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Author Topic: Are most power amp wattage specs given as RMS or Peak values?  (Read 13529 times)

Ivan Beaver

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #20 on: July 06, 2015, 07:17:10 PM »

Typical test signal? Surely not a sine wave which only has 3dB peak voltage to average voltage. Perhaps noise or music with 6db crest factor, but noise and music both come with different crest factors?
 
For voltages Peak is commonly contrasted with average or RMS, while most voltmeters measure average unless specified as true RMS. huh? I think I missed the rest of this conversation.
The "test signal" I was referring to is a 6dB crest factor limited noise.

The "peak" numbers" used in several very popular amplifiers, is not the "RMS of the maximum (peak) signal". but rather 3dB higher as measured/calculated at the very "peak" of the signal.

Or the "peak of the voltage waveform of the maximum RMS value the amplifier is capable of producing (or the speaker is rated at).

Does that help?

So it is actually 9dB higher than the "continuous" rating.
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Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Ivan Beaver

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #21 on: July 06, 2015, 07:22:24 PM »


HOWEVER...

Very few amps can actually do this for any length of time; time in this case meaning 1/2 second or longer, or so.  In this sense, amps are rated in some version of "peak" or short term power.  The "heater mode" output of a typical power amp - the amount of power the amp can make for more than a second or two - would be more like 1/4 - 1/3 of the nameplate rating of the amp.


For a large number of modern "high power amps", 1/2 second (500ms) is a musical eternity.

You will find that very often they can only produce the rated power for 20-80ms.  Quite a bit less than 1/2 second.

Yes some can produce the power for longer, but they are more rare than common.

FOr some types of music this is no big deal.

But for other types (that contain longer sine waves) having the extra power for longer time can make a difference.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Jeff Bankston

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #22 on: July 06, 2015, 11:38:01 PM »

Cough.... 

The difference between a 1kHz power spec and 20-20Khz power will be modest. For pure tones that are lower frequency that the refresh rate of the power supply reservoir caps (2x 60Hz ) will suffer some measurable power loss due to reservoir capacitor sag. The slightly less power at 20Hz due to saving pennies on the reservoir caps is far from a significant sonic problem (IMO). All else equal more is better, but more is always better. Know and understand what you are paying more for.

 JR
it may not matter to you but it matters to me.
« Last Edit: July 06, 2015, 11:42:13 PM by Jeff Bankston »
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John Roberts {JR}

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #23 on: July 07, 2015, 01:03:48 AM »

it may not matter to you but it matters to me.
EXACTLY... that's why the marketing guys do that.  ;D ;D

I used to be a product manager for power amps so i was one of the guys spinning those numbers.

The amount of power difference it takes to make an audibly perceptible difference, is far more than the power difference between I kHz power and 20Hz power, or the difference between rated power at 1% distortion vs .001% distortion, etc yadda yadda. it's all about hitting some nice round power number, or being x watts more than the next guy (even if nobody could ever hear the difference).

I could tell you stories, but the customer is always right, even when........ Ignore the man behind the curtain.  8)

JR
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Jeff Bankston

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #24 on: July 07, 2015, 02:30:16 AM »

EXACTLY... that's why the marketing guys do that.  ;D ;D

I used to be a product manager for power amps so i was one of the guys spinning those numbers.

The amount of power difference it takes to make an audibly perceptible difference, is far more than the power difference between I kHz power and 20Hz power, or the difference between rated power at 1% distortion vs .001% distortion, etc yadda yadda. it's all about hitting some nice round power number, or being x watts more than the next guy (even if nobody could ever hear the difference).

I could tell you stories, but the customer is always right, even when........ Ignore the man behind the curtain.  8)

JR
i want to know what the power output is at 30hz. i know my woofers are going to be running close to max amp output. my mids dont use a lot of power and they have higher sensitivity. my woofs are 97db 1 watt 1 meter , my mids are 102db 1 watt 1 meter. my QSC 3500 is 300 watts 8ohms 20-20k. its 350 watts at 1K. i have way more power than i need for my mids. QSC Series 3 has a power graph in the owners manual. They advertised RMS power is the lowest rating. The 3800 is rated 375 watts at 8ohms 20-20k. 375w is the lowest and its at 20hz. The power rises as the frequency goes up. The top graph is 8ohms and the bottom graph is 4ohms. This is the way amp power rating should be done. The graph below starts at 10hz and goes to 40k. Thats 10-40K. The top line is for the 3800, 3500, 3350, and the bottom line is for the 3200. 4 amps per graph. Also i can put ps caps with twice the mfd in place of the original size caps thanks to advancements in cap manufacturing.
« Last Edit: July 07, 2015, 02:36:35 AM by Jeff Bankston »
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Stephen Kirby

