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Old 20th Jun 2021, 11:40 pm   #2381
GrimJosef
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Default Re: The Audiophoolery Thread.

Quote:
Originally Posted by Synchrodyne View Post
... Let’s say for a specific example the curve oscillates between zero and 10 watts, for an average of 5 watts. But the points on the curve above 5 watts have a bigger effect on the RMS value than those below 5 watts. One may do a very rough approximation using just the (nine) pi/4 points of a cycle, namely 5, 8.5, 10, 8.5, 5, 2.5, 0, 2.5, 5 watts. The sum of the squares is 332, the mean of that is 36.89, and its square root is 6.07. That indicates that the RMS power number will be a little larger than the average power number ...
Analytically:

The power curve is 5*(sin(wt)+1)

So the square of this is 25*((sin(wt))^2 + 2*sin(wt) +1)

The mean of (sin(wt))^2 is 0.5, the mean of sin(wt) is zero and the mean of 1 is 1. So the mean of the whole term in brackets is 1.5, and the mean of the whole expression is 25 times this i.e. 37.5. The square root of that is 6.12.

Your estimate is very close indeed to this, but I'm afraid that we need a couple of corrections. We're really only supposed to count eight pi/4 points in the average (or, if you like, only half of the first and ninth points, as the other halves belong in the previous and subsequent cycles). This drops the total sum of the squares from 332 to 307. Also the power values at 5pi/4 and 7pi/4 are 1.5 watts in each case, rather than 2.5 watts. This drops the total by a further 8 to 299, the square root of which would be 5.76. That's still within 6% of the analytic result though, so not bad for what you said was 'a very rough approximation' .

Cheers,

GJ
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Old 21st Jun 2021, 3:53 am   #2382
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Default Re: The Audiophoolery Thread.

Thanks for that GJ. I must brush up on my simple subtraction – I couldn’t even get the right answer to 5 less 3.5….which made a very rough approximation rougher than rough.

It is interesting to note that if one does the approximation using only the pi/2 points, the answer comes out exactly. The points are 5, 10, 5, 0, for summed squares at 150, their mean at 37.5 and the root at 6.12. Funnily enough, that was my first thought, then I supposed that a closer approximation might be had by using the pi/4 points. I should know by now that endeavoring to simplify mathematics can be dangerous. Better to do the analysis properly from the start, as you have done.

Anyway, I think we have established that RMS power is a parameter that has little or no utility when it comes to measuring amplifiers, and that it is not the same as average power, and so not really a suitable proxy for the latter.


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Old 21st Jun 2021, 4:22 am   #2383
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Default Re: The Audiophoolery Thread.

Might I suggest that a MUCH more meaningful measurement of power is PMPO!!
After all an LM380 chip can output over 2000 watts.
Into what load I have no idea.
For what length of time I have no idea.
How many "samples" as they are digitally called I have no idea.
Using what power supply I have no idea.

RMS has served me well for close to 60 years!! I will continue using it, quoting the load
and the voltage
and the frequency.
That way I can avoid all those nasty big sums.

Joe
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Old 21st Jun 2021, 6:57 am   #2384
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Default Re: The Audiophoolery Thread.

Quote:
Originally Posted by merlinmaxwell View Post
Surly a square acre would be a volume, 9 million cubic feet ish.
I suspect that it would have to be four-dimensional.

It was something I once heard a reporter say on TV. A colleague called such statements 'Eejit markers'

David
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Old 21st Jun 2021, 7:23 am   #2385
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Default Re: The Audiophoolery Thread.

If you measure RMS voltage (and/or current) and use it to calculate power, then the sort of power you get is the mean power.

That's nice, it's simpler to understand and easier to say.

With AC signals, any talking of power needs a bit more saying about it to have any useful meaning. The power of an AC wave is continually changing. It goes to zero twice per cycle and it hits a peak value between these zeroes. If you want a number to define the power level, it has to be the peak or the mean. Peaks alone don't tell you how much work the signal can do without knowing the wave shape. So mean is probably the most useful if it has to be a single number and not a graph.

With a mean, you get the added complication of which particular time the mean is taken over. An integer number of cycles will produce the same result an integrating over infinite time. So that's a convenience. In ordinary instrumentation integrating time is simply slow enough that any uncertainty from truncated parts of cycles is small enough to neglect. Practicality 1, Mathematical nicety 0. Practicality goes on to the semi-final.

The M bit in RMS says that a mean has been taken, but by bracketing it between a root and a square says that the averaging was done in terms of power, not voltage. The S converts voltages to powers, the M takes the average, and the R puts you back to voltage.

