Long cable run for speakers

[twotoedsloth]

Hello and thanks for taking the time to read and/or respond to this query.

A friend of mine belongs to a church that wants to replace their speaker cables.

She is concerned that the cable run is too long - 140 feet. In the concert hall I work in we have cables running 200 feet with no problems.

Could she use 8 gauge cables instead of 12 gauge cables to mitigate this problem?

Please let me know what you think.

Peter

[Sam Spoons]

Assuming these are passive speakers then long cable runs will cost some power and maybe cause impedance issues, using larger conductors will mitigate the issue but without more info it's impossible to say exactly what cable cross section would be suitable.

[twotoedsloth]

Hello and thank you for your response.

Yes, these will be passive speakers. I remember reading somewhere that speaker wire can be 300 meters, can't recall where..

What info would you need to help me out here?

Many thanks

[Folderol]

It depends on the type of speaker too. Assuming you are talking about ones already in place, seeing as they are in a church (which will have all sorts of acoustic issues anyway) they may be 100V line types, in which case you can have much longer cables for the same cross section. Other than that, for any given speaker power, if you double the impedance the cable current is divided by 4.
Also are the runs daisy chained, or independent back to the amplifier?

[Wonks]

A lot is down to how much power (and therefore current) you are putting through the cables.

12 gauge is 4mm2 cable and 8 gauge is 10mm2 cable. 12 gauge should be more than OK if you aren't using too much power.

Let's say 500W into 8 ohms. That's a current of 7.9A.

140 feet is a round trip of 280 feet, so let's say it's a bit over 300 feet, or 100m.

Put that into a cable loss calculator for copper cables and you get a 26W loss from the cable, which isn't going to be noticeable. Going to 8 gauge/10mm2 only drops that to a 10W loss, a 15W difference, and you've got much more expensive cable to install and it's a lot stiffer, so won't go round bends so easily.

But 1000W into 8 ohms results in a 100W cable loss for 12 gauge cable, so you'd really want a bigger cable for that.

So it really depends on what sort of power you are putting out and your speaker resistance (as that determines the current, and power loss is based on the square of the current).

[James Perrett]

The first question I would ask is why do they want to replace their speaker cables? Reasonably modern (i.e. made in the last 50 years) installed cables don't wear out.

[twotoedsloth]

Hello and thanks again.

From what I have read, aside from stiffer, less manageable cables, there is no downside to used 8 gauge speaker cable. Do I understand this correctly?

The cables will be run under the floor, so a stiffer, recalcitrant cable isn't really an issue.

I don't know the specs on the power amp or speakers, but I will ask her right away.

Best regards

[Wonks]

Well, there is always the issue of being able to terminate 8 gauge cables at the speakers. Standard Neutrik Speakon connectors will accept up to 6mm2/10 gauge cable for termination and an overall maximum cable diameter of 14.5mm O/D.

If the speakers have Speakon connectors, then 10 gauge will be the largest cable that will fit them.

You can always run the final length at each end in 12 or 10 gauge, and run the main length in 8 gauge, but you then need some connection boxes at either end, and I'd prefer to avoid the risks of loose screws within the connection boxes if possible.

Find out what power you are talking about, the speaker make/type/resistance if possible and the connection details at the amp and the speakers. Then decide on the best speaker cable size.

[twotoedsloth]

Okay,

Apparently the amp is a Rogers S200B 140 watt amp with 4 ohms outputs to the speakers.

Does that help?

[Folderol]

Ouch! Bad combination. Oh well, what are the actual speaker impedances.
How may speakers are there?

[Wonks]

A bit. Not a lot of info out there that I can find on it apart from that it's a valve amp and looks to have hi-fi style banana plug/binding posts for the speaker lead terminations. As it's a valve amp you don't want to risk the speaker cables coming loose so I'd avoid having connection boxes if possible.

140W into 4 ohms gives 5.9 amps and a 14W (10%) loss over 100m for 12 gauge cable. If 140 feet is the actual cable run distance then that's 85m in total, so the cable loss is 12.4 Watts.

