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Replacing RCA s/pdif sockets with BNC's
Replacing RCA s/pdif sockets with BNC's
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Old 21st January 2020, 07:11 PM   #1
raueda1 is offline raueda1  United States
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Default Replacing RCA s/pdif sockets with BNC's

Most of my digital gear already has BNC s/pdif connectors. There are couple exceptions, however, and it seems to make sense to just replace the RCA connectors with proper 75 ohm BNC sockets. I'm tired of wondering what problems are introduced by RCA => BNC adapters, BNC => RCA cables and other dubious kludges. But I wonder if the equipment designers did anything special to accommodate the RCA sockets in terms of impedance or god-knows-what else?

So: Is there any reason not to just go ahead and replace the RCA connectors with BNC's? Is there any special lead dressing or positioning with the BNC sockets that I ought to be aware of? special wire from BNC to PC board?



The gear in question is an M-Audio CO3 (source selector and s/pdif <==> AES/SBU converter) and BluSound Node 2i. Maybe also Asus Xonar Essence STX sound card.

Thanks for any comments.
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Old 21st January 2020, 07:26 PM   #2
diyiggy is online now diyiggy
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Hi,
If your cable is 75 ohms and it's ok with that, sometimes the simpliest for tv antenna are the best...but as you are in USA Belden cables is an excellent choice. Now if yohr stuff has a 50 ohms rca you want to swap by a BNC, you have to check the passive parts before on the board just before the RCA. They could be here to show a 50 ohms to the RCA and ask you some resistors value tweaking.

However I have the feeling now most gears have optical spidf (toslink) and coaxial spidf with rca for mass markett becomes the exception at the opposit of the good BNC you see on pro stuffs (while rj45 is going to winn the battle for stuffs interlinks in studios)
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Old 21st January 2020, 07:34 PM   #3
diyiggy is online now diyiggy
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Edit, also, I don't know if it was what you asked for pc to external digital gear interlink, the good thing with toslink is the isolation between grounds...a short 75 bnc to toslink to bnc board could be a cool thing to improve over a noisy pc.. things as you wrote is to keep the correct and same impedance between boards... I don't know if it's a good idea and if such gadgets exist, I'm happy myself enough with BNC when I use it. I always try to short the distance between two digital gears I,m not speaking about diferential transmission in tje anqlogic domain but just about coqxial rca vs 75 homs spidf. Anyway the simple rule is to keep the rigjt impedance, so cables chosed with the same impedance than the jacks and sockets....
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Last edited by diyiggy; 21st January 2020 at 07:40 PM.
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Old 21st January 2020, 09:33 PM   #4
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by raueda1 View Post
Is there any special lead dressing or positioning with the BNC sockets that I ought to be aware of? special wire from BNC to PC board?
Use 75 ohm mini-coax between connector and board (though pcb-mount bnc sockets are ideal as they can go straight to a 75 ohm microstrip trace). Neat and tight layout is advised.
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Old 21st January 2020, 11:42 PM   #5
raueda1 is offline raueda1  United States
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Quote:
Originally Posted by diyiggy View Post
Edit, also, I don't know if it was what you asked for pc to external digital gear interlink, the good thing with toslink is the isolation between grounds...a short 75 bnc to toslink to bnc board could be a cool thing to improve over a noisy pc.. things as you wrote is to keep the correct and same impedance between boards... I don't know if it's a good idea and if such gadgets exist, I'm happy myself enough with BNC when I use it. I always try to short the distance between two digital gears I,m not speaking about diferential transmission in tje anqlogic domain but just about coqxial rca vs 75 homs spidf. Anyway the simple rule is to keep the rigjt impedance, so cables chosed with the same impedance than the jacks and sockets....
Sorry, I'm not sure what you're trying to say. First, I'm guessing by "PC" you mean Printed Circuit Board (aka "PCB"), not Personal Computer. Second, this is nothing to do with 50 ohms, TV antennas, cable selection etc. I have 75 ohm cables and connectors. Finally, TosLink doesn't enter into it at all. IMO TosLink is a cheap, flawed approach used in mass market gear and unsuitable in a high end system. I'm really just asking about swapping RCA for BNC sockets in 75 ohm s/pdif components and any associated issues.
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Old 21st January 2020, 11:45 PM   #6
raueda1 is offline raueda1  United States
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Quote:
Originally Posted by Mark Tillotson View Post
Use 75 ohm mini-coax between connector and board (though pcb-mount bnc sockets are ideal as they can go straight to a 75 ohm microstrip trace). Neat and tight layout is advised.
Makes sense. I'm going to take it all apart and see if I can source BNC PCB sockets with same (or close enough) dimensions as existing RCA sockets.
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Old 22nd January 2020, 12:06 PM   #7
diyiggy is online now diyiggy
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Hi raudea,

Yss I understood pc as personal computer. So I answered in my first post, forgett the second which was given about spidf noise between two gears if it's not your particular problem.There is no difficulties in your problem but the usual advices already given by me and Mark, so short, the less contacts you can acheive between the two pcb (signal weaked by more solder bridges with the tweak to swap rca for bnc, aka boards and foot print for the new connectors (indeed the less contacts the best or professional will just stand in the look of the bnc stuffs). And yes, impedance 75 ohms checking if the original board was made with 50 ohms rca purpose for rca cables. As I said use whatever 75 ohms cable you have, the Belden cable was a plus, forgett it if no need.

