Questioning a/c isolation for digital.?

Isolation of ANY component in your system means fewer ground-loops. Ground-loops are insidious things, not only creating hum, but also HF sibilance. Once they are eliminated, the noise floor drops considerably revealing much more of the music.

There are several opportunities for isolation, including:

1) USB interface - either with common-mode filter of isolated interface using the 5V on the cable
2) USB converter or transport - the S/PDIF output if transformer coupled eliminates another loop. The downside is that this usually doubles the jitter. It is actually better to have the transformer on the DAC S/PDIF input. This adds very little to the jitter.
3) DAC analog output to preamp or amps - if this is transformer coupled or isolated externally, this eliminates one of the most troublesome loops. Here is an external isolator/selector:
http://www.empiricalaudio.com/products/final-drive

Steve N.
Empirical Audio

Elizabeth - 99.9% of power conditioners are not isolation transformers. Only an isolation transformer can separate the ground connections and eliminate ground-loops. Even this is not optimum. Better to have isolation at each component.

Conditioners are great at preventing noise from switching supplies from getting back into the power line or preventing damage from lightning strikes, but that is about it. I don't like them for the most part. Never use them.

Steve N.
Empirical Audio

Ptss - you are better off to get some kind of isolation or volume-control transformer.

Some large AC power transformers are available, but they only solve the problem of isolating house ground from audio system ground, they don't solve the problem of audio system ground-loops.

Steve N.
Empirical Audio

Isolation does far more than eliminate hum.

Steve N.