Ground isolated A/B/Y

[toddvirgil]

Sounds like a standard A/B/Y can often cause hum.. found this transformer based one:

http://forum.orangeamps.com/viewtopic.php?t=13433

or this one from General Guitar Gadgets
http://www.geofex.com/FX_images/humfree2.gif

[Yehudi]

Check this layout from Sabrotone:
http://www.sabrotone.com/wp-content/uploads/2013/08/AmpSelector.gif

Can someone shed some light on how could I add a phase reverse switch to the layout?
Cheers