Author Topic: 75 Ohm for Digital cable or NOT  (Read 2782 times)

Offline rollo

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75 Ohm for Digital cable or NOT
« on: January 26, 2016, 11:00:19 AM »
   There seems to be some arguement lately over the 75Ohm standard.
    It appears not all receiving end components actually have a 75 Ohm input. ie DAC.
   Reflections of the signal going up and down the cable is not good. George Cardas recommends a 17'-6" cable so the resistance handles the reflections. Others say 37.5 Ohms is correct. Got me.
   Any thoughts.


charles
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Offline jimbones

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Re: 75 Ohm for Digital cable or NOT
« Reply #1 on: January 26, 2016, 12:33:22 PM »
I know that when it comes to 75 ohm cable it is better to have a cable that is longer. But never heard the recommendation for a 17 ft long cable. I have a short ELCO cable that is awesome that I purchased from Bill. My experience is to not to focus solely on impedance or length but demo multiple cables in your system and pick a winner.
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Offline Werd

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Re: 75 Ohm for Digital cable or NOT
« Reply #2 on: January 26, 2016, 01:29:47 PM »
Digital coax can sound laid back while not losing any prat. Especially with instruments that end up layered like fillers (back ground keyboards and stuff like that). If you go to short like a meter or less it can still have PRAT but it seems to be on edge and less laid back. Like it's in a rush somehow. Never have listened to longer than 1.5 that I recall. Also with complicated time signatures in electric jazz the PRAT gets lost with cheap coax. Seems better in 1.5 meter. It's about PRAT imo.
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Offline P.I.

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Re: 75 Ohm for Digital cable or NOT
« Reply #3 on: January 26, 2016, 04:10:40 PM »
Digital coax can sound laid back while not losing any prat. Especially with instruments that end up layered like fillers (back ground keyboards and stuff like that). If you go to short like a meter or less it can still have PRAT but it seems to be on edge and less laid back. Like it's in a rush somehow. Never have listened to longer than 1.5 that I recall. Also with complicated time signatures in electric jazz the PRAT gets lost with cheap coax. Seems better in 1.5 meter. It's about PRAT imo.
Agreed.  When I was working with coax cables, it was fairly obvious to me that right at 1.5m (6') was the best sounding compromise.  It didn't matter if it was standard RCA's, Canare 75ohm-ish RCAs or BNCs... they all sounded better at that length.  AES/EBU just sucked, no matter what.  So much for standards.  :duh

I'm finding the same kind of results with USB cables.  Seems like .75m and 1.5m sound better than 1m, but the jury is still out.  I have them listening to hear what they hear.  I've been trying to tie transmission line theory into all of this, but admittedly digital is not my forte, even though I build some good ones by the old cut'n'try method.

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Offline Werd

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Re: 75 Ohm for Digital cable or NOT
« Reply #4 on: January 26, 2016, 06:00:03 PM »
Steve Nugent said this recently and gives light as to the length of digital cables.

 "Therefore, make it as short as possible, given your risetime, but long enough to avoid the first reflection (there are attenuated second and third reflections)."

I am using the XLR on my Bryston dac from the BDP. It sounds better or as good as the BNC. I like it better but not sure if its the transformer coupled input on the BDA2 or the BDP has a better XLR out and are designed to work together. It has a very stiff sound stage with great bass. I can see how it fails since the impedance on xlr is never near 110 ohm when i measure it (could be a lousy meter though) if that matters?. But other dacs i have tried the XLR is ok but it seems to sag a bit.
« Last Edit: January 26, 2016, 06:02:09 PM by Werd »
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Offline rollo

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Re: 75 Ohm for Digital cable or NOT
« Reply #5 on: January 27, 2016, 11:36:27 AM »
   The BBC insisted on balanced for studio use and Sony pushed single ended which sounds better according to the inventor.
    However it appears Sony gave in. If the receiving DAC has a pulse transformer rated at 75 Ohms any cable fed in will supply a 75Ohm signal. If not then the cable and component connectors must be 75Ohm.
    One can resistance to eliminate reflections as well. Say halfing the 75Ohm for starters.
    Dave if I were you forget the USB and male a USB/SPDIF converter instead. USB is just flawed and the effort just not be worth it. My 2 cents.


charles
   
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Offline Werd

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Re: 75 Ohm for Digital cable or NOT
« Reply #6 on: January 27, 2016, 01:19:07 PM »
The Bryston XLR input is transformer coupled also. Everytime I see a dac I like and it's not coupled, it makes me cautious. Only because I think where all the Prat comes from is the dac. Getting 75  or 110 ohm
at the input is crucial IMO. This is why I believe the BNC and SPDIF inputs are different and cabling makes a bigger difference on the SPDIF  input as opposed to the BNC.

I just wish Bryston would drop the nickel plating on the dacs.
Nola Viper Reference iii, Nola Blue Thunder Subs, Chapter Couplet 400s, Chapter Précis 250 integrated set to pre, Bryston BDA2/BDP1.
Torus RM-20 240v

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