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max. length of cables

Discussion in 'Sound, Music, and Intercom' started by great_beyond, Jul 23, 2004.

  1. great_beyond

    great_beyond Member

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    I am suggesting to my theatre director that we move the sound board out of the booth and into the house. We already have 100-150' of cable running from the booth to the stage, then cables at a max of 50' connected from the stage to various microphones. If I connect all of the existing connections from the board and plugged them into a snake which would be 75', would there be any significant loss of signal strength, that I could not compensate for with the Gain controls on the individual channels? And what is the maximum cable length that can be used?

    Thank you,
     
  2. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    Assuming all of your cables are balanced, and everything is wired properly, you shouldn't have a problem doing this. Just make sure you test all of this out a few weeks before the show - there's nothing worse than having the whole sound system become inoperable the week of a show.
     
  3. The_Guest

    The_Guest Senior Team Emeritus Premium Member

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    It should be fine as long as your not using too many connectors/cables, everything is wired/grounded good, and the cables/snakes are in good shape. Right now I'm working on a musical were there are no mic jacks up in the fly space or grid area. The designer's design called for three hanging mics. Instead of running a long, painful "buzz-worthy" run of XLR from the stage to the grid and back down. And remember, thats not one run, not two, but three long XLR runs the grid and back down. The problem was solved by dropping a 200' snake down stage from the grid. Sounds and works fantastic, no buzzing or anything. Now think about this, from mic to console (start to end)...

    25' perm mic cable attached to Audio Technica AT853Rx Microphone
    25' XLR Cable (connected to junction box on grid)
    200' Snake Cable (from Junction Box on Grid to DS mic jack)
    About 70' XLR run (from the DS jacks to the patch rack at stage left)
    1.5' TRS Patch Cable (patch rack)
    100'-150'? XLR run (from patch rack to control booth muiti-pin jack)
    15' Muilti-Pin to XLR Snake in booth (from muiltipin jack to console)

    That's roughly a whopping 461.5 feet from the hanging mic to the consoles input. I didn't notice any problems at all. As long as your using good cables you should be fine. Think about it this way, just imagine the cable runs on tour. While some are going digital and serializing their analog signal, most aren't do to latency problems. Anyway, that's a smart idea mixing from the house, I wish I could make that permanent at my facility. Mixing from a tiny window just isn't fun. Even alone setting up the console and all it's gear is so much work to begin with, you gotta put it all away after a show. Security is important, you can't just leave it sitting out. It's a bummer, I'd only go permananent unless they made a secure booth that was not permanently accessable to the house.

    EDIT:

    Here are some pictures of the snake run...
    http://msnusers.com/techphotospeter/fiddlerontheroof.msnw?action=ShowPhoto&PhotoID=155
    http://msnusers.com/techphotospeter/fiddlerontheroof.msnw?Page=3
     
  4. MikeJM

    MikeJM Member

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    Yeah, you shouldn't have a problem with anything as long as everything is wired properly like mbenonis1 and The_Guest stated.

    You run into the problem with long cable runs more with speakers. You can run into problems like phase difference, loss of signal strength.
    (That is why amplifiers are normally placed by speakers in longer FOH distances, instead of by the sound board)

    Connections are always a big problem, more so than distances. I always say the less connectors and cable transfers you have to make, the better.
     
  5. DMXtools

    DMXtools Active Member

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    With high-quality balanced shielded cable in a low-impedance system (~300 ohms), you may notice a slight loss of highs if the cable length gets up over about half a mile or so. In your situation, it'll be just fine.

    John
     
  6. The_Guest

    The_Guest Senior Team Emeritus Premium Member

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    I added some shots of the snake drop to the original thread, check them out (if interested).
     
  7. zac850

    zac850 Well-Known Member

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    Now, say you were thinking of putting the amps 75 feet or so away from the speakers, would that be OK or not?
     
  8. DMXtools

    DMXtools Active Member

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    It depends a lot on how much power you're trying to push through what size wire, and the speaker impedance.

    Even the best wire has some resistance. For FOH speakers, I like to use #12 AWG or better and keep the wires as short as possible. I cringe when I see #22 AWG being used as speaker cable at any length.

    Let's do a little math (now you're the one to cringe :) ). Let's say your speaker is 4 ohms, a common value for fairly-high-power boxes, and your amplifier is rated to put 500 watts into a 4 ohm speaker. The first thing we need to know is how many volts the amplifier will put on the speaker wire. We can figure it out.

    We know power in watts = volts X amps: P=ExI. E stands for Electromotive Force, a fancy name for volts. Don't ask me why "I" was chosen to represent amps.

    Ohm's Law tells us that amps is volts divided by ohms: I=E/R. If we plug Ohm's Law into the power formula, we get P=ExE/R... we've gotten rid of amps. Simple algebra tells us we can do almost anything to an equation if we do exactly the same thing to both sides of the equation. Let's multiply both sides by R. We wind up with RxP = RxExE/R. The two Rs on the right side of the equation cancel each other out, so we end up wit RxP=ExE, or RxP=E squared. So E is the square root of RxP. Plugging real numbers in, E is the square root of 4x500 = square root of 2000, so E is about 44.72 volts.

