For anyone thinking of buying or making an external LPS here are a few thoughts to give perspective. First you need to ask what kind of device you are powering to know your objectives. If the device is a pure music server - not connected directly to the DAC but rather to a network, it really doesn’t matter. What maybe useful here is placing a filter between the noisy PC and its noisy switching supply and your mains/AC power to keep that noise OUT of other stuff.
On the other hand, for most streamers and bridges that spit out USB to a DAC directly, blocking power noise (most likely on the ground) is the goal. On the best devices the USB is fully isolated and powered by its own LPS. I do this on my home made stuff and prototypes. Otherwise, an LPS is very helpful. Generally there will be some form of regulation inside to down-convert the input DC to whatever the various chips use (often 5 and 3.3v). But those regulators are typically pretty noisy themselves - no one’s thinking "high end" when they design a computer, a Pi etc.
Any actual computer will draw a bunch of power, more at boot up.
Finally, lets talk DACs. DACs are largely analog devices and demand very quiet power. Most have internal regulation and filtering to accomplish this. Even if they have a wall wart or a switching supply externally, it feeds internal regulation and filtering. So bear in mind that what you are doing is providing a quieter input, including less ground noise, you hope.
DACs also draw a lot less power typically. The DAC chips themselves don’t draw much, so its any digital filtering circuitry,clocks, USB receivers, and finally the analog output stage. Its those last ones that matter most, and again, they are typically fed by their own dedicated "LDOs" (low dropout regulators).
Any external LPS is unlikely to be any better or even good enough, and the distance both in length and resistance to an external supply would render it in effective. So keep in mind that you are providing filtered, quieter input power, but are almost surely not powering the low level circuitry directly with whatever you get/build/conjure up.
If you read the datasheet on any regulator you will see that its noise is a function of the input noise. Maybe its 100:1 improvement, often less, sometimes more. Always specified at a big number that is never achieved across the frequency band. So the better the input, the better the output - simple as that. Its called the "PSRR" or power supply rejection ratio.
G
