Power Of Audio Power Amps?

crutschow

Joined Mar 14, 2008
25,690
.........................
There's still no such thing as an 'RMS Watt' though....or a 'peak Watt' or an 'average Watt'. All in common usage . All wrong.
I don't agree. The term RMS watt may be incorrect (or perhaps redundant) but you can certainly talk about an instantaneous peak watt if you desire. It's simply the instantaneous peak current times the instantaneous peak voltage. And if you average all the instantaneous voltage measurements by the instantaneous current measurements over a time period then you get the average power in watts over that period.
 

rogs

Joined Aug 28, 2009
279
rogs, if you are going to be in the business, you need to understand all of it.
Been there - done that!....:)

You're still wrong! A 'Watt' is a 'Watt'...no ifs, no buts ... no 'qualifications'...

Just because there has been many years of marketing 'hype' - from many quarters - trying to pretend otherwise, it doesn't make it right.

I'll leave it to the academics we have here on the forum to explain the details to you.... clearly, I've not been able to! :)
 

rogs

Joined Aug 28, 2009
279
I don't agree. The term RMS watt may be incorrect...
Not 'may be'.... is incorrect.

As I say, in common use for many years. Still wrong.

And if you average all the instantaneous voltage measurements by the instantaneous current measurements over a time period then you get the average power in watts over that period.
It's still incorrect to describe 'average power' as 'RMS Watts'
 

Lestraveled

Joined May 19, 2014
1,946
rogs, I like you. I am not going to try to convince you further. You are doing good electronics. Building fun stuff. You will always have my ear. Rock On!
 

MrChips

Joined Oct 2, 2009
22,113
When someone uses RMS Watts, we know what they mean. Don't get uptight over the semantics.

There is a simple way to estimate the max power output of a linear transistor amplifier based on the supply voltage.

Assume the load is 8Ω and the supply voltage is bipolar ±20V.
The maximum peak-to-peak voltage is 40V. The amplitude of a sinewave signal is 20V.
The RMS voltage is 0.707 x amplitude = 14V
The power = V x V / R = 14 x 14 / 8 = 200/ 8 = 25W

If you were to double the supply voltage to ±40V, max power output is 100W.

In summary, the max output power = (bipolar supply voltage squared) / (2 x load resistance).
 
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vk6zgo

Joined Jul 21, 2012
677
When someone uses RMS Watts, we know what they mean. Don't get uptight over the semantics.

There is a simple way to estimate the max power output of a linear transistor amplifier based on the supply voltage.

Assume the load is 8Ω and the supply voltage is bipolar ±20V.
The maximum peak-to-peak voltage is 40V. The amplitude of a sinewave signal is 20V.
The RMS voltage is 0.707 x amplitude = 14V
The power = V x V / R = 14 x 14 / 8 = 200/ 8 = 25W

If you were to double the supply voltage to ±40V, max power output is 100W.

In summary, the max output power = (bipolar supply voltage squared) / (2 x load resistance).
I can call my cat a chicken,but I better not rely on the eggs!:)

"RMS Power" is a silly term that shouldn't be given any encouragement,no matter whether "we know what they mean",or not.

rogs is right about terms like "average watts","peak watts",etc.

The thing being described is power,which is measured in watts.

The correct Technical (& English) usage is "average power","peak power",etc.

Thus,we may say:-"The average power output of this amplifier is 25 watts."
 

wayneh

Joined Sep 9, 2010
16,402
The correct Technical (& English) usage is "average power","peak power",etc.
Rubbish. The word "power" can be omitted because it is implied.

I fill my car's gas tank with an average of 10 gallons and a peak of 17 gallons. I don't need to use the word "volume" because it is implied. A gallon is, by definition, a measure of volume just as watts is defined as a measure of power. It's not our job to keep restating that definition; we can imply it.

Another example: "I sleep on average 8 hours and a peak of 9 hours." It would sound silly to say "I sleep 8 hours time".
 
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endolith

Joined Jun 21, 2010
27
Also, you most certainly can have RMS watts, peak watts and average watts.
Yes, that's the problem. "RMS power" and "average power" can both be calculated from the power waveform, but they are different values, and the RMS value is useless. Please stop saying "RMS power" when you mean "average power". They are not the same thing.

If you put 10 Vrms into an 8 ohm load, the average power is 12.5 W, while the RMS power is 15.3 W. The second number is meaningless, but can be calculated.
RMS (root mean square) power, would have to be defined as the square root of the time average of the square of the instantaneous power, since this is what 'RMS' means. This could be done, but it is not the power as measured, and furthermore, it would have no technical significance - Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been
The RMS value of power is not the equivalent heating power and, in fact, it doesn’t represent any useful physical quantity - “RMS Power”
When someone uses RMS Watts, we know what they mean. Don't get uptight over the semantics.
No, people do not know what you mean. For example, see this thread, where someone is told that they need to calculate the RMS of the power waveform in Excel in order to compute the power factor: http://forum.allaboutcircuits.com/threads/help-using-ltspice-to-calculate-power-factor.84491/
 

endolith

Joined Jun 21, 2010
27
Is there a point to resurrecting a thread that died a year and a half ago?
Is there a problem with posting something to a thread that's relevant to that thread? People still find them in searches and read them even if no one's posted in them lately. If the information is confusing/inaccurate, it should be corrected for their sake.
 
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