the isolation devices benefit well manufactureres.
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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:
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 I appreciate your input, don't get me wrong, but the thread is about isolation from AC. I think your topic deserves it own thread for simplicity. I'm also not talking about isolation or conditioning to eliminate gross hum, which is as you previously explained-but which basically shouldn't happen with decent equipment, grounding and i/c's & connections.
I found that basic conditioning, using chokes and caps, helped inexpensive players with inexpensive switching power supplies to a degree, but just sucked the life out of better built players and DAC's with more advanced power supplies. Using isolation transformers (aka Jon Rich's design) had mixed results but generally followed what I said above.
Now better power cords on the other hand seem to enhance most anything, but after a point it is all about synergy and tuning to your liking.
I have made an attempt to isolate digital from analog. I have Spectron class D mono block amps so we used 3 isolator transformers feeding plugs directly installed for my system. There are two 3 kilowatt isolators for each amp. Then there is a 5 kilowatt isolator divided with separate outs, one for analog and one for digital. Now that it is installed and working I am happy with the system. I started with a smaller isolator for the preamp and DAC but the preamp was starved since I only had 600VA and the sound was hard in the treble and less defined on the bass end. Changing to the bigger isolator relaxed the sound but produced better dynamics and much better bass.
The isolators set on a concrete pad under the house about 5 feet away from the music room so you do not hear the hum. There are dedicated 10 gauge home runs maybe 15 to 20 feet long under the house feeding the new copper wall plugs. I have found copper plugs sound much better than brass.
That sounds like ample power supply. I also went with 10 guage wire. I found larger than necessary transformers are a benefit. I think "relaxing" of the music by the additional power transformer is an appropriate adjective. I think cleaner sound has a more relaxed musical feel. Do your transformers have a duplex outlet receptacle?
ran through a series of experiments last night and ended up where i was:
when comparing putting my transport on 60/60 power (via 3kva signal DU tranny) vs adding a separate 1kva tranny downstream of the 60/60, preferred the latter.
when comparing the 1kva downstream of 60/60 vs 120/0, 60/60 preferred.
preferences largely defined in quick ABA testing to determine lowest noise floor / microdetail retrieval / treble tonality / dynamics.
oh, and the transport ground is floated (all others have ground)
point being: so long as the transformer is big enough, more is better when it comes to motors in your rig. --kind of like how your rig sounds better when the fridge /washerdryer /AC is on vs off.
the addition of isolation transformers will greatly help digital gear (best digital i ever heard was esoteric P/D02, and it had 4 transformers per chassis! i'm simply adding them outside, with good results), but make sure sizing is adequate...analog stage requires power more than digital circuitry.
and power cords always seem to help.