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Microphones Wire for XLR question...

Discussion in 'Sound, Music, and Intercom' started by Archer, Nov 30, 2008.

  1. Archer

    Archer Member

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    Greetings everyone.. My name is Steve, new to controlbooth. I am one of Tech Directors for the local high school. The wiring for our sound system is almost 40 years old and has deteriorated. I have plans on purchasing new wall plates with single XLRs, like these..
    Pro Co Wall Plate with Single Female XLR from zZounds.com!

    I was hoping someone could give me a recommendation on the wire to use, mainly wire size. It would be about 250 feet from the jacks to the Amp/mixer.

    If you need anymore info, please feel fee to ask..

    Thanks in advance!
    Steve
     
  2. Footer

    Footer Senior Team Senior Team Premium Member

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    Are the cables going to be pulled through conduit in the wall or are they going to be be exposed?
     
  3. Archer

    Archer Member

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    They will be pulled threw a conduit and would probably need to be derated..
     
  4. Footer

    Footer Senior Team Senior Team Premium Member

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  5. Archer

    Archer Member

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    Great thanks! Yeah, that would be ideal..
     
  6. mbenonis

    mbenonis Wireless Guy Administrator

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    I don't think there's any reason to derate the cable. Mic lines carry primarily voltage signals, not current - so the current that a mic would draw is miniscule (microamps or below). This, there is no risk of any kind of heating (unlike, say, speaker cable or dimmer circuits).

    I'll second the cable suggestion above. However, as far as wall-plates go, I might shop around a bit. You might be able to save a few bucks here (especially if you're handy with a drill press or have access to a punch).

    Mike

    PS - Welcome to the 'booth!
     
  7. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    In the interest of not being manufacturer specific, another alternative is West Penn 291, often listed as an equivalent alternative to Belden 8451.

    [​IMG]
     
  8. museav

    museav CBMod CB Mods Departed Member

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    250' is a long run and is getting to where the capacitance of the cable could affect the high frequency response. If you have very high quality condenser mics and a high quality system this might be a consideration. On more recent projects I often specify low capacitance, 110 Ohm digital audio cable rather than standard analog cable, this addresses the capacitance issue and potentially makes the cabling a little more 'future proof'. My favorite cable for critical lines is currently Belden 1800F.

    Some people prefer Belden 9451 over 8451, basically the same cable but the shield is turned inside out with the drain wire inside the shield making it a bit easier to work.
     
  9. mbenonis

    mbenonis Wireless Guy Administrator

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    Out of curiosity, I worked the numbers for the frequency response of the cable. Assuming a 250 foot run, we have a resistance of (0.0190 Ohms/foot)(250 feet) = 4.75 Ohms. For capacitance, we have (111 pF/foot)(250 feet) = 27.75 nF of capacitance to ground. This forms an RC low-pass network, with a transfer function of (1 + j*2*pi*f*R*C)^-1, where j is the imaginary unit, f is the frequency in Hz, R is the resistance given above, and C is the capacitance given above.

    Attached is a plot of the frequency response, from 1 Hz to 1 MHz. From the numbers, it looks like you'd have to have audio at 500 kHz to see a loss of 1 dB from the cable. :)
     

    Attached Files:

  10. museav

    museav CBMod CB Mods Departed Member

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    However the cable is not by itself, it is part of a circuit with an output (mic) and input (console) load. Many "150 Ohm" mics are actually 300 to 500 Ohms while a DI box output would typically be 600 Ohms. Based on that, I believe that with 100pF/ft capacitance cable and a 250' run you are getting into where the high frequency losses would be around 1dB.
     
  11. mbenonis

    mbenonis Wireless Guy Administrator

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    You bring up a good point here. I'll re-run the numbers when I get a chance to take that into account. Also worth noting is that the output impedance of the mic is likely not strictly resistive, but probably has an inductive component (for dynamic mics) or a capacitive component (for condenser mics).
     
  12. avare

    avare Active Member

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    This is getting complex on an ad hoc and theoretical basis. Jim Brown'sThe Effects Of Cable On Signal Quality is the most comprehensive I know of on microphones and cables.

