how to verify or cross check RMS watts of audio amplifiers

Discussion in 'General Electronics Chat' started by jatinparekh, Jan 2, 2012.

  1. jatinparekh

    Thread Starter New Member

    Jan 2, 2012
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    hello every one
    i am in process of manufacture transformer based amplifiers used in general PA systems
    how can i check that amplifier made is giving how much output in watts in RMS value ?
    any help will be very use full
    Thank You
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you have an oscilloscope and distortion analyzer??

    Since most don't, that is why companies have gotten away with ludicrous claims.

    Even a scope with knowledge of rail voltages will show when gross distortion sets in, you can do that with a signal generator on an audio range sweep.
     
    endolith likes this.
  3. crutschow

    Expert

    Mar 14, 2008
    13,030
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    If you are asking how you do the measurement, you use a resistor load equal to the rated speaker impedance and measure the maximum voltage across the resistance that does not exceed the rated distortion. You need to do this over the rated frequency range of the amp. The power is, of course, equal to V^{2}/R where V is the RMS voltage.
     
  4. AetherWerx

    New Member

    Dec 30, 2011
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    Also know there is no such thing as RMS power. If that is the claim then be highly suspicious of any rating.
     
    endolith likes this.
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The RMS (normally taken to be continuous sine wave) power rating for an amplifier is a long-established objective method of measurement, divorced from the advertising inflation and hype that gives us measures like Peak Instantaneous Music Power (acronym!).

    The RMS measurement has been in use for many decades: while it is true that normally power is power, and no reference to RMS is necessary, in this context we need to differentiate between the true average power (related to RMS voltage and current) and for instance the peak power.
     
  6. jimkeith

    Active Member

    Oct 26, 2011
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    You probably can do without a scope or distortion analyzer--this is how:
    Connect a dummy load, but also connect a small speaker or headphones via a series resistor to keep from blowing your brains out.
    Apply a low distortion 400hZ sine wave to input of amp.
    Increase signal level until you hear the sharp clipping sound (harmonics introduced)--yes, a trained ear can easily hear 1% distortion.
    At this point, measure output voltage and calculate power.
    RMS power is simply sales hype as power is power--much like "parts is parts"
     
  7. wmodavis

    Well-Known Member

    Oct 23, 2010
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    RMS power may in fact be a misnomer but it has a very important concept which differentiates it from the truly over stated peak power ratings used to define power in a way that marketing people prefer. Bigger number equals more sales equals more revenue.

    The idea behind 'RMS power' is that it more truly represents what the human ear is responsive to in a logarimithic fashion, i.e. the average power measured and computed using the RMS values of voltage and current in the load at or just before signal clipping occurs - or at a point defined where the total harmonic distortion (THD) reaches some stated value such as 0.1%, 1%, or even 10%. Of course you can achieve a higher stated power using a higher percent THD at the measurement values.

    If you measure the RMS current and RMS voltage into a resistive load and the THD is below the THD you specify, you will be able to compute the 'RMS' power. To convert that to peak power for advertising you double that value. If you want to go even further into inflated specsmanship you can double that again and call it peak-to-peak power. And another trick to get the number higher is to temporarily power the amplifier with a regulated DC supply that does not have voltage sag at max power output when you make the measurements.

    So the fictitious 'RMS power' does carry with it the more conservative and realistic method of rating power.
     
  8. jimkeith

    Active Member

    Oct 26, 2011
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    Of course you can use the term "PEP" that is a single sideband term for "Peak Envelope Power" This is significantly higher than average power that involves a modulated RF signal.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    At high SPL levels, many people don't notice until there is 5% or 10% harmonic distortion, such as at a concert or car audio.

    Simply opening it up and looking will give you a good idea of what it can do. Measure the rail voltages and you have it narrowed down quite a bit.

    If the rails show ±12V, you probably aren't going to get anything like 100 Watts from the 24V supplied, unless the amp is magically capable of driving a near short circuit load.

    Figure about $1/watt of decent sounding power when looking at a commercial product. If it's $25, it would be very odd for it to be 150Wx2.

    However, things are changing a bit in that area with Class D and Class G amplifiers, making subwoofer amplifiers cheaper, so it's a trust thing, and the audio makers haven't done much to gain buyer's trust lately.
     
  10. @android

    Member

    Dec 15, 2011
    178
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    RMS power is true. You must be talking about PMPO!
     
  11. AetherWerx

    New Member

    Dec 30, 2011
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    Sorry, there is no such thing as "RMS" power. It's average power and determined by the integral of the RMS current and voltage.

    If audio engineers wish to play games, and they often do, then that's their business. But they are technically incorrect and it does nothing but perpetuate incorrect information.

    I feel sorry for students leaning engineering.
     
    endolith likes this.
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Hey guys most of this 'discussion' is off topic.

    jatinparekh, welcome to All About Circuits.

