There is one thing about the SMSL D6S, and that is Michael Fidler's and other member's review of the power supply.
I have received my D-6s in the meantime, yet I did not have time to listen to it extensively.
About the SMPS:
On one hand it's quite handy to have the power supply inside the unit (no bulky wall-wart that blocks several positions in the mains stripe), on the other hand this always raises issues with "noise" - and yes, this applies to SMPS as well as old fashion transformer based power supplies:
Transformers suffer from:
- stray magnetic field (especially these nice flat molded transformers need a lot of distance or magnetic screening inside the unit and schottky diodes in the rectifier and maybe even snubbering)
- leakage current that generates I*R-drop in the signal cables downstream and thus "hum" (ideally they should have a foil- shield between the primary and the secondary windings)
SMPS suffer from:
- relatively high leakage current with a lot of harmonics of the mains frequency (I*R-drop just like above)
- high frequency content that usually is beyond the audio spectrum (might get mixed down in case it finds its way into clocks)
You can get rid of the leakage current by connecting the audio- ground to PE (protective earth). This is done in the D-6s as PyramidElectric just mentioned above: RCA- shield and XLR pin1 are connected to PE.
This however gives rise to ground-loops (e.g. if the amplifier is grounded as well) and when magnetic fields are around (big transformers) this will very likely cause audible hum when using an asymmetric connection (RCA). With XLR you will still be fine because the induced voltage shows up common-mode and thus gets subtracted; this is why the pro-stuff uses differential signalling.
In my system the D-6s is the only grounded unit so I'm fine and the D-6s is dead quiet indeed.
My preference would always be an external power supply, ideally transformer based with shield:
- even if it's an SMPS I can replace it by a good transformer based one
- the SMPS is usually the first thing that breaks down (usually caps dry out due to ripple current -> temperature)
I like the solution that Topping has chosen on the L30II. This unit has a (bulky) transformer as wall-wart (such that the stray magnetic fields are away from the unit) and the unit is fed by AC voltage. The leakage current is small compared to an SMPS and the harmonic content is less audible. When the wiring of rectifier and caps is done properly inside the unit, there will not be much to complain about.
Even better, rectifier and first set of caps would be in the transformer (such that the current spikes that recharge the caps generate their magnetic field outside the unit), but you'd need a 3-wire connection then, which adds cost.
