Is it a big issue if my speaker wires have an 4 ohm impedance? - AVS Forum
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post #1 of 18 Old 10-04-2012, 08:40 PM - Thread Starter
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My house is wired for outdoor speakers and they might be long, low quality wires (I didn't put them in). When testing for continuity I discovered that the wires have a 4-ohm impedance. Even with 8-ohm speakers, this would be a 33% voltage drop. What does this mean for the amplifier? How about the speakers? Is this wiring usable?

Amplifier: Pioneer VSX-1120 6-8 ohm
Speakers: Definitive Technology AW-5500 4-8 ohm

Thank you.
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post #2 of 18 Old 10-05-2012, 12:25 AM
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Well it's not a perfect situation, but for background music it won't be a problem. The amp won't care at all, the speakers may play a little less loud. If this were a home theater or serious listening room I'd probably tell you to rewire, but if it's must for background music it's not a big deal.
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post #3 of 18 Old 10-05-2012, 05:40 AM
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You should replace the wire, but first be sure that the meter reading is correct. Even if the wire were only 20 ga. it would have to be 200 feet long to be 4 ohms. If it's really 4 ohms not only would you have massive insertion loss but the lack of adequate current capacity could result in the wire heating to the point of insulation failure, possibly toasting the amp, if not the house. If you have to replace it use this to calculate the required gauge:
http://www.bcae1.com/images/swfs/speakerwireselectorassistant.swf

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post #4 of 18 Old 10-05-2012, 06:50 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

You should replace the wire, but first be sure that the meter reading is correct. Even if the wire were only 20 ga. it would have to be 200 feet long to be 4 ohms.
Agreed. Could be 22ga. I'd check the meter. Short the leads with no speaker wire, I'm guessing you'll see a couple of ohms. Low resistance accuracy of ohm meters is not reliable. By the way, small point, you're measuring DC resistance, no impedance that way. Still works for our discussion here, though.
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If it's really 4 ohms not only would you have massive insertion loss
Insertion loss assuming an 8 ohm speaker and the original 4 ohm wire is 1.8dB...not "massive".
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but the lack of adequate current capacity could result in the wire heating to the point of insulation failure, possibly toasting the amp, if not the house.
Assuming an 8 ohm speaker and 4 ohm total wire resistance, and a 100 watt amp, running the amp at full output with a sine wave (never happens in real life...ever), the wire would dissipate 33 watts over its entire length. Since the length in this case is unknown, but we might assume 100ft, that's .33 watts per foot, which might, if run with a full output continuous sine wave, warm up a little up eventually. But nobody I know, even bench techs, run full output sine waves into speakers for very long. Typical listening levels of music or movie sound, the average power is less than 10 watts. Background music is much lower. Sorry to disappoint, but there'll be no insulation failure, no smoke, no flames, the wires will remain ice cold. Never heard of a house fire cause by small speaker wires. Oh, and no toasting the amp either because with additional wire resistance the amp will work less hard, not harder. The amp will actually prefer the long wires.
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If you have to replace it use this to calculate the required gauge:
http://www.bcae1.com/images/swfs/speakerwireselectorassistant.swf

I'm not saying the wires shouldn't be replaced, I'm saying for most background music or distributed whole-house applications it's not necessary.
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post #5 of 18 Old 10-05-2012, 07:40 AM
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Quote:
Originally Posted by has7738 View Post

Never heard of a house fire cause by small speaker wires..
I have. It's not uncommon for electric guitar and bass players to inadvertently use a light gauge instrument cable instead of a speaker cable, have the insulation melt and the conductors short out. Fire has resulted on more than a few occasions. And that's typically with less than 20 feet of wire. True the fault is usually because the insulation of an instrument cord isn't rated for the heat that a speaker cable is, but that same scenario could apply to many light gauge cables.

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post #6 of 18 Old 10-05-2012, 07:58 AM
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Note 100 W into 8 ohms is 28.3 Vrms, or 80 Vpp. Much lower than house wiring but not insignificant.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #7 of 18 Old 10-05-2012, 10:30 AM - Thread Starter
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Update: The resistance is correct. I actually rounded it down to 4 ohm from 4.7 ohm. The resistance of the leads is 0.1 ohm. This is a fairly new Fluke multimeter, so I would expect it to be accurate.

The wiring shouldn't be longer than 80 feet (unless they ran circles in the attic). In the family room I see a 4-wire cable (16 gauge) and outside I see 2 2-wire cables (18 gauge). I went into the attic but I couldn't follow it all the way to where the cables are connected. The direction looked reasonable though.

Is there any other possible explication for the high resistance other than a very, very long wire?
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post #8 of 18 Old 10-05-2012, 11:34 AM
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Bad connections? How are you measuring the wire resistance? If you are tying them together at one end and measuring at the other, you are actually measuring the resistance of two wires (twice the length).

