Spdif Coax cable 10K parallel resistor

seasusa

Member
2016-05-02 7:48 pm
So I bought this expensive Tellurium spdif/coax cable from ebay. It kept dropping audio whenever something would switch on or off in my house (like my thermostat or fridge). So curious to see what might be the problem I check the connectors. Now on one end I have found a 10K ohm resistor soldered between the signal and ground. Now first I thought it might be to stop reflections coming back into the cable from the load, but that would require a maximum 100ohm resistor so that cant be it. It is a pull down resistor, but why is it being used in a coax cable? More importantly why is the audio dropping? Any advice will be much appreciated. Pic below:
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You could try using an ordinary cheap (but not too cheap!) 75ohm coaxial cable intended for video. That will have good screening. Check that the expensive cable is proper coax - you would be amazed what some people use for a wideband unbalanced connection.

The 10k resistor may be there to help them test for end-end continuity in production? Alternatively, it may be an unnecessary attempt to move the low frequency characteristic impedance back into low and resistive territory. For a coaxial cable at low frequencies the impedance is dominated by series resistance and shunt capacitance. Adding shunt resistance can swamp the capacitance, so in conjunction with the series resistance the cable becomes resistive again. Completely unnecessary, as the cable is too short to benefit from this. However, in the hands of a marketeer this could be a major selling point: "Subsonic Impedance Control" or "Completely Removes The Audio Effect Of Shunt Capacitance".
 

wwenze

Member
2008-03-07 12:46 pm
Now first I thought it might be to stop reflections coming back into the cable from the load, but that would require a maximum 100ohm resistor so that cant be it.

The receiving equipment would have a 75ohm resistor for the terminating load. Another 10k in parallel won't have much effect. I suspect continuity check, or an attempt at preventing noise when one end of the cable is unconnected.
 

seasusa

Member
2016-05-02 7:48 pm
No. There is nothing to pull down. Pulldown resistors are used in some logic circuits, not SPDIF cables.

So the consensus is that it is for a continuity check, or possibly a sign that the makers are confused about cables. Is the cable actually coax?

Yes the cable is a Digital BNC 1.0m, came with RCA-BNC connectors. So if there is no signal from the source wouldn't a pull down resistor hold the signal to ground. Making sure the load gets either a 0 or 1 and not something undefined. Anyhow I would expect that to be implemented on a logic circuit. So it's as simple as checking for continuity. Sometimes the simplest explanation are the correct ones.
 

seasusa

Member
2016-05-02 7:48 pm
But the question still remains as to why this cable is dropping audio? Not to point anyone is any direction but if feel I should explain a bit of my setup:

The source: Pioneer N50a, which a ground to the chassis. Usual tingling sensation when you touch the metal case. Two prong IEC inlet.

The receiver: Devialet 200, properly ground to mains earth. Three prong IEC inlet.

Everything works fine with an optical cable, and a 75ohm chord pro dac vee3.
 
Yes the cable is a Digital BNC 1.0m, came with RCA-BNC connectors.
BNC connectors on the end imply coaxial cable, but do not guarantee coaxial. People use all sorts of strange things for audio!

So if there is no signal from the source wouldn't a pull down resistor hold the signal to ground.
The termination resistor will do that, so the 10k achieves nothing in that respect.

But the question still remains as to why this cable is dropping audio?
Poor screening? Poor contacts?

Buy a cheap 75ohm video cable and try it.
 

wwenze

Member
2008-03-07 12:46 pm
Plot twist - "zero" in SPDIF is alternating high voltage low voltage
And "one" is a slower alternating high and low voltage

Tying it low will just cause the receiver to lose locking.

Either way this has little to do with the DAC cutting out when a light is switched on.

There is no real answer here, whether a better cable or worse cable would prevent that from happening.

But most of the time I take anectodal evidence with salt, as I have seen many troubleshooting attempts as well as DIY improvements to be caused by the magic of small sampling size and random variables. Things like time of day can have effect too, indirectly (Not via astrology). Suggest observing for a few days.
 
But the question still remains as to why this cable is dropping audio? Not to point anyone is any direction but if feel I should explain a bit of my setup:

The source: Pioneer N50a, which a ground to the chassis. Usual tingling sensation when you touch the metal case. Two prong IEC inlet.

The receiver: Devialet 200, properly ground to mains earth. Three prong IEC inlet.

Everything works fine with an optical cable, and a 75ohm chord pro dac vee3.

There should be no tingling sensation when you touch the case of the Pioneer N50a, this is worrying...
The problem is an EMC issue, when something is switching it is sending a pulse into your mains, your audio equipment is reacting to this pulse, it sounds like a conducted immunity problem. The pulse is probably entering via one of the mains leads and is either getting through the PSU and causing a glitch. The SPDIF cable you have is not good, and is probably already at the limits in terms of signal integrity (often a problem with audiophile digital cables) so the extra noise introduced by the mains switch on/off pulse is causing the receiver to miss read... it could be the ground potential rising. The problem is finding and solving the underlying issue or just get a decent cheep cable that will do the job, and sell this cable on....
This is how SPDIF works the bit rate is always the same there is no slower transition, it is a form of Manchester encoded signalling...
https://en.wikipedia.org/wiki/Differential_Manchester_encoding
 
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seasusa

Member
2016-05-02 7:48 pm
The cable is faulty. Not working at all now. Tried a cheap video coax everything working as should. Going to send it for independent testing to find out what exactly is going on. Tellurium has refused to tell me the reason behind the 40K resistor. They are not willing to say anything about the technicalities of the cable design. However they are willing to have a look at it. Does anyone know where I can get this cable checked in UK? They would need to supply some proper certification documentation.


PS: Aren't resistors are supposed to be properly colour coded under some international regulation?
 

wwenze

Member
2008-03-07 12:46 pm
There should be no tingling sensation when you touch the case of the Pioneer N50a

More common than you might think. My Marantz PM6005 has it too. So did Zhaolu D1.3. And so does my laptop's USB port's ground shield. The best one would be the RCA in of an old Thomson CRT TV.

When transformer secondaries are left floating, they don't actually float, but rather coupled to mains voltage via a high impedance - the whatever leakage through the transformer or somehow through the case or whatever.

Now, when a spike happens, let's say the Live wire is instantaneously reaching 600V and the Neutral staying at zero, if your secondary is left floating, or more correctly, if your secondary's potential is a function of AC voltage and transformer coupling, then for that instance, equipment A 's secondary potential would be at voltage X. Now, equipment B, having a different amount of coupling, reaches voltage Y. And if equipment A and B are connected together... normally that connection would equalize the voltage. But everything has finite resistance and speed.

One reason why I prefer connecting everything to ground. But the safety people actually think that is more dangerous, especially when done on 2-pin equipment certified with double insulation.
 
seasusa said:
Tellurium has refused to tell me the reason behind the 40K resistor. They are not willing to say anything about the technicalities of the cable design.
What a surprise!

Does anyone know where I can get this cable checked in UK?
You can do simple continuity and insulation tests with a multimeter from Maplins. My guess is that one of the connectors has a bad joint.

PS: Aren't resistors are supposed to be properly colour coded under some international regulation?
Modern resistors sometimes have a value in character form instead of colour codes. Nothing sinister in this.

My experience is that Japanese equipment often has too large capacitors in the mains filter, so you get significant (but still legal, and allegedly safe) leakage current. This can be alarming when you are not expecting it.