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Ok...Continuous, Program, RMS, Peak. Erm... Effects Pedals, Units & Accessories
Old 7th August 2014
  #1
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Ok...Continuous, Program, RMS, Peak. Erm...

Hi all,
I'm trying to find out what kind of power I need to drive a few different options of PA cabs.
I'm using a pair of Mackie powered subs with built in crossovers.
I have a number of options for tops, all of which are similar in that they are all passive 1x15" and horns that, for the sake of this post I want to run non bi amped, even though they all have that option, so basically I'm running a 2 way system with the LF crossover in the subs.
Also, just to be clear, I'm using this setup as a small pair of mains for rock bands in a 100 cap room.
I am getting increasingly frustrated trying to find a "rule of thumb" that seems consistent regarding what wattage of amplifier to use in order to drive the tops (or any speaker in general).
I understand the continuous and peak measurements (I think).
It's the program and RMS ones that are causing confusion for me.
In some manufacturer's literature program and RMS are the same number whereas in others they use one or the other. Some places say (for example) "300 watts continuos, 600 watts RMS, 1200 watts peak" whereas other say "300 watts continuos, 600 watts program, 1200 watts peak".
Others still only give an RMS rating. My understanding is that RMS aka Rout Mean Square should be 70.7% of the measured peak rating (not %50 of the peak rating as it says in the first example I gave).
Also, other documents give the continuos power rating as being the same as the RMS rating.
I'm totally confused!
Which of these varieties is correct and which can I use to determine how much power should I have to drive the box efficiently?
I've seen so many varying versions of this from having an amp that is rated at the same as the program value (is that the same as RMS Aaargh)!!!!
I've also read that your amp should be somewhat more powerful than the speaker rating (which rating though)? as more power means less need to drive the amp into clipping and therefore square waving and damaging the speaker.
How much more powerful, or is that not the way it works?
I hope some of you can shed some light on this in a way that makes it at least somewhat easy to make a decision.
Cheers!
Ps. I realize that the impedance of the speaker will change how much power the amp will drive it by.
For the sake of this thread, lets just take it that all impedances are matched (let's just say 8 Ohm).
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Old 7th August 2014
  #2
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Engineering spec were intended for engineers but they seem to be utilized even more by marketing departments. So looking at a single number for any representation of amplifier power is suspect. The first one is that amps don't even put out power, they put out voltage (but let's skip that one for now).

There are a number of methods of specifying continuous power output (aka RMS) and peak power. Namely how long and how frequently can they do it. And none of them really speak to how they will make your system sound. It's really not the most important thing anyway. But back to your RMS power ... which one do you have?

"Program" power is really a suggestion used by manufacturers for matching up amps to speakers. Manufacturers generally spec that you can/should used 2x the continuous power rating of the speaker and the reason is that music is never continuous but always dynamic(well most music). So using music as a source you will never drive your amp to deliver its full continuous rating but you will likely very frequently drive your amp to its full peak output. And at that moment when you do the average/continuous power output will be as many dB below that peak as your dynamic range is at that moment. That means you will likely be delivering about 10% to maybe 30% of the full continuous rating of the amp when the limit light comes on and says "no more power for you".

Back to Program power. So if you use an amp that delivers 2x the speaker's rating it is unlikely that in the normal course of things (horrendous accidents excluded) you will still never actually deliver even half of the speaker's rating.

Here's a little piece I wrote that I'll quote so I don't have to type it all again ...

http://peavey.com/support/technotes/...MUCH_POWER.pdf

Oh yeah, the reason you are getting messed up on the numbers is that the peak vs RMS powers are because most manufacturers test with pink noise and limit it to a 6dB crest factor.
Old 7th August 2014
  #3
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Quote:
Originally Posted by BlackLabStudio View Post
Some places say (for example) "300 watts continuos, 600 watts RMS, 1200 watts peak" whereas other say "300 watts continuos, 600 watts program, 1200 watts peak".
Others still only give an RMS rating. My understanding is that RMS aka Rout Mean Square should be 70.7% of the measured peak rating (not %50 of the peak rating as it says in the first example I gave).
Also, other documents give the continuos power rating as being the same as the RMS rating.
This is quite clear but heres some extra info that should help.

To start with, each value may or may not be different as it is taken from different input sources OR measured at different levels of operation and will depend on all these variables, the design as well as parts used in the design.

RMS... This is the rating (in watts) you will generate from an amplifier (and to the speakers) at the THD measurement on the speck-sheet of said amplifier. So, if you have a bad (high numbers) THD rating then you will get higher distortion at this RMS wattage. However, if the THD numbers are nice and low and the RMS rating is high then you will get cleaner power at this RMS level of operation.

Continuous .... This is the rating (in watts) that will run at a constant rate of time before peak levels (and clipping).

Peak This is what an amplifier will generate (in watts) at top output. Think of a system running at high levels and the drummer smacks the kick-drum with all his might. That will work a subwoofer amp to its max or close to max output.

Program... If your going to use this system for a live band with lots of dynamics and the possibility of extra dynamics processors in the signal chain then this rating is not what you want to look at. This rating (in watts) is usually for program music like CD's, DVD's and MP3's etc, that have been produced in a professional studio with professional tools, engineers and musicians for smooth dynamic output.

So its best to look at the THD first and then compare it with the RMS. Even if the THD is a bit high, as long as the RMS is well over your needs for your speakers peak input rating then it may still be a good match, you just don't want to push the amp on that speaker. This is of course only true if your speakers meet your rooms needs (coverage & required SPL's) but most sound guys don't like to run amps at lower levels as it will add a whole new level of dynamic rage & head-room can suffer.

