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Ohmn I see

Discussion in 'Home Cinema Speakers' started by rich tee, Jan 17, 2005.

  1. rich tee

    rich tee
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    What's the deal with Ohmns numbers?
    Do my speakers have to match the receivers Ohmn number and what difference does it make?

    For example my receiver states: 1KHZ, 0.7percent THD at 4 ohmn one channel driven, 80w + 80w. Effective power output during surround operation, for Front, Centre, Surround.

    :(
     
  2. Mason @ B&M

    Mason @ B&M
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    The ohm reading is the impedance your speakers show your amp.

    The lower the impedance the easier it is for the amp to put it's power into the speakers.

    So at full power and connected to a 4 ohm speaker you amp will produce 80w.
    If you connected an 8 ohm speaker it would produce 40W and so on.

    Amps do run hotter when driving speakers with lower ohm loads though, and you don't want to run speakers which are lower than the amp can handle..
     
  3. Londondecca

    Londondecca
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    Generally speaking, unless you are using some esoteric equipment it is not really an area for concern
     
  4. rich tee

    rich tee
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    Is it therefore essential to have The same ohmn rating for each speaker?
    My Left and right are 6ohmn.
    My centre and Surround are 8 ohmn.

    Out of interest
    How best for an equal sound should I set them up?
    Alternatively how do you set up your speakers, in terms of level of sound output (option), per speaker/channel in the amp setup?
     
  5. pragmatic

    pragmatic
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    My Mains are 8 (although supposidly they can go down to 3ish with big bass) as are my rears but my centre is 6. I have the switch set to 8ohm on the back of the amp and it sounds good, although the centre does have to be turned down compared to the rest.
     
  6. eviljohn2

    eviljohn2
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    It's all a bit complicated. The impedence shown on your speakers is the nominal impedence. This is what the manufacturer states BUT the impedence of a speaker changes significantly depending on what frequency is being played. The upmarket B&W models for example are rated at 8ohms but can swing down to below 2ohms at low frequencies! Conversely, my Mission 771e's are also rated at 8ohms but don't drop below 6ohms at any point.

    The numbers quoted by manufacturers are generally pretty useless. The ideal amplifier will double it's power output as the load impedence halves (ie. Provide 200W into an 8ohm load and 400W into a 4ohm load). This is very difficult to do and only the very high end models get anywhere close to this.

    What you have to remember is that the voltage supplied to a speaker is dependent on the volume so at a constant volume, twice the current must be supplied to the speaker to double the power provided. Current supply is where a lot of amplifiers fall down, they may be fine for 8ohm loads but they certainly won't be able to provide twice the power into a 4ohm rated speaker.

    There are far too many variations but remember that you need a better specified amplifier to drive speakers that have a low impedence.

    Your manufacturer, for example, states an 80W output into a single 4ohm load. If it were an ideal amplifier it would only provide 40W into an 8ohm load. This is also for a single speaker, if you're using more than one at a time you can expect these values to drop further. :suicide:

    Hope that helps a bit but as I said at the start, it's a complicated area and most manufacturers are not to be trusted in it! :)
     
  7. pragmatic

    pragmatic
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    Also cheaper speakers (very cheap) run at 4 ohms too, but i suppose people here wouldn't use them.

    Examples of these are usualy cheaper 5.1 sets.
     
  8. giantteabag

    giantteabag
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    Not all 4ohm speakers are cheap. eg KEF Reference 3. Why did this company use 4 not 8 ohm for that range?
     
  9. pragmatic

    pragmatic
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    They have their reasons! (don't actualy know but i bet they do!)
     
  10. eviljohn2

    eviljohn2
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    Many (perhaps the majority) of "expensive" speakers are rated at 4ohms. I don't know the reason but would guess it's an intrinsic property of higher end voice coils. :)
     
  11. Reiner

    Reiner
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    Do not worry about mixing impedance, in most setups you will have a mix (fronts different to center different to rears).

    However note that a low impedance causes a higher load to the amp, so if the amp is not very powerful (which applies to most budget AV amps/receivers) you better choose speakers with a high impedance (6 or better 8 Ohm).
    However impedance is not the only criteria, also the sensitivity (measured in dB) has an effect on this. Rule of thumb: the higher, the better. E.g. 86dB is a tough load, 90dB is easier to drive.

    This is done via the AV amp/receiver's setup menu, preferrably you use a SPL meter for this (not your ears).
    Sitting at your listening position you will balance the volume of each channel to the same level (front channels are usually used as the reference).
    As well there is a parameter for compensating for the time delay caused by different distances to each speaker, either input in ms or in meter (depending on make and model).

    Newer amps/receivers have an auto-setup / auto-calibration where you place a mic connected to the amp on your couch and the above is done automatically.
     
  12. rich tee

    rich tee
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    Thanks for the above,
    I've always set the Balance of the centre, higher than the left/ right speakers.
    The rears one number higher than the left/ right.

    Is there a general rule of thumb in how you set up the balance/sound level to each channel or is it personal preference or am I barking up the wrong tree, and should just set each speaker to the same level?
     
  13. eviljohn2

    eviljohn2
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    All the speakers need to be the same level in actual volume, not just what the numbers on your amplifier say. This is where an SPL meter is invaluable but it can be done with your own ears. :)
     
  14. rich tee

    rich tee
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    I mean't to add the set up on the amp is in + or - db.
     

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