ESS PEE DIFF. S/PDIF. Sony Philips Digital Interface Format. As a derivative of the pro audio world’s AES/EBU standard and designed to transmit digital audio signals over short distances, it comes in two flavours: coaxial and optical.
Not without its own shortcomings, coaxial connections are, by a large, the audiophile’s choice when connecting a USB-less streamer or CD spinner to a DAC. Toshiba’s fibre-optic-based TOSLINK is all too often looked down upon by audiophiles despite its ability to completely isolate one’s DAC from electrical noise spilling from any upstream device.
A 75 Ohm characteristic impedance guides the design and build of modern coaxial S/PDIF cables where the centre pin usually carries the voltage and the outer ring the ‘return’. It’s helpful to see digital cables as carriers not of ones and zeroes but of analogue representations of those ones and zeroes.
According to engineers like Chris Sommovigo, a cable with a characteristic impedance of 75 Ohms optimises the connection between transmitter and receiver to give us the best sounding connection.
(The background story of how Sommovigo and his ILLUMINATI brand arrived at this number, one that would, according to Stereomatic, eventually be adopted as an industry standard. Read their take here.)
The fly in this 75 Ohm ointment are the RCA plugs demanded by the majority of hardware devices. Their characteristic impedance is typically 50 Ohms. You might own a fancy-schmancy S/PDIF cable whose conductor fully complies to the 75 Ohm standard but is terminated with RCA plugs that do not.
Of course, as is common in the world of audiophiles, and especially when it comes to cables, whether or not this 25 Ohm differential audibly impacts sound quality is a matter of (sometimes white hot) debate.
One (theoretical) workaround is to terminate one’s S/PDIF cable with 75 Ohm-compliant BNC connectors as per Sommovigo’s own Black Cat and Silverstar-branded S/PDIF cables of the late noughties and early 2010s.
But what if we thumb our nose at the 75 Ohm specification altogether? That’s the question being asked by Bob Prangnell of New Zealand’s Mad Scientist Audio.
He writes: “I am sending you one of my Heretical digital cables – the world’s first and only 37 Ohm cable”.
Except it isn’t necessarily. Prangnell continues, “The 37 Ohm I mention is not the characteristic impedance of the cable, it’s the actual resistance”. Apples and oranges then.
Here we infer Bob’s point: that the characteristic impedance could be inconsequential if the carbon fibre core of the Heretical cable “absorbs the mass of reflections that bounce up and down the cable, causing jitter” as this Mad Scientist claims.
A supporting document supplied with the cable brings this Kiwi’s train of thought back into focus: “The Heretical Digital Cable (HDC) is very different to most digital cables. It makes no attempt to have a 75 Ohm characteristic impedance.”
The carbon fiber conductor means this cable’s resting state tends towards straight. Coil it too tightly risks damage. Standard RCA terminations are brass with a rhodium plate. Keith Eichmann’s (KLE) Innovations plugs are available as an order-time option.
“However, the carbon fiber construction means that it is immune from [sic] skin effects; even up to 250 MHz (a frequency reduces [sic] silver and copper skin depth to a few micrometers.) And because of its inherent resistance, HDC absorbs any reflections caused by impedance mismatches,” continues Prangnell.
Maybe. Maybe not. I’ll leave that to readers of a more engineering persuasion to politely discuss in the comments section below. My limited engineering nouse means I’ll be noping out on the tech talk in favour of moderation.
Then comes the kicker. “The result is a digital cable that makes your DAC sound twice the price”, concludes Pragnell’s marketing spiel. Yeah naaaah.
Wild claims like this are par for the course for marketing bods but reality at DAR HQ says otherwise. Swapping out an AudioQuest Forest S/PDIF coaxial cable (US$25) between AURALiC Aries and Devialet Expert 200 didn’t bring silence or a dose of the (literal) jitters. Perhaps a slightly smoother presentation from the HDC with lower levels of cymbal splash – but I wouldn’t stake my lunch money on it.
The differences proved to be super subtle too with the multibit version of Schiit’s Bifrost. A multibit Gungnir it does not make. Completing the picture was Vinnie Rossi’s LIO and Spatial Audio M4 loudspeakers.
Wanna try one for yourself? US$99 gets you through the checkout.
Lastly – Prangnell’s letter explicitly permits re-gifting of the accompanying HDC sample. If any reader 1) has the gear required to properly measure this cable’s characteristic impedance and 2) an inclination to report on their findings, speak up and I’ll wang it your way.
Further information: Mad Scientist Audio