Amp ratings: how to undrstand them.

LittleCar_w/12s

This has been bugging me for a while now, and I feel it needs to be said. I will post this as a supplement for either home or car audio enthusiasts who are interested.

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How the power ratings on amplifiers are expressed depends on the manufacturer. Most will use the rating that makes them look best, however you will find the better manufacturers will give you the rating that will show their continuous performance, and not just their peaks. Here I will help you understand exactly what these rating mean, so that you may make your decisions informed.

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There are three ratings commonly used to describe the power of an amplifier. These are:
Peak power output
RMS power output
Continuous power output

To describe these we have to understand that we are working with an AC signal, that is, if you have balanced your inputs correctly. We must also know what a watt is...

Watt: A measurement of the power used or dissipated by an appliance that is equivalent to one volt at a current of one ampere through an one ohm load. -- this is to say that a 2 volt supply will pass one ampere through a 2ohm load, but it will also be 2 watts, not 1. I will post some usefull equations below.

Understanding an AC signal is simple, yet complicated. Let's delve into the basics of how power is used in the amplifier to understand it better:
The power stage:
Each amplifier has a power supply that creates a higher voltage than 12 volts (for car audio) or a nominal high-voltage (home audio) that is supplied to the amplifier as positive and negative DC voltage. You will find they will range from 24v to 55v for 1KW or less in car audio (2-4ohm loads) and 55-240v for home audio (8-16 ohm loads). The Amplifier stage can only output a sine wave (given, say, a 55v+/- supply) that will peak at +55v and -55v.
[/b]How is the power used?[/b]
Additionally you should know that this power is both dissipated through your connected audio hardware and the internal components of your amplifier... what power does not go through your drivers gets dissipated by your amp. Though it seems odd, your amp dissipates the most power and creates the most heat when crossing the half-output thresholds. In the previous example, this is when the sine-wave crosses 27v and -27v. Therefore, a properly set amp running at it's maximum will dissipate more heat than an amp set at clipping, and an amp set at below it's maximum. Also if you have a clipped input to your amp, and set it at half-output (making the amp run mostly around it's half-output threshold) you will not have an amp for long. Remember your amp is designed to run set properly, and with clean inputs!

Now for what you want to know:
How do we rate power form an AC signal?
An AC, or Alternating Current, signal is just that, the current is constantly changing. Well then, how do we get a current with which to derive our wattage output from if it is constantly changing? Here we discover the two first ratings mentioned: peak and RMS.
Peak:The peak power is easiest to get. The peak voltage in our example is +/-55v. We would use the Ohm's Law (see attatched diagram) with our voltage and resistance to get the power. Let's use a 4 ohm driver: at 55v, a 4ohm will let 1/4 of the amperage go through as a 1ohm load would. so we have 55/4 is about 14 amperes. Now our wattage is 55 Volts x 14 Amps to equal 770 peak watts! Now don't get too excited, this is peak we're talking about!
RMS: Of the ratings, this is the most usefull for our amp user. The RMS, or 'root mean square' of a sine wave represents the equivalent DC output compared to a full sine wave (one that goes positive and negative equally, and is properly shaped). So if we have a +/-55v sine wave we want to know the equivalent DC, or constant, voltage. We do this by using the R.M.S. equation, which for sine-waves is 0.707x(ac.v) So our +/-55v is equivalent to running about 39v. Keep in mind that half of the time this is +39v, and other -39v. So now we have a constant voltage to run through our ohm's law... 39v/4ohms allows a little less than 10Amperes, so we have a RMS wattage output of only 390 watts. Where did all that peak power go?!?!? Well that's just it; Peak is PEAK, nothing more.

Let's talk about that last one, continuous power rating.
Continous Power: is the maximum constant current you can run through an amplifier for an unlimited time without damage. Let's discuss what it means... Continous output is the amount of amperage, and hence wattage output, that your amp can run without burning something up. We have two things to burn up here... the power supply and the amplifier. Either one may affect this rating, or both. This rating can NEVER be above the RMS; it would be impossible to do such without outputting a purposely clipped signal to your drivers, and we know what that leads to: dead drivers. This rating can, however be below your RMS... so be careful, you may not be able to run that amplifier at full-voltage output without hurting it!