There is a similar problem in using a spectrum analyser to measure noise. Most analysers don't tell you on the outside, you have to dig into applications notes, but most of them perform their mean function or averaging AFTER logarithmic conversion. This isn't the same thing as averaging the power in linear terms. Practically, it gives you a dB or so error in the displayed level of gaussian white noise. The levels of sinusoidal tones are not affected, so you need to know of this if measuring signal to noise ratios or phase noise.

David
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Old 21st Jun 2021, 8:15 pm   #2386
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Default Re: The Audiophoolery Thread.

OK, I concede most of you have concluded to your own satisfactions that RMS is a meaningless term.
Having said that, it has been in use for decades, and I think people generally had a feel for its meaning.
In the late '70s, my then wife had a Leak stereo 70, and it satisfied her ears and could fill our cottage with sound. I understood the "70" to mean 35 watts (rms) per channel, so decided to check it.
I connected a suitable resistor (15 ohms I think) across one channel with my scope across the resistor. I applied a 1kHz signal and wound it up until clipping set in. My calculation yielded 28 watts. I guess if the preceding stage had better biassed the output, something around 30 W would have been the result.
Considering it was heat generated, real, true watts, I reckon it more or less vindicated Leak's claims.
At a later date, I repeated this, loading both channels, but got lower readings, suggesting Leak's power supplies were not as good as their amplifiers.
Right or wrong, I had a feel for the numbers, as I am sure did many others. That lasted until the '90s when suddenly it became music power with a bigger number, then peak music power output with MUCH, MUCH, Much bigger numbers.
They tried to drag us to hell in their handcart. They lost the older ones, but captured all the children, and still have them.
Audiophoolery anybody?
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Old 21st Jun 2021, 8:35 pm   #2387
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Default Re: The Audiophoolery Thread.

Just as a note to this. In the late eighties I was working with audio, although in a field we don't discuss. My boss at the time was quite resolute regarding RMS watts not being a real measurement. I have to agree with him, and others on here that have discussed the maths, but then again I reckon being sniffy about the difference between RMS power (which I agree doesn't really exist) and power based on RMS volts (or amps) is way past what any consumer would ever understand, and is at least more honest than the PMPO or whatever other unit you might choose. Back in the late eighties or early nineties, there were amplifiers with local inverters into capacitors that could supply VERY short term voltage / current boosts that meant peak outputs could be way higher than the supply voltage and loudspeaker impedance could possibly support.
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Old 21st Jun 2021, 9:48 pm   #2388
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Default Re: The Audiophoolery Thread.

Regarding the Leak Stereo 70, for this Leak actually quoted a power output of 35 watts RMS into 8 ohms.

I suspect that that was derived from measuring the output (RMS) voltage across an 8 ohm resistor, and calculating power using V²/R. In that case the raw number would have been average power. But Leak just might have done the additional calculation to derive RMS power. 35 watts average translates to 43 watts RMS. Or alternatively, if the target were 35 watts RMS, that would require a continuous power output of (roundly) 28 watts. So perhaps a measured 28 watts continuous was consistent with Leak’s intentions?

In general, I am inclined to think that use of RMS power by amplifier makers back in the day was a misguided designation of what was really average power, in which case one could posit that there was actually an element of underselling. But the Leak Stereo 70 example makes me wonder. Quoting actual RMS power and calling it that is not overtly wrong, but in a world where end-users (and perhaps even reviewers, although they should have known better) de facto (and perhaps unwittingly) associated the term RMS power with the parameter average power (something that could have been known by amplifier makers), it could be somewhat misleading. Caveat emptor on the one hand, but – well - creeping into ‘phoolery on the other, perhaps.

A quick check of practice around 1970 plus/minus shows Leak, Rogers and Sugden all quoting RMS power, whilst Armstrong, Quad and Radford were quoting continuous sine wave power, using that or equivalent expressions. Radford also quoted volt-ampères for the SPA50.


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Old 21st Jun 2021, 10:22 pm   #2389
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Default Re: The Audiophoolery Thread.

Initially loudspeaker protection circuit for directly connected o/p stages monitored dc
imbalance or device failure, later what was termed "transient" protection was added which
monitored short duration signals of high amplitude. It could be argued were the relays
fast enough for the task.
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Old 21st Jun 2021, 10:56 pm   #2390
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Default Re: The Audiophoolery Thread.

I have always understood RMS to mean exactly "continuous sine wave power into a quoted and specific load and a particular frequency". Have I been wrong all these years?. At least when figures are quoted in this way they "should" be reproducible by anybody with the knowledge and equipment to do the tests.

Just as an aside, many amp manufacturers measured the power at the plate/s of the output bottles not the secondary of the (usually) transformer output. The Audiocyclopedia quotes it that way in some instances, and I have no problem understanding this either. Even if a silver wound audiophool output transformer was as good as could be it still has losses, LOTS of losses, so 35 watts may also be what is measured at the plate/s.

