Recently bought some big PA Mosfet amps off a mate of mine.

[Marcel]

Yep, basically how many dB per Watts SPL the speaker would put-out, taking into consideration how the load-impedance varies with frequency, speaker cab resonances, and the frequency response of the speakers, you're quite right, I forgot that each of the 20 450W amps were stereo so 450W per stereo channel.


So, a speaker that has an efficiency of say 105dB/W is going to be more efficient, and sound louder, than a similarly specced speaker with an efficiency rating of say 75dB/W, because it is more efficient at converting the electrical power (energy) output of the amp into acoustic power (energy).

Actually for the River Stage there were 23 Jands 920's per side.... 20 on the FOH each side (18kW + 18kW), and 3 more per side (2.7kW +2.7kW) on delayed stacks some 75m from the stage. (18 +18 + 2.7 + 2.7 = system total of 41400kW true RMS at onset of sine wave clipping)
As for speaker cabs do a search for "Concord PA cabinets" & "3way cabs" circa late '80's. There were 16 3 way cabs hung on motorized hoists in a 4x4 array per side on the stage with 2 of the 3 way cabs in each delay stack, and 4 sub cabs per side of stage for below 80Hz. ... I think the crossover points for the stereo 4 way system were 80Hz, 750Hz and 2.6kHz or somewhere near those frequencies.

As a side note....just days before the 6 month "show" started they had everything working including the 512 channel DMX controlled lights array.... PA was up making musical noise, and the lighting dude raised all his lighting channels and then the main dimmer on his console....
Next minit.... BZZZZ!!... the ground briefly shook at 50Hz, and all the lights suddenly went into uncontrollable auto disco mode with the PA making weird buzzing noises....within seconds everyone was scrambling to turn things off....
It tool a team of electricians over an hour to diagnose that there was nil Neutral connection from the river stage dedicated 250kVA supply transformer.... They were all baffled as to how a new power transformer of that size could literally burn out its Neutral link.... I laughed, and reminded them all how an SCR dimmer works.... and that there was over 200kW of incandescent lighting on that stage. 3 phase all on no problem, 3 phase all off no problem, 3 phase at 66% SCR dimming and damn huge Neutral problem. (and yeah there is another story on how I knew these things...lol)
A 750kVA transformer was fitted next day.... the next 6 months went just fine....

[DrNomis_44]

Actually for the River Stage there were 23 Jands 920's per side.... 20 on the FOH each side (18kW + 18kW), and 3 more per side (2.7kW +2.7kW) on delayed stacks some 75m from the stage. (18 +18 + 2.7 + 2.7 = system total of 41400kW true RMS at onset of sine wave clipping)
As for speaker cabs do a search for "Concord PA cabinets" & "3way cabs" circa late '80's. There were 16 3 way cabs hung on motorized hoists in a 4x4 array per side on the stage with 2 of the 3 way cabs in each delay stack, and 4 sub cabs per side of stage for below 80Hz. ... I think the crossover points for the stereo 4 way system were 80Hz, 750Hz and 2.6kHz or somewhere near those frequencies.

As a side note....just days before the 6 month "show" started they had everything working including the 512 channel DMX controlled lights array.... PA was up making musical noise, and the lighting dude raised all his lighting channels and then the main dimmer on his console....
Next minit.... BZZZZ!!... the ground briefly shook at 50Hz, and all the lights suddenly went into uncontrollable auto disco mode with the PA making weird buzzing noises....within seconds everyone was scrambling to turn things off....
It tool a team of electricians over an hour to diagnose that there was nil Neutral connection from the river stage dedicated 250kVA supply transformer.... They were all baffled as to how a new power transformer of that size could literally burn out its Neutral link.... I laughed, and reminded them all how an SCR dimmer works.... and that there was over 200kW of incandescent lighting on that stage. 3 phase all on no problem, 3 phase all off no problem, 3 phase at 66% SCR dimming and damn huge Neutral problem. (and yeah there is another story on how I knew these things...lol)
A 750kVA transformer was fitted next day.... the next 6 months went just fine....

Sounds like it must have been a fun time getting that all sorted-out, I can just imagine how loud that 50Hz buzz was at the time, and 50Hz must have been the natural resonant frequency of those light fittings too, or close to it anyway....lol.


I seem to remember studying SCRs while doing some electronics courses at uni more than ten years ago, when they are used to control AC power, the waveform isn't exactly sinusoidal all the time, it's a spiky looking waveform, and I seem to remember that snubber-circuits are sometimes added to reduce high-frequency harmonics, or, I could be totally wrong, it has been quite a while.

Still, great story you have there mate.


Note for everyone else, SCR is short for Silicon Controlled Rectifier, it's a special type of power diode used to control AC power, most often used in light dimmers, although nowadays you do see some stage lights that use Leds (Light Emitting Diodes) rather than incandescent lights, I think Led stage lights are powered by DC if I'm not mistaken.


I'm really amazed at how far P.A. sound technology has evolved since it's early days, who knows what the future has in store for us.