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How to calculate RMS on PA

Discussion in 'Pro Audio Equipment' started by Horace, Apr 21, 2005.

  1. Horace

    Horace Guest


    This is my first posting so bear with me........

    Just bought a Peavey Power Amp for Live Band use. It is rated at 2000W RMS going into 2 ohm speakers, and 1300W RMS going into 4 ohm speakers.

    We're using 8 ohm speakers, so what does that make our RMS rating?

    Thanks in advance.
  2. Massive Mastering

    Massive Mastering Distinguished Member

    Jul 18, 2004
    Chicago area, IL, USA
    Home Page:
    "Somewhere around" 6 or 700 WRMS - Give or take.
  3. Horace

    Horace Guest


    Are you sure, the previous figures suggest a logarithmic power scale rather than a linear?

    What I mean is, doubling the Ohmage (resistance) doesn't halve the power from 1300W to 650W?
  4. killersoundz

    killersoundz Guest

    It should state somewhere in the manual. If not, then shame on Peavey. I generally love Peavey stuff so if they don't state the power at 8 ohms then thats dumb.

    Every amp ever is always massively advertised as power at 2 ohms, just for "wows." I think thats really pathetic because very rarely are you going to be using speakers that run at 2 ohms. Maybe a sub here and there..
  5. Horace

    Horace Guest

    I agree. Unfortunatley, the amp was bought second-hand on ebay, so no manual.
  6. chriscavell

    chriscavell Guest

    what model?
  7. killersoundz

    killersoundz Guest

    You should be able to find a PDF version of the manual online somewhere. You could probably even find it on musicians friend, on the product page, if not Peavey's site should definitely have it.
  8. Kev

    Kev Well-Known Member

    Nov 13, 2001
    John gave you the answer
    ... it might push to 750 but the above is close enough

    and you don't know the 2000W RMS going into 2 was for both sides driven.

    2 ohm specs are good as it does tell you what the current capabilities of the amp are. It is the power supply and impedance of the output that is starting show when the load goes very low.

  9. Horace

    Horace Guest

    Got it. The majority of power amp figures are for stereo driven pairs

    2000 W into 2 ohm =
    1300 W into 4 ohm =
    825 W into 8 ohm

    Thanks for the input guys.

    By the way V=IR is a measure of the amps power consumption, not power output.
  10. Kev

    Kev Well-Known Member

    Nov 13, 2001
    are you sure you want to start this ?

    yes V=IR will give the current and once you have the current you will get the power


    if your power amp has +/-100 Volts ... the rail voltage.
    That's 100 Peak
    around 70 volts RMS

    If the load is 16 ohms
    so 70/16=4.375 amps
    P= 70x4.375= 306.25 Watts

    If the load is 8 ohms
    so 70/8=8.75 amps
    P= 70x8.75= 613 Watts

    things get tuff here as the current goes up and the output devices and the power supply just can't deliver the current.

    If the load is 4 ohms
    so 70/4=17.5 amps
    P= 70x17.5= 1225 Watts

    and here almost no amp will double again and the voltage drop on the rail and the current limiting of all the series path takes effect ... this include the speaker cables and all current carrying components

    If the load is 2 ohms
    so 70/2=35 amps
    P= 70x35= 2450 Watts

    AND here comes the whopper ... only the BIGGEST beasts even get close to this one. It will need a serious AC power connection as this will send us over the available current in the average domestic power outlet ... For my country that's 240/230V AC at 10 amps .... 2.4 KW (2400Watts)

    If the load is 2 ohms
    so 70/1=70 amps
    P= 70x70= 4900 Watts ( twice the grunt of my outlets ... IT CAN'T be done ... must head for Three Phase Power)

    There are amps that can do this ... Aircraft carrier stuff here.

    ALL this and very few amps that we will use will in fact double from 8 ohms to 4 ohms but that's what we should be looking for.
    I do tend to run the Bass and Subs at three units per power-amp and that's around 2.6666 ohms
    JBL 2226 is not 8 ohms and more like 5.8 ohms to 6.2 ohms.
    Use only TWO units per amp with these babies.

    I typed all that very fast ... did I make a mistake in there ???

    I'll shut up now
  11. killersoundz

    killersoundz Guest

    Hey Kev, I don't want to get off topic, but while you're at it... whats the best way to wire up 2 speakers in a cabinet? I clearly understand ohms for the general purpose, but I get confused when it comes to doubling up. If you wire 2 8 ohm speakers together will that give you a load of 4 ohms? It's been a while since i've wired and soldered stuff. I completely forget even the most simple schematic for parellel and series wiring, which is sad. I just need a refresh.

    Also, it would be cool if you could give examples of safe instances of daisy chaining speakers without hurting your amp or the speaker. Well i'll be more specific. What if you have 2 8 ohm speakers and you daisy chain those together on a channel of a power amp, does that just increase the load to 4 ohms and everything will be ok? Ah wait, I mean...if you have 4 8ohm speakers and daisy chain 2 on each channel, because if you did it on just one channel I suppose that wouldn't be too healthy. Or wait, there are probably seperate rails for each channel.

    Ahhhhhhhh i'm frustrated with how rusty I am with this.
  12. Kev

    Kev Well-Known Member

    Nov 13, 2001
    personally I prefer parallel
    so yes two 8 ohm speakers will become 4 ohms and put more current demand on the amp.
    This means that each speaker with have the same potential(voltage) across each.
    Series can end up with a different potential across each speaker and so the output can vary ... this is due to differences in voice coils. This difference can increase if the heat dissipated in each coil is different and that inturn causes more difference.

    there was a time when speaker efficiency was rated at a given volts and not a given wattage

    A typical solid state amp will probably be fine with a 4 ohm load.
    A transformer'd tube amp will need to be double checked to see it will cope without generating too much heat.

    Often it is a juggle between amp spec and the number of speakers you want to drive.
    Many Quad boxes these days are series/parallel and built off 8 ohm speakers.
    8 ohm speakers are the norm
    BUT it might be better to stick with old method or using 4 x 16 ohm speakers.

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