Joe
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Old 22nd Jun 2021, 3:50 am   #2391
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Default Re: The Audiophoolery Thread.

A look back through likely sources shows that Gordon J. King addressed the "RMS Power" issue in his book "The Audio Handbook" of 1975. A scan of the pertinent pages is attached.

King Audio Handbook Amplifier Power.pdf

Cheers,
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Old 22nd Jun 2021, 12:01 pm   #2392
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Default Re: The Audiophoolery Thread.

I seem to remember NAD as being a serial offender in terms of gilding the lily with regards to output. They claimed momentary peak outputs vastly in excess of the RMS rating. The peak OP made it look like a PA amp, until you read the smaller print. Not that I'm down on NAD - they have always come across as one of the few rational players in the market.
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Old 22nd Jun 2021, 12:52 pm   #2393
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Default Re: The Audiophoolery Thread.

'RMS' is to voltage or current exactly what 'Mean' is to power.

It's one of those things some people think make it sound better. A lot of people when thinking of screening always prefix it with 'Faraday'. They think it makes them sound more knowledgeable, but it's actually the reverse. A Faraday screen is a special type, designed to let ALL the magnetic component of an electromagnetic wave pass straight through/ Michael Faraday invented it to demonstrate that his new invention did not work by capacitive coupling, but by magnetism. Anyone experienced in designing screening will tell you that the electric fields are easy to screen, it's the magnetic ones which are right b'stards.

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Old 22nd Jun 2021, 1:22 pm   #2394
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Default Rf detector for audio

https://www.analogueseduction.net/sy.../vdhbleep.html

Thought this was quite clever albeit a tad expensive
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Old 22nd Jun 2021, 1:56 pm   #2395
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Default Re: The Audiophoolery Thread.

Quote:
Originally Posted by knobtwiddler View Post
I seem to remember NAD as being a serial offender in terms of gilding the lily with regards to output. They claimed momentary peak outputs vastly in excess of the RMS rating. The peak OP made it look like a PA amp, until you read the smaller print. Not that I'm down on NAD - they have always come across as one of the few rational players in the market.
I met Martin Borish back in Wharfedale days - he was visiting Cambridge Audio, which we were in the process of acquiring from the receiver. I think the undercurrent was he was trying to sell NAD, something he didn't succeed in for another 7 or 8 years.

I had no idea what NAD stood for, so I asked him "dead simple - New Acoustic Dimension".

Craig
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Old 22nd Jun 2021, 2:00 pm   #2396
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Default Re: The Audiophoolery Thread.

Quote:
Originally Posted by Synchrodyne View Post
A look back through likely sources shows that Gordon J. King addressed the "RMS Power" issue in his book "The Audio Handbook" of 1975. A scan of the pertinent pages is attached.

Attachment 236207

Cheers,
Great reference - thanks for posting that

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Old 22nd Jun 2021, 3:14 pm   #2397
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Default Re: The Audiophoolery Thread.

Quote:
Originally Posted by Synchrodyne View Post
A look back through likely sources shows that Gordon J. King addressed the "RMS Power" issue in his book "The Audio Handbook" of 1975. A scan of the pertinent pages is attached.

Attachment 236207
A well-written piece which tells it like it is, without any bias. King published it sufficiently long ago that it looks bad that people haven't yet all cottoned-on.

I suspect M G Scroggie had a few slightly sharper words to say on the subject.

David
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Old 22nd Jun 2021, 8:28 pm   #2398
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Default Re: The Audiophoolery Thread.

Reading #2388, I see that I was giving an output of "average power". It even looks as if my measurements (corrected by calling them average) were about right. That is a relief, after all these years.
Les.
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Old 22nd Jun 2021, 8:59 pm   #2399
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Default Re: The Audiophoolery Thread.

Quote:
A Faraday screen is a special type (of electrical screen)
I would go as far to say a Faraday screen is just that, no special type needed. As to the so called Faraday Cage, don't get me started. Unless he kept mice as pets.
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Old 22nd Jun 2021, 9:57 pm   #2400
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Default Re: The Audiophoolery Thread.

I was recently asked if this item was worth buying, a number of sellers have it, this one is priced at £309.68.
It is a degausser for CD's and DVD's, yes you read that correctly degausser!

Described as Degaussing of CD and DVD discs, it will revive high quality sound and image.
It is used by many recording companies.

I never knew of this, I must look in my old tool chest for my old Delta Gun degaussing wand and revive all of my CD collection?

https://www.ebay.co.uk/itm/124555900...Cclp%3A2334524

There is one for doing CD's, DVD's, Cables and Valves.
At 75, this degaussing of copper cables has made me rethink the old saying, never too old to learn something new.
https://www.ebay.co.uk/itm/274815129...Cclp%3A2334524
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