Hi 
…
A quick question asked by a friend.
Also,.have Naim or Chord a cable that makes him avoid said adapter.
I recently purchased a Chord Qutest. It has two options for inputs, BNC and optical. Currently I’m using a S/P DIF lead out of my Cambridge Audio CXN2 into one of the Qutest’s BNC inputs, with a BNC adapter on the S/P DIF lead. It sounds good, but I’m wondering if this is the optimal connection method.
No, a RCA to BNC would be an optimal connection. Adapters should be avoided as they alter the 75 ohm ideal.
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According to the designer optical is the best connection for all Chord dacs
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This is true.
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Not a direct answer, but you can gat a Van Damme 75 ohm cable RCA to BNC with good quality Neutrik connectors. A quick Google will find. I think better than RCA to BNC adapter would be, especially is the RCA to RCA cable is not 75 ohm, which RCA cables for general audio are not normally, and only would be if specified (S/PDIF specofication is 75 ohm). In the past I have bought RCAs plugs claiming to be 75 ohm, but my understanding since then is that despite the claim it might not be physically possible for RCA plugs to truly 75 ohm. I don’t recall what brand the ones were that I bought. However where RCA is avoidable, as with the person you mention, that with good quality 75 ohm cable and 75 ohm BCA plugs would be the nearest achievable.
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According to Naim it’s best to use electrical rather than optical SPDIF, so they can’t both be right. With my Hugo and Dave I’ve had better results with electrical when using them with a Naim streamer.
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Of course they can 
Both use very different DAC architecture. Chord dacs are supposidley immune to jitter, which is the big problem with optical.
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I guess one should align with what the manufacturer recommends, unless listening test prove differently.
If Chord say use toslink why would we disagree. One might suppose the manufacturer knows best. Perhaps they designed their equipment differently from naim.
We use a selection of connections here. naim solely in lounge so coax DC1, but in office our workaround for Tidal Connect and hires radio (PI) sounds perfectly adequate over toslink.
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Naim recommend coax, Chord recommend optical (apparently) so which manufacturers recommendation do you want to take? I used my ears and concluded that coax sounded a little better on devices that offer a choice. With a Naim streamer the only output option is coax and it works extremely well into Chord DACs so I won’t be losing any sleep over it.
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Well you were in a position of finally not being able to adhere to manufacturer recommended connection method to DAC, due to source (NDX2) not providing it.
Kinda makes it a moot point.
With a view to the future we’d be interested in which other streamers (with both optical and coax connections) you tried into Dave, concluding coax was best?
There is a third option. Shielded twisted pair.
To comply to S/PDIF standard, the cable must be coax or optical, but if you look at the application notes of the transformers used in the transmission of S/PDIF, it does not specify coaxial or 75 ohm.
I’m using a Primare streamer that uses a Pulse transformer ref PE-65612NL. Recommended interconnecting cable is single shielded twisted pair.
The S/PDIF transformer in the ndac is a Murata DA101MC. No recommendations in the data sheet to use coaxial or 75 ohm.
So, I’ve got a nice piece of shielded twisted pair and an RCA plug. All I need is a 75ohm BNC plug and I’m ready to go. 
@Simon-in-Suffolk may be the most informed person to advise on RF transmission requirements at the frequencies applicable to S/PDIF.
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I think there is… it says the transformer complies with IEC 958, which is the standard that incorporates SPDIF which is defined as using a 75 ohm transmission line system.
I guess the difference between engineering device information and consumer device information in terms of how the information is presented.
In a transmission line or the elements of the transmission line chain must be at the same characteristic impedance to work optimally and efficiently and with least degradation or distortion, ie 75 ohm CI for plugs, sockets, transformers, coax cable etc.
Though SPDIF was designed as a consumer transmission standard and so is relatively tolerant of this hence why standard phono plugs and sockets which physically can’t be 75 ohm are usually seen, however the cable CI would be more important to comply with. The commercial /professional equivalent is AES3.
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I don’t understand your discussion. You say optical is the best with Chord, Chrissu says Naim prefers coaxial. But in the case of the Ndx2, used as a transport into Dave, the former doesn’t have an optical output, does it ?
I think that was @IainO 's point, but a moot point for the OP as his question concerned a Cambridge Audio CXN2 i not any Naim component.
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So, yesterday a made a cable for connecting Primare NP5 streamer to Ndac.
The cable I had at hand enabled me to make up a twisted pair, screened 120ohms CI, 2 x RCA plugs, with screen connected at one end only. Bi-Laminate (Alum+Poly) inner shield.
This is obviously nothing like the S/DIF spec but it is very close to the AES3 standard (110ohms STP, with screen connected at both ends).
Looking at the different standards; AES3 with 110ohms STP, is good to 1000m, whereas the S/PDIF 75ohms coax only 15m. Although the AES3 voltage is a lot higher.
It sounds very good.
Better than the coax spdif cable I’ve been using. (Something costing £35 from maplins about 15 years ago). Not as bright.
Sounds the same as the glass toslink cable I’ve been using.
The question is. Will the 120ohm cable be causing any damage to the streamer or dac, which are designed to work with a 75ohms cable?
No will not damage the connected units. Whether the sound would be audibly degraded compared to 75 ohm I do not know.
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AES3 comes in two variants, balanced and unbalanced. Unbalanced uses coax and uses CI of 75 ohms, the balanced variant uses CI of 110 ohms, and has different physical characteristics to SPDIF.
If you create an AES3 balanced cable (110 ohm CI) , you clearly need to have AES3 balanced plugs and connectors, and AES3 balanced interface electronics in your devices. If you don’t do this you will be operating well outside specification and you may find your link becomes quite lossy, and/or the ends of the link are producing high levels of reflections. It is will almost certainly work still, especially over a short distance, but you may be introducing a higher framing error rate which may cause little millisecond dropouts or clicks occasionally.
Remember these sort of relative high frequency links use transmission line engineering principles in their design standards, and for minimum loss or distortion of the signal should have as consistent as possible CI from one electronic interface, connectors, plugs, cable, to other electronic interface, with SPDIF that CI is 75 ohms.
Many many years ago I experimented creating none standard SPDIF leads which were well outside specification, one such cable actually sounded quite ‘warm’ when used in my particular non Naim setup which I quite liked , but I did find later it was adding framing errors in the link from time to time… I do use cables suitable for SPDIF now, and yes they are not hard to make, especially with BNC connectors. There are many variants of 75 ohm coax cables that you can buy and tweak to your heart’s content if you decide to make your own.
Finally using incorrect cables and plugs at this extremely low power level will almost certainly not damage anything other than possibly performance.
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