Converting Average Power to "RMS Power"

[Cursitor Doom]

Hi all,

This subject continues to severely bug me. I've just read some audio amp
servicing/repair book where the author states that customers often find
it desirable to know an amp's actual output power, and to return an amp
having measured its output power and noted it down for the customer's
info. The trouble is, customers are familiar with "RMS power" - but
there's no such thing! Irms X Vrms = AVERAGE power and that is all that
matters. BUT - try telling that to a typical customer!

So it must all come down to perception. Customers have an idea in their
minds what say 70WRMS sounds like through listening to dozens of various
VRMS labelled amps over the years. So there ought to be some kind of
notional, informal 'conversion factor' between Waverage and "WRMS" we
could adopt to satisfy them without compromising on the accuracy of the
correct measurement, n'est pas? What would that conversion factor be?
Greater or less than unity for a start?
Your thoughts invited.

[jurb...@gmail.com]

Well, we drive on parkways and park on driveways. the macula in your eye used to be called the fovea. Double density floppy disks had half the capacity or the high density disks.

Chalk it up to that.

Other than that "RMS power", no matter how nonexistant, is;

V * 0.707 ^ 2 / R

Might piss you off, but hey, what does a five pound can of coffee weigh these days ? And they say you can use less, bullshit. Do that and you'll be drinking dishwater. You are not supposed to see through coffee. And lowfat milk, no thanks, I got water. Go ahead "Oh they don't just add water", damn right, they are selling what they take out of it. Kinda like salt, they make more money from what they refine out of the salt than the salt itself.

This mythical RMS power is the result of the FTC trying to put a lid on ridiculous power claims back in the 1970s. It only has any real teeth when the power is stated in a certain way. Thye can rate a unit 2,000 watts on a 2 amp fuse, as long as they do not claim it as an actual, legal power rating. Note that the regulations do not apply to commercial or auto equipment, or to other than the front two main channels of a surround amp.

Now there are commercial amps lying their (_|_) off about power. Says 1,000 watts and I get into it and there is a little TO-220-15 package IC with a fan on it smaller than the one in my PC.

Same with cars. Do you really think they are 5,000 watts ? At 100 % efficiency it would have to pull over 400 amps. Even my 12 : 1 455 luxury racecar engine with a sloppy timing chain didn't pull that to start hot. For a car battery, 400 amps is a "cold cranking amps" rating. And alternators ? Change one lately ? They cannot handle anywhere near the current they used to, it is right on the box "DO NOT ATTEMPT TO CHARGE A DEAD BATTERY AS PERMANENT DAMAGE WILL OCCUR, CHARGE BATTERY BEFORE USING ALTERNATOR. What ? Know what that means ? That means if you leave your light on and need a jumpstart, it might wreck your "new and improved" alternator. And that suits "them" just fine. (I think I prefer the old and shitty ones that worked reliably)

Now some of these guys DO have a second battery and alternator just for the amps, but still. Does anyone even know the gauge of wire needed to conduct 400 amps ? They got fat wires in them cars but not that fat.

So there is a formula for it though it does not exist. Well, neither does the square root of -1.

[Julian Barnes]

On Sun, 15 Nov 2015 08:20:09 -0800, jurb6006 wrote:

[...]

Oh boy. Are you going to stir up a hornet's nest with that post! ;->

[M Philbrook]

In article <n2a55v$5lf$2...@dont-email.me>, cu...@notformail.com says...

Don't get confused, RMS and AVG is not the same

While RMS is working in the 0.707 of the peak, AVG is
in the 0.637 or there abouts.

Since Watts = 1 J(joule)/ Second we have this to look at;

Inject a 1 Hz tone into an amp from a clean sinewave source with
1 Volt Peak coming out going to a 1 ohm load. How do you
measure Watts if Watts involves a J value over a 1 second period?

We know that the SINE wave isn't always at full level of 1 volt during
that time period so that would indicate that we don't have 1 WATT
of energy there, so what do you do ?