    There are various standards round the world, such as the german DIN standard or the american SMPT standard for specifying and measuring the power of audio (and other ) amplifiers.

    These standards specify the conditions of measurement so others may check.

    You need to get hold a copy of the one your company wishes to use and measure in accordance with that standard.

    post again for more detail.
     
  13. @android

    Member

    Dec 15, 2011
    178
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    Yeah. Yeah. But unfortunately it has now became convention to say 'RMS power'. Means if you are gonna buy some audio system then the power rating(average) to be considered is RMS(measured correctly, but misnamed). And 'peak music power' is another marketing gimmick as everybody knows
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    I recently bought a Sylvania 2.1 powered speaker system for $24.99 as a "special buy" at a local store. It sounds great, looks good and is fairly loud. I use it for the TV in my computer room and the digital cable TV converter also plays many FM radio stations.

    In the ad it says 150W. On the box and owner's manual it says 75W RMS. On the power label it says 75W (so it is 100% efficient!). Inside it has a small power transformer labelled 9VAC/1.1A (9.9W). The amplifier is probably 50% to 60% efficient but might be a modern class-D switcher that is 90% efficient.

    Its amplifiers are two 16 pins DIL ICs with a small heatsink glued on.
    One IC is probably a stereo amplifier feeding 1.5W to 2.0W real Watts to each 4 ohm satellite speaker.
    The other IC is probably bridged and feeds 4.0W to 5.0W real Watts to the 6 ohm sub-woofer.
    So the total output power is 5.5W to 7W.

    Each satellite speaker has two 1.5" 2 ohm speakers with foam surrounds and the "soft-dome tweeter" is not a speaker but is actually a vent covered with a soft dome.
    The sub-woofer is a 5" long-throw speaker with a foam surround in a pretty big wooden ported enclosure. It lights up with 6 blue LEDs.

    The amplifier and power supply are built into the sub-woofer enclosure and has a volume control and bass and treble tone controls.

    I have played it loudly for hours and it doesn't smoke. I have never heard its amplifiers clipping. I tested its frequency response with my 66 years old ears and it plays from 40Hz to 18kHz smoothly and very well.
     
  15. rogs

    Active Member

    Aug 28, 2009
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    Maybe 'convention' - still not true. Aetherwerx is quite right. There is no such thing as an 'RMS Watt'.

    'RMS VA', OK if you must, but a Watt is a Watt. No 'RMS' -'Peak' - 'Music Power' or even 'Yeah Yeah' Watts - just 'Watts'.

    And as 'Watts' do nothing to explain the 'volume' of a sound system ( the usual reason it's used for!) it's an even more stupid analogy!

    This'll do as well as any to explain:
    http://www.hifi-writer.com/he/misc/rmspower.htm

    All the 'yeah yeahs' in the world don't change the facts.....
     
    endolith likes this.
  16. Audioguru

    New Member

    Dec 20, 2007
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    An amplifier power rating must say the frequency range for the power and the amount of distortion when the amplifier plays a sine-wave continuously. The load impedance must be stated.
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    As he points out in the article, Watts RMS Was created so there would be some standard:



    The author states that Watts RMS doesn't exist, then posts the definition of the standard used for a while to determine RMS Watts. \reflectbox{?}

    However, as Audioguru stated, you don't know the true power unless the waveform, frequency/frequency range, impedance, and tolerated distortion used for the test are stated. Good luck finding that on any box, or most user manuals of big box store specials.

    The best test is your ears. Though remember that distorted music has a tendency to sound "louder", since it can physically hurt at higher SPL levels, leading some to think they have more power. Non-Distorted music with wide dynamic range can hit "ear damaging" levels, without being literally painful.

     
    Last edited: Jan 6, 2012
  18. Audioguru

    New Member

    Dec 20, 2007
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    I agree.
    Sometime in the late 70's or early 80's I took a sound level meter into a disco. It measured 120dB most of the time but did not sound too loud because it was not badly distorted.
    Today large stores in malls that sell expensive clothes to teenage girls play hi-fi loud music at close to 105dB. One store was giving away a little "cube" speaker with every purchase. They did not disclose that the little speaker would be lucky to produce a frequency as low as 200Hz at 85dB.
     
  19. rogs

    Active Member

    Aug 28, 2009
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    No - he starts with the words 'The idea was' and finishes with 'This was misguided thinking even at the time'

    Not really the definition of a 'definition' ? :)

    But, hey -- this one has run for years and years, and will probably continue to do so.

    It's still wrong.

    -- and comments like 'yeah, yeah' when someone points the error out, are not really very helpful....
     
    endolith likes this.
  20. @android

    Member

    Dec 15, 2011
    178
    9
    It's obvious that 'yeah' 'yeah' will not change the fact. But first read carefully what i have written.
     
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