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overpower/underpower monitors

Discussion in 'Monitoring / Headphones' started by rockstardave, Apr 11, 2008.

  1. rockstardave

    rockstardave Active Member

    everyone knows the old "better to overpower than underpower" saying when it comes to passive speakers.

    but WHY?

    when your amp is way overrated for speakers , you use the trim knob to keep from blowing speakers, right? (for example, if your amp pushes 1000w and your speakers handle 250, then you'd keep the amp's trim knob pretty low).

    but when your amp pushes less power than the speakers can handle, you dont want to clip, cant you just keep the trim knob pretty high. (for example, your amp pushes 300w and speakers handle 750, then you'd keep the trim just at the lower threshold of clipping, right??).

    right? the only downfall i see is that it just wont be as loud. or is there an electronic thing i'm missing?

    i want to hear from the pros on this one.

    -dg
     
  2. Cucco

    Cucco Distinguished Member

    I wonder if Ken Kantor will jump in on this one.

    Here's his reply from a previous post:

    It's from:

    {old-link-removed}
     
  3. sheet

    sheet Well-Known Member

    Turning the knob on your amp does not turn up or down the amount of power you amp produces. It increases or decreases your sensitivity.

    The things to consider. When matching power amps to speakers, you also must factor in the speaker's sensitivity, it's efficiency, cable awg, cable length, dseired headroom, etc.

    By underpowering a speaker, you are not allowing for the power consumed by the driver's heat, wasted by the cable's length/awg, and you will have zero headroom. You are not likely to blow a driver by underpowering. Not with a state of the art modern amp. Back in the old days maybe. Speaker protection is pretty good now.
     
  4. rockstardave

    rockstardave Active Member

    now here's what i mean though... is that "underpowering a speaker", or is it "overstressing an amp"? which phrase is more appropriate?
     
  5. sheet

    sheet Well-Known Member

    It is not underpowering a speaker. Think about the logic of that. How often in a music reproduction environment, that music is at a constant? It is not often. Music is dynamic. Are all of the drivers at their peak power capacity at all times No.

    When people speak of this phenom, they do so with the assumptions of an amp out of headroom, clipping, distorting, with no driver protection. This comes from a generation of JBL/Crown engineers. It was a way to sell power and better driver science. Back then when the DC150 and 300 were current, it was a crap shoot either way, as both were known for shooting straight DC to drivers.

    Consider guitar amps. Consider light bulbs. Case closed IMO. With modern drivers and modern amplification being what it is, the speaker is not harmed when you fail to provide it the maximum power before voice coil failure.
     
  6. rockstardave

    rockstardave Active Member

    sheet ok, thats great... thats exactly what i suspected.

    is it bad for an amp, assuming that the amp is not constantly clipping?
     
  7. bent

    bent No Bad Vibes! Well-Known Member

    One important thing to keep in mind, and this is very important where I work (lot's of bad grounds and ride power that leaks across to the amp rooms) - the lower you can set your sensitivity, the lower you're also putting the noise floor.

    I purchase amps that will, in most instances, overpower drivers by 200%, simply for this reason.
     
  8. bent

    bent No Bad Vibes! Well-Known Member

    For more info I suggest the Crown amp forums.

    Here's a good post in particular, though there are plenty more in-depth ones you can read...

    Octopre
     
  9. sheet

    sheet Well-Known Member

    Is it bad for an amp to underpower the speaker? No in principle. Why would you want to? You want headroom to handle transients, etc.
     
  10. Codemonkey

    Codemonkey Well-Known Member

    Q1: The power rating of an amp determines the maximum amount of "oomph" it can give to the speaker when the signal gets up to clipping? So when the signal is less than clipping the oomph provided is (Max_Power * Signal / Max_Signal)according to the signal level at any instant in time?

    Meaning that if the signal peaks around -20 and floats around -30dB on the mixer meter, the amp won't overdriver the speaker so long as the regular output roughly matches the Continuous Input of the speaker?
    (Almost the case with our FoH)

    And obviously if the amp is miles under the speaker rating, you don't get enough drive and the speaker isn't loud enough to drown out a fart.
    (Almost the case with our monitors)
     
  11. sheet

    sheet Well-Known Member

    Power ratings = smoke in mirrors. Power is a commodity now. Manufactuers lie about their power ratings, sometimes stating things that have not been tested and approved by the governing authorities, UL, etc. Books have been written on this. I will not reinvent the wheel.

    Power ratings give us the amp's power draw, efficiency, distortion and power output at various loads over time. Unfortunately, many manufacturers are providing ratings without listing some of these critical stats. There is rms power (which is not the correct term) and apparent power. Then there is the real power available, power factor:

    http://www.rane.com/par-p.html#power_factor

    Power amp specs will not tell us how much oomph a speaker will have. Speaker sensitivity tells us the dB-SPL at one watt/one meter. From that you work backwards, factoring in the desired headroom, make up power for that consumed by the cabling (distance, gauge, etc) and desired SPL of the system at a specific distance.

