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Are watts all the same?

Discussion in 'Archived Threads 2001-2004' started by Ed O, Feb 26, 2002.

  1. Ed O

    Ed O Agent

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    I've read that they are not all the same...that some are "cleaner" than others. For example, in this thread:
    http://www.hometheaterforum.com/htfo...threadid=52005
    Saying that the 100 watts/channel really aren't 100 and the HK 50 watts are apparently better. How can I read specs to know what I'm looking at. There is no standardization to how manufacturers present their specs?
    Any info in this area is appreciated.
     
  2. RobertR

    RobertR Lead Actor

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    I would phrase your thread title differently. Of course all watts are the same. A watt is a watt. What varies is how an amplifier behaves into different loads or how its power output is measured. Some can double the wattage into a 4 ohm load compared to an 8 ohm load. Others can't. Some are measured over the full 20 to 20 kHz bandwidth. Others aren't.
     
  3. John Doran

    John Doran Screenwriter

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    ed,
    when you're looking at wattage ratings for amplifiers or receivers, make sure that it says "RMS 20hz - 20khz". this means that the amp is capable of outputting that much power continuously (RMS - root mean square) into every frequency over the audible bandwidth.
    there's also usually a number of ohms quoted (e.g. 100w RMS 20hz - 20khz into 8ohms), which means the amp can put that much continuous power into a speaker whose nominal impedance is 8 ohms. that number is sometimes 4 or 6, but is usually 8, since most speakers offer 8 ohms of impedance.
    if your speakers have a lower impedance than the number given in the amp's specifications, then you have to be careful that the amplifier is capable of generating the power necessry to drive speakers with that impedance - the lower the impedance of a speaker, the more power is required to drive it. if the amp doesn't have the power to drive speakers with impedances that low, then you run the risk of clipping the amp (see below).
    some manufacturers display the peak power capability of their product - basically how much power the amp can put out for a very short period of time (usually less than a second, i think).
    also, some manufacturers display the power output of their product into a specific frequency, like 1khz, which is also misleading.
    as for the "cleanliness" of the power, when an amp tries to generate a signal but lacks the power required to do so, it sends signal the top of which is "clipped" off (where the power ran out) - imagine a sine-wave, but with a flat top - which damages speaker voice-coil assemblies when they try to reproduce it.
    beyond that, there's the basic issue of the quality of signal generated by different amplifiers, which makes different amplifiers sound different (so some say, at any rate [​IMG]). but that's got nothing to do with the wattage of an amplifier in and of itself.
    hope that helps. i'm sure others will chime in with stuff i've forgotten or mis-stated.
    - jd
     
  4. GordonL

    GordonL Supporting Actor

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    My observation has been that, when it comes to multi-channel receivers/amps, if a manufacturer does not specifically state specs with all channels driven from 20hz-20khz, then don't assume that you're getting the full rated power with all channels driven. Most high-end receiver/amp manufacturers do use "all channels driven" in their marketing brochures. Likewise, all THX certified receivers/amps do because that's part of the spec. For all others, you'll have to rely on reviews from the various audio/HT magazines.
     
  5. mubashir

    mubashir Agent

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    I don't agree with GordonL.My reciever the STR-DB1070 got good reviews from What Hi-Fi and Home-entertainment(5 star).Neither one mentioned it being underpowered which is what it really is.Now I regret buying it and am now looking for a power amp to drive my fronts before they become more damaged.The reciever is rated 100X6 Watts at 8ohms(20Hz-20khz)THD 0.09%
     
  6. Tony Lai

    Tony Lai Stunt Coordinator

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    >My reciever the STR-DB1070 got good reviews from What Hi-Fi and Home-entertainment(5 star).Neither one mentioned it being underpowered which is what it really is.

    Yes but you don't all use the same speakers. I'm sure this thing will die if hooked up to Martin Logan panels yet it'll drive Kelly horns to distraction.

    Some magazines do mention power deficits. A recent Australian magazine stated categorically that the Marantz 5200, 6200, 7200 had problems driving sub 4ohm and under 88dB SPL speakers to decent volume. And that is true...

    The 5200 has 90w x5. The 6200 has 100w x 6 (AFAIK).

    Another American magazine stated that the Parasound 885 five channel 85w amp was more powerful than any other receiver they had ever tested and that included such luminaries as the Denon 4802.

    And lets not get into how much those watts sound better than a receiver.

    Why does a 85w poweramp sound better and run harder than a 100w receiver?

    T.
     
  7. Patrick Sun

    Patrick Sun Studio Mogul

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    Sound and Vision did disclose that the Sony DB1070's power output was over-rated after performing testing on the amp section. It was in the 60W/ch territory (IIRC) though it boasts a 100W/ch or 110W/ch power rating across 20-20KHz.
     
  8. David John

    David John Stunt Coordinator

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    Many receiver manufacturers state the power of all channels, but not all channels driven simultaneously.

    Meaning each channel can output 100 watts, but if all 5 or all 6 are driven simultaneously, they can only output say 60 watts or so.

