I don't know if we have a spec in the files. However, a high current
1:1 toroidal transformer will work very well. The key specs are:
1. DC Resistance of primary and secondary should be less than .8 ohms and preferably less than .5 ohms. The lower the better.
2. Inductance of both primary and secondary should be at least 10mh.
Ideal is around 14mHy.
3. Current capacity to saturation should be 5 Amps minimum.
4. Of course the turns ratio is 1:1
Multiple transformers can be used in parallel to lower the DC resistance and increase the current capacity. Just remember that the inductance is half for two transformers in parallel and one-fourth for 4 transformers in parallel.
Regards, Ken, Polk Audio