Right Transformer for 12v ?

[KA4HJH]

>The 12VAC wall-wart in the standard voltage doubler configuration should
>give you about plus and minus 16 volts DC with 1N4001 diodes and 1000uf
>caps. The 7812/7915 regulators operate best with at least 3 volts
>difference between input and output pins.

What is "the standard voltage doubler configuration" for a bipolar supply
from an untapped AC source? Simple half-wave rectification will give an
unfiltered voltage less than 12Vrms x .707 (1/2 the square root of 2),
which is obviously way too low. This is why I grab all the 15V+ AC wall
warts I can. Voltage-doubling would be a big help. A somewhat underloaded
12v AC supply would just be sufficient for +/-12V DC.

Terry Bowman, KA4HJH
"The Mac Doctor"