Unbalanced RCA Cables as Digital Coax Cables

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Nikhil

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Earlier this month Jeff Day had blogged about his experience with Belden 8402 IC.
Towards the end of the article he mentioned about using the Belden 8402 digital coax cable.

I have been using my Belden 8402 as digital coax from my Mutec to my DAC over the past couple of days and the sound is superb! I'm not sure what is going on but the soundstage has expanded a little and the Belden has imparted it's trademark "texture" to the sound a little. Sound impressions aside - I'm surprised that it works so well given how much attention is given over the 50 ohm / 75 ohm digital cable specs. My Belden cable was made primarily as a SE unbalanced interconnect with no measurements taken to check their specs. Although I need to check the shield termination recommendation used.

Question to all the pundits - can all RCA ICs be used as digital coax cables?
Why fuss over the 50 ohm / 75 ohm specs?

Regards



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kartick

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Although I was unaware about this article, but ever since I got my 8402, i have already experimented using the same as a digi coax. I even used the Deulund 16ga as a coax. But i found the 8402 more musical between my cdp and dac. It just sounds good. For accuracy, i prefer Mogami 2964 as coax but in terms of musicality, Belden is quite good. I personally have not found the impedance numbers to be a limitation with such a small length of coax cables. I will also recommend trying the single core Lyrita IC as a coax cable for an airy and transparent presentation. It just disappears.
 

amit11

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I agree. I have also experienced that a normal ic cable works fine as a digital coax. To what extent the 50/75 makes a difference, needs to be experimented.
 

Nikhil

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I agree. I have also experienced that a normal ic cable works fine as a digital coax. To what extent the 50/75 makes a difference, needs to be experimented.

From what I've learned - what makes a coaxial cable 75 or 50 Ohm is the distance between the signal wire (core wire of the coax) and the shield. So the cables that meet the spec use a medium like a foam to maintain the distance between the core and the shield.



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IndianEars

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I too have experimented with using Non Digital (neither 50 Ohms or 75 Ohms) Coaxial cable in place of a digital coax between my CD Transport and DAC.

I have been VERY happy with the results and the sound quality.

In fact, years ago, I used a cheap Video RCA to RCA cable ... Yellow RCA that comes free with DVD Players and I was pleasantly surprised with the results. The sound got smoother and felt more "whole" as in all if the music "came together'.

On extended listening and comparison, I realised that there was a little less resolution, compared to my Rs 10,000 Transparent Audio Digital Coax Cable. The Digital Coax was more 'Hi Fi' and technically correct, but the Analog Cable could have certainly be used to advantage, to tame the system.

Infact, less than a week ago, I had the previledge to liostenm to a Very High end stereo setup. A Marantz SA 10 (US$ 7K MRP) used as a CD Transport, feeding a Lampilizator Pacific DAC, to Cary 2 Box Pre amp and ANK 300B monoblocks driving Avangarde Duo Horn Speakers. The system owner did not have a Digital Coax cable and was using a Van Den Hul Analog (1.5 meter length* RCA to RAC) interconnect between the CD Transport SPDIF out and the DAC Coax input. I had taken along my Transparent Audio Digital Coax Cable.

After more than an hour, of listening, it was clear to both of us (System owner and me) that the Van den Hul analog interconnect doing digital duty, provided a more pleasing sound in that setup!

I am sure that no general conclusions can be drawn on whether Analog Interconnects sound better or worse than Digital Coax, for carrying Digital signals, but the change is sound quality is certainly worth exploring.

* Would love to have feedback on whether the analog coax that you guys have deployed for digital duties, is atleast 1.5 meters in length.... There is a technical reason for that :)

Would be even better if you could try out analog interconnect cables of 1 meter length or less, vs the same of 1.5m to 2 meters (or more) in length.
 

keith_correa

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Question to all the pundits - can all RCA ICs be used as digital coax cables?
Am not a "pundit" - not by a long shot but, yes, one can. Whether one should is a different matter.
Why fuss over the 50 ohm / 75 ohm specs?
When one needs to transmit a signal efficiently, with minimal attenuation and over a wide frequency spectrum, it was determined that for the cost, 75 Ohm Coax provided low signal attenuation AND also low [relatively] capacitance. 50 Ohm Coax - more power handling.
From what I've learned - what makes a coaxial cable 75 or 50 Ohm is the distance between the signal wire (core wire of the coax) and the shield. So the cables that meet the spec use a medium like a foam to maintain the distance between the core and the shield.
Not limited to only what you specified. The dielectric constant of the insulation of the core and I think the radii of the conductors also plays a role.
After more than an hour, of listening, it was clear to both of us (System owner and me) that the Van den Hul analog interconnect doing digital duty, provided a more pleasing sound in that setup!