Heaven knows how the contractors routed the walls. I had to deal with one install ages ago where they brought all the wires into the mech room ("for access") then back out. Some of the runs from one room to the next were actually several hundred feet long.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #9 of 18 Old 10-05-2012, 04:26 PM - Thread Starter
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Right. I'm tying them together, so it's double the length.

There is reason to suspect that the actual run is much longer than needed. I've located the place in the attic where the one cable from the family room meets the two cables from outside. The are not connected right there. All three go down through a stud and I can't follow them further.

So then, can we say that it's normal for a run of 400 feet (how long can it be?) of combined 16 and 18 gauge to reach 4 ohm?
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post #10 of 18 Old 10-05-2012, 04:33 PM
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Do you have any volume control boxes outside?
Usually the builders will have a volume control box somewhere inline.
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post #11 of 18 Old 10-05-2012, 05:09 PM - Thread Starter
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I haven't seen one. Is it usually outside on the patio, close to the place where you'd put the speakers?
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post #12 of 18 Old 10-05-2012, 06:20 PM
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check the walls- or any strange outlet covers that you might see outside.
In one home I owned, there was a blank plate where the control could go(wires just dangling), and my current one actually had it installed on the wall.
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post #13 of 18 Old 10-05-2012, 07:00 PM
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Quote:
Originally Posted by calimark View Post

Do you have any volume control boxes outside?
Usually the builders will have a volume control box somewhere inline.
+1, an LPad could be what's being read.

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post #14 of 18 Old 10-05-2012, 09:17 PM
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Seems like a logical thing to look for.

16 AWG is about 0.4 ohms for 100'; it would take 1000' (a 500' run measuring both sides) to reach 4 ohms,

See e.g. http://www.cirris.com/testing/resistance/wire.html

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #15 of 18 Old 10-06-2012, 07:26 AM - Thread Starter
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You guys are life savers! I found a volume control knob in the nook area. When I turn it all the way up, the resistance is just 1.2 ohm.

Do I need to mention that I never noticed that knob in 1.5 years?

Thank you.
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post #16 of 18 Old 10-06-2012, 10:03 PM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

I have. It's not uncommon for electric guitar and bass players to inadvertently use a light gauge instrument cable instead of a speaker cable, have the insulation melt and the conductors short out. Fire has resulted on more than a few occasions. And that's typically with less than 20 feet of wire. True the fault is usually because the insulation of an instrument cord isn't rated for the heat that a speaker cable is, but that same scenario could apply to many light gauge cables.

Ha! That's pretty funny! And I though musicians only busted up their instruments at the end of a show.

You don't suppose that instrument cables might be 22ga or smaller, do you? Or perhaps a full on amps-to-eleven fuzz-tone guitar running the amp into clipping might just be a little higher average power than background music? Nah....
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post #17 of 18 Old 10-06-2012, 10:11 PM
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Quote:
Originally Posted by DonH50 View Post

Note 100 W into 8 ohms is 28.3 Vrms, or 80 Vpp. Much lower than house wiring but not insignificant.

...except...

You don't listen at 100 watts (into 8 ohms). Music or movie sound won't let you. It might...might...peak at 100watts for brief explosion, but remember that to double apparent volume takes 10X the power. So, half the apparent volume of 100 watts is 10 watts. And w'ere still talking peaks only, not continuous tones. I know it's hard to believe, but most of our listening is done at 10 watts or less, long-term average.

Not too much point in quoting peak-to-peak voltages in this context, though, unless we're worrying about insulation dielectric breakdown. It's the RMS that heats the wire. Which, in any case, it won't.

But glad the OP figured the problem out.
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post #18 of 18 Old 10-07-2012, 07:27 AM
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People sometimes think of speaker wires as being low-voltage and they are not. Using 25 V-rated, or even 100 V, low-voltage lighting wire, is a not a good idea with speakers. I was trying to throw in a reminder that, while current flow is the primary cause of heating as Bill said, ignoring voltage ratings is inadvisable.

I agree it is essentially current that heats the wire, however you want to measure it (peak, average, RMS, whatever -- I assume you meant to say "RMS current").

I understand the relationship among dB, power, voltage, SPL, phons, Fletcher-Munson and all that jazz.

And I was in the sound biz long enough doing live sound to see plenty of wires go up in smoke... Mic cables for speakers, baling wire, transformer outputs twisted together to "double the power" (transformers REALLY stink when they are turned into slag!), etc. At least once a year some geetar player had to wire his biggest speaker directly to an extension cord and plug it in to the wall outlet to prove it could take it and play louder than the next guy's. Most could not.

Most people use larger wire guage these days, but in the 70's and 80's it was not uncommon for cheapo systems to come with 24 - 20 gauge wire, and then kids to use that same wire to hook their speakers up to a high(er)-powered amp and stress the wire to failure. Some would upgrade amp and speakers but not get new wire, same result. Especially when they hauled those speakers out on the deck to use for the block party. I dare say the power was a little more in that case than your average living or media room, not to mention that the RMS value of a clipped wave is much higher than that of a pure sine wave.

Whatever - Don

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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