powering a 4 ohm load. how much power required?

Discussion in 'Live Sound [BG]' started by craigie, Oct 4, 2017.

  1. craigie


    Nov 11, 2015
    Just a general question here. Say i hot an amp that can supply 350 watts at 8 ohms program. I hook up an 8 ohm speaker that's rated for 350 watts, that's a pretty good match (although more amp headroom would be better).

    Now say an amp can supply 500W program at 4 ohms and I hook up two 8 ohm speakers (say 600W program) for a 4 ohm load. 500W is pretty close to the 600W speaker rating, so maybe not too bad if I don't push it. But wait, there's two speakers, so each much be drawing 250W max. So grossly underpowered. Maybe Ok if not pushed into clipping but certainly underutilized.

    Is my understanding correct?

    I could pick up some Yamaha SW118v subs (maybe) for very cheap. Passive subs without crossovers and only rated at 600W, so worth it IMO if I can trade some inexpensive stuff I was going to sell. The problem is how to power them without buying a crossover.

    I have a QSC amp that will put out 2x450W or it will put out 1x500W at 4ohms and has a built in crossover that sends the lows to only one channel. So I think I can get 500W sub 100hz with both subs in series. I know I'm better off with say one 720W program active 15" sub with built in crossover and DSP (which I just picked up). This is just fun to think about.
  2. Stumbo

    Stumbo Guest

    Feb 11, 2008
    Speaker ratings are usually thermal, not the same as amp watts,so there's no direct correlation. You also have the mechanical limits(Xmas) of the speaker to contend with.

    Also, the amp eq. settings play a part in maxing out speaker capabilities.

    For a sub, it will mostly add mud in most venues and compete with the PA.

    The more you know.

    I suggest reading the Amp an Cab forum stickies.

    It is fun! :bassist::hyper::roflmao:drool:cool::cautious:
  3. s0c9

    s0c9 Supporting Member

    Jan 9, 2014
    1964 Audio artist, Fractal Audio Beta Tester
    The general rule of thumb for powering passive speakers is for the amp to produce 1.5 to 2.25x the mean/program/RMS wattage rating of the speaker at the given impedance to allow for transients and headroom.
    a 600w RMS/Program 8-Ohm speaker - ought to be powered by an amp that produces (1.5x600) 900w to (2.25x600) 1350w.
    Obviously, you don't want to run 1350w full on into a 600w program speaker whose peak is probably ~1100w.

    On the flip side.. you CAN run an amp that will supply 500W program at 4 ohms and hook it up two 8 ohm speakers (say 600W program) for a 4 ohm load.. but each speaker will only get 250w and you'll run out of headroom and get into clipping pretty quickly. And remember, those ratings are PER channel, and many amps will run in bridged mode.

    If you can provide specifics on the amp and speakers.. that will help.
  4. craigie


    Nov 11, 2015
    Thanks! Will do.
    I agree with what you're saying about sub boominess. Thanks for the "sound" advice.
  5. craigie


    Nov 11, 2015
    That's exactly what I thought. Thanks for confirming! Amp is QSC GX3 (500W @4 ohms one channel driven) and subs are Yamaha SW118V. Funny thing about the QSC is they just give one number for power and don't specify program/peak etc, but reading powered speaker specs they give program/rms and peak to make the amp appear more powerful maybe. I will dig around more into the stickies to edumacate myself better.
    Last edited: Oct 4, 2017
  6. s0c9

    s0c9 Supporting Member

    Jan 9, 2014
    1964 Audio artist, Fractal Audio Beta Tester
    No, amps only give MAX power ratings at specific impedance levels per channel -- 2-ohm if supported -- then 4 and 8 ohm -- and bridged power -- if supported..
    Speaker specs usually give a program/RMS, a peak rating and a nominal impedance.
    Peak is usually close to TWICE program/RMS rating.

    That's neat that you have a GX3.
    I actually use a GX3 to run my 100w Selenium horns (8-ohm) in my bi-amped 15" tops. You can't run the GX3 bridged tho'.
    craigie likes this.
  7. Stumbo

    Stumbo Guest

    Feb 11, 2008
    An HPF can help tighten up the low end on your regular rig. The QSC may have an internal one.
  8. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    why would you worry about "pushing" it any more than you would if you had a bigger amp?

    if the amp is a little less than the speakers then you're fine unless you crank the amp into all-out terrible-sounding distortion. that's when it might produce too much power for the speakers.

    the speakers are only "underpowered" if they're not getting loud enough for the gig and are capable of taking more. my speakers are sitting in the trailer being underpowered as we speak :p
    Last edited: Oct 5, 2017
    craigie and Stumbo like this.
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