How To Utilize A Phono Preamp Board As Stand Alone

kosst is correct, that is about all there is to it.   Of course, you will need a chassis to put it in and all that goes with it.  

So...no comment on using a Walmart. 
And for the ground, I'm assuming there must be a frame it can be attached to ... that's an assumption I really don't know. 

Going to the local goodwill and finding a bi polar power supply wall wart. rare but they happen.Depends on the given outboard phono ..er.. board.

Bose outboard power supplies were many times +/-18 VDC@1A, which is very nice. I grab them when I see them, I’ve got about 4 of them, for experiments, if needed.

https://www.ebay.com/itm/Black-Bose-Sounddock-I-AC-Power-Supply-PSM36W-208-4-Prongs-for-SoundDock-1-...


some phono/RIAA boards/circuits use (+) and (-) lets say 15 or 18 volts, some might need a single positive (+)30 volt rail. It depends.

Depends on the given RIAA phono board.

Get the model number and go over to hifi-engine and download the schematics of that exact unit - in order to find out.

with that all important data in hand, one can then begin searching for a matching power supply.

exhibit a:
https://www.ebay.com/itm/Harman-Kardon-680i-Original-Phono-Power-Supply-Board-Parting-Out-680i-Recei...

those are good phono boards, IF they are recapped. the rest of the circuit was fairly complex and did not do the RIAA/phono boards any favours. the board is better than the receiver is/was, but it was buried under excessively complex ’other’ circuitry, wires, switching and pathways.

eg, look at this HK PM665 phono equalizer (RIAA) board. Crazy, just like it should be. explicit, directed, involved.

It would likely, after being cleaned up (new caps, etc), outperform anything op amp based, for the most part.

If you know the voltage / current specs you can find just about any wall wart transformer at Digikey or Mouser