QED Reference Digital Audio 40 Coaxial Cable (0.6m Single)

The QED Reference Digital Audio 40 sits within QED’s Reference tier — the middle rung of their good, better, best cable hierarchy, above the Performance range and below the Signature series. It is a dedicated 75-ohm coaxial S/PDIF interconnect designed to transfer digital audio signals between sources such as CD players, network streamers, and Blu-ray players and destinations including DACs, AV receivers, and digital pre-amplifiers.

Conductor & Dielectric

At its core, the Reference Digital Audio 40 uses a silver-plated 99.999% oxygen-free copper (OFC) central conductor. The silver plating provides a low-resistance path for the high-frequency components of the digital signal, while the high-purity OFC base ensures consistent conductivity. Surrounding the conductor is an extruded foamed polyethylene (PE) dielectric, chosen for its low permittivity — the air content within the foam reduces dielectric absorption, helping to maintain the cable’s characteristic impedance and minimise return loss.

Shielding

The cable employs a triple-layer electromagnetic shield coaxially spaced around the central signal carrier. This comprises a dual layer of closely woven 99.999% OFC braid plus an aluminium Mylar wrap, delivering 100% screening coverage. The multi-layer approach guards against electromagnetic and radio-frequency interference, which can introduce timing errors (jitter) into the digital bitstream.

Digiloc2™ Connectors

Termination is handled by QED’s twin-leaf 24-carat gold-plated Digiloc2™ RCA plugs. These feature high-purity copper contacts and a hollow central pin designed to eliminate the skin effect at high frequencies. The non-conductive, low-eddy-current body and improved locking barrel provide a firm ground connection and a more closely matched impedance to the cable itself. QED’s measurements show the Digiloc2 design achieves a 1.5dB reduction in insertion loss compared to conventional RCA plugs, reducing signal reflections at the connector interface.

Impedance & Performance

Maintaining a true 75-ohm characteristic impedance throughout the cable’s length is critical for S/PDIF digital audio transmission. The Reference Digital Audio 40 achieves this through the controlled geometry of its conductor, dielectric, and shielding layers, combined with the impedance-matched Digiloc2 connectors. The result is minimised return loss and reduced jitter, rated at typically 30ps.