[Late]

Hi Guys

I've got a bit of a conundrum that I hope you might be able to help with. I'm building a converted attic home cinema.

I have my projector, speakers and amp but no cabling as yet. Because the projector is positioned at one end of the room and the front speakers at the other, its difficult to work out where I should place the amp. The key difference is the length of cabling for video and audio.

If I put the amp near the projector, I can have a short HDMI cable to supply the projector with video. However, I have to then use speaker cables that are quite long - probably greater than 10 metres if I'm to have them nicely tucked away (that's a 20 metre round trip from amp to speaker and back if that's how these things are measured).

Alternatively, I could place the amp at the foot of the screen and have much short speaker cables. But that would mean an HDMI cable 10 metres or longer - and I understand HDMI has some problems at distances greater than 10 metres.

So which is best? Long speaker cables and short HDMI. Or. Short speaker cables and long HDMI.

The set up is:

Onkyo TX-SR606
Aego T speakers and subwoofer
Sanyo HD projector Z3/4 (can't recall which from where I'm posting)

Hope I've posted this in the right place. Your help is much appreciated.

[Para]

erm speaker cables will be cheaper as you may need a higher quality hdmi at that length..


plus the amp will be behind you so you wont have to stare at its display and leds..

[Late]

Thanks Para. Cost is indeed a consideration - feeling a bit poor after buying the kit - but I'm also concerned about sound and video quality. I'd hate to have this brilliant room with great kit, only to find it's let down by distances involved in cabling. I've read a few articles suggesting that the signal to speakers degrades at these kind of lengths - I would pretend to be an expert in AWG and the like. Equally, I understand there are inherent problems with HDMI cables and distance.

What I'm looking for really is the best configuration based on an approximate trade off of cost and performance.

[Late]

(Whoops! Double post)

[Jamie]

Personally I'd be happy with long speaker cables, as long as you use decentish cable (you don't have to spend a fortune mind) you wont have any problems. At the end of the day if you put the amp at the front your rear speaker cables will be just as long as well as the HDMI

I'd be more concerned about HDMI at that distance, you may even need a repeater/amp for such a long run. (probably not, I've heard of runs of 15m working fine but its an inexact science)Additionally A good long HDMI cable wouldn't be very flexible or easy to hide/tuck away.

I have approximately 11m of speaker cable to a set of MS floorstanders (old Ms25i's) in my dining room with no problems whatsoever.

If you really are concerned and have some extra cash you could look into sending the HDMI over cat 5, which is very flexible and easy to hide away, best of both worlds but may be fairly expensive and possibly overkill for just 10m

[Alan Mac]

A loudspeaker cable has a trivially simple task compared to that of an HDMI cable.

The only electrical parameter that is important for a speaker cable is the resistance of the wire. The resistance should be less than 5% of the loudspeaker’s nominal impedance.

Assuming your speakers are 8 ohms.
Then the cable wire resistance should be less than 0.4 ohms.

Your cable is about 10 m long, so the copper wire is 20 m long.

18 AWG (American Wire Gauge) copper wire has 20.9 ohms / km

So 20 m of 18 AWG wire has a resistance of:
(20.9 ohms / km) (20 m /1000) = 0.418 ohms
which is just above our resistance limit of 0.4 ohms

So, for a 10 m cable run, feeding an 8 ohm loudspeaker, use 17 AWG or thicker (lower AWG numbers correspond to thicker wires)


Alan

[mikim]

I'd say at only just over 10m (probably a 15M HDMI cable) then you will be fine. Allot of the good HDMI splitters etc have HDMI amps built in, so you could add one of them. You probably won’t notice any difference with a long HDMI cable, whereas there ‘may’ be a voltage drop issue with speaker runs?

To be honest though, I think you may as well decide where you want to put the stuff, and wire to that spec.
15m HDMI cable £30, distribution amp about £110 (price based on an Octava 1:2 splitter/amp). Add the 2 or 4 15m cables for the rear surrounds (depending on 5.1 or 7.1, although this will also come into consideration of the cost of bi-wire or not) at £3p/m... let's say 2 for the sake of bi-amping the fronts... £90
Total: £230 (although I doubt you will need the amp so start off without) Total: £130

15m x 3 for the LCR (possibly bi-amped?) with a good speaker cable (say £5p/m) £225.

So it's a bit cheaper to run 1 long cable that will suffer less losses than three speaker cables. The remote control will work better because it is in front of you, any fans on the equipment will be less noticeable as they are at the other end of the room, and you will probably find that you don't need the Distribution Amp anyway so that's 100 quid off the price.

I have a 10m run direct out of my Onkyo, it is a 1.3HDMI spec cable and cost £20 new... no issues what so ever I also have a 10m HDMI to the TV that comes out of a separate 1:2 splitter, no issues there either.

Pick where you want the equipment, as I don't think that kind of run distance should really make any noticeable difference in quality.

[Oli]

Originally Posted by mikim

To be honest though, I think you may as well decide where you want to put the stuff, and wire to that spec.
15m HDMI cable £30, distribution amp about £110 (price based on an Octava 1:2 splitter/amp). Add the 2 or 4 15m cables for the rear surrounds (depending on 5.1 or 7.1, although this will also come into consideration of the cost of bi-wire or not) at £3p/m... let's say 2 for the sake of bi-amping the fronts... £90
Total: £230 (although I doubt you will need the amp so start off without) Total: £130

15m x 3 for the LCR (possibly bi-amped?) with a good speaker cable (say £5p/m) £225.

So it's a bit cheaper to run 1 long cable that will suffer less losses than three speaker cables. The remote control will work better because it is in front of you, any fans on the equipment will be less noticeable as they are at the other end of the room, and you will probably find that you don't need the Distribution Amp anyway so that's 100 quid off the price.

