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Coax Out - Odd Issue. Some 75ohm Cables Work, Others Don't. All Analog RCA Cables Work Fine In Same Output/Input


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This one has me completely stumped.

Situation - Running Coaxial from source to DAC.  Distance 5m.

 

Running from my source using its coaxial out to a DAC using its coaxial in,  I use the Belden 1694A cable.  I have a few lengths of this cable and also some other 75ohm cables and was able to experiment/troubleshoot and the oddest thing happened.

When I run upto about 2.5m coaxial cable from the source to the DAC  there is no issue  ...After that no matter the type of Coaxial cable I use the signal plays up a bit and pauses every so often.  It plays a few min then has a second so pause and continues.  If i skip track or start a new track the DAC doesn't read the signal at all untill I back out of the track and start it again.

 

Here is where it gets weird IMO. 

I switched to one of those really thin cheap old yellow plug cables 5meters in length and things worked flawlessly.  I also then thought I'd switch in in a standard RCA cable I've got running from the same source to the same DAC which in this case is the Bluejeans LC-1 analog cable and it also works peferctly and that run is 6m. 

Switch back in any of the Coaxial cables of that length and the issue comes back but use a coaxial cable of around 1-2m and there is no issue again!!

 

Is there any technical answer for what is happening?

 

I am leaning down the path of using the single LC-1 "analog" cable as it runs the correct length and has good shielding.  I heard absolutely no SQ issue. I dont really know what jitter sounds like but figure as things sounded no different to the shorter coax things were good.  I could also use the basic yellow rca cable but IMO how can be sure that's actually meeting any technical spec that the Blue Jeans LC-1 isnt.

 

Some quick googling returned that 75ohm is preferrerd when running cable between coaxial output/input but I could find nothing that matched this oddity going on and further more some arjy barjy that the RCA connectors impact the actual cable ohm and that any RCA cable will in fact work safely and fine.  If there was any issue with the cable being used it would be immediately audible or not function at all.

 

Would love some thoughts on this

 

cheers.

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Could this be associated with the capacitance of the cable?  Capacitance increases by length, so a short cable will have low capacitance compared to the same cable in a longer length.  The BlueJeans LC-1 cable was designed to have very low capacitance, so it would still be low in a longer length. 

 

However, capacitance is usually discussed in the context of analogue cables (esp. phono cables).  I'm not sure it is of similar significance with digital signals, and will defer to other's expertise in this matter. 

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Specs for Belden 1694A

 

Centre conductor resistance 6.4 Ohm/1000ft

Braid resistance 3.8 Ohm/1000ft

Capacitance 16.2 pF/ft or 53 pf/metre

Nominal impedance 75 Ohm

Velocity factor 82%

 

Specs for a high quality single core shielded audio cable

 

Centre conductor resistance 47 Ohm/km

Capacitance <90 pf/metre

 

I'd have guessed excessive capacitance causing roll off of the signal when the distance is greater.

But it seems that the Belden coax has lower capacitance than a typical single core shielded audio cable.

It can't be rsistance, that's very low for any of these cables at this sort of length.

 

Is it possible that you are using the wrong connector for the digital connection?

Along the lines of a 75 Ohm BNC into a 50 Ohm BNC socket or similar?

I'm imagining a high resistance or capacitive coupling instead of direct electrical contact, this might work for a short distance, but fail over a longer cable.

Edited by pwstereo
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It has me stuffed.

The connectors are exactly as the cables were sold to me.  They are RCA socket type, none of the Coaxial input/ouput on the equipment has the BNC type fit.  I do have like foxtel coaxial here with an F type connection I could get adapters and try that cable at some stage but with lock down I cant get to bunnings or Jaycar etc to buy an adapter just yet.

 

The complete oddity  is that all tried coax works with lengths upto 2m and then spits and spats every minute or so if I use a longer run.

 

When I plugged in the basic thin yellow ended cable that you got with every VCR back in the day, it worked perfectly on a very long run, it made me think there is something hindering the information being transmitted properly on the Belden and other brands Coaxial Cable when going beyond 2m or so.

 

When using a stereo interconnect cable of say 4-5meters, ie take the white plugged cable and leave the red floating, it works with the LC-1 but not an Amazon cable.  It also works with the cheap cable matching the yellow plug.

I think there is definately something going on with capacitence and resistance and thats why the LC-1 is working.  The connectors on the Belden coaxial and the BJLC-1 are identical as they are both from Blue Jeans.

 

I checked their site today to see if I could buy a single length...Blue Jeans sell S/W cable and their recommended S/W cable is actually their LC-1, I think I will just buy a length of that as I know it works and I need the current LC-1 being used as a tester in it at the moment for a strereo interconnect from the same sources analog outs.

