Understanding power rating

Discussion in 'AV Receivers' started by Joe.P, Oct 30, 2003.

  1. Joe.P

    Joe.P Auditioning

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    I was just reading a review of a new Kenwood reciever that has rated power of 100wx6, however the article goes on to say that when all channels are driven power was limited to only 35w. Is this a bad thing, or is it typical of this price range($600).
    Some other questions for the more knowledgeable:
    How can you determine this kind of drop in power from the specs?
    Will a more expensive reciever with this same power rating not have this drop in power?
    This reciever has a THX Select rating. Doesn't that mean that the amplifier has to be of a certain quality or capability?

    I'm not interested in purchasing this model, just trying to learn about how to make the best choices for the money.
     
  2. John Garcia

    John Garcia Executive Producer

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    THX select I don't think certifies the amp section, only that it has certain processing features. Look it up at THX.com.

    This is an indicator of a relatively weak power section, but you also have to understand that it will be rare for a receiver to have to deliver it's full rated power to all channels for any length of time. Depending on the speakers, 35w may be enough to reach reference levels without distortion.

    A more expensvie receiver will definitely not have the same drop in power, but that does not mean it will not have a drop when all channels are driven.
     
  3. Michael R Price

    Michael R Price Screenwriter

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    You can't tell the real performance from specifications although some manufacturers are notorious for lying about them. Heavier and more expensive products tend to hold up better against continuous heavy usage, but there aren't really any guaranteed except official measurements. I would not pay much attention to THX classifications.
     
  4. Joe.P

    Joe.P Auditioning

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    So can you assume that the 100Wx6 power delivered by $400 system are not of the same quality as 100Wx6 power delivered by a $1000 system?

    At what price do you start to really realize the capability of the amp?
     
  5. altan

    altan Stunt Coordinator

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    I don't know if this is true... but I've always wondered how a 100Wx6 receiver can have a power supply rated at 300 Watts.

    You might consider looking at the watt rating on the back near the power supply. You mileage may vary...

    I had a Yamaha 52xx (cannot remember) and my left front speaker blew up when watching StarWars E2 (space ship flying by at the very beginning). That ticked me off. It was rated at 100 watts per channel, but I seriously doubt it was outputting near that.

    ... Altan
     
  6. John Garcia

    John Garcia Executive Producer

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    If your amp is rated to 100w, and you have a scene that required ALL of it, then you are almost certainly going to blow speakers because you will run out of current and clip. It is tough to blow a speaker by driving it with too much power, while it is very easy to do with not enough power.
     
  7. Bob McElfresh

    Bob McElfresh Producer

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    There is like 4/5 variables that come into effect when measuring power.

    Look on the Yamaha web site and you will see something like this:

    80 RMS watts-per-channel, all channels driven, 20-20,000 hz, 0.07% THD, 8 ohms resistive load.
    • RMS- Audio signals are a sine-wave. They move up & down. At what part of the curve do they measure Power? Some companies use the top and call it "Peak-to-Peak", "Peak Envelope", or just "Peak" power. This is a little sleezy. There is a way to calculate the "Average" power called RMS (Root-Mean Square). The better companies report RMS power rather than Peak power.
    • All Channels Driven - Your receiver is a "power plant". My old 120 watt stereo receiver was weaker than my new 80 watt HT receiver. The old receiver could produce (2 x 120 = 240 watts), but the new receiver could produce (5 x 80 = 400 watts). Better companies say 'all channels' while others say 'stereo' or 'stereo-mode'.
    • 20-20,000 Hz - Guess what? Low-frequency sounds take a lot-more power to produce. Poor companies measure with something like a 1,000 hz tone. Better companies sweep through a large range of tones and report what the minimum power was.
    • 0.07% THD - You can take a amplifier and crank the knob to produce higher-than-rated power. But damage is done to the signal. The amount of damage is called "Total Harmonic Distortion (THD)". Good receivers stay below about 0.1%. I have seen cheap receivers on EBay that brag about "High Power", but the fine print says "0.6% THD" which is huge/audible. Poor-quality companies do this to make their receivers appear more powerful.
    • 8 ohms - Your speakers are a load that "draws" power from your amp. A 4 ohm speaker draws twice the power that an 8 ohm speaker so it looks more powerful. Some companies report XXX power, and the fine-print says "4" or "6" ohms power. Other companies report power numbers with 8 ohm loads and appear 'weaker'. When comparing receivers, make sure you compare the numbers for the same load.
    • resistive load - An 8 ohm speaker is ... not always 8 ohms. The 'impedence' of a speaker is different at different frequencies. To avoid this variable, most companies use a fixed resistor to measure power. This is legitimate.

     
  8. Michael R Price

    Michael R Price Screenwriter

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    Bob, great post, but you have one thing backwards I think. In the old days people had LESS powerful amplifiers, and used larger, more efficient speakers. Today speakers are usually smaller and less efficient, most are 82-90 db/w, and large amounts of power (hundreds of watts) may be needed for realistic dynamics with such speakers - if the speakers can handle it at all. Of course most people still totally underestimate what "50 watts" truly means. Even with 84db/watt speakers, that is still a lot of sound. Putting the voltmeter on my speakers while playing pretty loud music shows that the power is around 1-4 watts continuous.
     
  9. Bob McElfresh

    Bob McElfresh Producer

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    I'm thinking of the Klipsch story: in the '70s speakers WERE power-hungry. Someone came up with a new horn design that would produce quite good db without 200 watts. This was what put Klipsch in business.

    And I'm looking at my/my family's old speaker collections - huge wood box's with 10" woofers - power hungry things. I'm comparing these to the more typical "Monitor Style" speakers w/o woofers.

    So thats where I come up with the idea that modern speakers are smaller, less-power hungry.
     

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