[sdiy] "backwards" connected positive regulator (was: Problem with 723 PSU)

[jhaible at t-online.de]

> 
> I'm still working on my Tiracon 6V synth and I've noticed that it 
uses the
> method of a "backwards" connected positive voltage regulator (LM317) 
for
> the -15V rail. I was and still am a little puzzled about how this 
method
> exactly works. 


Nothing special here - it's just like connecting two batteries
in series - the + output of one will become the "middle" (GND)
connection of the stack.
Just make sure that everything else is floating - you need two
separate secondary windings on the transformer, NOT just a
center tapped winding.

This approach has one big advantage over +/- regulators:
You have full control of defining your GND location on the
PCB. (With center tapped winding and 78xx / 79xx you have
one common trace from the big capacitors to regulators, and
you'll have some hum on the GND if you don't choose the right
point of this trace for GND distribution.

Disadvantage: Tracking regulators might be more difficult.

JH.