Hi all. I'm Pablo from Argentina.
I'm putting together a medium PA system.
4 x 350watts full range, 8 ohms, passive speakers (2 for each channel, in paralell)
Digital Power Amp with a 1200watts per channel at 4 ohms.
As I understand it, each channel will have a load of 4 ohms and add to a maximum of 700 watts.
That will leave a 500 watts headroom, taking into account the "rule" of "1.5 to 2 times the amp".
Is my math ok? Thanks for the help.
PS: sorry if my english isn't so good.
I'm putting together a medium PA system.
4 x 350watts full range, 8 ohms, passive speakers (2 for each channel, in paralell)
Digital Power Amp with a 1200watts per channel at 4 ohms.
As I understand it, each channel will have a load of 4 ohms and add to a maximum of 700 watts.
That will leave a 500 watts headroom, taking into account the "rule" of "1.5 to 2 times the amp".
Is my math ok? Thanks for the help.
PS: sorry if my english isn't so good.
Depends.. is that 350w speaker rating at rms, program, or peak?
Your math is good but unfortunately sizing amps is not that simple. That "rule" was developed when music had relatively high dynamic range with short duration peaks and the suggestion was to have an amp capable of up to 2 times the program rating of the speaker to be able to deliver music peaks without clipping the amp.
With the way the most popular genres of modern music are mastered for maximum loudness these days(almost zero dynamics), if you were to put that much power into a speaker it will only last a short period of time. So you need some way to limit amp output to something the speakers can survive on a case by case basis. Do your amps have adjustable limiters and are they simple peak limiters or are there rms limiters too?
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Damping factor = load impedance / output impedance
...and halving it is not a problem.
Not for anyone who understands it.
http://www.diyaudio.com/forums/tubes-valves/318538-explain-damping-factor-3.html#post5334438
Ok, so in one hand, my math is OK.
In the other hand, 1.5 to 2 times Amp power will fry my speakers.
I guess this is not a exact science for me...
My amps have 1 knob per channel. 1 to 10. Somewhere I asked if my amps are 1200 watts per channel, every notch was 10% of the full power.
The answer was: THAT IS NOT HOW ANY OF THIS WORKS.
And never elaborated on this.
Anyway, thanks for all the answers, but sadly I am as lost (or more) than before.
In the other hand, 1.5 to 2 times Amp power will fry my speakers.
I guess this is not a exact science for me...
My amps have 1 knob per channel. 1 to 10. Somewhere I asked if my amps are 1200 watts per channel, every notch was 10% of the full power.
The answer was: THAT IS NOT HOW ANY OF THIS WORKS.
And never elaborated on this.
Anyway, thanks for all the answers, but sadly I am as lost (or more) than before.
The reason you are confused is that it's not a simple matter, and the answers have all been simple so far. Nothing wrong with simple answers, but in this case they have left you confused.
Headroom is your friend because it allows you to hit the peaks without clipping. Clipping not only sounds bad, it means much, much more power at the peaks than with a clean waveform. That's where much of the danger to your speakers comes from, hard clipping.
Yes, if you continuously run higher higher average power than rated, you will burn up the speakers. You have to be careful with that. An extreme example for me was running speakers rated at about 50 WPC on a 700 watt amp. I did manage to kill one tweeter, but that was on a test sweep signal that has equal level from 20-20K Hz. Mostly I found that if I limited to amp to 150W, there was no problem on musical content. You can also limit your amp to were you want it, and it will never clip.
Limiting was mentioned in some other replies, but it can be as simple as turning down the volume at the power amp. How far to turn it down is not a simple answer, but there are ways to approach it.
Headroom is your friend because it allows you to hit the peaks without clipping. Clipping not only sounds bad, it means much, much more power at the peaks than with a clean waveform. That's where much of the danger to your speakers comes from, hard clipping.
Yes, if you continuously run higher higher average power than rated, you will burn up the speakers. You have to be careful with that. An extreme example for me was running speakers rated at about 50 WPC on a 700 watt amp. I did manage to kill one tweeter, but that was on a test sweep signal that has equal level from 20-20K Hz. Mostly I found that if I limited to amp to 150W, there was no problem on musical content. You can also limit your amp to were you want it, and it will never clip.
Limiting was mentioned in some other replies, but it can be as simple as turning down the volume at the power amp. How far to turn it down is not a simple answer, but there are ways to approach it.
Yes, yes it does. 
Remember, if your amp is rated at 1200 watts into 4 ohms, that means 600 watts into 8 ohms, and that's what each speaker will see at max power. They are still 8 ohm speakers, so each of the two on a channel would just "see" 600 watts maximum.
But think about this: Even if you are running as loud as the amp can go with music., each speaker is probably only at 60 watts average.
Remember, if your amp is rated at 1200 watts into 4 ohms, that means 600 watts into 8 ohms, and that's what each speaker will see at max power. They are still 8 ohm speakers, so each of the two on a channel would just "see" 600 watts maximum.
But think about this: Even if you are running as loud as the amp can go with music., each speaker is probably only at 60 watts average.
When you jump on a trampoline and and its membrane don't absorb your body mass kinetic energy enough (not enough damped) your body crash on the ceiling because of the excess of stored energy in the damping springs.
Maybe.. maybe not. Depends on the music or program type.
It is just much more complicated than you anticipate.
No... it doesn't work like that at all. That control knob does not in any way limit output power it just changes how much input signal is required to drive the amp to full output.
We can still help more but you didn't answer the question I posted in my first reply either and that information is needed to further the discussion.
The full range speakers are 350 watts RMS at 8ohm.
Since I first posted, I have added a fairly powerfull subwoofer, so the full range are now only for mids+highs.
Anyway, I'm aware that if the amp is bigger than the speakers I can blow them.
I just don't know if there is a way I can limit the power other than by hearing the clipping (or watching the red light) and then backing up a little.
So far I haven't blown anything.
I asked before if the red light means the speakers are clipping or the amps are insuficient for the speakers (I'm inclined for the first option), but don't know for sure.
Thanks a lot for your time!
Since I first posted, I have added a fairly powerfull subwoofer, so the full range are now only for mids+highs.
Anyway, I'm aware that if the amp is bigger than the speakers I can blow them.
I just don't know if there is a way I can limit the power other than by hearing the clipping (or watching the red light) and then backing up a little.
So far I haven't blown anything.
I asked before if the red light means the speakers are clipping or the amps are insuficient for the speakers (I'm inclined for the first option), but don't know for sure.
Thanks a lot for your time!
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