Let's have an example again: Our amplifier has a 55v power supply, the manufacturer rating it at 13.5v input from your vehicle (this is important) So we will have a peak of 700w, RMS max of 390w. Let's say the manufacturer rates their equipment at "350w continuous output" woah!! we've gone considerably down from 700. Also notice that this is about half of the peak output, which most manufacturers will design their continuous for 1/2 the peak. Also know that this is the reason our 350w (cont.) amp designed for 4ohms will NOT produce 700w at 2ohms... it just can't handle the power throughput!

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Now can we see why amp manf's of low status rate by peak?? You can also tell a good manufacturer buy one who will design their amps to handle a continous ABOVE the RMS. They have superior P.Supplies, and linear amp's. You will also notice that a good manufacturer may never state peak, also if they do it will be 1.414x the RMS (btw, 1.414 is the reciprocal of 0.707) Aditionally, amps by good manf's rated by con't power will almost always have a higher RMS, and these are the amps that are said to be "underrated" under normal conditions... be careful when turning that knob up if you don't know how to safely surpass that continuous rating!

If an amp is designed to power at 12v, it will outperform itself at 13.5-14.4, which your car will run, it may even blow up. ALL good man'fs rate at either 13.5 or 14.4 is levels, though they may have stats for 12v input. Your amp will also underperform at 12v when designed at 13.5-14.4v

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You may not obtain that higher-than-RMS continous rating without tweaking, which I may or may not discuss in another thread... if I feel like dropping my secrets to you. Also with amps that have less-than-RMS con't ratings, it IS possible to acheive that RMS safely and easilly ;-)

I hope I've helped here.
-Jerry

LittleCar_w/12s

MEMORIZE THIS IF YOU CARE!
It IS worth your time to do so!
edit: Additionally this image is from "the12volt.com" which I happened to run across, it is the best representation of Ohm's law I have on the comp. without scanning one of my electronics books...

bknauss

Here comes anal correction man

-You don't need a positive and negative DC voltage on the power stage. It can be a positive voltage (+55V or whatever) and ground. You get the same effect as long as the difference of the max and min voltage are the same (+55V and ground = +/-27.5V). Its actually easier to implement the positive voltage and ground, but its not all that much different.
-Power is also dissipated through speakers, not just in the amp.
-Although peak power theoretically can be calculated the way you state, its rarely calculated that way, if ever. Currently, there's no universal way to measure peak power.
-RMS in calculations (the correct use of RMS) and measured RMS are pretty much completely different things. RMS power as stated with car amps are continuous power at 1% THD (this is the primary rating with the CEA 2006... you're pretty much allowed to add any other ratings you want as long as you have the power output at 4 ohms/1% THD/14.4V voltage input).
-Continuous power, as described, is what RMS power is basically defined as in car audio.

LittleCar_w/12s

You are correct in the possitive-only supply. This can be found in plenty of amps, especially d-class amps (where it uses the full voltage for the positive, then effectively uses the full-voltage again, just reversed, for the negative) I was mainly, and generally, referring to class a or ab amplifiers, as they are easier to understand.
I really wanted to clear up what rms and continuous vs. peak are, though we cannot guarantee all amps will follow the exact calculations, or even be close.
As for the proper use of ratings, I guess I would have to say: good manf's use the correct use of the ratings. It is always possible a crappy manf'r has no clue what to put on the box, only the designers know that :-) And all manf'rs want their product to look best on the shelf ;-)

bknauss

Calculations are done on the design, but calculations aren't used at all (more or less) when stating power ratings if the company follows the CEA2006 standards.

AustinKP

Good article...thanks Jerry

neomagus00

me too... btw, anal correction man, the article mentioned that the power was dissipated through the speakers as well as the amp...

bknauss

Doh! You miss things like that when you post right after your mom came to visit. Talk about a draining experience...

neomagus00

oh, yeah, i hear that...

LittleCar_w/12s

"Thanks, Anal-Correction Man!"
I don't know what we could do without him.... It does help to have someone look it over, though, I type really slow so I might have completely forgot something by the time I finished writing that...
Hope It was a helpful post though.
And when I do an install or test equipment, I write down the actual outputs, etc, as I have oscilloscopes, etc to test them. I do need ot get a good freq generator though, so if anyone has an old one lying around....
-Jerry

neomagus00

i've been using a cd that i burned with maybe 50 tones on it... they're 31 third-octave tones, and maybe 15-20 more i threw in just for fun, mostly in the midbass-midrange area... i used a free program called GoldWave (www.goldwave.com) to generate the tones, made em 30 seconds long each, and burned em... works well... if you've got a clean soundcard you could skip the cd step and generate straight into whatever you wanted...