Hope that helped, if not, forgett it
Cheers
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Last edited by diyiggy; 22nd January 2020 at 12:11 PM.
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Old 22nd January 2020, 03:33 PM   #8
jean-paul is offline jean-paul  Netherlands
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RCA connectors were used as they were cheap. A bit like the RJ45 connector in network stuff. In the beginning of CD players no one cared if they were 75 Ohm or not as it just worked.

BNC 75 Ohm are way better. Mechanically more sturdy and you cannot mix up with analog. Just choose a version that can have 500 matings as many cheap ones are rated for only 50 matings. The silver plated Neutrik BNC are the shortest I have found and they have a larger diameter so they fit the existing hole in the chassis or you even need to widen that one a bit.

BTW please use insulated BNC connectors!
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Old 22nd January 2020, 08:45 PM   #9
aboos is offline aboos  Germany
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I can only fully agree on what jean-paul said. RCA connectors were and are not designed nor characterized for their chracteristic RF impedance. But SPDIF is an RF signal (and not an audio signal) and clean and distortion free signals are a must for the receiver to extract and recover the clock signal with lowest possible jitter. And the typical RCA cables used are undefined on top. The reason for sound differences between digital cables are simply differences resulting from RF signal distortions introduced by RF signal reflections due to impedance mismatches between board, cable and connectors imho.

BNC has been designed with RF stuff in mind and using 50 Ohm coax cable with BNC connectors ensures good impedance matching. (be aware that BNC normally is a 50 Ohm system while most TV stuff is 75 Ohm)

Same by the way is also true for RJ45 network stuff - this system was designed for RF signal transmission (which is the case for Ethernet LAN) via twisted pair cables with 110 to 120 Ohm impedance.

Last edited by aboos; 22nd January 2020 at 08:48 PM.
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Old 23rd January 2020, 03:27 AM   #10
diyiggy is online now diyiggy
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Hi,
Imho the important facfor is the cable, indeed it should be 75 ohms if gears manufactors has followed the spidf standard. Then this cable should be the less capacitive to reduce high frequencies filter effect due to dielectric loss. I'm not sure the impedance mismatch of the connectors to be very important nowadays with modern dacs qnd in your conf , but more the type of connectors to make simple and more direct links and also shorter (less capacitive), i.e. to avoid active connectors adaptators as the M-Audio. Less hassle indeed if it's the goal to make it plug n' play and simple and also less loss. I'm not sure what the most important here is the impedance break due to the rca socket (indeed few are 75 ohms while it exists) because it works as said by J-P enough with any non 75 ohms rca(while it doesn't hurt to stay 75 ohms if your bnc are true 75 ohms, as 50 or 110 ohms exist as said already).
I mean there is a chance that the tweak towards bnc makes it worse by extra soldering, ads of micro coax between the bnc and the pcb if mandatory due to pcb holes and socket feet mismatching. Ifnthe spidf cqble is less than 1 to 2 meter I will not bother. And there is a chance jitter not to be important as modern soundcard and dacs buffer this signal and often now reclock it before to send it in the modern dac chips.... I don't believe trully in data loss with short spidf coax length and ths 0,5 V mostly used for spidf in coax signal transport and with HF below the Ghz AES/EBU being a different story for spidf in the professional uses.

I may be wrong as I'm not a specialist but the op should not worry buf if it's simplier to use Bnc for him because interconnects and gears move often and stay away from the MAudio connectors adaptator BNC is good as said for HF, contacts are better and thiner for sure and I also believe it helps as aboos and Jean-Paul said. I'm also more worried by what are between the active part signal source and the connector in the same board and the same on the receiver side on the board of the other gears.
The only good think I could see in the MAudio connectors adaptator is to use the rca to a short toslink ability to break noise from the ground if a big personal computer is at the party. I'm not sure the Blusound node has an spidf rca inputt, it has a toslink imho, the opposit for the Asus stx dac headphone card for computer.

Hope that helped a little more, if I am wrong someone correct my inputt.
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Last edited by diyiggy; 23rd January 2020 at 03:32 AM.
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