raspberry pi

How to separate power rails? It's quite easy to do the necessary modifications on the pi. It just takes 10 minutes.

Main board Power Mod

(this is for the models with the 40 pin GPIO)

First we want to have a good and direct connection to the +5V & ground of the raspberry pi. 

We break away little pieces of the sockets plastic at pin 2 & 6 of the GPIO and bent the pins 90º outward so that they won't interfere with the socket of the DAC. It's easy now to solder a wire to those pins.

Second we want to cut all other power supplies from the raspberry pi to the DAC sitting on the GPIO pins. This would be pin 1, 4 & 17.

Generally (but depending on the design of the final player) ground form the DAC would still be connected to the pi. If a star ground is established somewhere else and the DAC is connected there also cut the ground pins 9,14,20,25,30,34 & 39.

USB Power Mod

To get usb its own +5V to the upper row of its plugs the rear cover of the socket has to be removed. When removing the cover just take care not destroy the small smd cap that sits just behind on the socket. Bend the cover a few times and it will come off.

Inside the first metal strip on the right is +5V of the upper socket. Cut it at the bottom, bend it outward and solder+heat shrink your supply wire to it.

Connectivity

It was mentioned somewhere that it's better to use a wifi dongle instead of the ethernet connection. In the last lines of this post is a hint. the main comment to that is here.