    That is what that sound and vision test was showing.

    It comes down to the power supply and how much it can put out.

    So for 2 channle music, sure you will have 100 watts per channel, but with 5 or 6 channels you will not.

    However, there are very few times that the receiver will be trying to output full signals to all 5 or 6 channels.

    Maybe on some powerful explosion scenes or such.

    This is the reason the H/K receiver offer only 5 channels of amplification. because they know the limits of the power supply and are making sure it can output the current needed for the rated watts.
     
  9. Ted Lee

    Ted Lee Lead Actor

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  10. John Doran

    John Doran Screenwriter

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    also, don't forget to make sure your amp is correctly matched with the sensitivity/efficiency (not literally the same thing, but colloquially the same[​IMG]) of the speaker.
    if a speaker is relatively INefficient or INsensitive, the amp powering it will have to work harder to make it achieve the same SPL as a speaker that is MORE sensitive/efficient.
    and not all amps are capable of working that hard (i.e. generating and/or sustaining that kind of wattage.
     
  11. Larry B

    Larry B Screenwriter

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    In my opinion, there is far too much emphasis on amplifier wattage, and far too little emphasis on amplifier sound quality.
    Larry
     
  12. MatthewJ S

    MatthewJ S Supporting Actor

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    Funny thing; I often suggest to people that they buy very efficeint speakers when they have (or are considering) a mid to low-end receiver ,especcially when their penchant is to run their system loud for movies or music. Of course very efficeint speakers ,while ussually not causing an amp to distort, will tend to show the quality differances between one receiver's amp and another's . I think that many receiver mnfgrs are expecting that most people are running the lower-end units with the speakers set to small, so they tend (the mnfgrs)to not worry as much about their output...too bad
     
  13. Ed O

    Ed O Agent

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    Thank you for the excellent information. I posted this in the linked thread but figured since the conversation went this way it's also appropriate here:

    Does speaker efficiency affect this by taxing an amp less if the speaker efficiency is higher (I wouldn't need to turn up the volume as much on the receiver with more efficient speakers to produce the same volume from less efficient speakers)? Or am I off base here and the only important specs are the ohms in this case?

    What would happen if there is a switch on the back of the receiver selecting the ohms reading on the speakers you are using?
     
  14. Lee-c

    Lee-c Second Unit

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    Yes, more efficient speakers will give you more volume per watt. Therefore asking less of your amp

    to get to a given volume than would less efficient speakers.
     
  15. ling_w

    ling_w Second Unit

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  16. Kerry Hackney

    Kerry Hackney Stunt Coordinator

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    FYI, here is a little table that shows the effect of impedence on current.

    watts-res-amps-volt

    500-8-7.91-63.25

    300-8-6.12-48.99

    150-8-4.33-34.64

    100-8-3.54-28.28

    50-8-2.50-20.00

    25-8-1.77-14.14

    12-8-1.22-9.80

    5-8-0.79-6.32

    4-8-0.71-5.66

    3-8-0.61-4.90

    2-8-0.50-4.00

    1-8-0.35-2.83

    0.5-8-0.25-2.00

    0.25-8-0.18-1.41

    500-4-11.18-44.72

    300-4-8.66-34.64

    150-4-6.12-24.49

    100-4-5.00-20.00

    50-4-3.54-14.14

    25-4-2.50-10.00

    12-4-1.73-6.93

    5-4-1.12-4.47

    4-4-1.00-4.00

    3-4-0.87-3.46

    2-4-0.71-2.83

    1-4-0.50-2.00

    0.5-4-0.35-1.41

    0.25-4-0.25-1.00

    500-2-15.81-31.62

    300-2-12.25-24.49

    150-2-8.66-17.32

    100-2-7.07-14.14

    50-2-5.00-10.00

    25-2-3.54-7.07

    12-2-2.45-4.90

    5-2-1.58-3.16

    4-2-1.41-2.83

    3-2-1.22-2.45

    2-2-1.00-2.00

    1-2-0.71-1.41

    0.5-2-0.50-1.00

    0.25-2-0.35-0.71

    As you can see, halving the impedence doesn't quite double the current demand because current (I) = sqrt of (Watt (W) / Res (R)) Ohms law is V = I * R... You can't use wattage and current interchangeably. When you reduce the resistance of a speaker you ask the amp to deliver more current or amperage.

    Let's see if I can make this more confusing... When you half the impedence, the amp will produce exactly half of the wattage at the same current. Or 50 watts at 8 ohms is 2.5 amps, 25 watts at 4 ohms is 2.5 amps. Now, an amp with a very robust voltage supply that can deliver 20 volts at both 8 ohms and 4 ohms can deliver twice the wattage at 4 ohms as at 8. Thats where you see that manufacturers rate their amps as more powerful at 6 ohms or at 4.

    Bottom line, if you want to drive a lower impedence, the amp has to be able to deliver more voltage/current and dissipate the associated heat without melting down.
     

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