BNC to RCA S/PDIF cable.

[Music or sound]

Thanks to all for the input here so far, good stuff.

Regarding the debate regarding impedance and RCA to BNC adaptors etc., I think it would help if I explain my motivation behind wanting to replace my adaptor for a proper BNC to RCA cable.

A while back there were some posts on Computer Audiophile regarding some experimental measurement kit that the Sonore guys are setting up, in part to to get some hard data regarding the benefits of audio clocks in various parts of the streaming chain.  There were some comments about the use of RCA to BNC adaptors when they were experimenting with the set up of this kit.  This is the key comment:

I had to use a BCN to RCA adapter in order to continue to use the same interconnect I had been using. When I switched to an interconnect that didn't need the adapter the jitter measurement was reduced by half.  

We still await the results of whatever it is that Sonnore are testing and experimenting with, but mindful of the comment above, I thought that ditching the adaptor might just be a good idea.....

I think it is likely the adapter but it could be also the different cables
There are also fancier adapters https://blackcatcable.com/products/xox-p...ca-adapter. That would require to use a BNC terminated cable. I am quite happy with my DIY approach.