But that's at a maximum of 140W and I doubt the amp will be pushed that hard, so speaker cable losses will be significantly less in practice at lower outputs. But if it is running at near maximum output, then 12 gauge is probably a bit too thin and 10 gauge would be better. You could use 8 gauge, but there could be termination issues with this size wire.

8 gauge/10mm2 cable has a diameter of around 3.6mm (without insulation) approx 1/8". I'd have a look at the connections on the amp and the speakers and see of they will accept that size wire. 10 gauge is about 1/10" or 2.5mm in diameter.

[twotoedsloth]

Thank you all for your help.

I will wait for my friend to get back to me with the speakers to be used, probably Sunday.

Your help is greatly appreciated.

Peter

[Hugh Robjohns]

A friend of mine belongs to a church that wants to replace their speaker cables. She is concerned that the cable run is too long - 140 feet.

Much depends on how these speakers are connected to the amplifiers and what kind of amplifiers are being used.

Most conventional church voice PA systems in the UK use the 100V line system (using special coupling transformers in the amps and speakers) which can handle extremely long cable runs with relatively small gauge cables.

But if you're talking about standard amplifiers and standard low Ohm speakers, you will need very heavy gauge cables for that kind of run.

In your situation it might be wise to find an experienced system installer to advise on an appropriate solution for the specific equipment and installation.

[ef37a]

I have been out of the PA* "'game' longer than some of you have been alive so I am not sure what is meant by "speaker cable" in this context?

If the meaning is a specialist, multi-strand cable sold as "hi fi speaker cable" then don't for heaven sake buy that! For a fixed 'factory' installation use the cheapest power cable you can find. Personally would use single wire 'conduit' cable.

Terminating grey 2.5mm (say) is a problem but a 13A back box and a PVC blanking plate and some 'choc-bloc' solves that one. Cheap as chips.

I would like to clarify as well a bit of a misunderstanding about 100 volt line installation? The power saving that is often quoted as the reason for its use is in fact rather secondary. Even a quite large factory would only use a 100W amplifier and in valve days a 200W amp was a fearsome brute! Such an amplifier would feed dozens of speakers but individually they would draw only 5W or so maximum and were often 'tapped down' to 1/2W.

The primary reason for 100V working was the freedom it gave you to hang shedloads of speakers on the line and not worry about impedance "matching" plus the power at individual points could be adjusted. 10W into a small horn in the warehouse say. Half a watt for offices.

I would also say the lumped inductance and capacitance of 140mtrs of cable is going to play ducks and drakes with the speaker's response and be noticed more than any power loss? I doubt however that any bad effects will arise anyway. Valve amps are rather forgiving things.

*When we needed to send audio to a speaker a couple of clicks away we used a pair of 30W 100V traffs each end. Hi Fi it was not but 'words' could be understood!

Dave.

Dave.

[resistorman]

The first question I would ask is why do they want to replace their speaker cables? Reasonably modern (i.e. made in the last 50 years) installed cables don't wear out.

I've been wondering this myself.

[Wonks]

It may be an insurance or code requirement, especially if there is other refurbishment work going on. The existing cable may be plain PVC insulated, but running in a public space it now needs to be ‘plenum’ rated i.e. LSOH/LSZH (low smoke zero halogen).

If this is not the reason, then the cable insulation type requirement still needs to be checked before purchase to avoid falling foul of code.

[Mike Stranks]

Lots of valid points...

My tuppence-worth as a former church installer... (some of this for previous commenters)...

The days of 'church' systems being mainly 100v line systems in dreadful acoustics are long gone. Many churches (even those very traditional in worship styles) now have sophisticated systems with decent speakers. Many systems utilise passive low-impedance speakers because of the problems of getting power to remote high places.

I've regularly run long cables to passive low-impedance speakers. The key is amplifier power, speaker impedance and (particularly) speaker efficiency. Manufacturer A's 4-ohm speaker may sound much 'quieter' than manufacturer B's because of the relative speaker efficiencies. I found JBL's 'Control' range to be much less-efficient than others.

