Does an amplifier run cooler at 4 ohms than it does at 8 ohms

Discussion in 'Amps and Cabs [BG]' started by um... yeah, Nov 25, 2004.

  1. The amp I own produces a lot of heat at 8 ohms (bridged) into an 8ohm 410 and it concerns me. Apparently these units (Hafler P series amps) are known for this. My question is this:

    Would my amp be more efficient (produce less heat) if I was using two 4 ohm cabs in stereo? How about two 8 ohm cabs? 210s? 410s?

    Curiously submitted,

    um... yeah
  2. hell no. it runs hotter.
  3. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Not really. It would run hotter at 4 ohms bridged than it would 8 ohms bridged, but that wasn't the question.

    Question was to compare 8 ohms bridged to 4 ohms stereo per side. Personally, I'd think it'd be a wash and be about the same. Can't hurt to hook it up and try it that way and see, right?
  4. Passinwind

    Passinwind I Know Nothing Supporting Member

    You're talking about your Hafler, right? I always run mine in stereo with 8 ohm loads, it'll run noticeabley cooler that way provided you don't clip the amp.

    Stereo 4 ohm operation = bridged mono 8 ohm, in terms of current load on the outputs, which is where heat comes from. None of the Haflers I'm familiar with are rated for 4 ohm bridged operation.

    The higher the sensitivity of your cabs, the less hard you need to push the amp for a given sound pressure level, so a 4 X10 could actually be more benign than a 2 X10, assuming both are equal in impedance. If you use a lot of compression, that'll make the amp run hotter too.
  5. 4 ohms will always run hotter than 8. You are expending more energy to drive the 4 and energy relates to heat dissapation.
    Like a light bulb...go up in wattage...heat goes up.
  6. Thanks everybody for the info it's much appreciated. ;) Maybe running this puppy bridged isn't such a great idea after all, and I'm not one to take risks with his gear. Looks like there's another cab on the horizon!
  7. Tim__x


    Aug 13, 2002
    Alberta, Canada
    No, listen to Eric Moesle, 4 ohms a side will be no cooler than 8 ohms bridged.
  8. +100000000

    If you want to reduce heat you have to reduce power output. You could run your 8 ohm cab on one side of the amp and it would cut the heat output considerably. But it would also only get 1/4 the power. If you run another 8 ohm cab on the other channel you'll have the same volume as running the single 8 ohm cab in bridged mode but with less heat produced.

    I will attempt to explain the physics behind all this:
    When you bridge an amp, it puts both sides together in series. Therefore the minumum impedance is twice what each side can run individually. The total power put out at that impedance is the sum of what the two sides would put out at half the impedance. So:

    8 ohms 500 watts bridged = 4 ohms 250 watts/side stereo in heat generation.

    If you run in stereo at 8 ohms you will put out approximately half the power with significantly less heat.

    8 ohms 125 watts/side, but you can only use one side b/c you only have one cabinet.

    If you get another 8 ohm cab you can get 125 watts/side from both channels, giving you 250 total watts.

    If your current cab has an efficiency of 100 dB at one watt one meter:

    8 ohms bridged 500 watts will put out 127 dB at one meter

    Two 4 ohm cabs run in stereo will be 130 dB at one meter with the same amount of heat.

    Two 8 ohm cabs run stereo will be 250 watts, so -3 dB for half the power, but +3 dB because you have 2 cabs. So that is:

    8 ohms stereo 250 watts 2 cabs 127 dB at one meter

    And if you just use one 8 ohm cab on one side of the amp it will be 121 dB at on meter.

    I made all the numbers up in this post, so you can fill in your own, but remember that every time you double your power you add 3 dB and every time you add another IDENTICAL cab you add 3 dB.

    My suggestion is to use two 8 ohm cabs in stereo.
  9. Thanks for clearing that up. :D :D :D
  10. the dude

    the dude Supporting Member

    Sep 19, 2004
    That was *very* cool. Thanks!
  11. So if you have a mono amp, and the specs say it produces (approx.) 240 watts power at 8 ohms and 300 watts at 4 ohms, what are the advantages/disadvantages of each set-up?
  12. Jack


    Sep 6, 2003
    Newcastle, UK
    8 ohms.
    Pro - Cooler
    Con - Less power/volume

    4 ohms.
    Pro - More power/volume
    Con - Amp runs slightly hotter.

    Thats about it really, *MOST* heads will do 4ohms no problem.