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RE: magnetic distortion from the transformer coils ?

Does each linear psu need to be individually boxed & grounded ??

It's a bit of a myth that linear PSUs are necessarily a noise-free panacea for the ills of the switcher. Quite the opposite. Way back in 1989, an article in Hi-Fi News on power supply design for pre-amplifiers put it well and, as I happened to build part of the design (I've still got it), I kept a copy. The author has a weakness for purple prose but this extract might be useful:

. . . the standard PSU circuit is a devil. It pollutes the supply by drawing current in swift packets as the voltage peaks. A sideways look reveals a multi-resonant network, snapped on and off by switches (disguised as rectifier diodes) and damped by a load which happens to be an audio circuit in series with a pair of regulators. Each diode commutates (switches) current at least twice in every cycle, producing a pattern of back-EMF spikes with a PRF of (say) 200Hz. Sags and surges in the mains voltage only add to the poly-rhythm. If the spike’s own (oscillatory) frequency is high enough, there’s no guarantee it’ll be spotted with anything short of a VHF lab’s sampling oscilloscope. Still, it won’t have any trouble making itself manifest once it’s inter-modulated with another signal containing components harmonically related to 200Hz - such as music. If this is so, the most likely symptom is modulation noise, noise which follows legitimate audio signals.

The bi-phase capacitor ‘smoothed’ supply’s output ripple has high harmonic content. Thanks to brute-force filtering, AC filtration is typically zero at 100Hz, becomes slightly effective (eg -5 to - lOdB) in the audio midband, only to return to nil above the reservoir capacitor’s resonant frequency, between 1 and 10kHz.

Small wonder dedicated audiophiles have taken to the hills, powering their pre-amplifiers and CD players from batteries. . . .

The mundane approach to building audio supplies crumbles further once we acknowledge that all conventional voltage regulators (whether discrete or IC) rely on NFB. It makes them rugged, predictable workers at low frequencies but their ability to ‘filter out’ incoming garbage is progressively impaired with ascending frequency and decreasing periodicity. Conventional regulators (whether IC or custom) are especially hapless in the face of VHF voltage spikes. They need protection as much as the op-amps they serve. Summing up, it seems likely that, if universal AC/DC power conversion techniques influence sonic quality, then they do so by offering cross-rhythms of their own making, as well as introducing others from down the line. Up to now, most ‘good sounding’ PSU’s have relied on empirical turning. That small changes to values, to component makes or mere layout has led to dramatic sonic differences is neatly explained by the exaggerated sensitivity of multi-resonant networks. It’s small wonder high resolution sound systems often sound radically different from day to day.

For a local but equally helpful discussion, see link. It doesn’t give you a recipe but it does explain things really rather well.

In short, I wonder if you’re not asking more of your kit than it is able at this point to give you. That’s not a very helpful answer of course.

Why not start by putting the units back where they were and making sure that that restores your earlier ‘inner peace’? Then take it from there. I have little experience in this area but I'm not convinced that just playing about with the grounds is going to be a fruitful approach. On the bright side, there's plenty folk round here who should be able to help.


Edits: 08/26/11
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