I am sure that no general conclusions can be drawn on whether Analog Interconnects sound better or worse than Digital Coax, for carrying Digital signals, but the change is sound quality is certainly worth exploring.
What was the characteristic impedance of the VDH IC? Do you know?
* Would love to have feedback on whether the analog coax that you guys have deployed for digital duties, is atleast 1.5 meters in length.... There is a technical reason for that :)

Would be even better if you could try out analog interconnect cables of 1 meter length or less, vs the same of 1.5m to 2 meters (or more) in length.
That minimum of 1.5 meter was thrown around in the early 2000's [I think] but then was determined to be a non issue if the interfaces at both ends along with other stuff [which I don't remember now] were all impedance matched. In short - bunkum! :p
 

Kannan

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Just to add, BNC or RG58 as they are termed are the real true 75ohm speced connectors. The closest RCA compromise are the Canare RCAP type crimp connectors
 

drkrack

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Just to add, BNC or RG58 as they are termed are the real true 75ohm speced connectors.
Yes BNC Connectors are truly 75ohm as opposed to regular RCA connectors.

too have experimented with using Non Digital (neither 50 Ohms or 75 Ohms) Coaxial cable in place of a digital coax between my CD Transport and DAC.
I have been using Belden 8402 digital cable with bnc termination as digital Coax cable it is one of the best musical sounding cable a real VFM. I have a Pair of Belden 8402 XLR ICs which are again very good for the price ; bettered only by high end cables from MIT, Kimber, Audioquest, Chord, Straightwire and Xlo ; albeit at a much higher price point.

I did try vdh rca cables as digital coax didn't find them any good, in fact the cheaper DAC Coax from Hifimart.com sounded way better than Vdh analog cable used as digital Coax.
 

Nikhil

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* Would love to have feedback on whether the analog coax that you guys have deployed for digital duties, is atleast 1.5 meters in length.... There is a technical reason for that :)

My Belden 8402 cable is a 1m cable.
Very interested in knowing more about this 1.5m length condition!



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IndianEars

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Great to see a fairly detailed conversation going here, on the subject of Cable impedance.

What was the characteristic impedance of the VDH IC? Do you know?

Analog interconnects do not pay attention to controlling / maintaining an (RF) or characteristic impedance.

(20 KHz is hardly RF... RF is generally referred to for frequencies Above 3 MHz.)

Analog signal cables pay no attention to even maintaining Concentricity between the centre conductor and the shield .... That is why RCA connectors are used.

Analog Signal cables are simply shielded cables and not 'proper' coaxial cables refering to coaxial as referred to for RF frequencies.


The closest RCA compromise are the Canare RCAP type crimp connectors
I totally agree.

Cables and connectors can maintain a specified impedance ONLY when their 2 conductors are maintained at a constant separation. For Coaxial cable, it means the cable's perfect concentric geometry MUST continue into the connector.

download.jpg


Note that the centre Copper conductor and the copper wire braid are separated by a Fixed distance, by the (White Colour) dielectric / insulator.
 

IndianEars

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I did try vdh rca cables as digital coax didn't find them any good, in fact the cheaper DAC Coax from Hifimart.com sounded way better than Vdh analog cable used as digital Coax.
Yes, as I said its system dependent and depends what sound modification you are trying to achieve. YMMV

Also, there is a wide range of Van Den Hul analog interconnects......

Incidentally, I personally do not like the sound of Most Van den Hul cables and in jest refer to them as Bland N Dul ;)
 

IndianEars

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Am not a "pundit" - not by a long shot but, yes, one can. Whether one should is a different matter.

When one needs to transmit a signal efficiently, with minimal attenuation and over a wide frequency spectrum, it was determined that for the cost, 75 Ohm Coax provided low signal attenuation AND also low [relatively] capacitance. 50 Ohm Coax - more power handling.

Not limited to only what you specified. The dielectric constant of the insulation of the core and I think the radii of the conductors also plays a role.

You are fairly close to the facts, Keith, on the how & why 50 Ohm & 75 ohm were widely adopted.

Wiki provides good historical and technical info
" The best coaxial cable impedances in high-power, high-voltage, and low-attenuation applications were experimentally determined at Bell Laboratories in 1929 to be 30, 60, and 77 Ω, respectively.

When more common dielectrics are considered, the best-loss impedance drops down to a value between 52–64 Ω. Maximum power handling is achieved at 30 Ω."
 

IndianEars

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There are 50 Ohm BNC's available too.

RG-58 is actually a 50 Ohm Coaxial Cable. It has a very well defined geometry.

its 75 ohm counterpart is RG-59.

The BNC that fits onto a RG-58 cable will be a 50 ohm BNC connector with a 50 ohm impedance... often referred to as a RG-58 BNC connector.

Similarly for RG-59 BNC connectors

The dielectric constant of the insulation of the core and I think the radii of the conductors also plays a role.


Cable Impedance is defined by:
The centre Conductor outer diameter.
The Material and diameter of the dielectric
The Shielding inner diameter.

The material of the centre conductor (usually copper) and the material of the shield (usually copper or tinned copper) also determine the the Resistance of the Coaxial cable. However, typically, the resistance is a very small part of the overall impedance of the Coax cable.
 

Kannan

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Technically speaking, these dedicated coax connectors are designed in a way so that the braiding is retained while termination (normally by crimping), which cannot be done properly with normal RCA connectors where the outer shield is unbraided to terminate.

How much of this translates to being deciphered while listening is another issue, maybe differences can be measured.
However staying within the technicalities is advisable, especially while transmitting digital signals (more so video).
 

IndianEars

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There is "Impedance" and there is "Characteristic Impedance". Different animals, both.
VERY Loosely speaking (for better understanding)

Characteristic Impedance is the Ideal or theoretical or designed impedance.... eg 75 Ohms

Impedance would be the ACTUAL, imperfect, real world impedance.... could vary between 70 Ohms to 78 ohms, depending on the frequency.
 
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