Interconnect cables

[John2003]

<font color='#000000'>Hello All,
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; I need some knowledgable advice here, i want to purchase a roll of coax cable to make my own connections (it's cheaper).
However i have noticed that most if not all video coax leads are 75ohm cables.
I'm thinking of buying a quad shielded OFC cable multi strand core.
It says in the description suit video signal transfer.
I just wanted to know can i use this cable to connect audio aswell? (to run from outputs to an Amp)
And also could you explain the diff between audio and video leads as to why video leads have 75ohm rating and audio don't.

Thanks
John</font>

 

[Guest]

<font color='#000000'>Check out the Klipsch forum and do a search on cables you will get tons of info</font>

 

[Audioholics]

<font color='#000080'>Uh... or you can do some research here and get some great advice.

Hey, davmar, we don't mind you promoting other websites, but how about registering?

Your cumulative posts sound a little like a walking billboard for Klipsch...</font>

 

[audioengr]

<font color='#000000'>You can certainly use 75 ohm Characteristic impedance cable for analog.  The disadvantages are:
1) it is usually coax, so the forward and return impedances are not balanced
2) because it is coax, the capacitance per foot will be higher than a good analog interconnect
3) because the RCA connectors are terminated to ground 360 degrees, the connectors usually have metal backshells which will add capacitance.  A good unshielded interconnect can utilize unshielded connectors with plastic backshells which have very low capacitance</font>

 

[gene]

<font color='#000000'><table border="0" align="center" width="95%" cellpadding="0" cellspacing="0"><tr><td>Quote </td></tr><tr><td id="QUOTE">You can certainly use 75 ohm Characteristic impedance cable for analog. The disadvantages are:
1) it is usually coax, so the forward and return impedances are not balanced</td></tr></table>

Impedance matching at audio frequencies, and given normal cable lengths is both not necessary and snake oil at its finest.  The electrical wavelength of 20kHz is about 15000 meters.  A good rule of thumb to start considering transmission line effects is at about 1/10th the wavelength, or in this case 1500 meters.  If one were to run 1500 meters of cables the lumped parameters would destroy the signal far sooner than “transmission line impedance mismatch effects”.  

<table border="0" align="center" width="95%" cellpadding="0" cellspacing="0"><tr><td>Quote </td></tr><tr><td id="QUOTE">
2) because it is coax, the capacitance per foot will be higher than a good analog interconnect
This is true, however decent coax cables can be made with relatively low capacitance.</td></tr></table>


Take for example the following coax cable from Blue Jean at 16.2 pf/ft:

Blue Jeans Cables

A typical length of say 6ft would equate to less than 100pF of capacitance.

Certainly any decent modern hifi equipment should be able to drive a few hundred picofarads of capacitance.  Most competent audio designers employ a series resistance after the feedback stage of the output amplifier to isolate it from capacitive loads while also limiting the bandwidth of the circuit by applying parasitic feedback compensation to further increase stability.  

<table border="0" align="center" width="95%" cellpadding="0" cellspacing="0"><tr><td>Quote </td></tr><tr><td id="QUOTE">
3) because the RCA connectors are terminated to ground 360 degrees, the connectors usually have metal backshells which will add capacitance. A good unshielded interconnect can utilize unshielded connectors with plastic backshells which have very low capacitance </td></tr></table>

The metal backshell metrics of an audio interconnect are mostly negligible until we reach frequencies in the 100’s of Megahertz.   I would almost never recommend an unshielded audio IC over a shielded IC just to save on a little capacitance.  There is no reason why any competent cable designer cannot produce a shielded twisted pair IC with reasonably low capacitance.  The benefit of shielding far outweighs the added capacitance, especially in high RFI/EMI environments typically found in today’s complex home theater systems. I personally use twisted pair Belden Shielded IC's from REDesigns that measure about 100pF/meter with excellent results with all test gear I review.

For more information on cable basics read the following Guidelines I have prepared at:

Cable Budget Guidelines

[edited: fixed link]</font>

 

[PaulF]

<font color='#000000'>Gene,

Dusting off the cobwebs from my transmission line theory, I seem to remember that a balanced cable such as coax needed to be of a minimum length before it reached its characteristic impedance (as seen by the connected equipment). I'm sure there's a formula for it (and you probably know it).

Paul</font>