Does shielding on speaker wire really mess with impedence? - diyAudio
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Old 15th November 2007, 09:14 PM   #1
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Default Does shielding on speaker wire really mess with impedence?

I am working on a sound reinforcement system in a dance studio that has been pre-wired with heavy-duty, probably between 8 and 16 gauge SHIELDED speaker wire, the longest run being less than 100 feet. Someone else who looked at the old system said that the wire should be replaced because the shielding will ruin the impedance of the speakers and blow the amp. Is this possible? I don't see how a shield, that is not making actual contact with anything in line with the speaker (it is not acting as the ground or the negative or anything) could cause such a problem. Any thoughts?
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Old 15th November 2007, 09:45 PM   #2
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I will answer this simply.

No, the shield will not mess with the speaker impedance. The shield is grounded at one end and left to float at the other. Take care that the shield does not short to the positive or negative.

I mention both polarities just in case you are using a bridge amp.
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Old 15th November 2007, 10:05 PM   #3
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What he said.
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Old 15th November 2007, 11:07 PM   #4
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Rats, I posted without reading the question, so I've deleted my pointless reply.
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Old 16th November 2007, 08:26 PM   #5
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It could affect the capacitance per length of the cable, but that could easily be measured and checked against what the amplifier can take. It could be a good idea to have the shielding there, since it may supress HF noise (from cell phones etc) that otherwise might introduce noise in the system.

Possibly.
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Old 18th November 2007, 12:58 AM   #6
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The whole reason for shielding is to supress noise.

Shielding goes back at least 50 years. I've never heard of any problems associated with it.
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Old 18th November 2007, 10:23 AM   #7
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The HF noise from speaker elements and wire causes problems in power amp. Depending on the specturum and level of the noise, it can cause unstability in the feedback loop and anyway it eats the amplification to be used to compensate amp's basic non linear function.

In the amps there is typically used small inductor at the output to make sure that amp never sees capacitive load on high fequencies. This same inductor of course filters most of the HF noise from speaker&wire to amp but not all.
In some speakers like Tannoy there is actually dedicated connector so that metal frames of the speaker elements can be grounded to AMP ground. The voice coil is very good HF receiver/antenna.

The inductor at the end of power amp is often neccessary but it has also side effect. It can be heard (most probably) and any way measured. In some high end amps the inductor is not used but this requires different HF compensation in amp so that amp does not oscillate because of capacitive load.

So if target it to go to extremely high, I would use shielded speaker wires and conect also speaker element frames to that same shield.

Couple of years ago I had very difficult oscillation problem in one of my amp proto. It oscillates continously at about 2MHz. Then I found that it oscillates a bit also when power is off!! The reason was DSL carrier in the in the phone line in my house. The level of the signal in amp was not very high but clear anyway. Before I realised the reason I also used several months with oscillating aftive filter problem before I gave up.
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Old 18th November 2007, 10:41 AM   #8
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Of course this all depends on whether the noise you want to keep out is actually kept out by whatever is used for shielding. That is a whole science in itself, and just assuming that a simple shield keeps out all that's out there is pretty naive.

How many shielded speaker cables are there on the market?

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Old 18th November 2007, 11:24 AM   #9
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It is true that finding twin-axial speaker cable is difficult but basic coaxial can be found easily, like this one
http://www.thomann.de/gb/sommer_cable_magellan_240.htm
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Old 18th November 2007, 11:59 AM   #10
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Hi,

Screened speaker cable example:
http://www.jenving.se/screened.htm
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