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XLR Cable Capacitance Help?
Old 13th June 2018
  #1
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XLR Cable Capacitance Help?

I need to run several hundred feet of balanced mic cable in a new building and I'm trying to decide which to go with. The cables with be run up to the attic and back down totalling about ~75ft each run.

Our past building suffered from some nasty RF/EMI noise and I found the solution to be really clean, high quality star quad cables (Canare). However in researching the best cable for this job, I discovered the capacitance of the Canare Star Quad cable is about double that of normal Belden/Mogami used elsewhere.

Is there really that much of a difference in fidelity resulting from the higher capacitance of a cable such as the Canare Star Quad versus that of a lower capacitance alternative like Mogami or Belden?
Old 14th June 2018
  #2
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BIG BUDDHA's Avatar
i have never run microphone cables as long as you are doing.

i do know that Data Cat-6 cable is twisted round and round many times per meter, and this twisting in a circular pattern is designed to reduce interferance from external sources. lower value Cat-5 cable has less twisting and picks up more noise.

some people are using Data system cabling, with a breakout box at both ends. but it would be a digital transmission i guess.

for analogue transmission, the theory is to stay away from 240 volt electrical cables, as much as possible,
and if you have to cross them do it at 90 degress. (ie dont run parallel) to reduce induced voltages.

you probablly need to find a rearly good tech and ask for suggestions

hope that helps . Buddha
Old 14th June 2018
  #3
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Quote:
Originally Posted by clanderson View Post
Is there really that much of a difference in fidelity resulting from the higher capacitance of a cable such as the Canare Star Quad versus that of a lower capacitance alternative like Mogami or Belden?
I would use the quad cable. BUT, since you concerned just rig up two test cabless and hear for yourself.

Andre
Old 14th June 2018
  #4
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fireberd's Avatar
I have a retired TV Engineer friend, who also designed and built some recording studios. He was also a contract engineer for NBC for the Atlanta Olympics. He says he only used Mogami.
Old 14th June 2018
  #5
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avare's Avatar
 

Quote:
Originally Posted by fireberd View Post
I have a retired TV Engineer friend, who also designed and built some recording studios. He was also a contract engineer for NBC for the Atlanta Olympics. He says he only used Mogami.
With all due respect, what does this have to do with the price of goldfish? Mogami makes quad mic cable. An example is part number 2534.


Andre
Old 14th June 2018
  #6
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fireberd's Avatar
Quote:
Originally Posted by avare View Post
With all due respect, what does this have to do with the price of goldfish? Mogami makes quad mic cable. An example is part number 2534.


Andre
Sorry I ruined your day and your snide remarks.
Old 15th June 2018
  #7
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Richard Crowley's Avatar
 

Does "balanced mic cable" imply that you are running microphone-level signals through this wiring? Or are these cables going to be used for line-level?

What is the source-impedance of your source(s)? Most modern microphones (and line-level sources) have quite low source impedance so that cable capacitance is essentially a non-issue for normal audio frequency band (20Hz to 20KHz). That is the major reason that modern audio gear uses low source impedance. Except for electric guitar/bass instruments with vintage high-impedance pickups. But you don't use ordinary mic cable (of any kind) for instrument connections.

75 ft is not a particularly long distance. For large location/venue and recording/broadcasting installations, that is rather short.

I have probably a total somewhere approaching 1/2 mile of various XLR cables. Some are star-quad, but most are ordinary shielded pair. I have never encountered any situation where the star-quad cable performed any differently than the ordinary 2-wire cable. Not a believer, sorry. YMMV.

If you are worried about RF/EMI environmental interference, then a good shield is far more important than star-quad. When we do permanent installation of mic and line twisted-pair cable it is very common to use "install-grade" cable. This is quite different than the kind of mic cable you would use in the studio or control room or on location. Install-grade cable typically uses foil-shield for absolute 100% coverage (much better than any kind of braided shield). And install-grade cable typically uses more rigid, thinner, and more shiny (slippery) outer sheath. Designed specifically for pulling through long conduit runs, and to be space-efficient since you don't need the thick outer sheath when the cable is inside a wall or a conduit pipe.
Old 17th June 2018
  #8
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Thomas W. Bethe's Avatar
At my church we ran some mic cables from the stage in the social hall to the back of the hall up through the attic and down the wall (about 300 feet) and had no problems with RFI even though we were right across from the police and fire station with high wattage transmitters. We used the wire that Richard was discussing.

