If you power it from a large DC supply then the relays may drop out after the amp starts to make any odd noises (as many do) as the rails collapse. Ideally you want the speakers to drop out pretty quickly once you turn it off.
If the transformer in the amp is an 18-0-18 and producing normal -/+ 24 volt DC rails then you simply use one of the 18 volt windings (either) to power the board.
If the transformer in the amp is an 18-0-18 and producing normal -/+ 24 volt DC rails then you simply use one of the 18 volt windings (either) to power the board.
It does make a bit of a difference to how it works depending how the transformers are wired.
If the windings are connected in series and you use a single bridge rectifier it works as expected.
If you keep the windings separate and use two bridges and form the amplifier ground connection by connecting the negative of one supply to the positive of the other then I think it will still all work OK but technically the voltage the board sees is 'less pure' due to the four diode bridge affecting the input to the board as seen from ground... its complicated
but I don't think it will make any difference at all because the board itself has no effect on sound quality and does care over how pure the supply is in absolute terms. I think it will be fine.
If the windings are connected in series and you use a single bridge rectifier it works as expected.
If you keep the windings separate and use two bridges and form the amplifier ground connection by connecting the negative of one supply to the positive of the other then I think it will still all work OK but technically the voltage the board sees is 'less pure' due to the four diode bridge affecting the input to the board as seen from ground... its complicated
but I don't think it will make any difference at all because the board itself has no effect on sound quality and does care over how pure the supply is in absolute terms. I think it will be fine.