In retrospect that, you need to use voltage in your calculations
to form a WATT value at some point. Wouldn't be logical to
use RMS or AVG as the figure, one being a little different than the
other?

Others will argue the fact that RMS power does not exist, I think they
may have fallen off the cliff, turret, edge, mountain what ever it is
they like to hang out on.

When I was schooled in electronics(Long time ago), starting at lower
levels, through highschool, Post grade in a trade shop, college and
adult lectures, there were never any discussion of RMS/AVG power ever
not existing, because of the roots of how it is derived.

I just flip my eye brown when I hear otherwise and move on.

There are some twisted use of Peak, Peak To Peak power for sales
only and they can get away with it. Only because in retrospect, it is
valid, just the end user is getting fooled by the lack of reallity.

To add to this, doing power calculations in AC does bring in the
phase angle but we get back to the power level of WATTS/second which
then puts a fork into it, if you really think about it. So summing all
the points of a sinewave when doing AC power, still comes out back
to what we were talking about above.

P = I cos(V);do that for all 360 points or even closer if you wish and
see where that leads to.

Just my opinion, really! :)


Jamie

[Jeff Liebermann]

On Sun, 15 Nov 2015 12:43:26 -0500, M Philbrook
<jamie_...@charter.net> wrote:

> Just my opinion, really! :)
>Jamie

Perhaps this might change your opinion:
<http://www.eznec.com/Amateur/RMS_Power.pdf>


--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Ralph Mowery]

<jurb...@gmail.com> wrote in message
news:cef93438-dab4-4001...@googlegroups.com...

>Well, we drive on parkways and park on driveways. the macula in your eye
>used to be called the fovea. Double density floppy disks had half the
>capacity or the high density disks.

Chalk it up to that.

Other than that "RMS power", no matter how nonexistant, is;

V * 0.707 ^ 2 / R

Might piss you off, but hey, what does a five pound can of coffee weigh
these days ? And they say you can use less, bullshit. Do that and you'll be
drinking dishwater. You are not supposed to see through coffee. And lowfat
milk, no thanks, I got water. Go ahead "Oh they don't just add water", damn
right, they are selling what they take out of it. Kinda like salt, they make
more money from what they refine out of the salt than the salt itself.

This mythical RMS power is the result of the FTC trying to put a lid on
ridiculous power claims back in the 1970s. It only has any real teeth when
the power is stated in a certain way. Thye can rate a unit 2,000 watts on a
2 amp fuse, as long as they do not claim it as an actual, legal power
rating. Note that the regulations do not apply to commercial or auto
equipment, or to other than the front two main channels of a surround amp.

Now there are commercial amps lying their (_|_) off about power. Says 1,000
watts and I get into it and there is a little TO-220-15 package IC with a
fan on >it smaller than the one in my PC.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

There seems not to be any real standard for audio power.

Someone needs to define a standrd something like input a sine wave into the
amp and get so much RMS volts (undistorted or a % distortion) across a
resistive load for a period of time like 60 seconds. Do that at several
frequencies across the audio range with one being the lowest and another the
highest frequency the amp is rated for.

The 5 pound bag of sugar is now 4 pounds and the 2x4 boards seem to be
srinking along with the thickness of plywood. A number of years ago I
bought some "2x4"s and nails that just would not go all the way through them
when the 2" sides were layed on top of each other. A while back I bought
some new 2x4s and the same nalis went through by about 1/16 of an inch.

Mother had recipeas that called for a can of something. Now the size of the
cans are smaller and difficult to convert so a cake comes out the same.

[et...@whidbey.com]

You are all wrong about those amps for cars. All you need to pull 400
amps from 60 watt 14 volt alternator is 4 gauge oxygen free gold
plated Litz wire. Don't be fooled by claims that other than Litz wire
can be used. And the gold must be 26 carat, that 24 carat stuff just
won't pass the current without high losses.
Airick

[M Philbrook]

In article <_-OdnWKC25EnetXLn...@earthlink.com>,
rmower...@earthlink.net says...

