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March 1, 2011

A Simple Recipe for a High Quality Digital Coax Cable

The audiophile world has really changed in the past decade. Most of us have moved from a stand-alone CD player to a computer-based music server. Whether you use iTunes, Foobar2000, WinAmp, MediaMonkey or some other program, you really need a quality DAC to extract your music from your PC to your audio rig. The industry has caught on and there have been myriad high-end DACs being released in recent years.

Carrying your precious 1s and 0s to your DAC should be a quality cable you can trust. A good digital cable should have an impedance of 75ohms. Cable impedance is important in digital audio because the signal runs at high frequencies. Because of this, impedance needs be matched from the source to the cable to the load. If it's not, then your end up with reflections. The concept seems to be that some of the signal is reflected back to the source along the cable. If the source isn't 75 ohms, it will return the reflection to the load, creating a time delayed reflection. I've also read some essays on the matter stating that the coax cable should be 1.5 meters (appx. 5 feet) or longer to further minimize the possibility of this effect. 

It seems that there are plenty of opinions on the above that spark long debates running thousands of pages through hundreds of threads, so that's why I'm only going to touch on it and move on ;)  The important thing for the DIYer to get out of the above is that one should use 75ohm connectors and 5 feet of 75ohm coax cable when possible (unless it's a balanced AES/EBU connector, which is a different story). 


There aren't a lot of audiophile quality DIY Coax connectors out there, so you have to spend a little bit to get the good ones. Furutech makes the FP-3-117(R) Audio grade BNC connector which is attractive and rhodium plated. There really isn't much else out at the moment that falls between this and the pro-audio quality Canare connectors. I believe Vampire used to make one but the usual suspects (PCX, Percy) don't carry it. A pair of the Furutechs will set you back approximately $40 at the time of this writing.

Furutech FP-3-117(R)

To connect these two lovely coax connectors together will be some nice quality wire. DHLabs, a company that has been around since 1992, is known for providing audiophile quality parts at reasonable prices. I'll be using the DHlabs D-75 wire, which has a solid silver-coated center conductor to maintain uniform impedance. From DHLab's website: The use of a solid center conductor minimizes signal reflections, which are very important in digital data cables. This center conductor is encased in a very uniform closed cell foam Teflon dielectric. The result of these efforts is a cable that can provide excellent performance to beyond 2 Gigahertz ... In continuing with our policy of providing maximum value, the D-75 costs much less than its performance level would suggest. I would tend to agree. I'd imagine if other companies were selling the same wire, it would be at least twice the price.


D-75 Cutaway
Using these parts together is actually fairly easy. One strips the jacket, folds back the braided shield, trims away a small amount of the dielectric, then inserts the cable into the connector. The solid core center conductor is soldered to the center pin of the Furutech, and the connector clamps down on the shield for a solderless connection. 


Example of a solderless connection


So you're thinking to yourself, "those BNC connectors look a little strange, I don't think my DAC / CD Player / etc. has those". It is actually a little less common to have nice BNC females on your gear than a generic RCA female to accept or send a digital signal, but more and more new gear seems to be including BNCs now. If one isn't already mounted on your gear but has room to mount it, one of the Vampire BNC female connectors below typically works:


Vampire BNC Female
One can either mount it free-hanging and solder wire to it or it's a drop in replacement depending on certain PCB layouts. PartsConnexion carries these, and I'm sure other retailers do as well. If one would rather not futz with it, DHLabs still makes a nice RCA plug that matches the D-75.


DH Labs RCA-3
It's certainly possible to have an RCA-3 on one side and a Furutech BNC on the other depending on the application. Here's what my completed Furutech / DHlabs coax came out like:

Furutech / DHlabs Digital Coax Cable

And here's an example of an RCA to BNC using the aforementioned DH Labs RCA-3. 


RCA to BNC Digital Coax Cable


If you'd like to have your own custom Digital Coax Cable made, please contact Zynsonix


I hope you enjoyed this detailed discussion of a Digital Coax cable. Please note that this post is for entertainment purposes only and not intended as a walkthrough. The owner of this blog, Blogger, and all associated parties will not be held responsible if you attempt to recreate the above cable and damage yourself or your surroundings. 




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