Questioning a/c isolation for digital.?

the isolation devices benefit well manufactureres.

Ptss - You're talking about either conditioners or isolation transformers, right? Please clarify your post.

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

I use a separate power conditioner for my digital stuff.
After reading about using alternative AC power frequencies in Stereophile, I searched for a conditioner which could provide alternate power frequencies.
Seettled on a PS Audio P600 with the optional board (required)
Settled on 110hz AC output.
Have all my two channel digital gear connected to it.
All the rest is connected to my Furman REF20i conditioner.

I do have one exception, as digital power supplies do not do as well as standard supplies.. So my Behringer DEQ2496 is not on the PS Audio P600 because it is not happy at 110Hz.