Hey Marty, glad I could help. So here's the deal: mu-metal + copper shield is an absolutely incredible way to kill environmental noise (hum or UHF). What you did with the transformer is what I also did with the Pass phono's own power transformer as well, regardless of the fact that it sits in a box far away from the main amplification unit, with the end result being much like your own experience here. I bet you will be doing a lot of exploration with shields in 2018
Just last night, I finished wrapping the Alpha DAC's analog output section in copper, as seen below (there is also a shield underneath; you may also notice the two mu-metal shields between the transformer and main circuit board). Also shown below is the MA-X phono's MIT network box wrapped in copper with mu-metal underneath (and the necessary ground wire cannot be seen, but it's there). Not shown are all of my other MIT cables' shielded boxes, including the speaker cables' - with all shields routed to a common ground. All in all, it would be an understatement to say that I cringe at the lack of proper noise guards especially within phono preamp and DACs; and wrt DACs, I fret at the layers and layers of noise-generating stuff piled on top of each other in some of them, with a helpless analog section right next, to wrap up that abomination of a circuit execution (can you hear me say Formula Aqua???).
Yes, the back of my rack looks quite unusual, if not ugly, but here form follows function, and the results are unmistakable. For example, the metallic nature of cymbals out of this DAC right at this moment is something I have not yet heard anywhere else, as is the distinct duration of long-running notes within a complex music passage (not just a trailing note at the end of a track). Just food for thought on how noise can mask the music; it would be an understatement to say I am re-discovering my entire music collection.
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