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How much headroom is enough?

Discussion in 'Amps and Cabs [BG]' started by spectorfish, Aug 24, 2004.


  1. spectorfish

    spectorfish

    Aug 21, 2004
    Hi everyone. I am planning to get a power amp to run my bass processor through. And I want to play consistently at 500 watts. How much headroom should I plan on having so I don't overwork my amp? plus how much slack (in terms of watts) should I give my speaker cabinet considering that I'll be running at 500 watts.
     
  2. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Unless you compress the crap out of your signal and play loud enough to deafen nearby water buffalo, you will not need to be "consistently" putting out 500 watts.

    The "slack" for your cabinets is roughly twice the RMS handling capability. So if the cabinet is rated for 500 watts RMS, 1000 watts.
     
  3. 1000 watts...yessiree
     
  4. Never had a need for more than 600w. Sometimes the desire, but not the need.
     
  5. BruceWane

    BruceWane

    Oct 31, 2002
    Houston, TX
    It kinda depends on the speaker you're using and the manufacturers method of calculating it's RMS rating. The better companies (JBL, Electrovoice, etc,) use an AES standard which tests the speaker using a pink noise signal that has occasional 6db "spikes" in it to simulate a more real-world scenario.

    So in order to have enough headroom to cleanly drive a speaker at the speakers limits, you'd want 6db of headroom in your power amp. For a 400 watt RMS speaker, that'd be a 1600 watt amp. For a 1000 watt speaker, that'd be 4000 watts of amplification. When I say "a 1000 watt speaker", that is the total capacity of all speakers to be driven by that amplifier.

    Obviously, you have to be VERY careful to avoid clipping if you push speakers this hard, and you have to be aware of anything more than moderate compression being used. Really, to push this close to the absolute limit safely, you need speaker systems that provide data feedback in order to monitor their condition. Really high end PA gear has the ability to actively monitor cone excursion, voice coil temperatures, and actual wattage being fed to the speaker. The data is sent to a computer system that controls the output of the amps. These systems get an amazing amount of volume out of a minimal number of cabs.

    The safest thing for us mere mortals living in the real world is to run 3db of headroom (twice the RMS rating of the speaker system) in the power amp, and avoid clipping.
     
  6. BruceWane

    BruceWane

    Oct 31, 2002
    Houston, TX
    To answer your questions more directly............

    An amplifier rated at 500 watts RMS will put out 500 watts continuously, but you have to allow for the transient peaks that we all have in our playing. Some folks play a lot more cleanly and evenly than others, so it all depends on your playing; even the best player will have transients of 6db, some will have 9db or 12db spikes. If you really wanted to put out 500 watts continuous, and do it cleanly without loads of compression, you'd need 2000 watts IF you can hold your transients to 6db or less; more likely, you'd need 4000 or so to have enough headroom to put out 500 watts continuous while allowing for 9db transients.

    As long as the manufacturer isn't fudging the RMS rating, a speaker rated at 500 watts will actually handle 500 watts continuous. BUT you really have to see how they come by that rating. A lot of times it's at a fairly high frequency, so if you hit it with 500 watts at 41Hz it'll cry uncle in a big way.

    All of this stuff can be broken down and measured, and specific conclusions can be had, but it takes a good bit of electronic gear to come up with absolute data.

    That's why you really have to use some general guidelines, and then use your ears for the rest. Learn the sound of a power amp clipping, and learn the sound of a speaker distorting from being driven too hard. If your power amp is clipping but your speakers are clean, you can run more wattage. If your power amp is clean but your speakers are distorting, you need more speaker capacity.

    You basically use the rule of having a power amp puts out 2x your speakers rating. If that isn't loud enough, you need more power AND more speaker capacity.

    There are other ways to boost volume, mainly by increasing the efficiency of your speaker system by using multiple drivers, but keeping your available power and speaker capacity properly matched will get you the most volume from the smallest rig.