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Author Topic: Amps, Subs and confusion [quick help needed!]  (Read 8044 times)

Nicolas Poisson

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Re: Amps, Subs and confusion [quick help needed!]
« Reply #10 on: January 02, 2013, 02:12:52 pm »

Oh... I guess I should make sure I am awaken before posting anything.

Concerning the 3dB more wattage, this is an error. Of course with 3dB more watts you get 3dB more voltage, because the "10" factor for power vs. "20" for amplitudes is made on purpose. Sorry.

Concerning AES, I always thought the power indicated was the "peak" power of the signal, i.e. peak voltage multiplied by impedance. So I was thinking a "500W AES" speaker was powered by an average 125W during testing. And I used to consider roughly 500W AES = 250W sine = 125W square. I even never thought it could be otherwise indeed... I'll sleep smarter tonight.

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Luke Geis

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Re: Amps, Subs and confusion [quick help needed!]
« Reply #11 on: January 02, 2013, 04:54:38 pm »

Oh boy....... Conversation : )

I created a hypothetical situation for no other reason than to provide a way of thinking for the OP. The numbers are farce and the calculations are general. This is why I presented two links in which both contain good material about the subject.

As far as the power ratings of the speakers, it doesn't really matter. All we can go by is what we are told. How that number was derived should be by a measured standard. If it's true or not is the big question. But we make are decisions based on that info. I myself am in the belief that the numbers are untruthful, but close enough to make a decision. I take my NAG calculations and build my system around that factoring a 25-30% safety cushion.

As for what SPL really is? A, or C weighting? Most is done in A weighting, but C weighting is more suited for determining what the system is putting out volume wise over a larger frequency range. If the Rider for your act says 120db to front of the venue, does it say A, or C weighting?  My guess is most don't specify, the ones I have seen didn't say either way, only that they wanted X volume at X position in the venue. If I need 105db at FOH position I use C weighting. When the volume police come around the measurement is done in A weighting. It's up to the designer/operator to determine what the numbers mean and what can be achieved. I think I gave good safe information that should direct someone to making an educated decision. At least I hope so.........
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Luke Geis

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Re: Amps, Subs and confusion [quick help needed!]
« Reply #12 on: January 02, 2013, 11:39:54 pm »

here is another link that has more information on the topic: http://www.prosoundweb.com/article/understanding_target_spl_and_required_amplifier_size/
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Brad Weber

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Re: Amps, Subs and confusion [quick help needed!]
« Reply #13 on: January 03, 2013, 01:57:39 pm »

If you think of it like this, you will see why it makes sense. A speaker who's rms ( or continuous )  power is 500 watts, can in essence be powered by an amp that will provide 500 watts. However you don't want to power it with an amp who's peak power is 500 watts. That means that the amp would be working pretty hard all the time......
Most music is not a constant signal, the rated power of an amplifier does not indicate that it always outputs that much power (actually voltage and current) and the rated power of a speaker does not define a required power.  A 500W Continuous rated speaker may work just fine with an amp rated at 5,000W, 1,000W, 500W, 100W, 1W or any other value, it all depends on the signal, the application and the operation.
 
Where the issue seems to come up with many is not having a defined use and/or having to serve a range of uses, musical genres, etc.  In that case there are some 'rules of thumb' that have been developed over the years to represent what is typically a reasonable mix of maximum output, reliability and headroom.
 
So I'm a hobbyist being asked to check a sound system for church and please correct me if I'm wrong, but as a rule of thumb shouldn't the amp be double the wattage as the speaker for efficiency? Then I saw the features of the amp and got a bit confused with the options

- I have 2 EAS S250Z that run at  1050W (4 ohm) and 525W (8 Ohms)
- The amp being used is a QSC PL325 that pushes 500W @8 Ohms/ 850W @4 Ohms/ 1250W @2 Ohms.

1) should we be getting a higher powered amp?
2) if budget doesn't permit, what is the best setting for max bass output?
As others have already noted, what really matters in that type of situation is not how the amp and speaker ratings compare but whether they amp and speaker combination can provide the desired results without damaging either one.  That pretty much answers your first question, if it currently gets loud enough without clipping the amp then you probably don't need more power.
 
I can't find any information on that speaker but based on the specification you noted I would guess that it is a dual driver model that can be run as two separate drivers (two 8 Ohm drivers rated at 525W each) or with both drivers in parallel (one 4 Ohm, 1050W rated box).  If that is true then assuming you would be running the box or both drivers off one amp channel it makes no difference.
 
What we don't know is whether the 525W and 1050W ratings represent the Continuous/RMS, Program or Peak rating.  The safest assumption is to assume that is the peak rating, thus continuous ratings of about 130W and 260W respectively.  In that case your amp which is rated at 500W into 8 Ohms and 850W into 4 Ohms at 1kHz is already probably about as much power as you would want to apply to those speakers.
 
Even if the 525W and 1050W were the continuous ratings for the speaker then you probably wouldn't want to go beyond an amp that might increase the maximum output (not the output with the same input signal but rather how loud it can get) by about 3dB.
 
Bottom line, unless you can verify that the speakers could handle the additional power and a 3dB or so increase in maximum potential output justifies the cost, I would not change amps.
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Steve Anderson

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Re: Amps, Subs and confusion [quick help needed!]
« Reply #14 on: January 04, 2013, 09:01:21 am »

I can't find any information on that speaker [...]

I'm guessing he's referring to an EAW SB250z
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Don Boomer

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Re: Amps, Subs and confusion [quick help needed!]
« Reply #15 on: January 04, 2013, 12:02:41 pm »

You have to understand the difference between the potential voltage a power amp could deliver and the the amount you actually "deliver" to the speaker.  Theoretically as long as you don't exceed the peak power or the continuous rating you should have a reasonable chance of not destroying the driver. But this leaves out the possibility especially on a sub of exceeding the mechanical limits of the  box by sending a frequency that is too low.  Again theoretically if you apply a frequency an octave below box cutoff at 1/4 of the rated power you could be in trouble.

In my experience many more drivers are trashed by exceeding the mechanical limit than being burned by too much power by an order of magnitude or two.  So setting the proper HP filter is much more important in my mind than worrying about too much average power.

Back to thermal issues... As others have pointed out this is heavily influenced by the dynamic range of your music. If you have 10 dB of dynamic range then an amp rated to deliver 1000w will be putting out 100w at the time the limit LEDs fire (or there about).  This is why manufacturers recommend "program power" as a good bet. Music almost never drives a power amp to deliver its full average continuous potential.  Even if you had a highly compressed baseline such as in electronic dance music with only 3dB of dynamic range the amp would likely only be outputting less than 500w on a continuous basis.

The other thing you have to be aware of is the method used to get the power rating of a speaker.  AES and EIA are the typical standards used.  Both methods used shaped pink noise.  The AES method however stresses the lower frequencies much more than the EIA method which concentrates most of the energy in the curve around 300Hz.  The EIA method however does it for a longer time period ... 8 hours as opposed to the 2 hour AES method.  Opinions with people who know much more about it than I do go both ways.  To me it would seem that AES would be a better way especially when considering subs.
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Don Boomer
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RF Venue, Inc.

ProSoundWeb Community

Re: Amps, Subs and confusion [quick help needed!]
« Reply #15 on: January 04, 2013, 12:02:41 pm »


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