# Underpowering Stage monitors



## Anonymous067 (Apr 26, 2009)

Our facility has QSC CX 302 amplifiers.

I have monitors that I want to use, but they are rated for 400 watts. They would be hooked up in a daisy chain style, two per side. (or so is my hope).

Problem...the 302's only are rated for 300 (something like that) per 4 ohm side. Thats providing 150 to each monitor....and...yeah....400 watt speakers.

HELP???
Thanks....

PS-buying new gear is out of the question. How can I make this work with what I've got?


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## Chris15 (Apr 26, 2009)

Blah067 said:


> How can I make this work with what I've got?



So long as you don't run the amps into clip you will be fine.


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## Anonymous067 (Apr 26, 2009)

So provided the amps don't show a clip, the speakers will be fine?
Where should I adjust the volume then if I try to get the most volume out of them...
on the board or at the amp?


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## waynehoskins (Apr 26, 2009)

Your loudspeakers are rated to dissipate up to 400 watts of power. Your amplifier will deliver up to 150 watts of clean power. No problems.

When you clip an amplifier, that's when nasty things happen. Don't do that, and you'll be fine.

400 watts coming out a monitor speaker that's pointed right at my face? Ouch. 400 watts is loud.


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## TimmyP1955 (Apr 27, 2009)

Our wedges are 8 Ohm, rated 350W, sensitivity is 99dB. The amps are rated 350W/channel into 8 Ohms, and we run one wedge per channel. The amps are barely big enough, and that's because we run limiters, and we make the bands keep their stage volume under control (owing to a loud & lively room). I'd like to have another 3dB of power at least.


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## rwhealey (Apr 27, 2009)

PDF Warning:
http://electrovoice.com/download_document.php?doc=2320

I found this guide to be helpful in figuring out how to power speakers.

You will be fine as long as you don't try to make up for the loss of volume (because of less power) by driving the smaller amp into clipping.


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## Les (Apr 27, 2009)

rwhealey said:


> PDF Warning:
> http://electrovoice.com/download_document.php?doc=2320
> 
> I found this guide to be helpful in figuring out how to power speakers.
> ...



Sorry for the hijack but I love the PDF warning. We should make that a more common courtesy. At least until I get a better computer that doesn't freeze while it loads a PDF.


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## museav (Apr 27, 2009)

First, what 'ratings' are being referenced? Is that 20-20kHz or 1kHz on the amps? Is the speaker power rating continuous, program or peak? This information, especially the differences in the speaker ratings, can be important.

Second, you cannot 'underpower' a speaker, only provide insufficient power for the application. As long as you can obtain the output levels and headroom desired, it doesn't matter if what that takes is less than the speaker rating. I like to say that the power ratings indicate what an amp can deliver and what a speaker can accept, they really have nothing to do with what is needed or appropriate, that is dependent upon the application (the 'rules of thumb' many use simply assume some typical application). As Wayne said, avoid clipping the amp input by trying to get it louder than it can go and you should be fine, it just may not be as loud as desired.

On the latter, keep in mind that doubling the power to the speaker results in a 3dB increase, barely noticeable to some people. Going from 162.5W per speaker, one half of the CX302's rated 325W/Channel at 4 Ohms, to 400W per speaker would be just under a 4dB increase. Definitely noticeable but does it matter in your application? That I can't tell you.


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## SjonRokz4u (Apr 28, 2009)

When matching amps to speakers the general rule of thumb is 1.5-2 times the speakers rms. Give or take 20% either way. Is the 400 watt rating rms, peak, or program?


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## David Ashton (Apr 28, 2009)

The gain structure of your system can improve the results of your sound, you need to understand this, it is crucial to an efficient system,
read and understand this pdf,
http://www.gain.pe.kr/spboard/board.cgi?id=audio&action=download&gul=14

hard work but worth it's weight in Db


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## 3dB (Apr 29, 2009)

David Ashton said:


> The gain structure of your system can improve the results of your sound ... hard work but worth it's weight in Db


Worth its weight in S/N ratio ... perhaps? 

Regards,
Mark


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## Anonymous067 (Apr 29, 2009)

As I stated in the beginning, I CANNOT CHANGE GEAR! It's what I have to work with, and it has to work.

I don't have the money/authority to get new/different gear.

Please read entire posts before insulting my intelligence. Thanks.


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## cprted (Apr 29, 2009)

Blah067 said:


> As I stated in the beginning, I CANNOT CHANGE GEAR! It's what I have to work with, and it has to work.
> 
> I don't have the money/authority to get new/different gear.
> 
> Please read entire posts before insulting my intelligence. Thanks.


Relax buddy. No one in this thread has insulted your intelligence or told you you need to buy to gear.


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## howlingwolf487 (Apr 29, 2009)

Blah067 said:


> As I stated in the beginning, I CANNOT CHANGE GEAR! It's what I have to work with, and it has to work.
> 
> I don't have the money/authority to get new/different gear.
> 
> Please read entire posts before insulting my intelligence. Thanks.


Do realize that everyone here is trying to help you. How does it benefit us to put you down in any way? I just looked at the past posts and saw nothing about changing gear out.

I'm sorry if you felt insulted because someone brought up a concept you might not be familiar/comfortable with yet. Now you have some reading to do and can try to apply it to your situation. If you already know what gain structure constitutes, all the better!

Use the information in this post as a learning experience that can be applied to systems down the road as well as to the one in question here.


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## David Ashton (Apr 29, 2009)

Blah067 said:


> So provided the amps don't show a clip, the speakers will be fine?
> Where should I adjust the volume then if I try to get the most volume out of them...
> on the board or at the amp?



