A DAC will sound it's best when isolated from all types of noise. In the case of a coaxial input signal, optical isolation eliminates galvanic conduction and source RF noise; well shielded coaxial cables limit their impairment as RF antennas; Faraday isolation of digital components prevents radiated RF noise effects; mechanical isolation of connections limits vibration noise; and ultra-low noise and low-impedance power at the DAC helps maximize sound quality.
Still to be considered is the potential for leakage currents at the signal input:
Coaxial S/PDIF is bi-phase-mark encoded for a zero average DC; however, symbol correlated fluctuations may transfer low frequency AC and DC energy over time
The upstream component source blocking capacitor may degrade with age and leak DC current to the DAC
Transfer of low energy DC may occur along the coaxial cable dielectric
50/60Hz AC sources radiate EMF and induce current in upstream components and the coax cable - no matter the amount of shielding.
DC•BLOCK™ is an in-line device located right at the DACs coaxial signal input. It's a 2nd order Butterworth filter integrated in a BNC adapter that blocks DC and AC mains induced current. Zero unwanted current flow into a DAC means better sound and lets you extract the last bit of transparency from your system. Install one DC•BLOCK on a single-coax 384k connection; install a pair on a dual-coax (DX) 768k connection. The performance of the digital transmission is not affected.