 

My only concern was the fact the LC-1 is not 75ohm but I've made a few technical enquiry today and have been assured there is no damage that can be done and any mismatch would be very audible with bad jitter, out of sync timing or no sound at all....to which none occurs.

I also think the LC-1 is going to serve better than the basic $2 shop yellow ended cable which is likely not be 75ohm anyway but is likely not to have the same level of sheilding.  Certainly the LC-1 has better connectors, I use quite a few of them through my system and they do their job very well.

 

Very odd. 

 

 

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10 hours ago, 08Boss302 said:

This one has me completely stumped.

Situation - Running Coaxial from source to DAC.  Distance 5m.

 

Running from my source using its coaxial out to a DAC using its coaxial in,  I use the Belden 1694A cable. 

 

Certainly weird.  Are you sure 1694A is correct for this type of application?  I use Belden 1506A for the 75 ohm digital cables that I make.

 

Andy

 

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Belden 1694A is 75 Ohm, so it should be fine.

RCA plugs don't maintain the characteristic impedance anyway, they were never designed for actual coaxial cable or RF.

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Thats a good question Andy, I actually dont!

Its running the coaxial out of the Streamer into my DAC. I just assumed the 1694A would be fine.  I also have another coaxial that is rated 75ohm and it functions fine at 2m length but plays up with a 5m length.

 

FWIW, toslink is fine no matter the length between the same devices.

 

Aside from the frustration and major head scratching my follow up main concern was that using one of the cables from one of my interconnects would casue issues as they're not marketed as coaxial or 75hohm but on the contrary its actually solved the issue!

 

 

Edited by 08Boss302
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The cable is rarely the problem. True 75 ohm connectors in RCA are actually very rare - soldering them ruins the 75 ohm impedance. To get 75 ohm impedance you need dedicated - usually crimp - connectors. The vast majority of cables do NOT do this. When they advertise that they're true 75 ohm cables, they're not lying as the cable is 75 ohms, but not the connectors. I would suggest just use any cable that works and is shielded, irrespective of the claims about their impedance.

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20 minutes ago, Ittaku said:

The cable is rarely the problem. True 75 ohm connectors in RCA are actually very rare - soldering them ruins the 75 ohm impedance. To get 75 ohm impedance you need dedicated - usually crimp - connectors. The vast majority of cables do NOT do this. When they advertise that they're true 75 ohm cables, they're not lying as the cable is 75 ohms, but not the connectors. I would suggest just use any cable that works and is shielded, irrespective of the claims about their impedance.

This exactly what I was advised today via email with some of the tech help I reached out too.   Thankfully just out of my own curiosity I tried the LC-1 cable it worked perfectly.  I have just now also tried another LC-1 which is 1m shorter (4m as opposed to 5m) and it works perfectly too.  Looks like I'll never understand what was going on but have found a solid work around.

I'll order the S/W version of the LC-1 which has the purple connector as opposed to Red/White and should be happy days. 

 

It is transmitting all files too, from 44/96/192 so everything seems spot on.

 

...Just one of those wierd audio anomolies that causes restless nights.  I had a ground loop hum once that disappeared after months of agony by grounding the table to of all things my VCR at the time!  These odd things just happen in this hobby!!

 

cheers.

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On 22/10/2020 at 8:15 AM, 08Boss302 said:

any RCA cable will in fact work safely and fine

Most RCA cable is close enough (not very close) to the right impedance and HF loss that SPDIF will work.    It is very robust.

 

14 hours ago, 08Boss302 said:

My only concern was the fact the LC-1 is not 75ohm but I've made a few technical enquiry today and have been assured there is no damage that can be done and any mismatch would be very audible with bad jitter, out of sync timing or no sound at all

No damage can be done.

 

It will work with even quite substantial "mismatch".

 

14 hours ago, 08Boss302 said:

Very odd. 

I'd have guess that your longer runs of coaxial cable have some sort of problem/fault..... this seems to be the only explanation.

 

I would get a new longer run of 1694A or similar to try.

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15 minutes ago, davewantsmoore said:

 

I would get a new longer run of 1694A or similar to try.

Thank you. 

I actually have x2 of these Belden cables, one is 4.2m and the other is 4.5m and has the same issue.  I tried an amazon Basics coaxial which is also 4.5m and also a Space HiFi coaxial which is 5m and same issue.  Using a few different brands (including a Space HiIFi) coaxial under 2m things are great.  I think there is some sort of data issue from a certain length from the source to the device.  To add further confusion using the exact same belden cables directly to my amp things work out well - and using that same cable from a different source to the DAC also works well.  hahaha, its the most insanely odd thing.

