Hum from DS Jr

I had used a direct XLR connection from DS Jr to my amp without hum.Recently I decided to ad a preamp and get hum affected by preamp volume pot when I connect DS Jr. Now all the connection are RCA (single ended). Preamp does not have XLR output/input.
Just amp,preamp and speakers,NO hum.Cheater plug don’t help. Any idea why?

5 posts were merged into an existing topic: DSjr Single-Ended Ground Loop Problem