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The Reference Room
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Wired for Sound: Steve Lampen
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| Do You Need Bigger Conductors? |
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by Steve Lampen,
3.29.2006
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Steve Lampen is Technology Specialist, Multimedia Products for Belden Electronics Division. Steve holds an FCC Lifetime General License (formerly a First Class FCC License) and is an SBE Certified Radio Broadcast Engineer. On the data side he is a BICSI Registered Communication Distribution Designer. His latest book, "The Audio-Video Cable Installer's Pocket Guide" is published by McGraw-Hill.
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If you've recently walked into a hi-fi store -
Oh, I'm sorry, they're not hi-fi stores anymore. They're "Home Theater Specialists" or "Entertainment Centers." Anyway, if you've spent more than five minutes in one, you probably had an argument about speaker cable.
Their philosophy is, "If you paid $50,000 for your 5.1 surround and plasma, how dare you run everything in cable that costs less than $10 a foot?"
Now I realize we broadcasters are a practical lot. If 18/2 zip cord works fine on your air monitors, why would you need anything else? The difference between you and the average consumer is, of course, that you actually know something.
You know that the resistance on 18/2 is very low, especially for a 10-foot piece. And if you wanted to go farther, or carry higher power, you might go to 14/2 or even 10/2.
Be aware that there is only one 10/2 that actually carries a UL approval, and it says "audio only" on the cable so you don't "accidentally" use it for power cable or lighting cable.
I believe that moving your speaker a half-inch, or hanging a rug on a wall, will have a more dramatic effect on the reproduced audio than all the speaker cable ever made put together. So why even consider bigger conductors?
Options
Well, there is this effect called slew rate. It's the ability of your power amp to follow high-frequency waveforms accurately and to drive the speaker cones in a similar fashion. The slew rate is determined by dividing the load on the amp by the actual output impedance of the power amp.
Most modern amplifiers have an output impedance of only a tiny fraction of an ohm, sometimes hundredths of an ohm. Compared to the speaker, that could be a ratio of 100 or even 1000. The load on the output of the amp is the sum of the speaker impedance and the resistance of the wire that is attached. The smaller the wire, or longer the run, the higher the resistance.
The impedance of a speaker, as you might know, isn't 4 ohms or 8 ohms but a varying value over frequency. In fact, it can be a real problem if a speaker drops to a very low impedance, like below 1 ohm at a specific frequency, because the current from the amplifier will peak with that small load. It's not at all uncommon that certain amp-speaker combinations blow output fuses or activate output protection circuits when a certain musical frequency is played. The problem is the speaker, not the amp.
In that instance, some 16/2 might actually save you, because the added resistance of the cable keeps you above that speaker impedance dip. Remember, the load on the amp is the speaker plus the resistance of the cable. If that resistance is a significant percentage of the load, it means that the speaker will not respond to the fastest waveforms very well.
One solution can be to move the amp close to the speaker. This was the original basis for amplified speakers. If you put the power amp in the speaker, then speaker cable is a non-issue. You just have to get line-level audio (and AC power) to the speaker, which are both easy to do. And line-level audio cable is a heck of a lot cheaper than 10 AWG speaker cable.
The other alternative is bigger wire. Shown here is a table of wire gage sizes and their resistance per foot. If you're calculating the added load on an amp, remember that you have two wires' worth of resistance at the given length, sometimes called the "loop resistance."
Sure, if you can find it
If you deal with high-power amps, that's another story altogether. There are now 5 kW, even 10 kW power amps.
Wire Gage Per Foot (68°F) | | AWG | Ohms | | 18 | .00692 | | 16 | .00435 | | 14 | .00273 | | 12 | .00171 | | 10 | .00108 | |
This brings up an interesting question. Unless these are wired up to 480V three-phase, I must assume they run in 120V single-phase, with a 20-amp breaker like everything else. And 120x20=2400VA. So how do you get 5,000 or 10,000? Surely this is not RMS watts, more like peak power.
Some of these amps suggest 8 AWG or 6 AWG cable, when going longer distances. That stuff is hard to find. (Check the "lighting cable" or "industrial cable" section of the catalog.) These super-power amps also have an interesting sticker on the back that says "Class 1 Wiring Shall Be Used" or words to that effect. Do you know what that means? I wonder if that rock band roadie does either.
What it means simply is that the speaker outputs of these amps can now kill you. So they need the same kind of wiring for your AC power.
Ask any manufacturer of speaker cable (especially in the "Entertainment Center") if they have Class 1 speaker cable. Huh? No, of course, they don't. And if you find yourself in this predicament, you will probably end up with someone who makes industrial cable, for wiring up factories. Tray Cable, a specific kind of factory power cable, is all Class 1. And you'd better get an electrician who does power wiring to do the speaker runs as well.
Hey, I can't wait to see a 10,000-watt, 70-volt transformer. I'm sure that's $1.59.
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