@mikem
Ideally a DAC would take care of ALL external negative "influences" presented by the peripherals (i.e. switches, routers, etc.). The reality is quite different, and while the DAC manufacturers have taken tremendous steps towards isolation, there is still work to be done.
Keep in mind, much of perceived digital audio performance is dependent upon the quality of the clock signal entering the
DAC chip and the presence of jitter.
Jitter causes distortions in the audio signal coming out of
the DAC.
John Swenson has done quite a bit of work on this, and identified two kinds of external influences that can cause noise on a DAC's internal ground-plane: 1) jitter carried through digital data, and 2) leakage current (i.e. from power supplies of the various networking devices, router, switch, in your network).
Leakage current can flow through power cables, audio interconnects, digital cables, including Ethernet and
USB cables. The leakage will flow from a power supply, through a cable to another device or component,
then through that device or component’s respective power supply back to the AC mains.
Alternatively it can
flow through a cable to another box and then through a safety ground (the “third pin” of an AC plug) and
return back to the AC mains.
In fact, there are two types of leakage current, traditional low-source-impedance leakage, which is
found in all power supplies, and high-source-impedance leakage, which only occurs in SMPSs. This high-source-impedance leakage is hard to block and can take surprising paths to get where it
wants to go.
These two different forms of leakage currents have a deleterious effect on Ethernet in audio systems. All
copper Ethernet connections have transformers in each device port, and these transformers will block DC
and low-source-impedance leakage. However, they DO NOT block high-source-impedance AC leakage. This is
particularly important for digital networks used for audio.
Most all network systems used with audio use switching supplies (SMPS) to power the digital devices,
routers, switches, computers, etc. The high-source-impedance leakage current from the SMPS will travel
through the network equipment, the Ethernet cables, the end-points, and into the DAC where they will
create noise on the ground-plane, and thus jitter in the DAC circuitry
