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Power amp technical question

Discussion in 'Amps and Cabs [BG]' started by MorganM, Sep 12, 2002.

  1. MorganM


    Dec 11, 1999
    This may sound stupid but,

    does the gain knob control the maximum output of the amp. I.e, say you have an amp with 1000W at one channel and it's gain knob is at 50%, this would mean the maximum output of the amp would be 500W(respective to tolerances of course)?

  2. Nope.

    The Gain knob controls how much the amplifier amplifies the signal. If the incoming signal is small, then you may have to set the gain knob at 90% to get full power. If the incoming signal is large, then you may only need to set the Gain knob at 30% to get full power.

    So in short, the Gain knob does not directly correlate to the output power. It only tells the amp how much to "multiply" the incoming signal. The output power will depend on the Gain knob setting and the incoming signal voltage level.

    If the incoming signal is "hot" enough, then you can get 1000 Watts out of the example amp even if the Gain knob is only set to 10%.

  3. MorganM


    Dec 11, 1999
    ok, another practical question,

    Say I have a bass cab rated @ 600W and the output power of the amp is 700W.
    How would I determine where to set the gain knob in reference from the signal from the preamp without blowing the cab?

  4. If that 600 W is the RMS rating of the cab, you can't possibly blow the cab with a 700 watts amp. The cab can probably take 1200 watts peak.

    If that 600 W is the peak rating, better be careful, or get an amp peak power meter.

    Most professional speakers have a crest factor of 4, meaning they can peak at four times (!) their rms rating. Some pro class series of speakers can take 10 (!!!!) times their rms value.

    So if your 600 watts cab is a good one, it may take 2400 watts peak, before it blows.
  5. To expand on what Joris was saying, it's unlikely that you'll blow your cab with a 700 watt amp by applying too much power, but it's still possible to blow the speakers by driving the amp too hard and clipping the signal.

    This taken from the JBL website.

    Ideally you should pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. This means that a speaker with a "nominal impedance" of 8 ohms and a continuous IEC power rating of 350 watts will require an amplifier that can produce 700 watts into an 8 ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 ohms.
    A quality professional loudspeaker can handle transient peaks in excess of its rated power if the amplifier can deliver those peaks without distortion. Using an amp with some extra "headroom" will help assure that only clean, undistorted power gets to your speakers.
  6. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Supporting Member

    Mar 24, 2001

    Cool, the missing bit of info!
  7. I also found this on the JBL site. It doesn't contradict what's quoted above, but it does provide a different perspective.

    "4. What about loudspeaker abuse in normal
    operation? Should there be any de-rating of systems to
    allow for this?
    Yes. JBL defines the following three categories
    of loudspeaker application, each requiring an
    adjustment of the system's IEC rating:
    A. For carefully monitored applications where
    peak transient capability must be maintained, a system
    should be powered with an amplifier capable of
    delivering twice its IEC rating. For example, a studio
    monitor system rated at 300 watts can be safely driven
    by an amplifier capable of 600 watts output.
    Discussion: Careful monitoring is the key here.
    High quality music production today demands high peak
    factors in the recorded signal. Such peak signals are
    normally of such short duration that they hardly stress
    the system's components. Thus, the extra 3-dB margin
    (times two) of power will result in cleaner overall
    operation of the system, with less listening fatigue.
    B. For routine application where high
    continuous, but non-distorted, output is likely to be
    encountered, a system should be powered with an
    amplifier capable of delivering the IEC rating of the
    Discussion: This case describes the bulk of
    sound reinforcement activities. Such systems can often
    be inadvertently overdriven, or can go into feedback.
    When powered with an amplifier equal to their IEC
    rating, the user is guaranteed of safe operation.
    C. For musical instrument application, where
    distorted (overdriven) output may be a musical
    requirement, the system should be powered with an
    amplifier capable of delivering only one-half of the IEC
    rating for the system.
    Discussion: Much rock music is produced at full
    output with the amplifier well into clipping, and this is a
    matter of musical choice. When an amplifier capable of,
    say, 300 watts of undistorted sinewave output is driven
    well into clipping, its output power can approach 600
    watts! So, Berating the system to one-half its IEC power
    will result in safe operation of the loudspeaker."
  8. Richard, you bring up a good point. Guitar speakers are commonly 2x the output of the amp because of the speaker abuse you mention. Guitar speakers typically have to deal with outrageous amounts of distortion which is rough on speakers. It is possible to use speakers with a rating so far above the amp output that the amp can't put out enough current to fry the speakers no matter how bad it clips.

    I remember in the 70's when the first analog synthesizers came out. Among other waveforms, they put out a wonderfully perfect square wave. Soon keyboardists were frying not only they're own speakers, but PA speakers as well until somebody put 2 and 2 together and fiqured out why. The answer was to apply way more speakers than the power amp could fry, an expensive solution, but it works.
  9. MorganM


    Dec 11, 1999
    These are my plans,

    I have an SVT-610HLF rated at 600WRMS and 1200W Peak.

    The power amp I am looking at is the QSC RMX2450 which puts out 750W per channel.

    WouldI want to then run the amp stereo @ 750W or bridged at 1500W?
    Or should I look into a more powerful amp?
  10. It all depends on how loud you want to play. If you play loud enough that you'd run that amp near it's limits, I'd get another cab. I think it's a mistake to focus *only* on the relationship of amp power to cab rating. You have to look at the relationship between what you have available to you (including amp power, cab power handling, and speaker efficiency) and what you actually need to get the job done the way you want it done. To me it kinda misses the point to just say, get an amp rated for twice your cab. What you really need is to get an amp that's at least twice what you need to get the volume you want, then make sure your cab can handle at least the level of power you'll be throwing at it. You get the real benefit from having more power than you need for the job, not from having more power than the cab is rated to handle.
  11. More power? God no. I wouldn't. If you run the 2450 bridged that's all the SVT can take plus some. I wouldn't think you'd have any clipping problems with that setup, but you might be a little over powered. If you start smelling smoke you might want to turn it down a tad. :)
  12. Word to your mother. Richard makes an good point.

    It's a big equation, don't just stare at one tiny piece of it, look at the whole picture.

    And by the way, I've never needed more than 100Watts. :D What the hell are you people doing? Are you deaf? You probably will be......

  13. MorganM


    Dec 11, 1999
    so what I am asking is, would the RMX2450 have enough single channel power to power the cab without losing headroom?
  14. Oh yeah. It goes back to what Richard was saying...how loud do you want to be? 750 watts through a 600 watt 6x10 cab is going to loud. It could be clipped with that arrangement, but you'd have to be awfully loud for it to become a problem.
  15. Headroom doesn't depend on what the amp is rated at, it depends on how hard you have to push it to get the job done. A 10 W amp has ample headroom if you never have to use more than 5 W. A 750 W amp has no headroom if your playing situation means you have to use every single watt it can crank out.

    So again it comes back to, how loud do you need to be?
  16. MorganM


    Dec 11, 1999
    yeah I understand how headroom works.
    I am saying if I were to puch the cab at 600W would that extra 150W be sufficient headroom?
  17. But see, the question remains, sufficient for what? It still comes back to how loud you want to play. If it's a quiet duo in a restaurant with a nylon-string guitarist, 100 W is usually plenty. if it's a stadium with two loud guitarists in a metal band, obviously you need more.

    There's no such thing as sufficient headroom in an absolute sense. There's only sufficient headroom *in your situation*. There are times when 600 W is several times what you need, and there are times when it's not nearly enough.

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