Long cable run for speakers

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Re: Long cable run for speakers

Post by Hugh Robjohns »

twotoedsloth wrote: Thu Nov 25, 2021 3:40 pmA 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.
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Re: Long cable run for speakers

Post by 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.

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Re: Long cable run for speakers

Post by resistorman »

James Perrett wrote: Thu Nov 25, 2021 4:49 pm 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.
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Re: Long cable run for speakers

Post by 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.
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Re: Long cable run for speakers

Post by 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...
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Re: Long cable run for speakers

Post by 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?

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Re: Long cable run for speakers

Post by 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
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Re: Long cable run for speakers

Post by 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
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Re: Long cable run for speakers

Post by 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.
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Re: Long cable run for speakers

Post by ef37a »

Folderol wrote: Mon Nov 29, 2021 7:38 pm 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.

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Re: Long cable run for speakers

Post by Folderol »

S200B - It's solid state. One of those monoblock module thingies by the looks of it.
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Re: Long cable run for speakers

Post by ef37a »

Folderol wrote: Mon Nov 29, 2021 8:01 pmS200B - 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.

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