PA System Help

[hydrargyrum]

Hey Everyone,
 
I've been asked to help one of our band members buy a PA, and I was hoping to get some advice here.  He has the speaker cabs already, and they are rated for four hundred watts (I'm not sure the ohmage, but have asked him to find out).  Does this mean that He could use up to an 800 Watt Head (400 Watts to each channel right?)?  I know that the ohmage of the speakers need to match that of the amp, but wasn't sure about this.  Also, what are some of the better brands you've used?  Do you guys recommend separate mixers and poweramps, or all in one systems?  Any help is appreciated.
 
Kevin

[keith_h]

The correct answer depends upon what your current and future needs are. I prefer separate mixer and amps. This allows an easy upgrade/growth path with any of the components. It also makes it easier to intermix powered and non-powered speakers. I will say what I have is typically overkill for a coffee house acoustic act where a powered mixer would be adequate. Of course upgrading means replacing the whole unit not just a component.  
 
My PA consists of a MixWiz 3 16:2 board and QSC power amps. With the MixWiz I can handle 16 discreet microphone channels, run up to 4 monitor mixes, run Aux fed subs and it has reasonable built-in  effects. I have always found QSC amps to be reliable and most have built in limiters, crossovers and the like which means you do not need additional processing right away.  
 
My subs are JBL MRX528's and each is driven by a bridged PLX2402. This is approximately 2400W program into 2000W program speakers (~1000W RMS). My mains are MRX515 and these get driven off of the two channels of a PLX3602. This gives me about 785W program into 800W program speakers (~400W RMS). Wattage wise these are all pretty good matches for my setup.  
 
Hope this helps.  
 
Keith

[lbpesq]

Kevin:
 
What is the P.A. going to be used for?  I used to have a rack system - mixer, power amp, EQ, etc., but found it was more than I needed as most venues we played that required as big a system already had one.  I sold it all and picked up a Mackie 808s.  I used it with a pair of JBL mains and a Carvin power amp and wedges for the monitor system.  I used it mostly in the studio.  I've since retired that system and now use a Bose PAS cylindrical radiator ( the tower) for the studio and small gigs.  Sounds great and eminently portable.  Since they came out with the second generation, the first generation can be had for a decent price used.
 
Bill, tgo

[hydrargyrum]

Hey Bill,
 
I suspect that this will at most be seeing small club use.  I took a look at that Mackie, and that looks pretty intriguing.  I was just looking at the PPM608 as well (http://www.mackie.com/products/ppm608/), which is a slightly smaller version of the 808s.  My concern is that even the smaller version seems a bit over powered for his speakers.  How easy is it to blow a speaker rated for 400 Watts with a 500 Watt signal?

[mario_farufyno]

Many companies recomends you to use a Power Amp with double power that is rated on Cabinets. Lets say using a 2000W Power Amp to match 1000W Cab (but you must use its master volume set to 50%).  
 
This is to give Amp enough headroom to work safely, since transients could blow your speakers if Amps is working in its limits. Something like driving your car during a overtaking, you must have to get that extra power if needed (but if you are already in full throtle, you simply can't speed up quick enough).
 
The idea is to never use any Amp in its maximum, since you adjust its levels hearing to perceived loudness (SPL RMS) and not by its real level (wich includes its Transients, its Peaks, that are several times more intense than its average level).
 
They say you can blow a Speaker using a 200W Amp working on its edge, even if the Cab is rated - lets say - in 300W, because Transients can really pop out hard if they surpass Amp's headroom. Don't know if this is the correct explanation, but I'm sure amp's makers sugests us to use Power Amp double rated than our Cabs are...

[mario_farufyno]

This is also due to clarity and fidelity sake, for sure. Here is a FAQ from Epifani:
 
http://www.epifani.com/support.php

[keith_h]

In general you try to match the Program Power rating of the speakers and the amplifier. This is typically, but not always, twice the wattage of the speakers RMS rating. What type of speakers are you dealing with?  
 
Keith

[hydrargyrum]

Thanks for the advice for far guys.  Unfortunately I still haven't received any real information regarding what kind of speakers my friend has, so I can't give you any real numbers to consider.  I'm feeling more educated on the topic though, and think I am beginning to get an idea of what he needs.

[benson_murrensun]

An explanation I have heard is that overdriving a solid-state amplifier results in a type of clipping that destroys speakers. Therefore it is better to have an amp that has enough headroom that it will not go into clipping.

[crgaston]

You are correct, Benson. It creates a square wave, which prevents the speaker from oscillating naturally, thus damaging it.

[hydrargyrum]

So it sounds like it would be better to have a amp rated higher than the speakers (within reason), than to have speakers rated higher than the amp.  Can someone describe for me how you bridge a system?  It's my understanding that this produces a mono signal with approximately four times the wattage of what normally would have been found in each half of the same system in a stereo setup.  Is this correct?

[benson_murrensun]

I have always heard that it is better to have an amp that is rated higher than the speakers.  
Some amps can be bridged, and for some it is not recommended (or possible, without damage). It seems as though the bridged power rating is about the same as the individual channels' ratings combined. And usually when you bridge an amp you must make sure the nominal resistance of the speaker load is double what it would be for each individual channel.

[lbpesq]

I, too, have always had p.a. power amps rated at a higher wattage than the speaker.  My bottom line rule of thumb has always been if it sounds like crap, turn it down.  As for bridging a stereo amp into mono, I've always had amps that were designed to do this, usually by re-setting some mini slider switches in the back, (I forget what they're called at the moment).  I'd be very cautious attempting to bridge a stereo amp that wasn't designed to do so.
 
Bill, tgo

[hydrargyrum]

Thanks guys.  I couldn't really imagine any applications where we would need to bridge the poweramp, but I thought I would try to be a little smarter on the topic.

[keith_h]

Bridging increases the output wattage. It also changes the minimum impedance an amplifier will handle. If the minimum impedance per channel is 2 ohms it will be a minimum of 4 ohms bridged. Bridging does put more load on the amp. Whether this matters or not depends upon the application. As I stated above I bridge the amps that drive my subs. This got me about as close as I could get to the rated program power of the speakers. I've been doing this for a long time without problems.  
 
On the amp size it really doesn't matter if the amp has less power than the speakers are capable of. There is no such thing as under powering speakers. Where amplifier size matters is how loud you get. A smaller amplifier is more likely to be run into clipping than a larger amplifier if you need to get loud.  
 
One last thing is a square wave does not damage speakers. Assuming you are within the various speaker specifications a speaker will reproduce a square wave just fine. The problem with clipping is the amp can actually be putting out more than it's rated power. Some amps also lose dampening control.  This can cause over excursion of the cone, over heating of the coil and other problems as the speaker is now being operated outside of it's specifications.  
 
Keith