DIY SPDIF Coax

[Hibster_2000]

I am tempted to try and make a coax cable myself for use on digital audio.
I am thinking of using Belden 1694a or Mogami 2694 for the cable.

The question I have is about the plugs. Should I fit some Canare rcap plugs, or will any reasonable quality RCA plug be ok?
The Canare are 75 ohm plugs, but look pretty cheap considering the price of them.

Has anybody done this before, and how important is the plug selection?

[Hibster_2000]

Thinking about it, I doubt that the RCA sockets used on my system are 75 ohm for the digital, so any old RCA of quality should be ok?

[NRG]

Yup, the socket won’t be 75ohm so no need to spend silly amounts on the plug. I’d go with the Belden cable as it’s foil and braid or something like Belden 8281 which is double braid. Make your cable in 1.5m lengths not 1m, this helps to offset signal reflections due to the impedance mismatch caused by the use of RCA plugs n sockets.

[struth]

I bought a mogami with a quality plug on. Weren't cheap but good value and works very well

[Hibster_2000]

Having just found a Mark Grant coax built with Canare LV-61S in my cable bag, I don’t think I will bother.

[Light Dependant Resistor]

I am tempted to try and make a coax cable myself for use on digital audio.
I am thinking of using Belden 1694a or Mogami 2694 for the cable.

The question I have is about the plugs. Should I fit some Canare rcap plugs, or will any reasonable quality RCA plug be ok?
The Canare are 75 ohm plugs, but look pretty cheap considering the price of them.

Has anybody done this before, and how important is the plug selection?

Adherence to 75 ohms sadly cannot occur with a RCA termination. Rather you need to install BNC fittings to
adhere to 75 ohms. The use of RCA with digital, historically has been a trade off between keeping the customer in the dark,
making a sale, and appeasing to something that has no scientific basis, whilst providing a incorrect type of termination and
making sure no one questions. Such is marketing vs what is better.

[struth]

https://www.amazon.co.uk/gp/product/B079J2VCVZ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

These are guaranteed to be within 10%

[Barry]

Adherence to 75 ohms sadly cannot occur with a RCA termination. Rather you need to install BNC fittings to
adhere to 75 ohms. The use of RCA with digital, historically has been a trade off between keeping the customer in the dark,
making a sale, and appeasing to something that has no scientific basis, whilst providing a incorrect type of termination and
making sure no one questions. Such is marketing vs what is better.

Whilst it would have been preferable to fit a true 75 Ohm BNC style connector for the SPDIF, the problem is that most BNC connectors used for RF have an impedance of 50 Ohm, and to the casual observer the two look very similar. Thus even if a 75Ohm BNC connector was fitted, an aftermarket coaxial interconnect between the CDT and the DAC might be fitted with 50 Ohm types, even if the cable itself had a characteristic impedance of 75 Ohm.

The impedance mismatch so created would be 1.5:1 (or -14dB), not that much better than that caused by using RCA connectors. Even worse, trying to mate a 50 Ohm impedance BNC plug to a 75 Ohm impedance BNC socket is quite likely to damage the latter.

The best that can be said about using an RCA connector is that the two mismatches created (the first between the 75 Ohm system within the CDT and the RCA socket, the second between the RCA plug and the 75 Ohm cable), is that the two reflection coefficients are of similar magnitude but have opposite sign. And since the two mismatches are electrically close (essentially the physical length of a mated RCA plug and socket), they will tend to cancel one another.

[Barry]

https://www.amazon.co.uk/gp/product/B079J2VCVZ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

These are guaranteed to be within 10%

Within 10% of what? Most RF cables have a reasonably tight specification on their characteristic impedance: +/-2 Ohm for 50 Ohm cables (or 4%) and +/- 3 Ohm for 75 Ohm cables (also 4%). As has been stated above, RCA style connectors (the same as are shown in the link) can never have an impedance of much more than about 40 Ohm; but practically this impedance mismatch has less of an effect due to the electrically 'short length' of a mated RCA connector pair.

I do like the way the connectors shown are crimped onto the outer screen of the cable. Does nothing for the impedance but does preserve the integrity of the shielding.

[Light Dependant Resistor]

Hi Barry
Instead of succumbing to RCA's which are wrong as we know - can we instead encourage readers to begin to know the differences
and what to look for with a 75 ohm BNC

75 ohm type BNC's are available here: https://www.pasternack.com/75-ohm-bnc-connectors-category.aspx
and explained here https://www.milestek.com/blog/index.php/2011/04/50-ohm-vs-75-ohm-bnc-connectors-explained/

" So what’s different?

Physically, the main differences can be found in the center pins and dialectric insulators. 75-ohm BNC connectors feature Teflon as a dialectric, and surround the outer spring fingers with air. Its center pin maintains a consistent diameter in both the front and rear areas (this is important—read on to find out why). 50-ohm connectors, on the other hand, use Delrin to surround the spring fingers, and its center pin is larger in the crimp area. You’ll need different crimp tools for each type of center pin.

What’s the difference in functionality?

Both connectors can mate without damage, but you can’t really mix and match and expect to get the same video quality. For example, if you installed a traditional 50-ohm BNC connector on a 75-ohm coaxial video cable, the digital signal would be significantly distorted.

Basically, 50-ohm BNC connectors are used for older analog video applications. But for higher performance A/V applications like HDTVs, 75-ohm BNC devices are necessary. 75-ohm BNC connectors can, however, be used for analog purposes."

[Barry]

Many BNC connectors use the same dielectric regardless of impedance. Thus the 75R connectors use a smaller diameter inner conductor compared with the 50R versions. So if one tried to mate a 50R BNC plug with a 75R BNC socket, the socket would be damaged.

That is why the BBC insisted on using only 50R BNCs, regardless of application. https://theartofsound.net/forum/showthread.php?3849-Basics-amp-General-Audio-Connectors

However some 75R BNC designs dispense with the dielectric in the region where the connectors mate, and can thus use the same diameter for the inner conductor as that of the 50R connectors (the dielectric will have a relative permittivity of ~ 2.25, so the impedance ratio is reduced by 1.5). With these designs the two impedance versions can be physically mated without damage.



But all RCA connectors are IMO lousy and would be better replaced with BNC, which are of a similar size and are a much more precise and rugged connector.

[Edward]

This thread got me looking at the cable between my Digi+ S/PDIF out which is connected to the DAC. The cable is a Belden 1694A with RCA Canare plugs.

The cable is marked 'Video Brilliance (R)'.

Is this cable rubbish and I need to go elsewhere? :scratch:

[Barry]

It looks fine to me Edward.

The Belden 1694A cable is the same as RG-6/U: https://catalog.belden.com/techdatam/1694A.pdf, and the plugs are the same as those on the lead mentioned by Grant: https://www.amazon.co.uk/gp/product/B079J2VCVZ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1, the latter of which are crimped onto the cable and is IMO the better way of attachment.

The Belden cable has slightly less loss compared with the Mogami cable.

[Edward]

It looks fine to me Edward.

The Belden 1694A cable is the same as RG-6/U: https://catalog.belden.com/techdatam/1694A.pdf, and the plugs are the same as those on the lead mentioned by Grant: https://www.amazon.co.uk/gp/product/B079J2VCVZ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1, the latter of which are crimped onto the cable and is IMO the better way of attachment.

The Belden cable has slightly less loss compared with the Mogami cable.

Ah thanks Barry for going through all of that for me. Appreciated. So I will not look further knowing all is fine. You saved me some time and money. Cheers. :)