Suggestions for DAC isolation?

I've only heard of one manufacturer who openly stated that one channel of their DAC will hum and that it couldn't be heard once music was played. That was Valab.
Now that they have the power transformer in a separate case, their is no noise at any volume setting with no music playing. Toroids are notorious for their nasty sounds and interference unless properly shielded. Heed all the advice given above until something works.

Very interesting Richard_stacy, thanks.
It is a Berkeley Alpha DAC. Any other users?

I like the sound quality of this DAC, one of the best I've heard, so no plans to get rid of it.

rja...mine will be here end of this week. i am very anxious to get it! what digital cable/power cable do you use with it?

I've been through a lot of digital cables in my day. Right now I'm happily using a Kubala Sosna Emotion 1.5m XLR cable that I bought used here on Audiogon. I'm not sure how critical it is with the Berkeley though.

If transformer in DAC behaves like one in power amp then read from Spectron Audio Web Site:

"Mechanical humming from the mains transformer is caused by a varying DC component on the mains grid's AC voltage. The DC component comes from unbalanced loads on the mains grid, such as hair dryers and is present in most households. The amplifier main transformer's vibrations degrade performance of the power supply and when audible can be annoyance as well. Spectron solved this problem by building appropriate filtering circuit below one watt, which is extremely low."

I had it until Spectron changed design of its amp to include appropriate circuitry. You can buy external devices which filter DC component out: one was sold by PS Audio...do search.

All The Best
Rafael