What makes speaker cable and coax optimized for speakers and subs? - AVS Forum
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post #1 of 6 Old 04-01-2014, 01:23 PM - Thread Starter
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The question sounds pretty straightforward, I know you could say because they are. What I mean is HOW. I'm dealing with a situation where I'm considering using 16/2 as subwoofer cable, by soldering on RCA fittings and making my sub connection that way. Aside from running new cable (coax) outside the wall, this is really my only option, because the room is completely finished with high end carpentry and molding. I've seen more than a few discussions where people say the connection is possible, physically, but this doesn't clarify whether or not it's actually practical.
Most of the professionals I've spoken with, either at Polk Audio or one of our AV distributors, completely advised against it. I know that coax runs at 75 ohms, but I'm not sure about speaker cable. So one of the things I was wondering was, why not just work a 75 ohm resistor into the speaker cable connection so that the resistance is equivalent to that of a coax run? A tech at Polk says that since speaker cable is suitable for high current signals, I would not want to use it for a line level connection. But the power of the signal is dictated by the signal coming from the receiver. So how would the capacity of the cable have any influence on a signal coming out of a line level output?
Aside from the resistance of the coax, what else makes coax a better conductor for line level signals versus speaker cable? And what makes speaker cable better suited for speaker level signals? I wouldn't want to do anything to compromise the sub or receiver. But if terminating the 16/2 with RCA fittings is a SAFE concept, should I just make the connections and go for it, and hope there's no noise or unacceptable performance? Or should I just go with all new runs outside the wall?
I'm no stranger to running and making connections, but I was just wondering about these connections from kind of an electrical theory point of view. Any descriptive links would be appreciated
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post #2 of 6 Old 04-01-2014, 01:25 PM
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What you want to do sounds fine to me.
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post #3 of 6 Old 04-01-2014, 02:10 PM
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Perfectly safe. Solder RCA connectors on your 16AWG wire. If it works... great. If you have noise/hum... do it right. wink.gif
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post #4 of 6 Old 04-01-2014, 03:11 PM
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sheilding theoretically makes coax superior for line level signals. If you don't pick up induced noise with unshielded cables, then it ain't no thing, as both of the above responses indicate.

Shielded cable is bad for speakers because the conductors are not sized for the current they will carry and because (AIUI) at speaker levels capacitance (?) of the shield-to=positive can cause audible frequency response anomalies. But doubtless many guitar and bass players have mis-used instrument (shielded) cable as speaker cable since the 1950s (they use the same connectors in most guitar and bass amps) and gotten through their gigs without noticing undue heat in the cables or audible weirdness. Of course, audible weirdness is relative, can be blamed on other causes and is masked as the evening goes on if the player is having a beer or two . . . not to mention the pesky drummer . . .
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post #5 of 6 Old 04-01-2014, 03:46 PM
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A few things:

1. Speakers carry high level signals to a speaker that has low impedance (and resistance). As such, they are not susceptible to interference. So what you were told about them carrying "high current" is generically true.

2. The input wires to a sub or amplifier however, operate at very low currents and high impedances. As such, they can easily pick up stray signals in the air and generate hum and noise.

3. The voltages involved are very safe so there is no safety issue with what you are doing.

4. Impedance is not the same as resistance. So you cannot add a resistor to a speaker wire and make it "75 ohm." This is a complicated topic that I won't go into. Suffice it to say, if you measure the resistance of a coax cable that is "75 ohms" your meter will actually read infinity.

5. 75 impedance and such only matters for Radio Frequency signals and such. For audio, that does not matter.

6. Coax wire has shielding which your speaker wire does not. This, combined with #2 means that it really is a bad idea to use speaker wire for this purpose.

In layman terms, you are asking if a tractor tire works fine on your car. Answer is that if it fits, you can probably drive it. But it really is not a good idea. Even if your sub works fine now, tomorrow it may not as you put in another piece of equipment around it and cause it to pick up noise and hum. Bite the bullet and do it right with a coax wire.

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post #6 of 6 Old 04-01-2014, 05:01 PM
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Quote:
Originally Posted by lougilman View Post

The question sounds pretty straightforward, I know you could say because they are. What I mean is HOW. I'm dealing with a situation where I'm considering using 16/2 as subwoofer cable, by soldering on RCA fittings and making my sub connection that way.

The first question is whether this is cable for the input to an active subwoofer, or cable for connecting a separate amplifier to a passive subwoofer. Sounds to me like this is the former, an input to an active subwoofer.

I've got a feeling that 16/2 might work, but it is inconsistent with accepted practice. Accepted practice would be to use shielded audio cable or shielded coax.

Generally speaking shielded coax is not terminated by soldering on connectors but rather special fittings called "Compression Fittings" are used. If you have to run it outside, not to worry because coax is rated for that, especially if you get special outdoors coax.

So here is your bill of materials:

Outdoor grade coax cable RG6 - only because that is most common and easiest to find. People on eBay will sell you it in whatever length you want
RCA compression fittings for RG6
Compression fitting stripping tool
Compression fitting crimping tool
Quote:
Aside from running new cable (coax) outside the wall, this is really my only option, because the room is completely finished with high end carpentry and molding. I've seen more than a few discussions where people say the connection is possible, physically, but this doesn't clarify whether or not it's actually practical.

Outside is fine.
Quote:
Most of the professionals I've spoken with, either at Polk Audio or one of our AV distributors, completely advised against it.

They probably advised coax like I am.
Quote:
I know that coax runs at 75 ohms, but I'm not sure about speaker cable.

Not an issue.
Quote:
A tech at Polk says that since speaker cable is suitable for high current signals, I would not want to use it for a line level connection.

Not an issue
Quote:
Aside from the resistance of the coax, what else makes coax a better conductor for line level signals versus speaker cable?

(1) Coax is shielded, and the usual convention is to use shielded cable for line level signals.

(2) You can get easy-to-install connectors for coax that are very durable, even robust and also weather proof.

(3) You can get coax that is rated for running outdoors. UV will attack regular cable that is always outdoors and make it brittle over the years.
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