My theory has run away so please bear with me if this is a dumb question.

I have a transformer 18-0-18 V feeding a BR to provide 40ish V to my DC power supply where I need +28 V, +12 V and +5 V. All good so far.

Then I want to power two circuits which run dual rail supplies, one 18-0-18 V, the other 12-0-12 V. I can't run the 12-0-12 V from the 18 V as the two 7812/7912 would be on their limits.

I don't like resistive voltage dividers, and changing the BR on the 12-0-12 V to half-wave will likely introduce too much ripple (it has 1000 µF before the regulators and 220 µF after).

Any suggestion on the best method to achieve this 1/3 voltage reduction?