SW Amp Ratings

Discussion in 'Speakers & Subwoofers' started by Steven_MB, Dec 11, 2003.

  1. Steven_MB

    Steven_MB Agent

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    Folks:

    Is there anything different about the way they rate power in SW amps, vs regular amps? I recently did some pathology on the failure of the '350W (RMS)' amp in my JBL PSW D115, and by the size of the heat sink really doubt that this thing could pump out 350w continuous for than a few cycles into a dummy load. My guess is that if you connected it to a dummy load at its design impedance, the most it could do continuously would be about 25W. I don't expect CCS kind of ratings, but even in Intermitent service (ICAS) expected to see more amp under there. Are we back to the old 'music power' rating system?

    Thanks, Steve
     
  2. Edward J M

    Edward J M Cinematographer

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    If the amp is rated for continuous power, as calculated with the RMS value of the sine wave voltage, into a given load, over a stated frequency range, with a distortion limit, then I'll bite. If not, who knows what it can really crank out?

    Sub amp specs are still loosey goosey, and are not yet regulated like the FCC requires for AVRs, etc. I hope they knuckle down and hold sub amp manufacturers to the same type of standards, similar to what you are referencing above.
     
  3. Steven_MB

    Steven_MB Agent

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    Ed:

    Thanks. You seem to confirm what I noticed. I just got a new 200 w replacement from parts express and it seems more substantial. It's too bad they don't let us pull out the electronics in the speaker store to inspect.

    Steve
     
  4. BrianWoerndle

    BrianWoerndle Supporting Actor

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    Most sub amps are digital class D amps, which do not genetate as much heat, so the size of the heat sink is not a very good judge of power.
     
  5. DavidLW

    DavidLW Stunt Coordinator

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    "Is there anything different about the way they rate power in SW amps vs regular amps?"

    If by regular amp you mean the amp in an average AVR , then no. "Watts RMS" is derived using the same method. If by regular amp you mean High End amps like Krell, M.Levenson, BAT, Cary, VTL and the likes then the answer is yes.

    "Watts RMS" was created, in the 70's, to standardize the way the maker of ampifiers rated thier amps. It really has no bearing on real life usage. The amp only had to output the rated watts into a known load (usually 8 or 4 ohms) using a 1khz sine wave for more than five minutes with less than .1% THD. Another word, your 350 watts rms SW only had to produce 350 watts for five minutes and 1 second to get a 350 watts rms rating. One of the reason why "watts rms" rated amps may be different is that this is a minimum standard. One manufacturer may test thier amps to output the rated watts for an hour while another may only test thier's for 10 minutes. Either way, they get to put the "watts rms" rating on it.

    High End amps, on the other hand, never use the watts rms rating system. Thier amps are rated to output thier rated wattage ,continuously, into one channel, for as long as the amp is on, with .001% THD from 20hz to 20khz. This is why a 500 watts rms (100 x 5) JVC AVR weighs 25 lbs while a 500 watts (250 x 2) Krell amp weighs 125lbs. Not to mention that one these will cost less than $500.00 while the other, in the neighborhood of $12,000.00.
     
  6. Edward J M

    Edward J M Cinematographer

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    Whether an amp maker uses the average, RMS, or peak value of the voltage sine wave to calculate the continuous power in watts is ultimately a moot point. What really matters are all the other qualifiers that surround the continuous power rating, like bandwidth, impedance load, and distortion.

    Most amp makers use the RMS value (which is .707 for sine waves) of the voltage, just to be consistent with the FTC. But watts are watts, there is no such thing as "watts RMS", it is just a nomenclature shortcut (and an incorrect one at that).

    What they really mean is the continuous amplifier power in watts, as calculated with the RMS value of the voltage sine wave. But that doesn't roll off the tongue as nicely, so we soldier on with the silly "watts RMS" shortcut.
     
  7. Steven_MB

    Steven_MB Agent

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    I concur - a real watt is a real watt, hopefully meausured with a dummy load and calorimiter to get around the peak nonsense for non sine-wave signals. Thanks for the clarification on the 5 minute period. It sounds like those high end amps use a standard like CCS = Continuous commercial service. As to the Class D and digital comment, I had thought about that as a possible way to get high efficiency by driving the output transistors into saturation, but there is no evidence of any kind of L.P. filtering or tank at the amp output, so the class D or any kind of Pulse Width Mod. design would produce lots of harmonics (undesirable clipping distortion).

    I received a new 200w SW amp and note that it's heat sink is around 40 sq. in. vs less than 10 for the 350W burned out JBL amp. Assuming that they both use class B or AB designs, and run at similar efficiencies, one might deduce that the 350w rating if correct, might have been sustainable for only a few seconds. While it is possible to lie with heat sinks (eg put a big one on just to sell a product),it does seem to be a useful first order way to screen products, assuming that they will show you an open unit in a showroom. In the future I will be more questioning of products with large ratings and tiny heat sinks.

    Thanks, Steve
     

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