The reality of all this is, a clean sinewave at 1 Hz over a 1 second
period to get WATTS is the average of the sinewave.

If you sum all values from the base line to the peak of a sinewave then
divide by the number of samples you used, you'll get 63% or 0.634 which
is really not too far from using RMS which is 0.707.

Although Technically, RMS is really not a proper measure in this case
because it is not truely accurate for use of power but the average is.

Like I said, if some one is using the RMS term for power over the
average it really isn't that much more, even though its a little higher
than it should be.

In Ac power measurements, you specify the phase angle, however, that is
only a fraction and must be considered when calculating Watts. You need
to consider the whole frame time which is why using the average is the
apperent answer.

I prefer to blow it off and just accept the fact that I know the
difference and that isn't enough to start a war over.

So when some one says RMS power, * that by 0.9

now to really thow you off, the RMS of a squarewave is the same
as the Peak and also the average is the same.

And if you have triangles, the RMS = Pk*0.577 and AVG := 0.5 * PK.

You can see it's obvious there with the AVG how that works out.


Go figure :)

[Cursitor Doom]

On Sun, 15 Nov 2015 12:43:26 -0500, M Philbrook wrote:

> Others will argue the fact that RMS power does not exist, I think they
> may have fallen off the cliff, turret, edge, mountain what ever it is
> they like to hang out on.

"RMS power" is a misnomer and doesn't represent anything useful (unlike
RMS volts and RMS amps).
It sounds to me like it's YOU who's fallen off the cliff of reality on
this one. ;->

[Phil Allison]

Cursitor Doom wrote:
>
>
> This subject continues to severely bug me. I've just read some audio amp
> servicing/repair book where the author states that customers often find
> it desirable to know an amp's actual output power, and to return an amp
> having measured its output power and noted it down for the customer's
> info. The trouble is, customers are familiar with "RMS power" - but
> there's no such thing! Irms X Vrms = AVERAGE power and that is all that
> matters. BUT - try telling that to a typical customer!

** The term "watts RMS" has been used in relation to audio amplifier specs for many decades, since the 1960s at least.

It is a shortened expression which indicates the long accepted method of audio power measurement using a continuous sine wave into a resistive load. The figure quoted is AVERAGE watts.

Other tests exist using either the peak value of the sine wave or short bursts of sine wave, generally know as "peak power" or "music power" - the figures quotes are not average watts, but represent the maximum power available on a short term basis for music and speech.

In nearly all cases,"RMS power" and "Watts RMS" have the same meaning.

Those who say otherwise must think that "Steak Sauce" contains actual steak.


... Phil

[M Philbrook]

In article <n2b8tj$jqp$1...@dont-email.me>, cu...@notformail.com says...

What a puts...

You don't get out much do you?


Jamie

[gregz]

There is a very good output measurement called voltage. Power is just a
byproduct. Power more related to linear cost of product. Forget power.

Greg

[stra...@yahoo.com]

You are correct on 'average' power but SO WHAT? Measure the average power and tell him it's RMS power. The method you're using is what all the reviewers use so where is the problem? You're being a 'purist' and it will get you NOTHING. Don't shoot yourself in the foot and then complain about the pain.

FWIW I have a little Technics receiver that the manual says is 40 Watts / channel. When I ran it up it sure SOUNDED like more than 40 Watts. When I measured the power supply Voltage of +/- 42 I had my answer. It IS more than 40 Watts under 'music' conditions. Am I upset because the 'lied' about the power? Nope, I'm just fine with it.

G²

[jurb...@gmail.com]

>"You are all wrong about those amps for cars. All you need to pull >400
>amps from 60 watt 14 volt alternator is 4 gauge oxygen free gold
>plated Litz wire. Don't be fooled by claims that other than Litz wire
>can be used. And the gold must be 26 carat, that 24 carat stuff just
>won't pass the current without high losses.
>Airick "

So you are well versed in new math. How's them boffo low unemployment numbers and non-existent inflation !

