Hi,
I have built two DIY battery eliminators.
One used off the shelf back to back Jaycar transformers, the other I rewound the secondary (easy as split bobbins).
If you scroll to the ends of the two articles you can see the designs/schematics there:
http://worldphaco.com/uploads/THE_GREBE_MU-1.pdf
http://worldphaco.com/uploads/UX-171...amplifier..pdf