, compatible with your other, similarly grounded, equipment.

Then there are other designs which don't use a center tapped
transformer, one output acts as ground and the other output seems to
feed both the plus and minus conditioning circuitry of the supply.  I
have not seen one of these with a mains ground wire connected in to the
circuit.  What is the reason for this?  Wouldn't the "virtual" ground
created have the potential to cause ground loop problems when the
circuit was connected to other equipment?  

The latter design does seem attractive because you could use a simple AC
output wall wart (cheap, easy to find) to drop the voltage to a safer
level before it even enters the enclosure.  But without a mains ground
connection it seems the potential for grounding problems is increased?

Thanks,

Chris MacDonald