I also got called to a large church that was having all kinds of RFI problems. Turns out that someone thought they could save some money so they put microphone cords through the floor on both ends of the sanctuary but in-between they ran lamp cord wrapped in aluminum foil. It did not work and we had to replace the lamp cord with microphone cables and put in XLR connectors. Cost the church a whole lot more than if they had put in the correct cable to begin with. (The mic lines ran above a dropped ceiling in the recreation hall under the church and it was a large time consuming task to replace all the individual cables since the person who put in the cables got them all tangled around AC ducts and electrical conduits)

When I worked for a PBS TV station we did the annual "horse jumping show" and had literally a mile of mic cable out in the field for the presenters with no major RFI problems. We used Belden 8412 for all our mic cables.
Old 17th June 2018
  #9
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nosebleedaudio's Avatar
 

If it was me I would use Mogami 2549..
Old 17th June 2018
  #10
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Richard Crowley's Avatar
 

Note that if you are running these cables through an attic/ceiling space in a commercial building, you must use "plenum rated" cable unless they are running through a metal conduit (pipe). Else you will be in trouble from the building inspector or fire marshal in your jurisdiction. Fortunately, there are several examples of plenum-rated, install-grade cable available. And note that install-grade cable is not only easier to install, and better shielded than ordinary mic cable, but it is also typically considerably cheaper than ordinary mic cable. For example:

West Penn 25291B Plenum Install Cable is 16 cents per foot, or 14 cents per foot over 1000 ft.
West Penn 25291B Plenum Install Cable | Redco Audio
Old 18th June 2018
  #11
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Thomas W. Bethe's Avatar
Quote:
Originally Posted by Richard Crowley View Post
Note that if you are running these cables through an attic/ceiling space in a commercial building, you must use "plenum rated" cable unless they are running through a metal conduit (pipe). Else you will be in trouble from the building inspector or fire marshal in your jurisdiction. Fortunately, there are several examples of plenum-rated, install-grade cable available. And note that install-grade cable is not only easier to install, and better shielded than ordinary mic cable, but it is also typically considerably cheaper than ordinary mic cable. For example:

West Penn 25291B Plenum Install Cable is 16 cents per foot, or 14 cents per foot over 1000 ft.
West Penn 25291B Plenum Install Cable | Redco Audio
Good point. Thanks for pointing this out.

The microphone cables I ran at our church were "Plenum install approved"

The ones in the church with the RFI problem were not approved for anything. <GRIN>. I think the person who put in the aluminum foil covered zipcord used a whole roll of aluminum foil. For some reason aluminum foil is not recyclable so we had to toss it. How and why he ran the cables in and around electrical conduit and AC ducts is still a mystery. FWIW
Old 18th June 2018
  #12
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Quote:
Originally Posted by Thomas W. Bethe View Post
.............................................
When I worked for a PBS TV station we did the annual "horse jumping show" and had literally a mile of mic cable out in the field for the presenters with no major RFI problems. We used Belden 8412 for all our mic cables.
I never did a horse show with you, but do you remember the nasty arcing noise from the factory next door to the channel 25 studio? (this was before star-quad cable). We were thinking about using telescoping shielded cable, but then they moved the studio.
Old 18th June 2018
  #13
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Thomas W. Bethe's Avatar
Yes I remember all the problems with the studio on Brookpark Road. At one point someone suggested a "Faraday" shield over the whole building. I cannot imagine what that would have cost. At the new studios in downtown Cleveland I understand there are no engineers left just "operators" . Too bad it was a nice place to work. I think when Dale started rewiring he found literally miles of cabling that went no where. Good memories.
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