Late 1970s and thereabout I used to build car amps. The design was like the mindblower amps but without the problems. The original amps would fry an output which would catch on fire and burn the cone of the speaker. I didn't care, all I wanted was the driver transformer. I found output chokes in old tube amps. Anything that had 4, 8 AND 16 ohm taps was fair game for me. I bet I scrapped some valuable amps, I mean maybe could retire on today.

Basically push pull with the center tapped output choke, one output would pull to the 12 volt rail and the action of the choke sent the other side to negative 12 volts or as close as possible depending on the efficiency of the transformer.

Theoretically that is 24 volts peak which by the math gets you about 30 REAL watts per channel. Not really efficient either, the ten amp fuses would blow from fatigue maybe once a month.

But it blew people's speakers. They say "How is that possible, these things are rated 90 watts each" to which I replied "Well these are OMWs, Old Marantz Watts". I went on to tell then that these amps 30 WPC is like the 30 WPC in your house stereo. And that was alot for a car. Remember, most people were listening to 2½ WPC, or if they were lucky, 9 WPC.

The BTL outputs had about 8.99 WPC, the single ended choke output DElcos did anywhere from 5 WPC to maybe 7 or so depending on how they're biased. The ones with the big ass DS-501 (IIRC) transistors came closer to the 9 WPC but really, the THD got pretty high being single ended.

Hmm, I bet the people who like tube sound and will discard a nice ass Sansui solid state amp for a single ended 6BQ5 based amp would really like the sound of those old Delcos.

Later, Delco got their shit together and threw together some really nice radios. Excellent everything, the DM-165 and high output chips with dynamic power limiting, and then IIRC the DM-185 had the power limiting independent for each of the FOUR channels. Made it sound a hell of alot better. And the tuners were unparalleled, seriously. If I want s tuner for in my house I want one of them. Pick up a stick in the mud from China, in fucking stereo ! And the cassette decks were good as well, with one variety in the higher end cars built by Blaupunkt. (type "D" they called it) Also, the ones with the EQ built in had the low and high controls still as normal bass and treble, which is how I think it should be and have modified EQs to be like that.

Now that I think of it, there are a few EQs out there I modified that should have the frequencies relabeled. Anyone ogt a Dymo label maker ? LOL

[jurb...@gmail.com]

>"FWIW I have a little Technics receiver that the manual says is 40 >Watts / channel. When I ran it up it sure SOUNDED like more than 40 >Watts. When I measured the power supply Voltage of +/- 42 I had my >answer."

Yup, 42 volt rails will give you a hair over 100 WPC, as long as they maintain 42 volts. But you do have it for a matter of milliseconds. that is called dynamic headroom. Some manufacturers gave that rating and it was based aon a short term bust of a sine wave, specified in milliseconds. However there is another factor about these "Old Marantz Watts" or OMWs as I like to call them sometimes.

The older amp topologies and the devices available limited them somewhat because of non-linearities. These happened worst when you got near clipping. Bottom line, your 40 WPC Tecnics or whater really IS about 75 WPC, but at like 2 or 3 % THD.

Later, they made really good square waves. By that I mean they got the linearity closer to the rails and that is why the older stuff sounds better. A 40 WPC amp now might only really be 50 WPC, because they could keep it linear though more of its high power range. That's why if I still had my old Marantz I could sell it for a grand, which I do believe is more than it cost new. (but not by all that much)

But yes, when you get older amps and receivers, don't be surprised if they even have double the power claimed. That would be 3 dB, that is all. The ones, maybe I can find some, that gave dynamic and clipping headroom specs could showya. If it has 1.8 dB clipping headroom and then on top of that 2 dB dynamic headroom, that amp is going to sound louder than its RMS power rating would allude to.

Only thing you can do is crank it up and enjoy, and hope your speakers can handle it. You really can't sue them for putting too much power in the amp.