I used 1.5mm core two-core cable as a minimum and sometimes went to 2.5mm. If the connectors are jacks you can get plugs with large diameter cable entry points specifically for speaker connection duties.

As for amplifier power, much depends on the intended use and volume expectations of the congregation/audience. I've known churches that regard anything where you're aware that there is a system as 'too loud' and others where levels are routinely at rock-concert volume. (I suspect that in this case it's veering towards the former.)

From the briefest of descriptions we have, as a former sound consultant to churches I'd, at least, be recommending a new amplifier... If the cables are needing replacement it might be time for an overall modest upgrade...

[ef37a]

As I said Mike, I have been 'out of it' a long time but I have kept up with some things...

For a factory +offices install I think I would go for CAT5+ cable and send balanced audio* down that and have active local speakers. For the few/fractional watts I mentioned the power could come from PoE. For the really noisy Bs mains power might be easier and cheaper to provide than long, lossy cables. The CAT wires also provide some interesting control/zoning possibilities. Same could apply for the 'head bangin" speakers in churches? Modern class D modules and SMPSUs are now common and cheap.

*100V systems are, or should be, inherently balanced, another good thing I forgot about. There tends to be an idea that '100V' is always low audio quality? Does not follow. Yes the speakers are usually single cone 1960s 'radio' grade or 'guitar' type 12s for noisier places but electronically the quality can be very good if decent transformers are used of adequate rating.
I understand the BBC distributed programme around their buildings with a 100V line system?

Dave.

[twotoedsloth]

Hello again,

Thanks again for tolerating my questions.

The speakers are Rogers FR1.7LXA Tone Cabinets

75 Watts power, 8 ohms impedance.

The Amp is a Rogers S200B

140 Watts power, 4 ohms output

Can someone please enlighten me on the correct cable specifications? The cables will need to be 140 feet long.

Many thanks,

Peter

[twotoedsloth]

Sorry!

The PA equipment in question is RODGERS, not Rogers...

Sincere apologies.

To confirm:

The speakers are Rodgers FR1.7LXA Tone Cabinets

75 Watts power, 8 ohms impedance.

The Amp is a Rodgers S200B

Sorry, and many thanks,

Peter

[Folderol]

OK, that looks quite respectable now
At that stated max power for the speakers and in 8 ohms that would be a max current of just over 3A each, so any decent came of 1.5mm or greater should be fine.

[ef37a]

OK, that looks quite respectable now
At that stated max power for the speakers and in 8 ohms that would be a max current of just over 3A each, so any decent came of 1.5mm or greater should be fine.

Yes, 16 AWG copper cable will have a loop resistance of around one Ohm. Yer hi fi bod would not be happy but little you can do in that situation and I doubt you would hear any degradation.

Now, I might not have got the right speakers and amp but I see a problem. Speakers rated at 75W amp 200 W? If that was the 4R rating of a solid state amps (especially class 'D') I would only be slightly worried but the Rodgers 200b seems to be a valve amp? The 'impedance' difference does not save you with valves! That amp will pump 200W, maybe much more into those speakers. However, I think you said you have had the system running in the past? Ok but I think you have been lucky and no one has tried to drive the beans off the kit!

I still say save money and buy 2 core, bog standard house cable. (must of course meet 'code')

If that IS a valve amp, get a set of 6550s and a couple of 12BH7 valves in stock. They are only ever going to get rarer and more expensive. Note, KT88s are not made the way GEC did any more so avoid those. Personally I would flog the amp and buy a decent, modern transistor jobby.

Dave.

[Folderol]

S200B - It's solid state. One of those monoblock module thingies by the looks of it.

[ef37a]

S200B - It's solid state. One of those monoblock module thingies by the looks of it.

Yes Will I found both that and the valve jobby. Even so 200W into 4R means a likely 120-140W into 8 R. I know many speakers are tougher than their ratings and amplifier adpuff depts like to 'massage' power figures but I still think OP is sailing close to the wind? Especially with 'remote' speakers where the person at the source probably won't be able to hear them going into distress.

Dave.