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fardan

Joined: 12/03/12
Posts: 2
need help, sound for long distance
#975325 - 12/03/12 02:56 PM
at beginning, thanks for everybody for sharing knowledge.
my case like that:

(side A)<<------2KM-------(PA1 Main Control PA2)-------2KM------>>(side B)

1-I have fixed two High Power PA Amplifiers with 1000 WATTS for each.
2-I have used 1.5mm wiring to connect PA Horn Speakers.
3-I have connect 130 PA Horn Speakers at each side from center to side A and side B.
4-The Horn Speaker setting at 5WATTS.
5-The PA Amplifiers Speaker Outputs settings at 100V
--------------
PA Specification
1-The PA Amplifiers Speaker Outputs 2ohm & 4ohm, 70V & 100V
2-The PA Amplifiers Input Channels
8 × Mic 0.8mV/4.7k ohm, 3 × Aux 100mV/50k ohm
1 × Aux 250mV/50k ohm
1 × Line 1V/50k ohm
--------------

The Problem:-
above 1KM from the center, the sound becomes boor.

how to solve this problem? and make the sound reach the full 2 km at each side.

At the end Thanks again

dmills

Joined: 25/08/06
Posts: 2129
Re: need help, sound for long distance [Re: fardan]
#975380 - 12/03/12 08:33 PM
5 watts per speaker, 130 speakers, so 650W, 100V line so 6.5A at the source dropping to ~3.3A at 1Km.

Resistivity of copper at ambient temperature is somewhere around ~ 17 * 10^-9 ohms meters.
So for 1.5mm^2 cable, the resistance per km is 1000 * 17 * 10^-9 / 1.5 * 10^-6 = 17/1.5 = ~ 11 ohms, however your circuit is a loop, so the resistance per km is actually 22 ohms (Someone check my working).

If for simplicities sake we model as the entire load at 1km, then for 650W the effective load impedance is 100V/6.5A, say 15 ohms, and you have 22 ohms of series resistance, so most of your power is going on heating up the cable, and the situation actually becomes worse as you get further away.

2.5mm (or even 4mm for the first few hundred metres) each way would be good, or move the amplifiers so they feed the string half way down, half the load each way and half the cable (Of course this requires a 2km balanced line (could easily be cheap cat 5 or FST or whatever), but maybe less painful then ripping out all that 1.5mm).....

Push comes to shove you can get 100V to line level 'DI' boxes, an extra pair of amps positioned halfway down would then just need power to provide the drive for the outer half of the rig, and the first half would not be carrying the current needed for the second half, do the math it helps.

On the big jobs it pays to run the trivial numbers before you start pulling in wire.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

Hugh Robjohns
SOS Technical Editor

Joined: 25/07/03
Posts: 18348
Loc: Worcestershire
Re: need help, sound for long distance [Re: fardan]
#975404 - 12/03/12 10:32 PM
Instant gut reaction is that the cable is much too thin for that distance and power dissipation, as Dan says. The sums above make sense to me. Much thicker cable, or reduce the cable lengths by repositioning the amps, are your only options.

Hugh

--------------------
Technical Editor, Sound On Sound

fardan

Joined: 12/03/12
Posts: 2
Re: need help, sound for long distance [Re: fardan]
#977403 - 22/03/12 05:46 AM
thanks for reply dear ((dmills))and((Hugh Robjohns)).
you are very helpfull to provide usfull information.based on what you say we decide to use 100v line to line level --'DI' boxes-- .
thank you very much indeed.

sory for late of replying beacuse of exam

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