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Old 14th December 2012, 11:51 PM   #21
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Quote:
RMS watts, or Root Mean Square, is a more reliable form of watts in an amplifier.

RMS power ?
Often stated, there is some misconception when mentioning RMS for Watts. They have no real meaning.
Just write Watts, nothing else.

Peak power ?
Speaking of Watts, many people forget that they are only calculated, the values which are really measured are Volts. And the conversion of Volts to Watts is is often misleading .

Lets say a 8 V RMS voltage applied to an 8 Ohm load.
The power is 8/8 = 8 W
The instanteneous peak voltage is 8*2^0.5 = 11.3 V.
Can we say the the peak power is 11.3/8 = 16 W ?
I don't think so. Power needs some time to show its heating effect.
There is no reference of time elapsing in an instantaneous voltage.
Why? If I have 1V and 1A running through a resistor, 1W will dissipate no matter how long it stays that way. P=J/s so even if the time is tiny and the total energy is tiny, the wattage is still the same. Am I missing something here?

Since the impedance of the speaker is varying with frequency, a graph showing output avarage continous wattage vs. impedance would give pretty correct values? Can it get more honest than that?
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Old 15th December 2012, 12:02 AM   #22
wahab is offline wahab  Algeria
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Originally Posted by DF96 View Post
By combining all these measures you can boost amplifier power by a factor of between 5 and 10, without actually telling any lies.
I Guess that it wasnt enough , a sine , even distorted , has less area
than a square signal , hence , PMPO , pulse mode or pure marketing
power output , will enhance 5W amplifiers or so to a 80W rating...
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Old 15th December 2012, 12:56 AM   #23
Pano is offline Pano  United States
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You can set all the good standards you want (they do exist) but it will never stop the salesmen from lying. That the way of business.

But just because salesmen lie, that does not mean you have to abandon or ignore the standards. Most of the standards are very useful and well thought out. Most salesmen lie. All the hand wringing in the world will not change either fact.
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Old 15th December 2012, 10:12 AM   #24
AndrewT is offline AndrewT  Scotland
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Originally Posted by donpetru View Post
Your existing amplifier power calculation .............
Why have you quoted my post?
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Old 15th December 2012, 11:48 AM   #25
DF96 is offline DF96  England
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Quote:
Originally Posted by forr
Can we say the the peak power is 11.3/8 = 16 W ?
Yes we can. It may be misleading (perhaps deliberately) but it is certainly true. If the sine wave was very low in frequency when compared with typical thermal time constants of the load then we would need to ensure that the load could handle 16W without damage.
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Old 15th December 2012, 12:18 PM   #26
AndrewT is offline AndrewT  Scotland
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Can we assume that when quoting peak or instantaneous powers that the unit should be Wpk?

In the above post, 11.3Vpk giving rise to 16Wpk.
Is that the correct way to describe?
Is that a way to avoid ambiguity?
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Old 15th December 2012, 01:09 PM   #27
DF96 is offline DF96  England
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The unit of power is watts - W, whatever power is being discussed. I think I would call it peak instantaneous power, as that is unambiguous. The pk is not part of the unit, but a helpful descriptor.

Strictly speaking, 11.3Vpk is incorrect. It should be Vpk=11.3 V. Vpk is the name of a particular voltage. V is the unit in which it is measured. So we could say Ppk=16 W.
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Old 15th December 2012, 01:32 PM   #28
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One channel driven, both channels driven, continuous or 3 cycle tone burst, 1 kHz or over the whole audio band, too much wiggle room for cheating. If you really want to get clever you can apply a tolerance to the voltage measurement so you have a nominal and a minus band, then spec everything at the nominal. Real units will all operate at the minus limit of voltage, and thus far fewer watts because of the squared term. You just have to use your imagination in this business.
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Old 15th December 2012, 01:40 PM   #29
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Worst still, i have seen amplifier rated only its input power, nothing mentioned about output.
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Old 15th December 2012, 01:46 PM   #30
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Quote:
Originally Posted by AndrewT View Post
Why have you quoted my post?
I quoted your post because I wanted to refer to the formula that you recommended to use after reading the voltage on the speaker. I'm talking about mathematical formula P = V ^ 2 / R (formula where do you refer, somewhat indirectly).

In the formula above, some manufacturers of professional amplifiers employ the notion of peak to peak voltage instead of rms voltage. For example, I have a stereo FFH-886 manufactured by LG. LG says it has 2 x 233Wrms. I measured the output power and it is 2x80Wrms. It is based amplifier STK 412-040.
And there are many such examples.
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