[sdiy] Isolating a +-15v/+5v PSU?

[Jeff Farr]

I'm working on PSU schematic for my 808 clone today.  I've been
sticking with the design of the CGS supply and adapting it to provide
+5v as well, which seems as simple as adding a 7805 to the mix.  While
researching I found this interesting little snippet about how to keep
these two supplies seperate from each other at piclist.com:

"Here is a simple idea that works really well to isolate noise
generating sections of a circuit from sections that need clean power -
it uses just 3 diodes and 1 extra capacitor. In the common leg of a 3
terminal regulator you add a diode that raises the regulator's output
voltage by the drop across the diode. You then fit 2 identical diodes
to the output from the regulator to drop the voltage back down to
where it would have been. Separate smoothing capacitors are then
fitted to the 2 separated outputs."

Seems logical enough, but I still have a few questions.  First can the
7805 really handle +15DC at Vi?  The national datasheet specifies a
working range of 7-20v (AC?), but I seem to remember having to be
careful with how much juice I applied to some Midibox projects (which
this is powering), it was likely DC voltage tho, and I figure that is
why.  So, if the 7805 can't handle high Vin's than the above technique
won't work, right?  In this case I have to connect the '05 before the
other regulators and add smoothing caps to it, but where to connect?
Will I get better isolation by giving the digial PSU it's own
rectifier, or will I be fine using the same rectification for both?