    Stay with me on this. We're going to use that voltage to figure out how much power is actually getting to the speaker and how much is being wasted in the wire.

    Let's assume your 75 feet of speaker wire is the cheap #22 AWG zip-cord a lot of places sell by the spool for home-stereo installations. I have a table that tells me the resistance per foot of various wire sizes. 22 AWG copper is .0165 ohms per foot. Doesn't sound like much, but we've got 150 feet of it, 75 from the amplifier to the speaker and another 75 back to the amplifier. 150x.0165=2.475 ohms! That's in series with the 4 ohms of the speaker, so what the amplifier sees is 6.475 ohms instead of the 4 ohms it was expecting.

    What effect will that have on the amplifier? Amplifiers are generally voltage devices. That's why most amplifiers are rated for different power levels with different speaker impedances - the amplifier's fairly steady voltage output will push more power through a lower resistance. Now we're showing it a higher resistance.

    44.72 volts will push 500 watts through 4 ohms, but what will it push through 6.475 ohms? Back when we were massaging the power formula, we got to a variation that said P=ExE/R. Let's plug the numbers in: P=44.72x44.72/6.475 = 2000/6.475 = 308 watts. We're only getting 308 watts out of our 500 watt amplifier.

    But it gets worse. Let's go back to Ohm's law for a moment. How much current is the amplifier pushing through the speaker (and wire)? I=E/R = 44.72/6.475 = 6.9 amps. That,in turn, lets us figure out the voltage that the speaker sees and the voltage that shows up across the wire. Another form of Ohm's law is E=IxR. The speaker is still 4 ohms, so it sees about 27.6 volts (6.9x4). The rest of the 44.72 volts, 17.12 is spent on the wire.

    Now, let's go back to our original power formula, P=IxE, and plug those numbers in. The power the speaker actually sees is 6.9x27.6, or 190 watts. About 118 watts (6.9x17.12) is being wasted in the wire. Because we used long speaker wires of a small AWG size, our 500 watt amplifier is only putting out 308 watts, and of that 308 watts, only 190 is actually getting through to our speaker. We're wasting 118 watts just heating up the cable.

    A lot of ready-made speaker cables sold at Guitar Center, Sam Ash and other music stores are 14AWG. It's resistance is .00258 ohms per foot, so our 75-foot cable (150 feet of wire) is now just .387 ohms. I'm not going to show all the math again, but the result is that the amplifier sees 4.387 ohms and puts out about 456 watts. Of that, about 415 watts gets to the speaker and only about 41 watts are wasted in the wire.

    In my own system, my amplifiers are right at the speakers. The cables are 5 feet long. The cable is #12 AWG, at .00162 ohms per foot. The cable resistance, then, is only .0162 ohms. The amplifier sees 4.0162 ohms and actually puts out 498 watts. Of that, about 496 watts actually get through to the speakers and only two watts are wasted in the cable.

    Bottom line is that long speaker wires are NOT a good idea. Small speaker wires are NOT a good idea. Long, small speaker wires are a very BAD idea. Short, big speaker wires are a very GOOD idea.

    John
     
  9. zac850

    zac850 Well-Known Member

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    wow, lots of math. I sort of remember a bit of that from physics class (which I took last year, dang me and my nonexistent memory).

    OK, so, anyway, with that info i'm going to talk to people who know stuff, and figure out what I need for my FOH speakers.
     
  10. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Great post DMXtools.

    My understanding is that a speaker is primarily an inductive load and this is why when you meter a 4 ohm speaker you read about 6 ohms on the meter. However, if I remember correctly, at different frequencies, the speaker will display different characteristics. Thus the amp can see a speaker as being a resistive load, inductive load and capacitance load (if that is the correct term) at various frequencies.

    In light of the calculations, does this actually make any difference? I am correct that the load type has implications with regards to power. However, this is getting a thin in my knowledge area so I may be confusing what is a very simple and straightforward mathematical problem. :?

    Just one other question, is the resistance chart something that you got off of the web (a link would be great) or is it one from a book? And would you trust a low ohms meter to give an accurate reading of a cable or would you advise the use of such a chart?

    Cheers,
     
  11. DMXtools

    DMXtools Active Member

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    Correct, except that the DC resistance will usually be lower than the A.C. impedance. 6 ohms resistance is typical for a speaker with an 8 ohm impedance.
    Again, correct... with a little clarification. It's mostly an inductive load. A speaker system can appear as a capacitive load at high frequencies due to capacitors in the crossover. A piezoelectric tweeter is, by it's very nature, a capacitive load. A well-designed crossover will limit the capacitive loading the amplifier sees, because a lot of amps, especially direct-coupled circuits, become unstable and begin to oscillate (usually at frequencies above the range of human hearing) if they see too much of a capacitive load.

