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power amp technical question

Discussion in 'Amps and Cabs [BG]' started by velvetphil, Nov 16, 2006.


  1. Hey all,

    So here's my situation: I have a QSC PLX2 3102 power amp, and an Acme B2 Series II at 4 ohms. One side of the QSC is sending 1000 watts into this cab, and it sounds great! I understand that the "gain" knobs on the QSC are more like input attenuators, and that's fine. What I'm wondering is: there's no way to "attenuate" watts coming out of the amp, so does that mean that the cabinet is seeing a consistent 1000 watts at all times, no matter how loudly or quietly I'm playing? I'm wondering because the Acme is rated at something like 350 or 400 watts (though I know it can take a lot of power). Is that a continuous rating or an RMS rating? What's the difference between the two? Does the wattage put out depend on the AC coming into the amp (120 volts ideally, right)? Also, how does headroom fit in to all of this? Headroom is how much signal the power amp can take before it starts clipping, correct? Doesn't that have to do with the voltage going into the amp?

    Wow. Lots of questions here. I've just recently switched to this pre/power configuration, and I'd like to understand as much about the signal flow in the system as possible. I don't want to damage my gear, and there are just way too many gain stages here for my little brain to comprehend! Maybe Bob or someone else can give a brief rundown? I'd really appreciate it!

    Thanks,

    Phil
     
  2. Jim Carr

    Jim Carr Dr. Jim Gold Supporting Member

    Jan 21, 2006
    Denton, TX or Kailua, HI
    fEARful Kool-Aid dispensing liberal academic card-carrying union member Musicians Local 72-147
    Well, this is how I think it works, somebody PLEASE correct me if I'm wrong:
    You won't get maximum acoustic power from the rig unless you are providing an input of a high enough level to reach what the amp can deliver. The amp always amplifies its input as much as it can. Turning down the preamp keeps the input lower, as does "playing quietly," or turning down the amp levels. They all do the same thing--the amp just takes a variable input and increases its amplitude by however much ampification it can deliver. A lower level input results in less output waveform amplitude (thus less total acoustic power giving less loudness).

    Headroom is having way more waveform amplitude increase available than you need to reach a given average loudness. If you can run at a lower input level for that average loudness, the peaks in your sound will not bump their little heads on the ceiling of your amp's ability to increase the waveform's size, thus preserving its actual dynamic shape (unclipped). It is obviously best if those peaks and your average loudness at a given frquency are within the capabilites of your speakers to deliver air movement. Otherwise, the sound will be distorted and your speakers may suffer a mechanical failure. (My apologies to the real engineers out there for my informal terminology.) :D
     
  3. OK, I think I understand that, but let me ask this: "loudness" as you use it indicates a signal level that doesn't involve the actual output wattage of the amp. So we've got the input level sorted out (and I think I am showing the amp an optimal signal level, with plenty of room for the amplitude of the input signal to vary). Now to the output stage of the amp - does the amp deliver a continuous wattage to the cabinet, or does the amount of power vary with the amplitude peaks in the input signal? In other words, does the cabinet "feel" 1000 watts regardless of how loudly or softly I'm playing? Or does the "power" delivered to the amp vary based on the amplitude of the input signal? I believe that the Acme can deliver air movement all the way down to a low B, but can a loud low B with 1000 watts behind it damage the woofers, or the midrange drivers in the cab? That's really what I'm worried about.

    See, my confusion stems from the fact that I always believed that if I had a 400 watt combo amp and I turned it down, the speaker was getting less than 400 watts because the volume was down. On a combo amp then, does this mean that there is no knob that controls the "power" of the amplifier? What does a volume knob on an amp actually control?
     
  4. ggunn

    ggunn

    Aug 30, 2006
    Austin, TX
    The power a speaker receives is directly related to the volume you are playing at. If you are not playing, the amp is not putting out (and the speaker is not receiving) any power.

    The way that power ratings on amplifiers are determined varies, but generally it refers to the most power an amp is capable of delivering into a certain load (usually 8 oms) without clipping a pure sine wave. How much it is actually delivering at any point in time depends on how loud you are playing.
     
  5. Something else about the concept of headroom, it takes less power to produce a high pitched loud note than a low pitched loud note.

