Phono without ground wire?

Hi all,

I have an old vintage turntable and stereo receiver from the same era (late 70s / early 80s). I was initially using a pretty standard phono hookup - RCAs with attached ground wire. However once I upgraded to better RCAs (van den hul D501 silvers) and bought a separate ground wire I ran into a weird “issue.” 
Basically if I use the ground wire alongside the van den huls it GENERATES a loud buzz but if I remove the ground wire the buzz goes away. 

Does anyone know what’s going on and whether it’s ok to keep using the turntable without a ground wire? I also started using a power conditioner around the same time so not sure if that could be contributing. 

Thanks,
Hauie