Why send signal through the shield?

R

rathpy

New member
Why don’t standard mic cables use a third core instead of the shield--so that all the signals are protected? (eg. 2 send cores as in a normal balanced line, and 1 core for the common return). What’s the use of the shield if it is carrying a signal that is not itself shielded?

On a more practical level, would standard Canare 2-core L-2T2S cable be a good choice for both my mic and line-level patchbay cables?

Regards,
rathpy
 
But in a standard mic cable doesn't the shield also act as a common signal return path for each of the core sends? Have I got this wrong or what???

Even with star quad, the shield is used the same way, i.e. as a common return path. The only difference is that the send cores are wired in parallel pairs--according to http://www.canare.com/gifs2/starquaddialogbox1.gif

Regards,
rathpy
 
You got it wrong... The shield is just the shield. The other are negative and positive. The real signal is the difference of both. There is no need for ground.

I think you're confused about the signal not having a ground. But the signal is not seen in relation with the ground, but with the negative signal!

You just feed it in the plus and minus of a differential amplifier, any common offset or common noise will be eliminated by the subtraction....
 
RFI and EMI hitting the shield needs to go someplace. Ground is the best place for it to go! I used to know the technical "why" of it, but that was long ago.

I think Canare Star Quad sounds better than your average Horizon or Whirlwind cable, but it isn't nearly as pretty sounding as the Mogami Quad cable. If you only need to make a few good cables, spend your money on the Mogami Quad. If you need a lot of cables, and you have a restricted budget, then the Canare is better than Hosa, Horizon, etc. But I think in the end, the Mogami is worth the money. I found about 3 different models of Belden to sound better than the Canare too. It is still a bit more expensive, but again, if you are only making a handful of cables, the increased cost is not much of an issue. Canare goes for about .40 a foot, while the Belden and Mogami is closer to .70 a foot. On a 20' cable, you are talking a difference of about $6 per cable in price. Not really that much of a difference really if you are only going to make like 8 cables when you consider the better sound that passes through it. But, you will buy what you are going to buy, and I am sure Canare owners around here will disagree (although I bet few of them have ever compared Canare to other mid priced brands of cable before!)

I am a Creep!
 
a good wiring for mic cables or any balanced connection is to open the ground on one side (only one connector has ground connected). this way you never get current flow through the ground and for connecting two pieces of electrical equipment this eliminates any possibility of ground loops.
 
Interesting sonixx, so you connect the shield at the mic for example and leave it unsoldered and cut short at the XLR jack or visa-vesra???

Would this also apply for balance outputs/inputs on souncards,mixers...etc
 
well...i don't have my cables that i use for mics wired this way because generally there's no possibility of ground loops. but, for all of my balanced cables that are used for connecting two pieces of equipment i use this wiring method.
 
Roel said:
You got it wrong... The shield is just the shield. The other are negative and positive. The real signal is the difference of both. There is no need for ground.

I think you're confused about the signal not having a ground. But the signal is not seen in relation with the ground, but with the negative signal!

You just feed it in the plus and minus of a differential amplifier, any common offset or common noise will be eliminated by the subtraction....
Click to expand...
Roel, I was wrong but couldn't see why. I understood how the 2 differential signals are combined and any common-mode interference is eliminated. What I couldn’t wrap my head around was where the return path was for each of the cores. After all, in an unbalanced cable the shield is used as the return path for the single core. So I was thinking that the shield in a balanced cable might be used as a common return path, but I kinda knew it probably wasn’t, because it is semi-common practice to not terminate the shield at one end. Adding to my confusion was the common terms “positive” and “negative” for the core differential signals--It's not as if the signal goes up one and down the other as in the conventional meanings of positive and negative.

Anyhoo... I found an explanation that I am fairly happy with at http://www.procosound.com/whtepapr.htm - read online or download the Acrobat file “Cable Anatomy I: Understanding the microphone” (past paragraph two of the second page – Anyone like me will need to read it in context to make head-nor-tail of it)...

“Because of this differential signal transmission, two very valuable things happen when using balanced lines. First of all, each piece of equipment can have its circuitry referenced to its own common, because the interconnection of the equipment does not require that the commons are connected in order to move the signal around. This eliminates the major cause of a lot of noisy audio gremlins, ground loops.”

Thanks to all,
rathpy

The following sites talk about solving ground hum problems:
http://www.yamaha.com/ycaservice/techdocs/Sound/ground/ground.htm
http://www.saecollege.de/reference_material/pages/Audio.htm
http://www.dself.demon.co.uk/balanced.htm#XLRphono
 
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