sorry, what i have is a small board with a TDA 7293, no preamp, just the amp, a transformer, and a small board with a relay to switch on the speaker with a three second delay, all mounted on a 8" x 8" piece of plywood and close to one another (inch apart), I noticed the input (shielded 5 inch lead wire with a 1/4" female) was sensitive to placement and would induce more or less hum when moved. I've since spread things out more and the hum did drop a fair amount. I guess my question would be is...is there a preferred transformer for these two rail amps that utilize these IC power amps that run on anywhere from 13 to 33 vac, possibly a shielded torroid?