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Need help with Ohms

Discussion in 'Home Cinema Speakers' started by John-D, Dec 15, 2002.

  1. John-D

    John-D
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    Hiya,

    I've just bought a Denon AVC-A11SR, and at the moment I'm using it with my old B&W 602's, CC6, DS6 THX Surrounds. I wanna buy the KEF THX set-up (EX-Demo) but am worried about the ohms. My amp can run at 180Watts at 6ohms, but KEF's site states, that the speakers run at 4ohms.

    Is this the lowest these speakers can run at? Or does it mean they can only be coupled to a 4ohm amp?

    Thanks for your time,
    JD
    PS. If this systems not for my amp, can anyone recomend a good system and sub? I can go up to around £2500
     
  2. GaryG

    GaryG
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    John

    Speaker manufacturers quote the 'nominal' impedance of their speakers, in actual fact, the impedance of a speaker varies with frequency, can be lower and higher than the stated impedance.

    The output stages of your amplifier have a maximun current delivery capability which if exceeded can result in either shutdown of the amp or, if there is no built in protection, destruction of the output stage (usually blown output transisitors). The lower the impedance of the speaker the more current it is capably of drawing, so a 4 ohm speaker will be capable of drawing more current than a 6 ohm speaker.

    So, providing you don't turn the volume up too high you should be able to use the Kef speakers without any problems, go overboard with the volume and you could either shut-down the amp, blow the output transistors or blow the tweeters in the speakers if the amp is driven into 'clipping'. Ensure you have plenty of ventilation around your amp to aid heat dissipation from the heatsinks and keep the volume at a reasonable level and you will be okay.


    Edited to include the point about clipping raised by NicolasB
     
  3. alexs2

    alexs2
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    With this receiver,you should have no problems running 4 ohm loads,as in the Stereophile tests of the US model equivalent,the Denon delivered up to 220W into 4 ohm loads.
    The Uk version,the AVC-A11SR,is THX Ultra certified,meaning amongst other things,that it is certified to deliver current into 3.2 ohm loads.
    The only likely problem you would encounter would come from running 2 sets of speakers in parallel,if the impedance of each set was below 6 ohms...the resultant load would be lower than the units minimum spec.....you'd need rather a lot of speakers for that!
    It's basically not so much the speakers you need to worry about,in terms of what load they present,as whether or not the amplifier is capable of running that load.....most amplifiers tend to run out of current delivery capacity into decreasing loads,and only very few,such as some Krells and Levinsons,will continue to deliver increasing power safely into 2 ohm loads or less...in the case of the speakers you've talked about,there should be no problems.
     
  4. NicolasB

    NicolasB
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    You can also damage your speakers doing this. If the output from the amplifier "clips" then the signal going into the speaker will try to move the speaker cone almost almost instantaneously from smooth motion to a dead stop and back again, which will trash the speaker. It's far easier to blow up a speaker by using an amp that isn't powerful enough than one that's too powerful.
     
  5. John-D

    John-D
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    Wow! Thanks everyone.

    I've learnt more about what Ohms do from you guy's, than what's in the booklet that came with amp.

    So the KEF speakers will push the power amps up to 220 watts per channel. I've looked through my manual, and it states that speakers below 4Ohms will trigger the built-in-protection to shut down the amp. So it looks like the KEF's will be OK!

    Only thing that's worrying me, is will the 4Ohm speakers shorten the life of the internial power amps?

    Sorry to be pain. :D

    Thanks again
    JD
     
  6. alexs2

    alexs2
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    No...shouldnt shorten the life of the power amps at all aslong as you dont utterly thrash the thing!
     
  7. NicolasB

    NicolasB
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    If the amplifier is beginning to clip then you should hear this as distortion in the output. If that happens, turn the volume down. :) If the output sounds undistorted even when the music is at its loudest then I don't think you'll do any damage to anything.
     
  8. John-D

    John-D
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    Cheers everyone.

    Thanks for your quick replies and help.

    Thanks again,
    John
     

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