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Digital interconnect: RCA, BNC and adaptors?

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My current source (Bryston BDP-2) has BNC output. My current DAC (Hugo) has coaxial RCA input that's not compatible with chunky plugs. My current interconnect (Audusa) is RCA both ends with v. chunky collared plugs.


So at the moment I'm running BDP-2> bnc/rca adaptor>interconnect>chord supplied rca/rca adaptor> Hugo. Using any adaptors doesn't seem like a good idea on this level of equipment let alone 2. So should I:


1. get a new i/c terminated both ends with BNC and then use a BNC to RCA adaptor at Hugo input (I got the idea from somewhere that BNC is the preferable connection for coaxial digital)


2. get a new i/c with BNC and RCA termination respectively (latter with Hugo-friendly plug)


3. stop worrying as the current adaptors can't make a difference?


I can connect the items via USB too (and to be honest at present they sound pretty much identical either way), but I'm just interested in whether my current set up will hinder coaxial input.



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For very long digital interconnects (10meters/30feet)

The best choice is:

2. get a new i/c with BNC and RCA termination respectively (latter with Hugo-friendly plug)


But if what you have works, why worry about it?

Whit digital cables it a rather go/no go situation. Either the sound is good or the sound is bad.

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Impedance matching is an important concern with S/PDIF coax systems, the official spec is 75 ohms. RCA plugs/jacks are nowhere near 75 ohms, so anything with an RCA is going to have a significant mismatch. BNCs come in both 50 and 75 ohm versions, which is why they are frequently used, if you get the right one it can really be 75 ohms.


Unfortunately 95% of the BNC connectors out in the wild are 50 ohm versions, so you have to be really careful to get the right ones. I am assuming that Bryston got it right and used a 75 ohm connector.


If you are going to have a mismatch it is best to have it at one end only, so since the DAC has the RCA, that should be the only side with an RCA. So the best way to go is a 75ohm cable with a 75ohm BNC on one end, and an RCA on the other. Unfortunately a large percentage of the places that sell 75ohm cable with BNC and RCA use 50 ohm BNC connectors! Several years ago I bought samples from 10 different companies and only two actually had 75 ohm BNCs.


One company that gets it right and has very good cables at reasonable cost is Blue Jeans Cables. Their 75 ohm BNC to RCA is superb.


John S.

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S/PDIF is a robust system that operates at a rather low frequency for Transmission Line Theory to come into play. For cables less than 10meters/30feet long the Radio Frequency Characteristic Impedance just doesn't matter. Characteristic Impedance impact is a cable distance to cable length thing. An RCA connector at less than an inch long doesn't have a Characteristic Impedance at these low frequencies.

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Impedance matching is an important concern with S/PDIF coax systems... <SNIP>


John S.


Here's Steve Nugent, another highly technically adept high-end audio designer saying basically the same thing as John:


Breakthrough mod to reduce jitter for OR5 and SM


With test data to show the jitter reduction.


Greg in Mississippi

Everything Matters!

2 systems... Well-Tempered Refs->ET-2.5->DIY or Lounge LCR MkII phono stages

Standalone digital Sony HAP Z1-ES or SDTrans384/Soekris DAM DAC

Networked digital Zotac PI320-W2 LMS Server -> EtherRegen -> USBBridge Sig -> Katana / Ian GB / Soerkis / Buffalo-IIIPro DACs

Passive S&B TX102 TVC or ladder attenuators -> BHK-250 -> Eminent Tech LFT-VIII / IV / VI

ALL gear modified / DIY'd; cables MIT;  all supplies DIY’d or LPS-1.2s w/HUGE Ultracaps; Audio gear on DIY AC filters + PS Aud P15s; misc gear on separate AC w/filters

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