    Most audio power amplifier designs take into account the fact that the load will vary from inductive to resistive to capacitive as the frequency changes. The math for that is a bit more complex.. and not really needed to figure out what size cable you need.

    It's from a book called Pocket Ref, published by Sequoia Publishing, Littleton, Colorado, ISBN 1-885071-00-0. My copy was swag from a vendor.
    Yes... both.
    The chart will tell you what wire you need, the meter will tell you what wire you've got.

    John
     
  12. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Thanks for those responses. I had a quick search on the web for resistance/length charts and the one that I did find was significantly different to yours. It took me a while but I worked out that their values were approximately double the ones that you quoted - thus eliminating the need to double the length of the cable in the calculations.

    As I am somewhat forgetful when it comes to math and often misplace the scraps of paper with the calculations on them, I have put together a very simple calculator in Excel into which I only need to enter the Amp power, speaker impedance, wire gage and length.

    The calculator looks up the wire gage from the reference table and doubles the cable length.

    My plan is to make a series of different calculators to keep on the PC in my workshop. Rather a good use of my lunch break I thought :)
     
  13. DMXtools

    DMXtools Active Member

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    Except for the square roots, I did the calculations in my head... It's how you can tell I've been an engineer too long. I'm no great mathematician, but I've been playing with these formulas and calculations for over 40 years. Some of it stuck, I guess.
     
  14. The_Guest

    The_Guest Senior Team Emeritus Premium Member

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    I once heard that for every 10' of guitar cable, you lose a dB of gain. Is this true? With any long run you'll lose gain but I was curious about that quote. Some of the runs in my rig live (no effect loops) have been as long as 70', effects patching and amp/guitar cables really add up.
     
  15. DMXtools

    DMXtools Active Member

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    Actually, simple loss of gain isn't much of a problem. Cable resistance, even for cheap cable with a really thin center conductor (#26 AWG), adds up to less than 5 ohms over that distance, while the input impedance of most guitar amps is upwards of 20,000 ohms. You only lose 0.025% of the signal - the typical guitar amp has more than enough gain to compensate.

    The real problems are capacitive loading and noise pick-up.

    A capacitor is just two conductors separated by an insulator. So is a shielded cable - the capacitive reactance between the inner conductor and the shield is the real signal killer. Capacitive reactance looks like a resistor, bypassing some of the signal to ground before it can get to the amplifier... and is frequency dependent. At DC, the reactance is infinite. At low frequencies it's still very high, but it drops as the frequency rises, shorting out more of the signal. Because the total capacitance rises (and the capacitive reactance falls) with cable length, the longer a cable is, the more it kills the high frequencies. You may notice that the sound is a little muddy and lacks definition in the high frequencies, or needs more boost on the treble control to sound good with a long cable. Lowering the impedance of both the signal source and the amplifier input reduces this problem.

    Guitar cables are generally unbalanced: the signal is sent on a single center conductor as a voltage with respect to the grounded shield. This makes it more sensitive to noise-pickup than the balanced system used for long runs of microphone cable.

    Balanced cables send the signal as a difference between two center conductors. Noise may still get through the shield, but it gets to both center conductors and affects them equally. Let's say that at a given instant, conductor A is at +.25 volts and conductor B is at -.25 volts. The amplifier sees only the difference - A is .5 volts more positive than B. Then a noise pulse gets into the cable and drives both conductors positive by 7 volts. Now A is at +7.25 volts and B is at +6.75 volts - big changes. But since the amplifier isn't looking at the absolute voltages, only the difference, all it sees is that A is still .5 volts more positive than B.

    A good rule of thumb is that both noise and loss of high frequency definition start to become noticeable (not necessarily objectionable - it depends on a lot of other, more subjective, factors) when the guitar-cable is longer than about 18 and a half feet. It's not a brick wall, where the guitar stops working. It's just the point at which the extra noise and the loss of highs just start to be noticeable. Other factors, such as active pickups in the guitar, can help handle longer cables.

    At one time, Peavey produced some extra-long cables with a balun built into each plug. A balun is a transformer that provides a balanced connection on one side and an unbalanced connection on the other. Both the guitar and amplifier would see the high-impedance unbalanced connection they expected, but the cable in-between was low-impedance, balanced mic. cable. I'm not sure if they still make them, or if anyone else does, but I always thought it was a neat idea.

    John
     
  16. soundman

    soundman Well-Known Member

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    You should be fine with a 75 foot cable run from the amps to the speakers as long as you use good cable, a point that can not be stressed to much.
     
  17. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    It is funny that you should mention this. I am just in the process of making some of these. At the moment, they will be more adaptors than specific leads. I have some great little 1:1 SMD transformers that are about the 1cm x 1cm x 0.5cm in size, so they will fit nicely into the body of an XLR. I also plat to make up a couple of “earth lifts” using the double-ended XLR adaptors. The tricky part is going to be fitting the switch to the body. I could sure use your milling machine!
     

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