looking for help load matching my PA

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Member Since: Jul 31, 2009

hi,

i've been trawling the web attempting to understand load matching before my band buy a PA, i saw this forum and some knowledgable people and I was wondering if you folks could help me as well.

i've been offered some speakers that are prosound 400w
www.maplin.co.uk/Module.a...strat15#specifi
they are marketed at 400W but looking at the specs they are 400W peak and 200W RMS (continuous). and they are 8ohm.

the PA we're looking at is the behringer pmp2000
www.bluearan.co.uk/sales/...r_(PMH2000).htm
this is 165 watts RMS per channel into 8 ohms, 250 watts RMS per channel into 4 ohms. Peak Power, both channels driven; 8 Ohm per channel: 225 watts; 4 Ohm per channel: 350 watts


so that's 165W RMS per side from PA going into 200W RMS speakers at 8ohm?

alternatively, the PA has main/monitor function, so could i daisy-chain the two speakers into the main side (thereby wiring them in parallel and halving the ohms) and have 250W RMS from one channel of the PA driving 2 x 200W speakers at 4ohm? (i've got some speakers from our current PA that we'd like to use on the monitor side if possible).

have i come to the right conclusions there? and which, if either of those setups will work?

thanks!
all help very gratfeully appreciated :o)

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The Czar of BS
Member
Since: Dec 31, 2007


Jul 31, 2009 08:09 pm

Yes it will work, but your a little light on the amp side of things.

Having a speaker that can absorb more then what the speaker can dole out is a little dangerous.

You can square wave your speakers very quickly at that point. Just make sure that you don't drive the amp into the red constantly.

Other wise, I might have to buy stock in the speaker manufactures that you buy from. :)

Member
Since: Jul 31, 2009


Aug 01, 2009 07:30 am

hi rob,

thanks for the reply. when you say the amp is a little light, do you mean

a) 165W RMS per side from PA going into 200W RMS speakers at 8ohm?

or
b) just using main rather than monitor out, daisy-chain the two speakers and have 250W RMS from one channel of the PA driving 2 x 200W speakers at 4ohm? (other speakers used to monitor)


generally, i'm a bit confused by different things i've read on the net about gain matching - some people seem to say that the amp should be rated a bit higher than the combined speaker RMS (because the speakers won't come to life until they are putting out their average max power), and some people say the speakers should be a bit higher than the amps RMS (so that you cant overload the speakers with a too-hot amp) and some people say the RMS of the amp should be about the same as the combined RMS of the speakers - which of these is correct??

thanks again for the help, i nearly bought a PA thinking it was 1kW, only to find out it was 1kW peak, and actually 400W RMS! - seems manufacturers make this whole area deliberately difficult and non-uniform so they can sell lots of gear to replace blown speakers/amps from uninformed and confused people like myself!

The Czar of BS
Member
Since: Dec 31, 2007


Aug 01, 2009 11:15 am

Yeah, I know what your coming across. Manufactures today, come up with different ratings just so it can sound more powerful then what it is.

A good example is the new switching amps that are out there. Such as the Crown I series. Most will claim these huge output numbers. And, they do....... But for the shortest time period. If they list 2500 watts, it's 2500 watts for 1/2 of a cycle of 40 hz. If you where to check it linearly, you would find it only to be 150 watts.

OK, for my taste in matching the amp to the speakers, I like to have the amp produce more then the max value of the rms rating. The reason I suggest this, is so that you do not come across the situation of needing more out of your system, and it just not being there. Now your clipping the input of the amp. And the amp responds by giving you a square wave out. And a square wave destroys all in it's path.

It is better to over power the speaker driver, then to feed it a square wave!

Over powering will give you distortion and a little heat. Square wave will give very small distortion, and a huge amount of heat!!!!!! And this heat is not just limited to the speaker driver, it also effects the amp as well. Which ultimately will bring your system to it's knee's. Causing hundreds, if not thousands of dollars in repairs.

And this is what I meant by your a little light. I mean this all the way around. For your FOH, and monitors. The monitors especially. Being that your drivers are in parallel, then can handle up to 400 watts. But your amp is only delivering 250. Remember two 200 watt drivers in parallel will absorb 400. Watts being current, and the drivers acting very much as a resistor. Two resistors in parallel will handle more current.

You seem to have a grasp of electrical engineering. So, I'll try to put things on those terms for now on. And that is the best way to view all of this. The speaker is a resistor. The amp is the power supply. And you know that watts are current. And when you design a power supply for a project, you always try to design it so that you have a little more power then what your circuit needs. That way your not at full demand all of the time. Which will casue the supply to over heat and fail.

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