    To put it another way,

    depending on a combination of many factors (power available ,volume of play, the pitch and sonic makeup of the note played, the efficiency and sonic ability of the speaker),

    the more power you have available to the speaker cabinet, the more likely the tone of each note will be as full as it can be in reproducing the sound of your bass. The low frequencies suffer first in the power shortage.

    Obviously you can blow a cab with too much power, but a 1000 watt amp only puts out 1000 watts when pushed to do so.
     
  6. Luis Fabara

    Luis Fabara

    Aug 13, 2000
    Ecuador (South America)
    Audio Pro - Ecuador
    Uhmm. No. It can put many times more during a peak without a limiter.
     
  7. robb.

    robb. Gold Supporting Member

    that's technically true, but i think it would be more educational to explain how.

    amplifiers are rated by a type of measurement called RMS. RMS is a way of averaging a sine wave that is very useful for audio. so a 1,000W amplifier is actually a 1,000W RMS amplifier. but the highest voltage level of 1,000W RMS is actually 1.414 times bigger, or 1,414W peak.

    many cabinets are not actually rated for peak voltage, but as long as your cabinet is, it should be no less than 1,400W peak. that corresponds to ratings of 700W program and 350W RMS. which is what your acme is, and why your acme sounds so good with a 1,000W amplifier. ;)

    robb.
     
  8. Thor

    Thor Gold Supporting Member In Memoriam

    I think you would learn a lot from the Ohms FAQ thread.
    A lot of your questions have been addressed there as well
    as on other FAQ threads and answered in much better detail
    than I could hope to provide.

    I would explore those resources, learn all you can, and
    come back for the topics you want clarified. Happy reading!
    :)
     
  9. Jim Carr

    Jim Carr Dr. Jim Gold Supporting Member

    Jan 21, 2006
    Denton, TX or Kailua, HI
    fEARful Kool-Aid dispensing liberal academic card-carrying union member Musicians Local 72-147
    Again, real engineers, PLEASE feel free to correct this:
    Loudness is a term meaning "how loud does it sound to a person." This takes into account all the variables of a rig and just turns it into a human perception. See, http://hyperphysics.phy-astr.gsu.edu/hbase/sound/loud.html It is in effect "how loud does it sound." When it comes to "volume," this is all that really matters in the long run.

    An amp does not deliver a continuous level of power to a cabinet unless it has a unvarying level of input (like a sine wave generator in a lab). Think of the amp's power rating like magnification power of a magnifying glass. If you have a pin head you are looking at with a 5X magnifying glass, it is five times bigger to your eye at a given distance. If you have a penny under examination at the same distance, it looks 5 times bigger, too. But because the penny is so much larger than the head of a pin, the resulting image is way way larger overall, even though it is only magnified 5X, just like the pin head.

    Its basically the same with amplifiers. They take their input and increase the size of it by some multiplier. If the input waveform is very tiny (small amplitude), the output is less than if the same waveform comes in at a higher amplitude (larger signal). If you put nothing in, nothing comes out (except a tiny bit of noise).

    Amplifiers are rated with some constant amplitude input signal at some constant sine frequency or noise band/center freq. So the answer is "No, the cab does not feel the '1000 watts' regardless of how loud or soft you are playing." The cab gets power from the amp based on how you play, set your bass, your pre, and your amp levels. The amp levels are (AFAIK) controlling how big the input looks to the amplification part of the device--like changing the size of the thing the magnifying glass is looking at.

    You CAN damage things with a low B or even a very high note. But if it does not sound distorted, you are fine. If things sound rough, buzzy, hoarse, or overdriven without an effect to produce those sounds, you are either clipping the amp (pushing the peaks beyond what it can manage) or over-powering your speakers. Turn down. However, if it sounds clean and lovely, just smile and play your best. :D :D :D
     
  10. Actually this is only true if the limiting factor in the RMS rating is the power supply rail voltages. If the limiting factor is the amp's thermal capacity but the power supply and output stage can deliver enough voltage and current, I believe the peaks can exceed 1.414 times the RMS for short periods of time.

    It is important to note that a bass looks nothing like a sine wave generator. There are big transients and (in terms of the circuit's response) lots of time when the signal is relatively low because of the logarithmic nature of hearing and also the way a string decays. Just a little softer uses a lot less power. Of course, if you compress the heck out of the signal and play fast the average power dissipation goes way up.


