Hm, I have to retract my recommendation for power supplies...
To cut the log story short, I realized that you should NOT use grounded (earthed) power supplies (the ones with 3 pins going to mains power)....
I tried a couple of those, and was horrified at first that the sound coming out from speakers was full of garbage - by garbage I mean high-pitched whining, clicks, pops, "scratching" noises, etc...
After two days of head-scratching, soldering additional power decoupling capacitors, etc., I narrowed it down to grounding.
Once I connected the same power supply, but with DISCONNECTED ground, everything was OK.
So, in short, to avoid potential problems, use UNgrounded power supplies. After some more reading I realized that Raspberry uses "floating ground", and that introducing another ground point (in addition to grounded amplifier) might (and probably will) lead to trouble....
I wonded how DigiAmp (and other DACs with integrated amplifiers) behave in this respect? - they should not have such problems, I guess.
So, the first linked power supply in my above post should be OK:
But the second one (with three pins from mains) should be avoided (unless you disconnect ground wire, which is NOT recommended for safety reasons - so best to avoid these)...
PS: while we're on the subject of power supplies and potential problems, another problem I came across: if you're using a "USB" power supply - i.e. one which has a "pluggable" USB cable (or the one which ends in micro USB connector), check the quality of the USB cable: if it's very thin and flexible, it may cause problems. Namely, I've had two USB power supplies which were problematic (although declared as being 1.5A or similar): turns out the USB cables from the power supply (charger) to RasPi were garbage
- which resulted in distorted audio in the speakers (hum, whining, "scratching" noises, etc...).
So, in general - first check the power supply (switch it for another, just to be sure of the culprit), and if you're using a USB power supply, also check it with a different (better quality) USB cable....
A possible check for the USB cable is to measure the voltage coming out on the end of the USB cable that's plugged in the RasPi (some of the poor quality USB cables I've come across actually act as resistors instead of conductors, resulting in a significant drop of voltage - so you get about 4.7 - 4.8V from the cable, instead of 5.0 - 5.1V)
