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.
