Phono Grounding & preamp lug

[MacL]

Hi all,
Looking at building or buying a phono preamp so I can use a turntable with another amplifier without phono inputs
Looking for comments and input-
OK - most turntables have a separate ground wire (for ac motor? or what?) -I assume it is a carry over from having two prong plugs and/or "old" style electrical receptacles in their homes - but needing some kind of grounding to maybe help prevent hum. Is it a chassis or signal ground?
All of the phono preamps I have looked at have a ground lug to attach the phono wire- but I don't understand how that helps because all of the phono preamps have a two prong wall wart for power and no way for the ground lug to return anything to ground.
If I build a phono preamp and have a ground lug would it be ok to use the chassis ground if I have on board power (PT inside chassis)?
If I mod a factory built phono preamp - I thought maybe about running a ground wire from the lug and tie strap it to the wall wart cable and then install a three prong plug.
Thanks

[dcriner]

I wouldn't add a three-prong grounded plug. You could try to connect the two chassis together and see what happens, but better to leave them unconnected. There is a danger of creating a ground loop which may cause hum and other problems.