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Permanent wiring for surround sound

Discussion in 'Audio Visual' started by Nozzle, 18 Feb 2017.

  1. Nozzle

    Nozzle

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    I'm having the iving room replastered as much of it is shot. I've decided to put CAT5e cable in for the router and permanently wire in some surround sound satellites too, terminating them at RCA plates at either end. I don't want to go to all this trouble and just put in some pantsy fragile bell wire - so I was thinking thicker stuff would be better. Not oxygenfreefancypants stuff, probably still bell wire type stuff, but thicker 42 stand or similar.

    Does anyone have tips doing this? (Not running them parallel to mains cables seems like a good idea). I assume the routes have to follow the same rules as mains T&E though - i.e. in safe zones up to the ceiling and across?

    Nozzle
     
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  3. oldbutnotdead

    oldbutnotdead

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    I've laid in similar cabling for possible future use. Yes keep them away from power cables but no need to be too paranoid about it. Yes route the cables logically in safe zones but again due to them being ELV don't be paranoid about it, best bet is take pics of the walls before you fill the chases in & stick them in an album marked 'WHERE THE CABLES ARE'.

    I've used 1mm T & E cable (for the surround) and 1.5 for the front L & R- cheap, mains-rated for where it comes anywhere near other cabling, mechanically robust, readily available. I'd use push-block faceplates rather than phono sockets- nasty things to terminate decent size cables to. Things like these http://cpc.farnell.com/vision-av/tc3-speaker/techconnect3-speaker-module/dp/AV26442 , sure you can get single circuit versions. Or Spekon sockets- excellent connectors but the plugs are a bit big in the domestic environment.
     
  4. Lucid

    Lucid

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    Speaker wire isn't carrying the voltage of mains cable. Unless the amp is stupidly powerful - and I'm talking real current driving capacity and not the magic Watts claims of a lot of manufacturers - it's going to max out at roughly 30V DC and probably a lot less than that in reality. What you should pay attention to though is the current the wire will be carrying.

    There are two areas to think about. The first is the power loss through the cable.

    This is determined by cable construction i.e. is it pure copper (good) or the cheaper copper coated aluminium (CCA = bad cable), and then by length and the thickness. e.g. If you only need to run 100W through 10m of cable within a single room then it's not going to make much of a difference if you run 1.5mm or 4mm cable, so spending the extra probably isn't justified. However, if you're starting to run cable all around a house like I do for multi-room speakers then the power loss ramps up and becomes significant. 100W in an 8 Ohm rated system will deliver over 90% of the power to the speaker over 40m of pure copper cable @ 4mm, but only a shade over 70W through a 1mm cable. It's far cheaper to buy thicker cable and avoid the power loss than it is to buy a much more powerful amp to compensate for the power loss in thin cables. The current in both cases is over 3 Amps. That's why you don't use phono plugs. They're not rated for the sorts of Amperages; 2A is commonly the quoted maximum and that presumes a well made plug. There's a lot of rubbish out there that won't deal well with speaker-level Amperages because phono plugs are really a line-level connection where the voltage rarely exceeds 1V and the input impedance of each item in the chain is so high (10,000+ Ohms) that the current is negligible.

    The second area is system and speaker matching. This can have a big impact on the current flowing through the speaker cables.

    Good quality home audio gear runs at 6-8 Ohms. That means the speakers present a 6-8 Ohm nominal load to the amp, and the amp is designed to drive an 8 Ohm load. With a system matched in this way your 10m 1.5mm pure copper cable is carrying around 3.5 A presuming it's a full frequency signal and the amp is pumping out 100W. Now watch what happens if the system matching changes. Let's say you have one of the all-in-one home cinema kits - you know, it has a built-in DVD/Blu-ray drive and a million Watts output according to the manufacturers specs. The speakers and amp are rated at (typically) 3-4 Ohm. The same 10m 1.5mm is now carrying anything up to 5.5 A. It gets even worse if you mix and match. Put 3 Ohm speakers on the end of an 8 Ohm amp and the current rises to almost 9 Amps.

    So, big question - Every all-in-one home cinema kit comes with crappy thin bell wire and generally use 3-4 Ohm loadings, so why don't the cables melt and the amp erupt in a ball of flames?

    Simple, the amp isn't outputting anything like the claimed power and the speakers aren't full range so they aren't drawing the current. Manufacturer's power claims are a master class in smoke and mirrors. The figures you'll see on the spec sheet or box are based on driving one single channel - therefore all the power is funnelled to that one channel - and it's working at a very narrow frequency band (1kHz) and for a fraction of a second (peak power output) so the amp doesn't have to recover and continue supplying power. The result is that you're lucky to see 20% of the claimed power output in real world conditions. Add to that that the amp will rarely be running at full power so the current being drawn by the speaker will be even lower.

    What are the take-aways then from all this guff?

    Speaker cables don't have the same safe zone requirements as mains voltage cables. Sticking a nail through a speaker wire is highly unlikely to kill someone unless something is very very wrong with the amplification or the way it is connected

    Decent cable is worth it - to a point - and that point is governed more by the cheapness of the gear (not a great phrase, I know; but accurate) than you might think. 1~1.5 will be adequate unless you plan to hang some full range surrounds off the ends of the cables in future

    CCA cable is crap. No two ways about it. Aluminium isn't a good conductor for low voltage signals. Pure copper is what you want. You mentioned 42 strand. That's a good shout. Yes you can use T&E but the difference in price is literally £3-£4 on a 25m roll and I prefer to see something that looks like speaker wire if I lift a board or open a wall

    Don't use phono plugs. Go for speaker wall plates with binding posts



    Good luck with the job :)

    Did we help? Then use the THANKS button. It's free and it marks good advice for other readers
     
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  5. foxhole

    foxhole

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    My old surround system uses wireless box at rear of room to receive audio for rear speakers.Do modern systems still use wired speakers.?
     
  6. Lucid

    Lucid

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    For quality, yes.

    Wireless transmitter systems simply swap two wires front to back of the room for an extra amp and other speaker cables running across the back of the room, and there's an extra power socket taken up; so they aren't really "wireless speakers" in the true sense. They also use compression of the audio stream. That's not so much of an issue with speakers using limited bandwidth such as small satellites, but you can hear the compression when the rears are large and/or full range speakers. It's the same kind of sound quality bottleneck you get from Bluetooth streaming. KEF brought out a nice solution about 8-10 years ago that used lower compression tech but it wasn't cheap: IIRC about £400-£600 just for the rear amp and a pair of small satellite speakers. That's about the same or more than a lot of folk spend on their whole all-in-one kit with wireless rears. From that you can work out that the A-i-O kit makers are cutting quite a few corners if they can include all the rest of a home cinema kit for the same sort of money.

    Philips has got closest to the ideal of wireless rears with their Fidelio B5 soundbar. It included detachable speakers that were charged from the main bar and could then be used either as rears or as multi-room speakers. They still couldn't crack the need for compression though. The system used Bluetooth for the link and it's stereo performance wasn't great. £600 all in last time it was available.
     
  7. Sam Gangee

    Sam Gangee

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  8. DIYnot Local

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