Use of isolation transformer

For about a week now I've been using a 160 watt isolation transformer over my NuForce DAC-9 D/A converter(power consumption: 9 watts). I was recommend this by an audio-friend who said that my HTPC and "analogue switching amp," a NuForce Stereo 8.5V3, could possibly "contaminate" the power to the DAC-9 (via the return-wires, I suppose?) being that they're all coupled through the same power distributor(i.e., I don't use a power distributor per se - the bare wires are connected directly; leadwire to leadwire, return- to return, and earth to earth). Actually, about a few months ago I started out by placing an isolation transformer over my HTPC, to good sonic effect, and then proceeded with an extra one over my DAC-9 to possibly "shield" it from whatever noise may be emitted backwards through the power cord from the power amp - or even from the power wall-outlet. There was nothing conclusive or overly presumptive about the use of isolation transformers as proposed by my friend(who has incorporated the same tweak in his, in some respects, roughly similar stereo setup) in regards to the hypothesized effect or its theoretical (dis-)advantages; it was simply a suggestive "what if?"

Now, the use of an isolation transformer over the DAC-9 has a definitive sonic effect, and one that I would also call an overall improvement. My immediate, and remaining impressions are that a more finely resolved/differentiated top end, a more full sounding and clearly outlined midrange(more organic, even), and a better integrated and "coherently distributed" low end. In fact, coming about these three frequency spectrums in a rather disparate fashion, necessary it may be, feels a bit awkward in that the overall impression is that of a rather homogenous presentation.

However, another friend of mine who came over some days ago, remarked, upon listening to my setup, that while he conceded the mids had become more full and present sounding since his last listen(i.e., prior to the iso. transformer over the DAC), and better integrated with the low end as well, he found that the reverberative nature of the top end had become somewhat constricted - and to be honest, I had thougt about this as well. Moreover, he now found the width of the soundstage had been narrowed, instead hearing a more center-focused soundstage. This, also, I could confirm via my own impressions, though contrary to him I find it to be a more distinct quality, and even disagreed on the narrowing of the soundstage width.

Being that my friend had made verbal the observations on the top end's reverberative less lively nature, and that I found this to be in tandem with my own impressions, I thought it possibly illuminating to remove the isolation transformer from the DAC powerline and see how its omission would affect the reproduction in this area. In short, after a few seconds of listening with the transformer removed it was clear to me that the sound had not only changed, but also (subjectively) deteriorated; the overall presentation seemed to have fallen apart as if a connective (invisible) glue was missing, the center-fill and fullness of the mids lacked sorely, the ease of flow likewise, and the top end appeared less resolved and now exhibiting a white-ish or pale "color" patina to it; a more withdrawn, less spacious and anonymous sound - simply not as involving. That is also to say: the top end energy or reverberative nature seemed in no discernable way to improve or correct the beforementioned "shortcoming," if indeed that's what it really is.

Having now re-inserted the isolation transformer over my DAC the sonic presentation has yet again fallen well into place, though periodically the round-ish nature of the highs, extremely well differentiated and resolved they truly are, continue to come off a tad uninvolving with some music as if the slightest "edge" is missing. Everything in me tells me the general imprinting of the isolation transformer is that of leading to a definate sonic improvement save for this small issue, making me believe that another issue in my audio chain could have been addressed this way. It could also be that over time one has become more or less accustomed to a slight excess of energy in the highs, or simply a specific sonic nature here, that the more finely resolved and organic presentation leads me to believe sometimes that something is missing. Or, that the fullness of the mids and (soundstage-)center presence has somehow reverted attention from the highs to the lower frequencies. Anyhow, it's an interesting and overall satisfying developement of the sound, but also one that may have exposed improvements or changes could be made elsewhere - if indeed over more time I should conclude that more top end reverberative energy is needed.

I would appreciate the input of others who're using isolation transformers in front of their stereo setups as well - be it either (though preferably) with smaller and separately placed isolation transformers, or large singular ones used over the outlet group feeding the entire setup - possibly even on a related note to what I'm writing above both with regard the general nature of the perceived sonic changes isolation transformers lead to, as well as more specifically, where noticed, the reverberative or overall nature of the highs.