First - £3/meter is totally unnescassary imo - as Alan points out, 17AWG cable will be fine and will be nowhere near that price.

Second - The point about one long cable will suffer less loss than three speaker cables, how do you come to such a conclusion?

[mikim]

The point I'm trying to make is that there really is little difference between running long speaker run or HDMI (at the length he has mentioned 10m or so), therefore he may as well decide where he wants to put the gear, and connect accordingly

I agree, I didn't spend anywhere near 3p/m on my surround cables (I believe it worked out at about 90p p/m), but I thought I'd put an average price between what I spent, and what some people might want to spend.. QED etc etc. In fact thanks to the wonderfull classifieds on here I didn't spend £3 p/m on my QED bi-wire either
HDMI is a digital medium, and only suffers digital packet errors due to medium long runs, this is due to possible interference/resistance of cable/drop in HDMI transport voltage... these errors are generally very minor and the circuits in the PJ should iron any error of delivery. Very unlikely that we would ever see it unless there is a serious problem, or the cable run is very very long, then there will be HDCP errors etc anyway.

Long cable runs for speakers can suffer from the resistance of the cable, voltage drop etc... again, very unlikely that you or I could hear the difference.

But it's a simple science... if you can hear a difference, buy better speaker cables.

[Alan Mac]

Originally Posted by mikim

Long cable runs for speakers can suffer from the resistance of the cable, voltage drop etc...

Why should a long cable run suffer more from a given resistance than a short run would for the same resistance?

Surely “voltage drop” is directly related to resistance?
Why do you consider “voltage drop” to be a separate issue?


Alan

[-Ad-]

Originally Posted by Alan Mac

Assuming your speakers are 8 ohms.
Then the cable wire resistance should be less than 0.4 ohms.

Quick question and Il promise not to derail anymore.

Why 0.4ohm resistance needed for 8ohm speakers? Was wondering as il be making some small diameter speaker cable for my ATC speakers to amp, but only 2.5m length so I assume nothing to worry about.

Other than that the maths and theory is all good.

Ta

[mikim]

Originally Posted by Alan Mac

Why should a long cable run suffer more from a given resistance than a short run would for the same resistance?

Surely “voltage drop” is directly related to resistance?
Why do you consider “voltage drop” to be a separate issue?


Alan

Yes, voltage drop is directly related to resistance.
A cable run long or short with the same resistance will have the same resistance. I don't think I wrote anything to suggest otherwise?
I don't consider V.drop to be a seperate issue... probably just my wording that has thrown you there Sorry about that.

[Alan Mac]

Originally Posted by -Ad-

Why 0.4ohm resistance needed for 8ohm speakers?

It’s not that we need 0.4 ohms, but that the speaker cable wire resistance should not EXCEED 5% of the nominal impedance of the speaker.

For an 8 ohm speaker that limit is :

8 ohms x (5 / 100) = 0.4 ohms


Similarly, for a nominally 4 ohm speaker the limit would be 0.2 ohms
and for a 15 ohm speaker the limit would be 0.75 ohms.


The 5% limit is chosen primarily to ensure that the loudspeaker impedance variations across the audio-frequency band do not have an audible effect on the speaker’s performance.


Alan

[Late]

Guys, I've just got back from work and read all your replies. They've been really helpful. I think I'm going to go with the long speaker cables because I think they're going to be easier to hide. Plus I want the noisy Xbox 360 to be well away from my ears!

Thanks also for the help on AWG stuff. I think I'll go one lower - 16 - and double check for resistance lower than 0.4 ohms.

[-Ad-]

Originally Posted by Alan Mac

snip

Excellent post, thanks for the lesson alan

Now calculations can be done whilst actually understanding the theory.

[mikim]

You might find this website helpfull too, it can help with AWG and cross sectional area etc:

HERE

[Alan Mac]

An observation regarding Loudspeaker Damping Factor:

(It is often claimed that a good reason for having a very low resistance speaker cable is the need maintain a high damping factor).


In the case of a loudspeaker driven by an amplifier via a speaker cable, the effective damping factor denominator is dominated by the loudspeaker voice-coil resistance, which is probably 5 ohms or more, for an 8 ohm speaker. The amplifier might have an output impedance of 0.1 ohms and our speaker cable we have limited to 0.4 ohms, making a total of 5.5 ohms.

The numerator, (which in this case is the modulus of the speaker impedance at a particular frequency) is unlikely to exceed 30 ohms.

So the highest effective damping factor we can expect is:

30 ohms / 5.5 ohms = 5.45

Even if we were to reduce the cable resistance to zero ohms and the amplifier output resistance to zero ohms, the highest effective damping factor would still only be:

30 ohms / 5 ohms = 6

Conclusion:

For practical purposes, the speaker cable resistance has negligible effect on the effective damping factor.


Alan

[farmerpalmer]

I would use longer cables for digital (hdmi) and shorter cables for analogue (speakers).

You have less (nil) chance of losing anything over a digital link and lots over an analogue link.

just my 2p worth

[Alan Mac]

Originally Posted by farmerpalmer

I would use longer cables for digital (hdmi) and shorter cables for analogue (speakers).

You have less (nil) chance of losing anything over a digital link and lots over an analogue link.

I think that ignores the practical reality.

Long HDMI cables are large in diameter, relatively heavy and require a large bending radius. Unless care is taken, damage to the HDMI connectors may occur, with intermittent (or permanent) loss of signal.

There is no audible “loss” (that I am aware of) caused by a speaker cable of the sort of length we are talking about here (less than 20 m), provided the resistance is adequately low.


Alan