Author Topic: 75ohm for digital co-ax, 50 ohm for analogue signals  (Read 1200 times)

Offline chrisc

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75ohm for digital co-ax, 50 ohm for analogue signals
« on: April 26, 2021, 11:06:35 AM »
A well-off Danish friend recently purchased a Wireworld Gold digital cable to link his media bridge to the DAC.  The cable cost €425.  It is said to be a 75 ohm cable yet has RCA phono plugs each end as opposed to BNC connectors which I thought was a requirement to maintain the 75 ohm impedance

He had to use RCA connectors since both the DAC and media bridge were equipped with RCA socket

He was using regular Audioquest phono cable before.   He maintains the sound has opened up and has more detail

How much might be his imagination?  I have no idea


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Offline kamikazi

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #1 on: April 26, 2021, 11:53:28 AM »
I experienced a similar situation with a Tributaries digital coax cable which is also terminated for 75ohm, but was recommended by HFX Systems to use for stereo analogue audio, though I was a bit wary about the 75ohm and admittedly I also never liked them that much for analogue playback so they kind of ended up in the cupboard. Yesterday experimented with digital coax cable options for a CDP in a friend's audio system and this cable ended up being quite a bit better than the options we tried which was quite a surprise to me. Was also the only 0.5m cable we had where the other were 1.5m which is said to be optimal in terms of signal reflections.

Supposedly the 50-75ohm impedance mismatch is too low to be really an issue, but with the above I'm not really sure about it either.

Offline Hi-Phibian

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Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #2 on: April 26, 2021, 12:11:23 PM »
Was also the only 0.5m cable we had where the other were 1.5m which is said to be optimal in terms of signal reflections.



Both 0.5m or less and 1.5m are good. 


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Offline sajunky

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #3 on: April 26, 2021, 07:42:00 PM »
Supposedly the 50-75ohm impedance mismatch is too low to be really an issue, but with the above I'm not really sure about it either.
There is no impedance mismatch when:
1. Source driver and a receiver impedance matches cable impedance.
2. A right connector is used. There are two two types of BNC connectors: 50 and 75 Ohm. They look similar, but can be visually identified by a professional.

S/PDIF coax specification is very lose and electrically inferior to the AES/EBU coax type AES-3id. Driver impedance matching is also not followed in many implementation. In the end, results are unpredictable.
If it measures good and sounds bad—it's bad. If it measures bad and sounds good, you've measured the wrong thing.
/ Daniel R. von Recklinghausen, Chief Engineer, H.H. Scott/

Offline Orcish75

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #4 on: April 27, 2021, 10:50:24 PM »
I'd say it's the other way round, The old 10Base2 ethernet (digital) uses 50 ohm cables and just about all analogue video equipment is 75ohms. I'd even try using the T-Piece on each end with the terminating resistors to prevent signal reflections.

https://www.telecomworld101.com/Thinnet.html

The frequency of the S/PDIF signal is not going to be an issue on such a short run of cable anyway, regardless whether it's 50 ohms or 75 ohms.

Offline chrisc

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Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #5 on: April 28, 2021, 08:03:01 AM »
I dare not tell him that he may have wasted his money.   I made up a 1-metre SPDIF cable using vh Hul 75 ohm cable and 2, Neutrik RCA phono plugs, which are well made, have a rubber strain-relief and a zinc-plated screw on body.

This cable works so well I have made up a further 4 sets for other people.   The cost was about R330,00

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Offline Agaton Sax

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #6 on: April 28, 2021, 08:30:59 AM »
I dare not tell him that he may have wasted his money.   I made up a 1-metre SPDIF cable using vh Hul 75 ohm cable and 2, Neutrik RCA phono plugs, which are well made, have a rubber strain-relief and a zinc-plated screw on body.

This cable works so well I have made up a further 4 sets for other people.   The cost was about R330,00



I own a number of lengths of that cable with those very plugs

I find it a moderately good  composite video cable.. As an spdif cable it is IMHO worth R330 .