    Also, if you have inferior summing amps and/or gain structure, you might not get the bang that you desire either.
     
  12. Codemonkey

    Codemonkey Well-Known Member

    OK so basically it's all guesswork and sales talk...
    Well thanks for that, at least now I know.
     
  13. rockstardave

    rockstardave Active Member

    so if i have an amp that pushes 75w, i can hook it up to a speaker that handles 250w ... without worrying?
     
  14. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    Yes, but I dunno about "without worrying".

    It depends on what you do. It also depend on the speakers' efficiency and how loud they get.

    Huh? :shock:

    If you can be disciplined enough to accept the level that a clean signal will provide to the amp and speakers, then you'll be OK. But you won't be using the full capabilities of that speaker.

    But, if you keep trying to goose "just a little more" volume out of it by boosting the input signal to the amp, or nudging up the EQ settings, etc., then you'll start asking for trouble. Even though you'll likely not feed it too much power, you may start feeding it too much distortion. Distortion can fry speakers, especially high-frequency drivers. And, it's even possible to damage the amp, especially if you try to make it louder by pumping something in that is more than the input was designed for.

    Where on one hand you can blow speakers by allowing too much power, on the other, it's possible to fry them with a nasty signal at a lower power level. A higher powered amp is not bad if caution is exercised.

    You may not need to "worry", but only a fool would abandon all caution with an over-powered, under-powered, or even a perfectly matched amp/speaker setup.

    You just need to be aware and accept the limitations and capabilities of whatever is in your setup. If it isn't doing what you want, you can't make it do it. You just need to find a setup that works better for you.

    (Hmmm..how did I come to know about blowing things up?) :wink:

    Kapt.Krunch
     
  15. bent

    bent No Bad Vibes! Well-Known Member

    Along with that, think about the gain structure - too small of an amp will cause you to run the preamps and mains beyond unity, too large an amp will cause the opposite (unless you properly set the sensitivity of the larger amp(s) that is...).
     
  16. rockstardave

    rockstardave Active Member

    guys guys - you are getting way too far ahead.

    i simply asked if it would damage the amp or speaker.

    we all know our gain structures,
    we all know that it wouldnt maximize the speaker,

    that wasnt the question. i knew you guys would get carried away.

    so it's fine for my speaker to be rigged up. thanks for the answer.

    </thread>
     
  17. bent

    bent No Bad Vibes! Well-Known Member

    Glad you got your answer, but keep in mind that this site is viewed by people from all walks who may not know or understand proper gain structure (I work with a number of them), and who don't understand paltry mathematical equations such as Ohms Law - or what RMS stands for, for that matter.

    Let's not close the door on a thread that may be of use to others lurking here!

    (We have not yet begun to get carried away)
    :wink:
     
  18. Codemonkey

    Codemonkey Well-Known Member

    Ohms law --> V=IR ??
    RMS is Root Mean Square and is Peak / root(2)

    Gain structure...I understand it, I just don't (want to) implement it.
    If I do, and PFL something, I get blasted by the suddenly HUGE signal compared with the cut down main mix. The cabs are powerful enough to blow the hall away, the peak meter never gets beyond -20. The faders are at 0 so I don't get my ears blown out when soloing.

    Would proper gain structure MASSIVELY improve the mashing together of stuff on buses? Going from about -20/25dB peak per channel to about 0 peak on a low end mixer, which will always sound lousy anyway.
     
  19. bent

    bent No Bad Vibes! Well-Known Member

    Wasn't your first foray on this site about the noise induced into your system at the church?

    If you're running everything that far below unity then naturally you're gonna get that nasty mess in the mains and monitors!

    Set your channels at unity and gain the amps accordingly.

    And lower the PFL volume pot, to avoid getting blasted in the future (your board does have a pot for the PFL, doesn't it?).
     
  20. Codemonkey

    Codemonkey Well-Known Member

    "Wasn't your first foray on this site about the noise induced into your system at the church?"
    Yes it was. I can't see the wood for the trees now (or can't hear the noise for the hissing).

    I started setting channels properly at a rehearsal last night, recorded it as well. Just got no way of getting those recordings off the comp and into my ears until Sunday.

    "Set your channels at unity and gain the amps accordingly."
    Will do. Although the main amp is in the mixer and has a power setting/input switch but no sensitivity, gain etc. It just runs off the relevant faders.

    The big problem is that the Main has no PFL and is by default AFL'd. With the main fader part-down I turn up the Ctrl/Phones and thus, hear it properly. *BUT*
    "(your board does have a pot for the PFL, doesn't it?)"
    It doesn't. The Ctrl/Phones IS the said pot. So the PFL is like 10dB or more above the normal volume.
     

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