Cables... advice on what digital cables to use

[Ianmac]

Chris
75 Ohm RCA connectors and sockets have been available in the UK for some years. I bought mine from Canford Audio and my standard BNC crimp worked with them. I think they are offered on Ebay.
I agree 75 Ohm BNC is the best solution if properly engineered.
Most of the issues come from the various manufacturers not adhering to an audio industry standard- none exists- and therefore a mixed bag of RCA sockets appearing for SPDIF inputs.
Making it a field day for the cable vendors !

[Barwell1992]

Just as a bit of an update to this..

I decided to make a punt on the coveted Fisual Hollywood optical cable in 0.5m long, linked it up with my Cambridge Audio CXUHD (using it as UPnP renderer) and Beresford SEG 30 DAC and gave it a run.... If I'm honest wasn't sold by the sound.. in fact my chrome cast audio over a QED performance optical with adapter sounded better and it defiantly sounded better previously with my QED performance coax, so i had a good look and it seems that the poor sound quality is most likely down to a fairly crap Toslink connector on the back on the Cambridge, its just not a good fit and allow's quite a lot of movement, not helped by the stiff nature of the fisual, bit of a shame given the Cambridge its a £700 unit.

So that leads me to the next cable, a blue jeans coax cable, this cable uses Canare RCAP crimped on plugs,these are a true 75 Ohm rca plug mated to a Belden 1694A 75ohm cable. So knowing exactly what i was getting 12 hours and £20 later its running in my system, well so far all i can say is its a great cable for the money all the detail is back that was missing from the Fisual and it feels very well made, its not the best looking cable but seems like it will do exactly what its supposed to, the fisual has been moved over to chrome cast duty and sounds good but to me it sounds no better than my plastic QED cable..

I'll update again once i've got some more hours on the cable.

Blue jeans
https://www.bluejeanscable.com/store...udio/index.htm

Connectors
https://www.bluejeanscable.com/pages...canarercap.pdf

cable
https://www.bluejeanscable.com/pages...s/1694tech.htm

Canare RCAP.jpg

[NRG]

The dimensions of an RCA plug mean it can’t meet 75 ohm impedance. Somebody measured the canare plugs on diy audio ages ago and they didn’t meet 75ohms...also if it’s plugged into a standard RCA socket then theres no chance of it being 75ohm. Then there’s the consideration of the pcb traces and termination of the DAC itself, does that meet 75ohm. It’s a folly to chase this requirement.

[Barwell1992]

The dimensions of an RCA plug mean it can’t meet 75 ohm impedance. Somebody measured the canare plugs on diy audio ages ago and they didn’t meet 75ohms...also if it’s plugged into a standard RCA socket then theres no chance of it being 75ohm. Then there’s the consideration of the pcb traces and termination of the DAC itself, does that meet 75ohm. It’s a folly to chase this requirement.

Very interesting, I guess the closer to 75ohm the better. Beresford recommends 75ohm for the DAC but I can’t be sure that it meets 75ohm’s.

[Barry]

The dimensions of an RCA plug mean it can’t meet 75 ohm impedance. Somebody measured the canare plugs on diy audio ages ago and they didn’t meet 75ohms...also if it’s plugged into a standard RCA socket then theres no chance of it being 75ohm. Then there’s the consideration of the pcb traces and termination of the DAC itself, does that meet 75ohm. It’s a folly to chase this requirement.

Agreed, the RCA phono connector cannot have a characteristic impedance of any more than 57 Ohm (assuming the two conductors of the coaxial pair are air spaced). With a typical dielectric support for the centre conductor, the characteristic impedance will in practice lay somewhere between 30 - 40 Ohm.

It has to be remembered the RCA phono was never intended to be a precision connector, unlike BNC, CAMAC, SMA, SMB, etc. connectors. So one wonders why Sony/Philips chose the RCA connector for their SPDIF. Well the reason was the RCA phono is a cheap connector that has unfortunately been adopted as the hi-fi industry standard. The saving grace is that the connection is electrically 'short'. So the electrical mismatch between 75 Ohm and the ~ 35 Ohm of the RCA connection will have a voltage reflection coefficient of -0.36, whereas the mismatch between ~35 Ohm and 75 Ohm will have a voltage reflection coefficient of +0.36. Since these two mismatches are electrically close to one another (~1.5cm at the most), the two mismatches largely cancel one another.