In review.... Low THD numbers and the same RMS numbers as your speakers can handle along with proper SPL's/coverage and you're power amps will be golden. You can avoid all these numbers games by using powered speakers with an output rating thats good with the SPLs and coverage you need.
Old 7th August 2014
  #4
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First of all.. the crossovers in the subs are electronic... and forget about RMS etc... Pay attention to the "long term" power spec. (not the peak)....that applies to the boxes as well as the power amplifiers. Allow at least 20% (should be more) headroom on the amps. A decent two way passive speaker will be rated at "continuous power"...Good power amps are rated at 2, 4, or 8 ohms,,, but pay attention to the frequency range that those rating can sometimes be attached to.

Not enough power will fry voice coils due to the results of clipping / power compression, etc.

Too much power into a passive crossover can burn up resistors and cause a failure, and also cause over excursions and voice coil melt downs.

Protect your equipment with properly set processing / limiters,,, but use common sense also. Too many people just wing it and then poof!....keep in mind that just a few more db of SPL requires a lot more gear.

Also....clipping a signal even before the power amplifier when you are running a system at high output levels or near the threshold of the rated power capability of the speaker system can also destroy drivers.

pay attention to clipping indicators and know what the warning lights mean. Some amplifiers will indicate a "clip" light 3db before the amp actually clips... kind of a pre warning.. Like setting your watch back 10 minutes so that you won't be late for a meeting.
Old 7th August 2014
  #5
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Quote:
Originally Posted by Teknobeam
Not enough power will fry voice coils due to the results of clipping / power compression, etc.
Quote:
Originally Posted by Teknobeam
Also....clipping a signal even before the power amplifier when you are running a system at high output levels or near the threshold of the rated power capability of the speaker system can also destroy drivers.
Both these statements are wrong. They always have been and always will be, yet people keep repeating this rubbish.

A loudspeaker is an electro-mechanical device. To damage it you need to exceed its thermal or mechanical rating. The loudspeaker doesn’t care what shape the signal is and will produce clipped signals indefinitely as long as the power it is expected to dissipate is within its capabilities and the amount the diaphragm moves is less than its limits of excursion.

When you power a loudspeaker, some of the power is dissipated as heat. This causes the voice coil and magnet temperature to increase. If the power input is constant, at some temperature the amount of heat dissipated to the surrounding environment will equal the heat coming in in the form of electrical power. At this point the temperature will stabilise.

If you increase the input power the temperature required to reach equilibrium goes up. In a loudspeaker this will keep happening until either the glue or insulation on the wire melts. On old drive units, with paper voice coil formers, I have seen the former set on fire.

Mechanical damage occurs when the diaphragm moves far enough to impact on stationary parts of the loudspeaker, cause the voice coil to become misaligned, or over longer periods cause the suspension to deteriorate.

In an AES presentation, Michael Miles (of Electro-Voice) stated that clipping the signal was the best way to limit over excursion due to excessive voltage. It is not a subtle form of limiting, but it only affects the part of the signal that would damage the drive unit.

If clipping does not cause damage, the question is why do people think that it does? Ironically it is because it acts like a limiter, which is what the same people advocate to prevent damage. The ouput of an amplifier is usually proportional to the input signal. Low input, low output. Increase the input and the output goes up. This continues until the required output signal exceeds the voltage of the supply rails which is why an amplifier clips.

Music signals tend to be all over the place, so clipping is not a constant. If you push the signal gain up past the point of clipping, some of the smaller signals are sill getting larger. This is the same action as a limiter. The effect is to increase the average signal level relative to the peak level, and it is the average level that determines the amount of heat needed to be dissipated by the drive unit.

The problem with suggesting rules of thumb is that there are too many combinations of amplifier to loudspeaker. Power a 50W drive unit with a 3KW amplifier and the drive unit will be smoking before the amplifier gets anywhere near clipping. Plug your ipod into a 1KW rated drive unit and it will reproduce heavily clipped signals until the battery goes flat.
Old 7th August 2014
  #6
Here for the gear
 

Thanks for the responses guys. It's clear there's no single answer to this.
The top boxes I'm looking to use are Peavey SP2Gs. They're rated at 300 continuous/600 program/1200 peak @ 8 ohm.
So - with dboomers suggestion I should run 600 watts as side into them (2 x continuous aka program).
This seems like the simplest solution so I'm going to run with that.
I'm scraping this little rig together bit by bit on very little cash (scored the Mackie SRS1500 active subs on Craigslist for $150 for the pair)!!! so you all may see other questions from me that may be somewhat naive.
I thank you all in advance for any assistance
Next on the purchase list is a dbx drive rack PA or something similar - any suggestions for alternatives that dwell in the bargain basement?
Old 7th August 2014
  #7
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Quote:
Originally Posted by BlackLabStudio View Post
Thanks for the responses guys. It's clear there's no single answer to this.
The top boxes I'm looking to use are Peavey SP2Gs. They're rated at 300 continuous/600 program/1200 peak @ 8 ohm.
So - with dboomers suggestion I should run 600 watts as side into them (2 x continuous aka program).
So just to be clear ... you will not actually be running 600W into them but rather you will have 600W available. At the point your clip limiter comes on you will likely be running between 60W and 180W into them if you hook up to a 600W amp.

But you also have to use proper HP filtering. And avoid doing anything stupid
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