This may seem a simple question, it is not, and that is why you need to know about gain structure because it is the way to get the best out of whatever gear you have, in fact a lot of gear is totally wasted because without correct gain structure you cannot get the best level, dynamic range and lowest noise from your system.


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## museav (Apr 29, 2009)

SjonRokz4u said:


> When matching amps to speakers the general rule of thumb is 1.5-2 times the speakers rms. Give or take 20% either way.


This 'rule of thumb' is a suggestion for the maximum amplifier you would typically want to drive a speaker with assuming some a generic music application. It is very useful, but grossly over applied, often to applications for which it is not really intended, theatre where spoken word and orchestral music may be heavily involved being one. The related EAW application note (http://www.eaw.com/info/EAW/Technical_Papers/AmplifierPower.pdf) has been changed to help clarify this. I wish the related Crown white paper (How much amplifier power do I need?) more clearly differentiated between their "How much power can my speakers handle?" and "Power vs. Application" sections, people seem to read the first section and then either not read or not understand the difference represented by the second section.

I agree with David that many systems can benefit more from improved setup than from new equipment. Ironically, optimizing existing systems is often more difficult to sell than new equipment even though any such optimization would potentially also benefit any new equipment.


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## 3dB (Apr 29, 2009)

Blah067 said:


> Problem...the 302's only are rated for 300 (something like that) per 4 ohm side. Thats providing 150 to each monitor....and...yeah....400 watt speakers. ... How can I make this work with what I've got?



In the interest of making do with what you've got - it will help to a small degree to roll off the low end going to the monitors (if you aren't already doing so). This would lessen the more power-hungry lows from being fed through the monitor system, thus requiring (somewhat) less power from the monitor amp at any given drive level. It will also help the monitor mix to "cut through" the overall stage volume and is helpful to vocalists and other musicians who have a more crucial need for clarity - and perhaps, will allow the monitors to be driven less hard and help your monitor amplification from being overly taxed. You could roll off low frequencies below 125Hz for starters and then experiment with a higher roll off point if it seems helpful. Onstage performers will (should) still get reasonable low end from the FOH speaker system to fill in below the monitor system roll-off. 

Regards,
Mark


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## Anonymous067 (Apr 29, 2009)

All I'm saying is that I do infact know about how speakers and amplifiers are supposed to be selected. This I don't need to be reminded.

All I'm asking about is...since I've always worked with MY gear, or gear PROPERLY purchased...will this work with the "not a perfect world setup"?
That's all...

And yeah...low end I believe is already being rolled off at the amps.
I don't have any EQ/Limiting (unfortunately) on the aux sends. The monitor system (in all seriousness) is used all of three times a year, which is why the amplifier system for the monitors wasn't really (in my opinion) thought out very well.


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## museav (Apr 30, 2009)

Blah067 said:


> All I'm saying is that I do infact know about how speakers and amplifiers are supposed to be selected. This I don't need to be reminded.
> 
> All I'm asking about is...since I've always worked with MY gear, or gear PROPERLY purchased...will this work with the "not a perfect world setup"?
> That's all...


I'm not sure what you are looking for. One aspect of "will this work" seems to be whether it will work at all and your knowing how to select speakers and amps along with the first few responses should answer that (yes, provided that you operate within the limits of the system). The other aspect of "will this work" would typically be whether it will work for the specific application and answering that requires knowing what defines that it "works" or not for the application. Without something to go on everyone is pretty much left basing any comments on generic situations and "rule of thumb" guidelines.

If we knew what the speakers were, how you were using them, possibly even some idea of the desired output level, etc., then there might be some more specific answers.

On your earlier question of where to control the volume, it is usually better to adjust the system for optimum gain structure and then adjust the amp input levels for the desired volume. In this application you want to be careful that you don't overdrive the amp inputs trying to get the desired level. I would turn on the clip limiters in the amp and verify that the high pass filters are engaged and set for 75Hz.


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## 3dB (Apr 30, 2009)

Blah067 said:


> And yeah...low end I believe is already being rolled off at the amps.
> I don't have any EQ/Limiting (unfortunately) on the aux sends.



Ahh, paying for the "sins of the father"... We've all been there haven't we? Looks like you have a choice of setting the CX 302's low freq. filter at either 33Hz or 75Hz (12dB per octave rolloff). Sounds like ensuring it is set to 75Hz would be the best you could do with respect to rolloff. Regardless, the effects of rolling off at a higher frequecy would be marginal...

Regards,
Mark


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## TimmyP1955 (May 15, 2009)

David Ashton said:


> The gain structure of your system can improve the results of your sound, you need to understand this, it is crucial to an efficient system,
> read and understand this pdf,
> http://www.gain.pe.kr/spboard/board.cgi?id=audio&action=download&gul=14
> 
> hard work but worth it's weight in Db



Gain structure should have no significant effect unless something is clipping before the amp.


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## museav (May 16, 2009)

TimmyP1955 said:


> Gain structure should have no significant effect unless something is clipping before the amp.


Proper gain structure helps regardless of the signal level. Consider hissing speakers with no signal present as an example of an audible problem caused by poor gain structure without clipping in any way being a factor. Or that heavily clipping the amp input when you barely see output on the console isn't poor gain structure and a problem just waiting to happen?


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## TimmyP1955 (May 17, 2009)

museav said:


> Proper gain structure helps regardless of the signal level. Consider hissing speakers with no signal present as an example of an audible problem caused by poor gain structure without clipping in any way being a factor. Or that heavily clipping the amp input when you barely see output on the console isn't poor gain structure and a problem just waiting to happen?



Certainly the gain structure should not be way out of wack. But if nothing is clipping, it is not a problem per say. A lack of optimization, yes.


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## David Ashton (May 18, 2009)

Would you care to explain exactly what 
"optimization" might be?


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