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #25 on: July 07, 2015, 04:50:28 AM »

However the idea a bigger amp is safer is yet another can of worms, yes it will deliver more clean power but it's too much power that kills drivers(not distortion)

Almost got it.  It isn't distortion per se, but the nature of distortion.  A clipped waveform contains more "power" even if the peak voltage is the same as an unclipped waveform.  What J.R. referred to as the "heat" of the signal.  The area under the waveform curve.  It is that power that the voice coil is dissipating.  When a lower power amp is driven into severe clipping it's output is nearly continuously on for the full swing of the waveform, forcing the driver to dissipate all that energy leading to voice coil burn out.  A high power amp will produce a higher peak voltage but the area under the curve won't necessarily be as large.  And the voice coil will have an easier time recovering thermally from the excessive peak than it will from being held at near it's maximum rating for an 80% duty cycle.
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Jeff Bankston

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #26 on: July 07, 2015, 05:18:09 AM »

  When a lower power amp is driven into severe clipping
and people should know better than to clip an amp. its not going to go louder. you have to double the power for every 3db spl increase. buy the right equipment to start with. the amps are part of your "tool kit". would you want an electrician to show up with an eyeglass screwdriver to change a light switch. i have gone to see bands with awful sound. i look for the power amps and the clip lights are a steady glow. during the set break i tell the sound guys if they back it down just enough so the clip light goes out the spl will stay the same. over the years a few guys took my advice and the sound was much better and the spl didnt seem to be any lower. i use enough rig so the -6db signal light is flashing on and off. those lights are not there to impress the audience , they are there so you know how much power the amp has left before clip. running the amps into full clip is the thing beginners do.
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TJ (Tom) Cornish

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #27 on: July 07, 2015, 07:09:57 AM »

Almost got it.  It isn't distortion per se, but the nature of distortion.  A clipped waveform contains more "power" even if the peak voltage is the same as an unclipped waveform.  What J.R. referred to as the "heat" of the signal.  The area under the waveform curve.  It is that power that the voice coil is dissipating.  When a lower power amp is driven into severe clipping it's output is nearly continuously on for the full swing of the waveform, forcing the driver to dissipate all that energy leading to voice coil burn out.  A high power amp will produce a higher peak voltage but the area under the curve won't necessarily be as large.  And the voice coil will have an easier time recovering thermally from the excessive peak than it will from being held at near it's maximum rating for an 80% duty cycle.
True, but i have never seen a person who was willing to run a small amp into severe clipping who would not do the same with a larger amp, causing speaker death even faster.
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Bob Leonard

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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #28 on: July 07, 2015, 04:07:27 PM »

Once again everything old is new. Now that all of the math lessons have been given, which is always appreciated, there are some simple rules to follow, rules that are at least 50 years old, and rules that will help you throughout the course of your life working with sound. I've never been one to promote complexity or extraneous detail when not needed. However, knowing the detail behind any subject is a good thing, and a good thing that can always be used for reference at any later date. Thankfully we have the Ivan's and JR's of the world located right here on this web site. Their knowledge of the detail is not only welcome to any subject, but can always be found to be accurate and truthful.

That being said there are also some basic rules of thumb that can be followed when sizing an amp that require no detailed explanation, only a basic understanding of why the rule works, and that has already been given.

The single most important rule of thumb you can use, now that the reasons behind the rule have been explained, is to use an amplifier capable of providing 1.5 and up to 2 times the cabinet's rated long term power handling capability. Simple and effective you'll always have enough power to properly push the cabinet to it's rated maximum if needed, without clipping and while having enough power in reserve to meet the needs of program material and the peak signals that are a part of all sound.

Please keep in mind that cabinet ratings can be whatever a manufacturer thinks they should be in many cases. Some cabinet manufacturers will show peak power handling capabilities, disregarding the fact the peak figure is what the speaker(s) can handle for milliseconds. But those who don't know better will bite at the bait, and once again it becomes a buyer beware experience,

However, honest and trusted manufacturers will generally provide figures based on testing that follows a specific test model. For instance, if JBL states they test their cabinets for 100 hours using a specific sound at a specific level and then use that test to provide the figure representing long term power handling capability, then that is a figure you can trust when applying the 1.5/2x rule of thumb. Another point to remember is this. Generally speaking the peak and program ratings for speakers are useless figures used to influence noobs, IMHO.

And while we're at it, let's make sure we show appreciation to the people above who have taken the time to explain science behind the rule.
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Re: Are most power amp wattage specs given as RMS or Peak values?
« Reply #28 on: July 07, 2015, 04:07:27 PM »


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