Thanks for the nice words, Kijanki. But you’ve explained it well, IMO, as did the summary quoted above by Imhififan. And I of course agree with your point. As you indicated, the references to RMS power or watts RMS that are often seen are actually references to average power, which are calculated as the product (multiplication) of RMS voltage and RMS current (assuming voltage and current are in phase with each other). And as you and the reference provided by Imhififan both said, RMS power is certainly something that can be calculated for a given power waveform, but it would be a different number than average power, and it would be a quantity that has no relevance or usefulness. While average power, being proportional to heat generation, is relevant and useful.
Widespread usage of incorrect terminology doesn’t make it correct. Just as widespread misuse of the English language doesn’t make it correct. For example, many people would say that they could care less about this topic, even though that is the opposite of what they mean :-)
Best regards,
-- Al
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Hi Roger, I found the Wikipedia page from which Kijanki quoted (the link opens near the paragraph he quoted): https://en.wikipedia.org/wiki/Audio_power#Continuous_power_and_%22RMS_power%22 RMS Power doesn’t vary as a function of time, instaneous power does. To be sure it’s clear, I absolutely did not say that "RMS power" varies as a function of time, although I can see how my statement might have been misread. I said that... ... what the words "RMS power" literally mean is the RMS value of a waveform that defines how power varies as a function of time. (The "waveform defines how power varies as a function of time"; the RMS value of that waveform of course does not define how power varies as a function of time). Again, what Kijanki has been addressing is terminology, nothing more. Regards, -- Al |
Hi Roger,
To use your example of 28.28 volts RMS into 8 ohms, corresponding to 100 watts, the corresponding RMS current is 28.28/8 = 3.54 amps.
Assuming a sine wave, the peak voltage is 28.28 x 1.414 = about 40 volts.
The corresponding peak current is 3.54 x 1.414 = 40/8 = about 5 amps.
The corresponding instantaneous peak power is 40 x 5 = 200 watts.
What Kijanki is saying is that the term "RMS power," if strictly interpreted, would imply 200 watts peak x 0.707 = 141.4 watts RMS. But of course what is really being referred to when that term is used is the product of RMS voltage and RMS current, which as you indicated is 100 watts in this example.
So the widespread use of the term "RMS power" is, strictly speaking, a misnomer. That is Kijanki’s point, with which I agree.
Regards,
-- Al
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That would be instaneous power or instaneous heating, agreed? Yes. The RMS ... defines its DC heating value. Yes. The RMS also defines how power varies as a function of time ... Well, if the waveshape is known, for example if it is known to be a sine wave, then of course one can determine the amplitude of that waveform from the RMS value. But I don’t see how my statement that you were referring to is contradictory, as you claimed. My statement, once again, being as follows: ... what the words "RMS power" literally mean is the RMS value of a waveform that defines how power varies as a function of time. Honestly, I think you may be misreading my statement. Would you care to tell me why an amplifier producing 100 watts RMS is any different than an AC generator producing 100 watts RMS. The 100 watts is the same in both cases, of course. I have never said anything that is in any way to the contrary, and as far as I can recall neither has Kijanki. And then please where 141 watts comes from. What kind of inbetween power is that to be called? As I have said, it is the RMS value of a sinusoidal power waveform having a peak value of 200 watts. There are 18 complaints about this article on the talk page of which 13 and 14 pretty much kill the whole thing. If kijanki learned this from articles like this then he is in the same mistaken camp. Shall we leave him there? The fact that the article has numerous flaws has nothing to do with the subject matter of this thread. And Kijanki is a very experienced engineer who doesn’t need any such help. I have nothing further to say on this matter. Regards, -- Al |
Hi Roger,
All of us, certainly including Kijanki who is very knowledgeable technically, completely understand and are in complete agreement about the technical aspects of what is being discussed, i.e., the "basic electronics" and the math you referred to.
What he is going on about is simply the terminology that is being used.
And his point is that strictly speaking what the words "RMS power" literally mean is the RMS value of a waveform that defines how power varies as a function of time. Which would be 141.4 watts in your example, while of course 100 watts (the product of RMS voltage and RMS current) is what is actually being referred to when that phrase is used.
Regards,
-- Al
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Hi Roger,
I haven't read any of the Wikipedia article besides the paragraph Kijanki quoted in his post, and I don't consider whatever flaws the article may have to be relevant to Kijanki's point about terminology, especially given the further explanations of his point that I have provided.
