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QSC Amp spec question

Discussion in 'Amps and Cabs [BG]' started by Scotius2001, Nov 27, 2002.

  1. This will probably be a question for Bob Lee but any help is appreciated. Why are the QSC PLX series amps rated for 4 ohms bridged at 1khz only while higher impedances are rated at full bandwidth? Just curious how to interpret this spec for a bass application. Should I expect full rated power when pounding a B-string? Also, are they rated for peak power? Thanks in advance
  2. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    There are two power measurement techniques in common use in the US: FTC and EIA. Both involve continuous sine wave signals at the threshold of clipping.

    The FTC spec is the more stringent and rigorous of the two. It involves driving all the amp's channels simultaneously and requires a preconditioning period of operating for 60 minutes at 1/8 of full power (until recently, it was 1/3 power) into the load with a 1 kHz sine wave, followed by 5 minutes at full power. During this preconditioning the amp cannot current limit, thermal, or in any other way shut down or cut back. Then the power test can be done, over a specified frequency range (usually 20 Hz to 20 kHz), at the onset of clipping.

    Because clipping is gradual, you have to pick an arbitrary definition of its onset. For example, if the sine wave flattens enough to cause a certain amount of harmonic distortion. At QSC, we typically use 0.1%, 0.05%, or 0.03% THD, and it's specified on the spec sheet or manual. Different manufacturers may pick different definitions of the onset of clipping, as long as they specify it. That THD figure also has to be good on the amp down to 250 milliwatts--where crossover distortion comes into play--which can be difficult on amps with poorly designed bias circuitry.

    Typically in an amp power sweep test, at low and mid frequencies the amp will reach a higher voltage before hitting the THD threshold than at high frequencies. Thus, FTC amp specs are mostly indicative of what the amp can put out at maybe 10 to 20 kHz, while at lower frequencies the amp can usually put out a little more.

    To calculate the power, measure the output voltage with an RMS voltmeter, then use a variation of Ohm's Law: P = V^2 / R.

    EIA specs are a lot simpler: no preconditioning, and at just a single frequency (usually 1 kHz). EIA also allows the manufacturer to test (load and drive) just one channel at a time. That's cheating! ;) Actually, that loophole could conceal a really wimpy power supply that is adequate for one channel but inadequate for two. QSC's EIA specs, unless they state otherwise, are with all channels driven to full power at the same time.

    Because of the 1/3-power+full-power preconditioning required in the FTC spec, only a couple power amp models out of hundreds on the market ever had legitimate FTC specs for 2 ohm/channel or 4-ohm bridged mono operation. The new 1/8-power preconditioning should make it easier, but as of yet we haven't retested any amp models.

    QSC gives both FTC and EIA specs so that you can compare the amps on an apples-to-apples and oranges-to-oranges basis to other brands that only offer one or the other. That's why you see only EIA specs on 2-ohm/channel and 4-ohm bridged operation and both FTC and EIA specs on other load impedances.

    "Peak" power is meaningless and no reputable amp manufacturer bothers with it.

    You can get full rated power regardless of what string you're playing.

    Please copy or archive this so that I never have to write this essay again. ;)

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