    Peace,
    S
     
  11. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I have a PLX 3002 and use it to push one or two Low-B2 cabs at 900W/ch. Even when the amp is bouncing off the limiters they handle all the power just fine - they only complain if you apply too much bass boost at too high a volume but it's easy to rectify that - turn down the lows or turn down the volume.

    Now regarding the power your amp is putting out. See the LEDs on the front? When the -20dB LED lights up that channel is putting out 1% power (i.e. 10W in this case). When the -10dB LED lights up that channel is putting out 10% power (i.e. 100W) When then clip LED lights up that channel is putting out 100% power (i.e. 1000W). If an LED remains on constantly whilst you're playing a phrase but the next LED up does not light then your phrase is putting out between the two amounts of power.

    For instance my rock band has really got its practice volume way down and my -20dB LED flickers whilst I'm playing and when things get really loud the -10dB LED lights up. So most of the time I'm using about 10W but when it gets loud I do go over 100W.

    With my previous drummer the -20dB LED stayed on all the time and the -10dB light flickered quite a bit but only once in a blue moon did the clip LED light - so most of the time I was using between 10W and somewhere over 100W.

    The RMS power handling rating on cabinets is pretty irrelevant, as it's a rating of average thermal power handling and with bass your average signal tends to be way lower than your peak signal, so the voicecoil doesn't have a chance to heat up much.

    The mechanical power handling of a cabinet is down to the excursion of the speakers and the cabinet tuning - Acmes are very well designed with huge excursion speakers and intelligent cab tuning for basses with a low B. Consequently they can handle far more power than either their RMS rating suggests or many other cabinets with far higher RMS power ratings.

    Alex
     
  12. robb.

    robb. Gold Supporting Member

    i think it's pretty clear by my post that i was avoiding what is technically true in order to give a more palatable answer to the original question, which was asked by someone who clearly doesn't have the technical acumen to appreciate the intricacies.

    i mean, technically, there's no such things as RMS power, but we all say it, anyway, because it makes sense and is easy, even if it isn't technically true.

    besides, if an amp is providing a 1,000W RMS sine wave, the peak of that wave most definitely will be 1,414W, regardless of how much power the amp is capable of sourcing. thermal capacity will not limit output power before the power supply, anyway.

    robb.
     
  13. Rune Bivrin

    Rune Bivrin Supporting Member

    Oct 2, 2006
    Huddinge, Sweden
    This is incorrect. At 1000W (RMS), the peaks will be at 2000watts:
    Peak voltage is RMS voltage * 1.414, and this will lead to a peak current which is RMS current * 1.414. Since P=UI, peak power will be (RMS) power * 1.414 * 1.414 = (RMS) power * 2.

    However, that is pretty irrelevant. A speaker will blow if:
    • It's over heated, which takes time.
    • It's forced to move outside of its mechanical limits, which will depend on both frequency and power

    Since the power provided by an amplifier is depends on the maximum output voltage and the wave form, you could get 2000 watts average from a 1000 watt amp if it's fed with a square wave, rather than a sinusoidal wave form. However, for a bass, I'd contend very few do that, since it would make the bass sound like crap to most ears. Also, many amplifiers have insufficient power supplies, and so would not be able to keep that up.

    Driving a speaker with thermally safe levels, but at a frequency below resonance MAY cause the cone to move so much that it literally detaches from the suspension or crashes the voice coil. But this will be very audible:spit:, even at lower levels.

    Rune
     
  14. Hey all,

    Thanks for the replies. You've given me a great deal to think about. I really do appreciate it!

    Phil
     
  15. robb.

    robb. Gold Supporting Member

    oops. you caught me. you're right. and that illustrates why using RMS to describe power is not technically correct. if it were, the peak power would be 1,414W. :help:

    most amps aren't designed to be able to source that much current (not enough output devices), so current-limiting protection usually prevents such a state. the power supply is another concern when it comes to sourcing current. and like others have mentioned, bass guitar signals are not pure sines, so it is very unlikely scenario, indeed.

    there are many ways to exceed the capabilities of a speaker cabinet, but the primary failure is still thermal. that is using more average power over time than the speaker driver can dissipate in the form of movement and escape heat.

    so to get out of the purely technical, with which many of us are intimately familiar, and to go back to the original questions in this thread, RMS and continuous are the same rating. the power output is determined by the input signal level. it can affected by the AC from the wall in the sense that most power supplies are not regulated. you will not hear this difference, though.

    robb.
     
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