 

It would seem that even with a potential mismatch of using the "wrong" cable it is actually "better" than the "correct" cable should be. I have done a bit of critical listening last night and am hard pressed to hear anything odd with regard to glare or clipped highs. On songs I am very well familiar with there was absolutely nothing happening negatively that I noticed.  Even using some of the digital filters I have on the DACs behaved correctly.  It is a very weird one

 

 

 

 

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Is it possible that the transciever interface/driver is faulty on one or the other peice of equipment?

For a number of different cables to be showing the same issue seems to pint to something other than the cable and its connectors.

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The odd thing is each device works perfectly with the same cables but not connected to each other.

For example

I can run the coax (any length) out of the source directly into the amp no issue.

I can run the coax (any length) from another source directly into the DAC no issue.

 

BUT...join those two devices together with coax above 2m and there is hostility! (unless I use the really thin cheap yellow plugged coax, then its all good at any length also)

 

 

Edited by 08Boss302
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I feel your pain, I recently needed to buy a coax cable to connect my new DAC when the USB connection refused to work with my streamer, as best as I could work out a Coax cable should be 75ohm coax but when I tried to buy a coax cable there is no mention on any of them that theyare running 75ohm coax.   I ended up purchasing a cable but for all intents and purposes it looked like any standard analogue RCA cable it worked fine and I was able to run 384Khz files without any problems !

 

I am quite dubious about the whole 75ohm coax, does any DAC or streamer really run a true 75ohm input or output or are they just standard TTL  ?  

 

I suspect in your instance that putting a true 75ohm cable is actually detrimental to running a standard piece of wire, the point of running 75Ohm coax in CCTV applications is for running high frequency signals over a big distance in a transmission line so you have minimal losses but if the transmit or receive device is not terminated with 75ohm then there is no point and adding a 75ohm cable to a non terminated I/O will in theory create data problems which will be made worse by longer cable runs. 

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1 minute ago, U_J said:

I feel your pain, I recently needed to buy a coax cable to connect my new DAC when the USB connection refused to work with my streamer, as best as I could work out a Coax cable should be 75ohm coax but when I tried to buy a coax cable there is no mention on any of them that theyare running 75ohm coax.   I ended up purchasing a cable but for all intents and purposes it looked like any standard analogue RCA cable it worked fine and I was able to run 384Khz files without any problems !

 

I am quite dubious about the whole 75ohm coax, does any DAC or streamer really run a true 75ohm input or output or are they just standard TTL  ?  

 

I suspect in your instance that putting a true 75ohm cable is actually detrimental to running a standard piece of wire, the point of running 75Ohm coax in CCTV applications is for running high frequency signals over a big distance in a transmission line so you have minimal losses but if the transmit or receive device is not terminated with 75ohm then there is no point and adding a 75ohm cable to a non terminated I/O will in theory create data problems which will be made worse by longer cable runs. 

Glad to hear my experience is not stand alone!

Exactly what you have found is what i've found too right down the bandwidth I am streaming too.

 

I'd still like to know what is causing the issue and if as you suggested above the streamer is in fact not truly rated for 75ohm output.  I have no fear the DAC is rated appropriately but it may also not be.

 

Right now I have the $2 shop yellow plugged cable running and will order a decent LC-1 this week from Blue Jeans as it will provide better shielding and I believe is a better made cable etc. 

 

I am just glad I found a way to make it function correctly.  Glad you worked through your similar issue too!

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4 hours ago, 08Boss302 said:

I actually have x2 of these Belden cables, one is 4.2m and the other is 4.5m and has the same issue.  I tried an amazon Basics coaxial which is also 4.5m and also a Space HiFi coaxial which is 5m and same issue.

Then I don't know what to say....  If a ~5m "normal" RCA cable works.

 

 

4 hours ago, 08Boss302 said:

It would seem that even with a potential mismatch of using the "wrong" cable it is actually "better" than the "correct" cable should be.

 

Something might be "broken" in the source  (if another source works fine)

 

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2 hours ago, U_J said:

does any DAC or streamer really run a true 75ohm input or output

Yes.... but in general SPDIF is quite robust.    It would probably work with wet string.  ;)

 

2 hours ago, U_J said:

I suspect in your instance that putting a true 75ohm cable is actually detrimental

No.  SPDIF should be 75 ohms.    If the long SPDIF cables are not faulty (which they're not given they work with another source).... then there is something up with the source.   It may be faulty, or just designed to be partiularly picky re: reflected signal.

 

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Maybe it was mentioned already - there might be an issue with the connector termination if they did not crimp Canare connectors correctly (assuming you have Canare RCAPs). 5m is short for those cables even for shiitty SPDIF signal. 

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