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Anyone here able to give any advice on the varying ratings to run different systems at?...I'm going for a job interview as a pa installer and am a bit unsure bout it. Looked on the net at Prof. PA etc, but still unsure.

I'm bet I'm worrying about nothing but I'd like some one to help refresh my hazy memory!

Ohms are a measure of electrical resistance.

loudspeaker drivers are resistive loads.

ohms law states that Current is proportional to Voltage
I= V/R, V=I x R, R=V/I, where R is resistance(measured in Ohms), I is current (measured in Amps). V, unsurpisingly, is Voltage.

1 Volt will push 1 Amp through a 1 Ohm resistor.

10 Volts will push 1 Amp through a 10 Ohm resistor.

So, the lower the resistance the more current will flow from a given voltage.

Power (P) is a function of current, V x I= P or (I ^2) x R = P measured in watts, W.

with regards to loudpeakers most drivers have a nominal resistance of 8 or 4 Ohms. If you connect two 8 ohm loudspeakers in series, the amplifier will see a total load of 16 ohms...

Where R is the resistance of a loudspeaker:

Series wiring: R+R=Total Resistance (8+8=16ohms)
This will work for anynumber of loudspeakers. R+R+R+R+R= RTotal...

if you connect two 8 ohm loudspeakers in parallel, the amplifier will see a total load of 4 ohms.

Parallel wiring: For two loudspeakers only!!!
use product/sum rule

( 8 x 8 ) / ( 8 + 8 ) =64/16 = 4 Ohms

http://en.wikipedia.org/wiki/Series_and ... l_circuits

Amplifiers deliver a fixed amount of power into different load resistances...
Most PA systems take advantage of the lowered resistance offered by parallel wiring, as an amp will deliver more power into a 4Ohm load than it will into an 8ohm load(approx twice as much).

Below is taken from the QSC RMX 4050HD amplifier spec sheet:
Power at:
8 ohms
FTC 20 Hz - 20 kHz, 0.1% THD
800 Watts

4 ohms
FTC 20 Hz - 20 kHz, 0.1% THD
1300 Watts

2 ohms
FTC 20 Hz-20 kHz 0.1% THD
1600 Watts


HOWEVER!!!!! if the load resistance presented to an amplifier drops too low, ie by connecting too many loudspeakers in parallel or if a conncecting lead has a short, too much current will be drawn and the amplifier will either go into protect mode or be damaged...
Most amplifiers will be able to supply a 4ohm load and its becoming more common for amplifiers to be 2ohm stable...

Hope this is of some help and not to confused, ive just got up and not had a cup of tea yet...
Good luck with the interview, i wouldnt worry too much, to start with you'll probably just be lugging boxes about, the glamour of pro audio

edit: have they got any other jobs going? lol
edit2: if i've made any errors please let me know and i'll correct it...

................I THOUGHT AS MUCH ....I'LL FIND OUT BOUT OTHER JOBS FOR YOU IF YOU'RE IN LEEDS!

hope that was of some help...

sERIOUSLY MATE.... NICE ONE FOR THE THOROUGH ANSWER TWAS PERFEKT!

also dont forget about bridging stereo amps. thats good shit to know too, as well as how to invert the phase of one channel. when you bridge an amp, one channel is used to amplify the positive side of the signal, another amplifies the negative side of the signal......

i think so long as you understand ohms law and the power law you should be all set. how to bridge your amp is usually covered in the manual

when the amp is in bridge mode, MAKE SURE that you have NOT connected too low a resistance to the outputs, when in bridge mode the minimum resistance that you can hang off the amp will be higher than when in normal stereo or mono/parallel mode.

As a rule of thumb most amps will be capable of powering a 4 ohm load, if this is the case they will only be able to operate at 8 ohm bridged, for amps that can power 2 ohm loads in normal operation, they should be able to power 4 ohm bridged loads....

As skunk says, bridging information will be in the manual. when an amp is bridged the power available can be quite scarily large, easily enough to destroy loudspeakers... so dont lick the bridge out terminals...

skunk, you from boston UK or US? if its US is there much sound/technician work over there? its somewhere i've always fancied visiting....

Boston US...

There's a good amount of work out here, its just super hard to get. Theres too many college kids that will work for food and beer.

pesky college kids!!

i think im just going to have to be happy running my system as a hobby and getting a proper job...

hope the interview went well ludofuzz!!