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SB Touch to Metrum Octave: toslink, coax RCA, BNC?

I'm finally pulling the trigger on an Octave, and while I wait for it to arrive I'm thinking about the digital link to the SB Touch. I don't have a good digital cable (using Touch analog out), so need to buy one. Should I go for toslink or coax?

If toslink, it would be a glass "cable".
If coax, I know about the inability of the RCA plugs to achieve 75 ohm cables, and have seen comments about replacing the RCA on the Octave with a BNC connector. Good idea if a Touch is on the other end?

Input from experienced users is highly appreciated!

Thanks!
lewinskih01
lewinskih01 OP
909 posts
 
Thanks guys. So I'm leaning towards coax with RCAs.

Should I DIY with true 75 ohm RCA plugs? BTW, which RCAs are "true" 75 ohm? WBT?

I have Purist Audio Museaus analog ICs, and doubt the RCA plugs on their Genesis RCA coax is truly 75 ohm.
kijanki's avatar
kijanki
4,432 posts
 
I used Canare 75ohm RCA plugs.
vhiner
544 posts
 
My vote is for a good coax...never heard a toslink beat a coax of equal quality. Depending on your budget, I've had good experience with the following in descending order of quality: Shunyata Python digital, Shunyata Cobra digital, Moray James digital ( very rare but inexpensive and incredible), Harmonich Tech Magic digital, and the ever-affordable Kimber Illuminati D-60.
lewinskih01 OP
909 posts
 
Kijanki,

Did you replace the RCA female plug on the Octave as well?
What did you use for the cable?

thanks!
kijanki's avatar
kijanki
4,432 posts
 
It wasn't Octave, just short coax between CDP and DAC without changing female socket. I used Canare coax and connectors but there is a lot of different choices. Quality of shielding is extremely important. Buy or build either very short <12" or >1.5m because of transmission line effects (reflections in the cable affecting edges).

Connection becomes transmission line (relections on impedance boundaries) when propagation time is less than 1/8 of transition time. Signal travels about 1.5ns/ft (5ns/m) allowing for 8x1.5=12ns transitions. Typical transport has 25ns transitions - we're safe.

For 1.5m cable reflections might happen, but they will just miss original edge. Reflection starts from the beginning of transition and takes with 1.5m cable (both ways) 15ns to come back (3mx5ns/m) and add to original edge and it's just missed threshold point that is roughly in the middle of transition (12.5ns).
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