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; and cables should be of short-length and well-shielded to limit their impairment as antennas. Still to be considered is the potential for leakage currents at the coaxial signal input:
S/PDIF is bi-phase-mark encoded for a zero average DC; however, symbol correlated fluctuations may transfer low frequency AC and DC energy
The upstream component source blocking capacitor may degrade and leak DC
Transfer of DC may occur along the coaxial cable dielectric itself
Proximate AC sources radiate EMF and induce currents in upstream components and connected cables
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 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.