Less than a week ago, I re installed my CD transport. In that process , I went through 12 spdif and 3 AES cables  that I own. Those included 3 different lengths of the van den Hul discussed above. A few years ago I tried all the spdif and AES that Croak had. My conclusion was ( and is)  that one gets the sound you pay for. Also that ,as Croak said, either 0.5m or 1.5.

The reason for the spdif over AES was that I wanted to try my reclocker again . That device takes spdif to the reclocker and spdif out to the DAC.  . Even the spdif from  transport to reclocker makes a considerable difference.

My ears , my system, my conclusion.Yours will be different.
« Last Edit: April 28, 2021, 08:35:38 AM by Agaton Sax »

Online pwatts

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #7 on: April 28, 2021, 08:40:54 AM »
S/PDIF will work over a coathanger. It sometimes will lock even without the ground return connected. It's not a binary situation about having a valid/locked signal or not though; mismatches because of waveform distortion will lead to jitter in the clock recovery circuit which effectively is what causes better cables to sound better.
IMO more gains are to be had from improving improper or textbook line drivers and receivers in the electronics used than the cables itself (see my thread on the Gustard U12 mods for reference, and that is by no means the worst I've seen across several price brackets), but of course hacking away at electronics is not everyone's cup of tea. Of all the implementations I've seen so far, Aurender intrigues me the most - sadly I can't do a full reverse engineer but from high-res images they seem to take extraordinary care with it.

Offline Agaton Sax

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #8 on: April 28, 2021, 09:17:22 AM »
Some of the sdif I tried was very little more than a coat hanger. One surprise was a simple twisted, unshielded pair of siver in teflon. In the final conclusion it was the dedicated designs that came through though.

TheWyred for  Sound reclocker  was initially bought to use between an Apple Airport Express and Dac. When I abandoned that route it became redundant. My CD transport is a Studer A 727, so I assume, technically well designed . A little lateral thinking brought  a jaw-drop moment with reclocking between transport and dac .  Have done this now with 4 Dacs. Strangely enough, the Berkeley DAC reclocks all incoming signals itself and even here it made a difference.

Reporting the audible only not qualified to debate the technical

Offline Hi-Phibian

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75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #9 on: April 28, 2021, 04:30:20 PM »
Spdif termination I have found very important too (it can “break” a good cable) and for co ax digital I try use connectors that allow me to fold back the screen and maintain its circular geometry. 

For that reason, good as those Neutrik Rean 373 plugs are, they can be improved upon with for eg amphenol that has a metal cable clamp to grab the folded over screen and cable.

A pig tail twist of the screen should be avoided if possible. 
Croak Audio
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Offline scrarfussi

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #10 on: April 28, 2021, 05:25:02 PM »
i have an old xlo coax cable
sounded way better than anything else
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Offline Nidri

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #11 on: May 05, 2021, 10:54:05 AM »
The Canare RCAP plugs are supposed to be true 75 Ohm. If it matters.

https://www.bluejeanscable.com/pages/technicaldocs/canarercap.pdf

I have a few solid-conductor Belden 1694A and multi-stranded 1505F (Flexible) cables terminated with either BNC/BNC or BNC/RCA Canare plugs. They work. Couldn't say if they're any better or worse than any other digital cables I have, e.g. Cardas Video/High Speed Data.

In theory one could use the Canare plugs to terminate other, more upmarket cables like the Wireworld mentioned I suppose. G you'd know better how easy they are to work with (or not).
Whilst Nidri takes reasonable care to ensure that the information in this message is correct and up to date, this might not always be the case. Nidri does not give any warranties as to the accuracy and safety of any information appearing in this message.

Online pwatts

Re: 75ohm for digital co-ax, 50 ohm for analogue signals
« Reply #12 on: May 05, 2021, 12:53:03 PM »
The Canare RCAP plugs are supposed to be true 75 Ohm. If it matters.

The geometry of the plug means it will still not be 75Ohms although they did an admirable job of addressing other issues with RCA. In addition, the mating female socket of course has to have a similar construction which usually isn't.