max. length of cables

[great_beyond]

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,

[mbenonis]

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.

[The_Guest]

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/...to&PhotoID=155
http://msnusers.com/techphotospeter/...of.msnw?Page=3

[MikeJM]

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.

[DMXtools]

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

[The_Guest]

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

[zac850]

Originally Posted by MikeJM

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)

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

[DMXtools]

Originally Posted by zac850

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

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

[zac850]

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.

[Mayhem]

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,