So I was over at headfi and asked something about the Singxer SA1 and R2R DACs there.
The result was that someone mentioned the power supply unit in the SA1 would introduce jitter/noise because of the way it's designed. And this would be the case in the Topping A90 as well.
I asked how to get rid of this problem and the answer was:
"All the battle is to keep noise outside. To do it properly, power supplies start to take lot of space and DACs like this (expensive amp linked here) or pre-amps like this use regenerative power supplies. So, my answer is 'yes', power supplies need to be designed different, but as you see on examples, it is expensive. The whole idea is to increase impedance for ground loops that would start flowing inside equipment through a power supply, closing a loop on the mains power outlet.
It is always more effective to fight a noise at the source. The amp in question is violating a fundamental design rule of using single grounding point. These two sealed SMPS's have a grounding point on a negative power rail. When there are connected in series, one grounding point is located properly on the system ground, but the other one is on the negative (-) power rail and it creates a current noise flow through the entire internal electronics. The same 'invention' had been implemented on a Topping A90 and in a result RCA inputs are unusable on many systems. XLR inputs are more immune to the noise, you can't hear a hum, but a ground a shield wires transfer internal noise to the associated equipment. Besides, SMPS noise is mostly high frequency, it is not normally heard as a hum, but spreads around much easier than a typical mains hum."
