Ah, no. Sry, simply forget it

The facts are: you have a 18VAC transformer, the LMs and the TDAs.

If you use the two LM1875 on 1 board (OCL) - you don't need a further supply. The supply is "on board" and that's the easiest way to go.
That's also the best choice of chips. The transformer is directly hooked up to the boards at "AC - GND - AC". Simple as simple can.
The little NULL is the "ground point" on the amp board. When you use the 18VAC with the LMs, the NULL is taken from the transformers
centre tap (even if you have a 2*18VAC transformer).

There are many ways to get a NULL (Neutral / GND rail) from / for a supply...
A NULL rail can be build from 1 rectifier, 2 rectifiers, centre taps, buffer caps, centre tapped trannies, dual secondary trannies and by 2 diodes.
It is just a neutral point between two different potentials.

If you really want to build the 12VDC supply, you can take the NULL from the circuit, by using 2 rectifier bridges and 2 positive regulators.
When a centre tapped tranny is engaged, you'll get the NULL from the transformer (centre tap). Then, you have to use one rectifier and
a positive + negative regulator (pos. for + and neg. for -). IMO - that's not worth the effort... ( i didn't think about that).

I hope, that the supply section for the "TDA support page" is ready tonight. Just take a look on it later...

Have fun

Hej Tom,
Thnx for a great answer and this little (problem) is solved by taking my tranny and just take out 2
windings as serial and Voila ! Ac - 0 - Ac
Hooked'it up with a fuse and plugged in and power on and
no smoke - no bang - no blown fuse
works perfect and later on this evening i have a small tone controller with preamp done and maybe
if i can stay awake i'll gonna try out my new find in form of 2 small Bose 2-way bookshelf speakers
i got from a local flea market for €5

Regards,
Kjell.

Hej!

Great Kjell, you got it.

Have fun