    Andre
     
  13. FMEng

    FMEng Well-Known Member Fight Leukemia

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    I build radio stations stations for a living. I recommend Belden 9451 with gray jacket. It is identical to 8451, but the foil shield is bonded so it comes off with the jacket during stripping. They both pull well in conduits due to a slick jacket. Light grey allows easy temporary marking with a Sharpy.

    I cringe when I see West Penn and others mentioned as equivalents. Sometimes they are fine, but I've also encountered many cables whose insulation shrinks or melts at the mere suggestion of a soldering iron, and the conductors are not tinned and don't solder well. I have installed many miles of cable and never regretted using any Belden product.
     
  14. museav

    museav CBMod CB Mods Departed Member

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    I'm sure there are problems with many off brand products but I've seen West Penn 291, 22291 (plenum rated) and other 'name' manufacturer (Liberty, Clark, Mogami, Canare, etc.) equivalents used on hundreds of projects over 25 years without any problems. I worked for a firm that did installs such as network broadcast centers and the International Broadcast Center for several Olympic games and we successfully employed a variety of these manufacturer's products for mic and line level audio over the years.

    FWIW, tinning may be nice for soldering but for conductors going to Phoenix type terminal strips the tinning tends to flow under the pressure of the connection and the connections can work themselves loose. Ideally these are addressed using bare copper rather than tinned conductors.

    The main reason I currently lean towards using digital audio cable for many applications is the lower capacitance and it being more flexible (functionally, not physically) in its use, thus potentially being a little more 'future proof'.
     
  15. jkowtko

    jkowtko Active Member

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    I may create some TRS box snakes to reach the bulk of the distance from the sound booth to the stage in our theater .. if I want to put 4-8 channels in a single run, would you recommend using multiple single-pair cables or buying a multi-pair cable instead? (The multi-pair don't look to be that cost-effective though).

    Thanks. John
     
  16. FMEng

    FMEng Well-Known Member Fight Leukemia

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    If the run is in conduit I'd probably use single pair cables for smaller quantities. Multipair cable can be nice to used where it's exposed, but they take more preparation to terminate. If the individual shielded pairs are not jacketed, then you have to put shrink tubing on each one to protect the shield. It's not hard, but the time can add up.
     
  17. museav

    museav CBMod CB Mods Departed Member

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    Just keep in mind that the "cost effectiveness" is not just in the cable itself. Consider if you want 8 runs, you can pull one multipair cable or simultaneously pull 8 individual ones. Do you have the experience and equipment to simultaneously pull 8 cables? What might that cost in time and equipment?

    I personally like pulling multiple individual runs where possible because it provides a lot more flexibility in where things run and how you terminate them, but in cases where you are running all the connections from one device to another, multipair can be simpler to install. And as FMEng noted, by multipair I am referring to multiple individual jacketed, shielded twisted pairs inside one overall jacket and not just muliconductor cable.
     
  18. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Down here, we tend to like Canare and Eurocable, Mogami also falls on that list but from memory is a touch more pricy. I personally despise Belden AES multicore. It is utterly unpleasant to work, with especially into DB25s that some brainiac decided was an appropriate connector for this industry.

    Terminating multicore takes skill. It's substantially easier to terminate it in an install than for portable use, you don't have to factor in so much twisting, turning and other nasty strains on the cables.

    If you run individual cables, then remember to indeliably mark both ends of the cable with some form of numbering else you WILL be guaranteed of fun when working out what's what.

    Agreed that a tinned conductor should be nowhere near a pressure connection of any type, screw terminals, phoenix connectors, I would also say crimp connections. Particularly if actual current is moving through it, the solder warms and the connection loosens. The solution is a solder pot :p.

    I find AES spec cable to be, in general, less rugged than a good analog grade cable. But given we're talking about an install that's less important. I'm yet to find an AES snake in Aus that isn't foil shielded rather than braid shielded. Sure it's easier to work with, but I prefer braid for flexibility. I also find the AES stuff has thinner conductors in the main and insulation on those that is allergic to heat.

    A note, if you are multipinning stuff, some people will common grounds to squeeze more channels into a connector. You can get away with this in most situations with analog, but it WILL bone you with AES. We've run 50 metres on good analog cable multicore with AES when it's only rated to 30. You won't get 30m on common grounded core unless you are REALLY lucky.
     

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