Distributed Pa for background music/announcements

[Shaggy]

I'm looking at putting in some PA for a cricket match, where coverage is required around part of the edge of a cricket pitch for the crowd to hear announcements and some loudish music. There is no seating and the distance between the edge of the field and the edge of the pitch is between 5m and 25m. The need for music means a 70v horn system isn't sensible, so I feel the best solution is powered speakers at intervals around the edge of the field pointing towards the pitch.

 

The cable runs are going to be rather long if this setup is followed, and while I don't expect this to be a problem for a balanced signal run, the power situation is obviously going to be more difficult as the distance for the cable run is going to be 250m - 300m plus. To stay legal and safe the cable run has to be uprated significantly. Would I be better off splitting the speakers into two and running the second lot off a generator, or is there another solution to this problem that I am missing? Has anyone else tried a different solution for a similar event?

[David Duffy]

You could use 70V (100V) distribution to box type speakers. I know of an event company that does this all the time in show grounds. It's easy to deploy and sounds good.

 

Most of the boxes they use are switchable between 100V and 8 Ohms from memory. They also have 100V - 4 Ohm transformers (in small boxes) that they rig with pairs of 8 ohm speakers.

 

Doing it this way means you can hang 100V horns off the same lines if you need to. All this is much easier than running power + audio over long distances.

[donald463]

There are hundreds of 100v loudspeaker systems available which are suitable for music reproduction. Not all 100v speakers have to be 'nasal' horns.

Look at wholesalers like HB Litherland or direct to manufaturers like TOA etc.

It is also possible to add a 100v transformer to any speaker system either internally or externally but this can be costly if high power levels are required.

[Pyramid]

Hi

If you already own the said small powered speakers I wouldn't worry too much about the power situation. Hiring or buying cable, if needed would probably be cheaper than hiring 100v gear.

We have done a few jobs using Mackie 450s on similar sounding outdoor jobs , but just for speech. We use 16A cable (3x2.5mm2) or 32A cable(3x6mm2), depending how many cabs we're using and what power they are running at.

 

hth

 

Andy

[paulears]

One point - you say it's for the crowd, but the speakers point towards the pitch? you mean around the boundary facing out, presumably?

[Stan Hope-Streeter]

One point - you say it's for the crowd, but the speakers point towards the pitch? you mean around the boundary facing out, presumably?

 

It's quite common to have the announcement speakers behind the crowd at these events - often it's the only practical place to put them. Nobody minds, if it's just announcements and background music.

 

The alternative is wedge monitors, or rectangular boxes on suitably angled blocks / mountings, around the edge of the pitch facing the crowd - as is done for a lot of sports events including the 20/20 Cricket matches.

 

Self-powered are best avoided if they are going to be exposed to the rain (it always rains). 4mm2 cable helps here, but it's best to keep the furthest ones on a separate circuit as the voltage drop will cause several dB of dimming. Don't let anybody hassle you about the damping factor. For these applications it really doesn't matter.

 

100V line transformers genuinely rated for music quality are very expensive.

[Shaggy]

The idea of speakers from behind the crowd was to avoid blocking the view of the pitch and putting cables over walkways. I hadn't thought of putting the speakers wedge style on the floor but that's definately an option. I have no idea where to hire 100v line music quality speakers locally, so I think that idea is out, although it would be a good choice.

[dbuckley]

You can, of course, put a number of identical ordinary cabinets in series / parallel to make up the approriate impedence to directly match an amp.

 

So, if you had 100W cabs with 8 ohm impedence, four paralleled chains of four in series will give a 8 ohm load with sixteen speakers for a rated output of 1600. Find an amp that can do 2K into 8 ohms (as many bigger amps can do in bridge mode) and you're all set. Given each chain of speakers will be 32 ohms, and the current per chain at 400W will be about 4A you can probably use 1.5mm lawn mower cable with not to much losses.

 

Not enough speakers - eight 100w boxes in series for 64 ohms, and eight chains in parallel, for 8 ohms, sixty four boxes, and a macrotech that can do 7KW into 8 ohms.

 

Edited to add that as there's no saturatable transformers in the way you can go down to whatever frequency your speakers will be happy with.

[paulears]

Nowadays, many people have large capacity amps for their music systems, so if you don't need ear shattering volume, why not just accept feeder loss, and stick a load of wedges around the boundary. You'd need to be a little crafty with feeder lengths, but When I did this kind of thing about three years ago - a one-off never to be done again favour - I bought 400m of 2.5 for the long runs and used the big amps in the racks for the long runs, linking straight into the inputs, and the smaller amps for the shorter runs using my existing collection of speakons. I lost plenty down the cable, but it didn't matter.

 

A cheap (just cable cost) job - and as they paid for the cable, which I kept of course - it worked out rather well.

[donald463]

There's a company in Leeds ( Fairbank Harding ) that specialise in 100v hire

[Rob_Beech]

The good thing about the OLD macrotechs (Not the I series that David refers to) is the current limiting feature. You can hang as many speakers off of them as you wish, the amp just senses the load and limits the current. Of course. Into 0.5ohm loads there isn't as much power as a 1ohm load, or as a 2ohm load as not enough can be drawn from the mains, nor can the amplifier keep itself cool enough but it is stable. The issue that IS pointed out however is that long cable runs suffer more with lower impedance loads, more is lost down a 100m 2x2.5mm cable with a 2ohm load than an 8ohm load.

[dbuckley]

Using the example I noted earlier of a Macrotech (modern) doing 7KW into 8 ohm, a little math shows the voltage across the amp terminals to be 237V, rather more than 100V.

 

Time was when a 100V amp was different to low impedence amp, but as amplifier powers have gone up, many amps can drive 100V stuff directly, and more than that, with a little crafty wiring as I suggested above you can get the low loss attributes of 100V withouut having to go near transformers with all the expense, saturation and low frequncy uglyness that is involved with them. Anything that can do 1200W into 8R is a 100V line amplifier! In the USA where 70V is prevalent there are even more amplifiers that can drive a 70V line, as its just 600W into 8R.

 

Ok, lets add a disclaimer: assuming the amplifier is a decent amplifier; many modern audio designers will have forgotten how to drive an inductive load like a 100V tranny.