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How much power of Amp you'll ask for?

Discussion in 'Pro Audio Equipment' started by ray1018, Dec 26, 2009.

  1. ray1018

    ray1018 Active Member

    Wondering s/thing!
    Say you got a Speaker - 250w 8ohm
    how much power of amp will you ask for it? 500w 8ohm? 750w 8ohm or 1000w 8ohm?

    P/s: situation on a Channel (Left or Right) & without Sub Woofer

  2. Kev

    Kev Well-Known Member

    I like lots of power and lots of headroom
    but I guess it depends on the job and the average volume levels you want to achieve
    and perhaps the dynamic range of program source
  3. ray1018

    ray1018 Active Member

    Thanks for reply!

    When come across of Head Room..how to ensure it?! The Amp without Peak while as loud as you want?

    I use Bose Speaker recently & set up with Parallel, that means 500w with 4 ohm each side! Then I just grab an Power Amp fr Wharfedale MP2800 which is 1400w with 4 ohm! Wow..I love the fullness & the Loudness..

    When I apply the MP2800 to the Wharfedale Titan 15" (400w 8 ohm), however I push sound it was weak & the meter level fr Mixer was Peak..Btw, the Amp MP2800 provide only 800 with 8 ohm!

    Besides, I try the same Amp MP2800 with Wharfedale Titan 12" (250w 8 ohm), the sound was sweet & Full enough..Meter Level fr Mixer shown below 0!

    Huh..what do you say with that situation? How s'ld I try to match the perfect power for my speaker in the future? Mind to show your points?!

    Thanks & appreciate
  4. Kev

    Kev Well-Known Member

    it could be all about the efficiencies of the speaker system

    The Bose speaker in parallel will draw more current
    IF placed on top of each other will give some Mutual Coupling
    this is good for mids and lows
    it's the reason quad boxes are loud
    google it

    I don't know the Titan 15 and it may be a lower output for 1 Watt
    It may be ready to handle a great deal more power .... and then come alive

    perfect power for your speaker ?
    I go back to my original answer
  5. BobRogers

    BobRogers Well-Known Member

    That's the idea. You want to be able to get enough power from the amp to drive the speaker to your desired volume levels, and you want the amp to be able to deliver that with power to spare. Now, there is no simple formula that guarantees that an amp with a given power has enough power for a given speaker. This is because there are more factors that go into the calculation than the simple wattage ratings of the speaker and the amp, and the standards for measuring the power ratings vary somewhat from company to company. HOWEVER, the general "rule of thumb" is to use an amp with a rating at least twice the rating of the speaker system. Some people say 2.5 or 3 times. As I say, this is a very approximate "rule." Note that this corresponds pretty well with your experiment. You had trouble with the 800W amp into the 400W speaker. Good results with 1400W into 500W and 800W into 250W.
  6. ray1018

    ray1018 Active Member

    Thanks to all of your kind replies!!
    Atleast I got some idea now!!
    Btw, I'll go for 3x-4x of my speaker power..
    Say, speaker 250w 8ohm, I'll go for the amp with 800-1000w with 8 ohm! If there are 2x250w speaker = 4ohm, I'll go for the amp with 1500-2000w with 4ohm!

    I think this is the better way for simple calculation..otherwise the person of the store might advise you a power which is not really power enough for your speaker purposely to protect the speaker of overloading..

    Any others experiences to choose anp out there?

    Thanks and appreciate! ;)
  7. Kev

    Kev Well-Known Member

    one last point

    when you do have that 3x-4x of speaker power
    you now have the ability to OUT thermal you speaker
    and you usually have the ability to over pole a speak even with low power
    and clipping is bad - especially for the HF units

    get to the point Kev

    feedback !!!!

    you now have the grunt to do damage FAST
    just be aware
    know where your master fader or mutes are
  8. ray1018

    ray1018 Active Member

    Haaa..YES! This is what mist of the Supplier affraid about, there for they always provide the 2x power of the speaker for the Amp! And they forgot bout the point you did point out..though 3x-4x power of the Speaker for amp will damage the Speaker but the Lower Power wil Damage the Amp or Speaker as well when it's CONTINUOUS CLIPPING of Amp!! That's why their Meter Level on Mixer was always Peak but still c'ld not get the Fullness & Loudness supposingly the Speaker Loudness can be! Too bad..

    Thanks for the Warning, haaa..and now I got the confirmation !! ;)
  9. sheet

    sheet Well-Known Member

    What is the speaker's sensitivity?
    Will it be indoors or outdoors?
    How far will it be from the audience and how far will it have to throw to the back of the audience?

