[sdiy] +/- 15v from a wall wart...

[Seb Francis]

 > Depending on the capacity of the wallwart and the load you can 
usually get away with 9VAC for +/-12VDC, 12VAC for +/-15VDC, etc.

I agree with the comments of Paul and Tony that this statement is 
perhaps risky.  In particular I hadn't considered Tony's point that UK 
mains voltage is at the high end of the 220-240 range that most 
wall-warts are designed to work with.

So I definitely should have said 'sometimes' instead of 'usually'!

Using larger smoothing caps will help.  As a check I always look at the 
voltage across the smoothing caps on the scope and check that the low 
point of the ripple is still comfortably above what the voltage 
regulator specs require (do this under full load of course).

Seb