It may be a grounding issue.

Does the Sansui have a 3 prong plug?

Does the DAC?

It would make sense that if the Sansui did not (which I'm sure it doesn't) but the DAC did, it might travel to the DAC.

You could probably solve it by wiring in a new 3-prong power cord and wire the ground plug to the chassis.

As with all cable, it's just not going to be right for audio unless the price of said power cable replacement approaches your rent (or mortgage) payments.

I hope that helps!!!