I provided a link to the article simply because you requested it, and I was able to find it quickly.
Regards,
-- Al
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Al, who you appear to agree with. thinks 141 watts is the average power of 200 watts peak. He has said so. No, I have not said that. I have said that 141 watts is the RMS value of a sinusoidal power waveform having a peak of 200 watts. "RMS" in the sense of a mathematically calculated root-mean-square. And in saying that I certainly recognize that the heating which occurs in that example corresponds to 100 watts, not to 141 watts. On the other hand, though, in citing the 141 watt figure I overlooked the fact that the product of a sinusoidal voltage and a sinusoidal current is not sinusoidal, since it never goes negative in the case of a resistive load. And correspondingly positive power is being delivered to the load at all times, other than at the zero crossings. So I believe the 141 watt figure should be, per one of Kijanki’s posts early in the thread, 0.61 x 200 = 122 watts. Also, Roger, a **much** better paper on the subject than the Wikipedia writeup Kijanki referred to is the one Imhififan linked to in a post early in the thread: http://eznec.com/Amateur/RMS_Power.pdfThat author’s conclusions: It should be noted that the term “RMS power” is (mis)used in the consumer audio industry. In that context, it means the average power when reproducing a single tone, but it’s not actually the RMS value of the power. Summary: I’ve shown that: -- The equivalent heating power of a waveform is the average power. -- The RMS power is different than the average power, and therefore isn’t the equivalent heating power. In fact, the RMS value of the power doesn’t represent anything useful.
--The RMS values of voltage and current are useful because they can be used to calculate the average power.
Imhififan also provided the following reference early in the thread, which again is highly supportive of Kijanki’s position: http://www.n4lcd.com/RMS.pdf In any event, I agree with PTSS that Ralph’s (Atmasphere’s) advocacy of a pragmatic outlook on this issue (see his post dated 6-30-2017) is well stated and appropriate. But in going forward on that basis it seems to me that at the very least we should also acknowledge the legitimacy of Kijanki’s point, that based on a strict interpretation "RMS power" does not equal the product of RMS voltage and RMS current, and therefore does not correspond to the heating effect of a given amount of power. Regards, -- Al |
Ramtubes 12-10-2018:
We still have to agree on one thing. The heating watts of a 100 watt amplifier is 100 watts. It is measured by V rms sq/R load.
Yes, of course. As I said earlier: Almarg 12-10-2018:
I certainly recognize that the heating which occurs in that example corresponds to 100 watts, not to 141 watts. Ramtubes 12-10-2018:
There is no RMS of power.... Power determined by RMS voltage. We dont RMS it again....
... If we want to be perfectly clear we should say:
100 Watts (measured by RMS voltage of an undistorted sinewave into a resistive load) .Then then there is no abiguity....
Agreed. Imhififan, thanks for providing the additional link. Regards, -- Al |
Thanks for the gracious post, Roger.
I wish we were sitting around a table drinking and having more fun with
this. These topics are interesting to discuss, we all learn from those
who can best express their ideas calmly and rationally.
I
would like to have Imhififan at the table. I have enjoyed how he said
little but kept coming back a source that cleared it up for me.
+1. And regarding Imhififan's posts, I've participated in quite a few threads to which he has contributed, many involving technical matters, and as far as I can recall every one of his posts has been spot on. And yes, it would be fun to kick these things around while "hoisting a few," as the saying goes. And if I weren't on the opposite coast I would certainly be tempted to attend some of your presentations. Best regards, -- Al |
bdp24 12-12-2018:
Al, as you can’t attend any of Roger’s presentations, have you watched
the two videos on You Tube of his seminars at The Burning Amp Festival?
His and Nelson Pass’, that is. Great stuff! It's a pleasure to own
products by both these great designers. Thanks, Eric. I had seen those mentioned in Roger's "Ask an amplifier designer" thread, and I've watched the video of Roger's presentation, as well as the several short videos on his Berkeleyhifischool.com site. All very informative and well presented. I haven't watched Nelson's yet, although I certainly intend to, in part because I recently purchased an XA25. (I've been delighted with it, btw). Roger's hifi school videos in particular pack an amazing amount of good information into their short lengths. Thanks again. Best regards, -- Al |