Let us assume the rise time for the sampling rate is 25ns, that is a signal of 10 MHz, the overall mismatch reflection coefficient can be shown to be 0.015 (or -36dB return loss). Thus despite the large impedance mismatch, the use of an RCA 'phono' connector is not quite as bad as it might appear.

Yes, a true 75 Ohm BNC would be ideal, but the 75 Ohm version is easily confused with the 50 Ohm version, and mating a 50 Ohm BNC plug to a 75 Ohm BNC socket will damage the latter. And regarding those RCA phono connectors sold as being designed to have a 75Ohm impedance, once they are mated with a standard RCA phono connector, the impedance mismatch is just the same (and just as bad) as mentioned above.

However the most important thing is to use coaxial cable that has a 75 Ohm characteristic impedance. Regardless of connectors used, this cable will 'sound' better than say 50Ohm impedance cable or any other audio interconnect cable. There are several 75 Ohm cables that one can use: RG59/CU, URM70, Trompeter TC75-2 etc.

[Light Dependant Resistor]

The dimensions of an RCA plug mean it can’t meet 75 ohm impedance. Somebody measured the canare plugs on diy audio ages ago and they didn’t meet 75ohms...also if it’s plugged into a standard RCA socket then theres no chance of it being 75ohm. Then there’s the consideration of the pcb traces and termination of the DAC itself, does that meet 75ohm. It’s a folly to chase this requirement.

No its not a folly at all, rather it is a folly NOT to. What you get by not abiding by the characteristic impedance of the cable including the circuit board traces is reflections of the required signal interfering with the same signal
known as standing wave ratio (SWR). Whilst some manufacturers cannot be bothered by providing RCA terminations, rather than BNC, this does not mean the problem magically disappears.rather it just remains
to detract from ideal transfer.

[Light Dependant Resistor]

Reflections on cables caused by not adhering strictly to 75 ohms then causes the reflection voltage to superimpose itself on the needed
transition, in turn causing jitter.

[Ianmac]

The dimensions of an RCA plug mean it can’t meet 75 ohm impedance. Somebody measured the canare plugs on diy audio ages ago and they didn’t meet 75ohms...also if it’s plugged into a standard RCA socket then theres no chance of it being 75ohm. Then there’s the consideration of the pcb traces and termination of the DAC itself, does that meet 75ohm. It’s a folly to chase this requirement.

Im surprised to read that a company with Canare's reputation are not making a product that meets their published specification.
I use Benchmark Dacs and their specification for input impedance on the RCA type connectors used on the DAC2 is 75 Ohms. From past experience, I have a great respect for Benchmarks engineering standards and would consider it unlikely that they would publish 75 Ohm input Z if it wasnt.
Now retired and no longer have access to the correct equipment needed to accurately measure these Zs but I suggest caution re believing everything one reads on DIY audio.

It might be interesting to look at the various quality DACs in the Hi Fi market and see how many actually use BNC.

Dan
So that leads me to the next cable, a blue jeans coax cable, this cable uses Canare RCAP crimped on plugs,these are a true 75 Ohm rca plug mated to a Belden 1694A 75ohm cable. So knowing exactly what i was getting 12 hours and £20 later its running in my system, well so far all i can say is its a great cable for the money all the detail is back that was missing from the Fisual and it feels very well made, its not the best looking cable but seems like it will do exactly what its supposed to, the fisual has been moved over to chrome cast duty and sounds good but to me it sounds no better than my plastic QED cable..

Good to read that progress is being made

[Pharos]

These posts are very interesting, and largely right in principle, (it was my specialist subject), but is this a significant factor in these contexts of audio?

There have been numerous articles on Z matches over the years, and in particular, although considering audio frequencies, ones on power amp into speakers comes to mind, most of which state that it is not significant.

[struth]

Ive got a Mogami 2964 Digital Coaxial Cable, 75 Ohm S/PDIF | Canare Gold RCA version.