    85dB SPL at 10M, sensitivity 98dB 1W/1M, 3dB of headroom = 10W
    90dB SPL at 10M, sensitivity 98dB 1W/1M, 3dB of headroom = 32W
    95dB SPL at 10M, sensitivity 98dB 1W/1M, 3dB of headroom = 100W
    100dB SPL at 10M, sensitivity 98dB 1W/1M, 3dB of headroom = 316W

    85dB SPL at 10M, sensitivity 98dB 1W/1M, 6dB of headroom = 20W
    90dB SPL at 10M, sensitivity 98dB 1W/1M, 6dB of headroom = 63W
    95dB SPL at 10M, sensitivity 98dB 1W/1M, 6dB of headroom = 200W
    100dB SPL at 10M, sensitivity 98dB 1W/1M, 6dB of headroom = 631W

    85dB SPL at 10M, sensitivity 89dB 1W/1M, 3dB of headroom = 79W
    90dB SPL at 10M, sensitivity 89dB 1W/1M, 3dB of headroom = 251W
    95dB SPL at 10M, sensitivity 89dB 1W/1M, 3dB of headroom = 794W
    100dB SPL at 10M, sensitivity 89dB 1W/1M, 3dB of headroom = 2512W

    Of course, none of this factors in the cable induced power loss.
  10. Kev

    Kev Well-Known Member

    I wondered when sheet would arrive
    he likes details
    I did mention " ... efficiencies of the speaker system "

    and sheet shows us the consequences in numbers
    100db at 10m .... 316W or 2512W

    I like horn loaded stuff
    puts the LOUD into loudspeakers
  11. Codemonkey

    Codemonkey Well-Known Member

    Sheet, might I ask, are there (or rather, what are the) formulae behind that?
    Or what is it known as, and I'll look it up.
  12. sheet

    sheet Well-Known Member

    You may ask and there IS a formula. You can find a user friendly spreadsheet version of it here:


    BTW, church sound guys are some of the hardest working sound people and they have no idea how easy their secular counterparts have it. Keep pushing buttons.
  13. sheet

    sheet Well-Known Member

    Sorry, I speed read through as usual. I too like horn loaded stuff most of the time, just where I am listening within 30 feet. haha. I have come across several churches lately with EAW AS hornloaded hi/mid and subs, throwing all of 45 feet! GAWD AWFUL SOUND.
  14. Kev

    Kev Well-Known Member

    very basically it's db per 1W per 1M

    3 db cost twice the power
    double the distance costs twice the power

    to get twice the power into the system
    halve the ohms
    double the volts
  15. Kev

    Kev Well-Known Member

    OH !
    and about the horn loading

    a friend ( my live music mentor )

    Horn loaded speakers
    ... LOUD down the front
    ... LOUD down the back

    Front loaded speakers
    ... LOUD down the front
  16. Codemonkey

    Codemonkey Well-Known Member

    Thanks. Not sure I want to see the formula now, 10 to the power of 20 log something over 10. Yeesh.

    I would debate that statement...

    Church guys get fixed installs, a three hour rehearsal and a one hour gig, per week. You do a few hours overtime at Christmas and Easter.

    Hire guys get to set up on a tight schedule, tear down late at night, have to deal with drunk crowds and runaway camel rides, and do that 2, 3, 7 times a week.

    The only benefits I see to the latter: having a source of income for new kit; and actually having plans and riders for special events.

    YOU guys are the ones who should keep pushing buttons, and keeping guys like me on a learning curve.
  17. Boswell

    Boswell Moderator Distinguished Member

    Doubling the volts would get you four times the power.
  18. Kev

    Kev Well-Known Member

    yeah the 6 db

    sorry !!!
    trying to over simplify
    and not use math

    but you get the point
    more volts
    more current
    more power

    efficiency lets you start from a MORE LOUD start

    when in db mode I usually check my thinking with this

    use 8 ohms
    rms volts
    try 10 volts = 20dbV
    try 20 volts = 26.02dbV

    helps to put things into perspective

    now try some numbers suited to the headphones and typical audio outputs

    start with the 600 ohm load
    ohms = 600
    now try peak volts
    30 volts
    this represents the very greatest peak to peak swing a simple +/-15volts supply in a typical audio output with normal chips
    and remember there will be basic junction losses anyway
    and you get a little over
    28 dbu & dbm and 26dbv
    and 750mW

    increase the load by lowering the ohms to typical headphone and speaker values
    600 - 250 - 100 - 75 - 64 - 32 - 8 - 4
    forget the dbu and know the dbu and dbv remains the same
    but watch the power climb
    until you get to 4 ohms
    1.125e5 mW

    e5 - move the decimal to the right 5 times
    mW move the decimal to the left 3 times

    and you get 112.5 W
    100 W into 4 ohms audio amp

    those that use a 50 to 100 W amps for headphones don't seem so crazy anymore ( I use a 50 - DIY off course )

    Still only 30 volt swing and 28dbu ( typical audio line level )
    only adding current for the load
    use a passive distribution box and plug in many headphones as you like
    not less than 4 ohm total load


    wait for it

    Crown 50, Crown 75, Crown 100,
    good for headphone distribution
    now where is Remy
  19. dvdhawk

    dvdhawk Well-Known Member

    Thanks for the knowledge and great external link! Above and beyond the call of duty.

    This is what I love about RO, people who know what they're talking about (Sheet, Kev, Boswell <and we don't want to leave out Remy. As noted, she will probably be along shortly>) sharing some important technical information that is often an after-thought or overlooked completely.

    There's more to this than, "will this plug fit in there?"

    This information is waaaaaaaay too good to get lost in this thread.

    Please sticky this AudioGaff !!

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