zac850 said:

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