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What's harder for a receiver: 6 ohm or 8 ohm speakers?

Discussion in 'AV Receivers' started by Stephen Gladwin, Oct 25, 2005.

  1. Stephen Gladwin

    Stephen Gladwin Stunt Coordinator

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    Simple question I know, but I've been wondering this for a while. Is it harder for a receiver to drive 6 ohm or 8 ohm speakers? It's been my assumption that the higher the impedence, the easier it is to drive, but I may very well be wrong. I'm wondering b/c I'm about to replace my 6 ohm surround speakers with 8 ohm JBL E10 speakers, and was thinking if they are easier to drive, my receiver wouldn't be as taxed. BTW: I'm using a Denon AVR-1705 receiver (75 watts/channel). Thanks!
     
  2. John Garcia

    John Garcia Executive Producer

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    Lower the impedance, the harder it is to drive because it draws (allows to flow) more current. Obviously there are other variables, but given two similar spec'ed speakers, the 6 Ohm will cause the amp to work harder. Impedance varies with frequency too, and what will be of significance is how low the speaker actually drops, but generically speaking, starting with a lower nominal impedance means the lowest it will drop will probably be lower than a speaker with a higher nominal impedance, but that's not a guarantee though.

    In your case, you may actually benefit from using 6 Ohm speakers if they are not stressing the receiver out, because they give you a little boost in output on those channels; the side effect of which though is the amp is working a little harder. If there's nothing wrong with the sound, I wouldn't bother switching them out.
     
  3. Steve_L

    Steve_L Stunt Coordinator

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    I went to electronics school a long time ago, but I remember the basic rule that maximum power transfer and efficiency was achieved when the output impedance of one circuit and the input impedance of the other were matched. Amplifiers are designed to drive speaker loads that ideally present a consistent 8 ohms load (impedance) across the audio spectrum.

    In practice, speakers will provide a much more complex input load to the amplifier, varying widely over the audio spectrum in impedance from something greater than 8 ohms all the way down to 2 ohms.

    Quality amplifiers can handle this variation in impedance while lessor amplifiers don't do so well. (Your Denon prolly will do just fine.)

    I wouldn't worry about the 6 ohm rating too much, but also wouldn't count on noticing much difference in output based on the impedance rating.

    What is much more likely to cause a noticeable difference in your perceived performance is speaker efficiency. Efficient speakers providing much more sound pressure level (volume) with less required output power from your amplifiers. Look for a high sensitivity rating in choosing speakers.
     
  4. Shane Martin

    Shane Martin Producer

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    Ding Ding ding. This is the biggest difference. Check your sensitivity ratings.
     
  5. John Garcia

    John Garcia Executive Producer

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    That would be a speaker's sensitivity, which is not the same as efficiency.
     
  6. Steve_L

    Steve_L Stunt Coordinator

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    >>> That would be a speaker's sensitivity, which is not the same as efficiency.
     
  7. John Garcia

    John Garcia Executive Producer

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  8. Chu Gai

    Chu Gai Lead Actor

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    Well, efficiency is generally represented as a percentage but one can also express it as so many dB @ 1 watt @ 1 meter. Imagine a hypothetical source that is able to radiate 1 watt of acoustical power. One can calculate that the SPL on the surface of a sphere that is 1 meter from the imaginary source will be approximately 109 dB. This assumes 100% efficiency. Now take an actual speaker and pump that same 1 watt into it and let's say the measured SPL at one meter from the speaker is now 89 dB. Well, that's 20 dB less which means that this speaker is 1% efficient. This though, applies only to direct radiator type speakers and not horns.

    Sensitivity is a convention that states that one uses 2.83 volts and measures the SPL at one meter from the speaker. Into an 8 ohm load, 2.83 volts happens to correspond to 1 watt of input.

    Therefore, simplistically, one can use sensitivity to get a handle on which of two speakers can play louder. There really is no easy answer to which places less strain on your receiver and in fact, depending upon how you've set your crossover for your sub, where you sit in relation to the speakers, where you've placed them, how loud you listen to them, etc. can all have an effect on your receiver's ability to drive your speakers successfully. It's not hard to imagine scenarios where either of the two speakers present difficulty. OTOH, it's not hard to imagine scenarios where it doesn't matter a hill of beans.
     
  9. Steve_L

    Steve_L Stunt Coordinator

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    Here's two definitions of efficiency plucked from the Home theater Glossary of Terms. http://white.hometheatertalk.com/glossary.htm#e

    Efficiency - The ability of an audio device to turn mechanical energy to electrical (microphones, phonograph cartridges) or vice versa (loudspeakers, amplifiers). For example, the more efficient a loudspeaker is, the louder it will play with a given input. A typical acoustic-suspension speaker may be anywhere from 0.5% to 2% efficient; some horn speaker systems surpass 20%. The leftover energy is dissipated as heat. Under most conditions, efficiency has little to do with sound quality, but with speakers, high efficiency allows one to use a lower-powered amplifier.

    Efficiency Rating - The loudspeaker parameter that shows the level of sound output when measured at a prescribed distance with a standard amount of energy fed into the speaker. Efficiency rating standard is 1 watt at 1 meter and is measured in decibels.
     
  10. John Garcia

    John Garcia Executive Producer

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    The second definition is incorrect. There is no such thing as an efficiency rating for a speaker. The correct term for what they are describing is sensitivity as measured. Efficiency can be calculated from that measurement, but it is generally not used to describe speaker characteristics. IMO, I'd submit a correction to the author of that glossary.
     
  11. Shane Martin

    Shane Martin Producer

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    Stephen,
    Something to consider efficiency and whatnot. Klipsch are well known to be an efficient speaker and some question why Klipsch folks would use an external amp but it's been shown they have a major ohm dip to the point where it will drain you amp of its power pretty quick if you crank it.

    Your receiver should be ok for the JBL's but if you decide to crank it up, you'll hit the limits of your amp pretty quick. If you don't listen very loud, then I wouldn't worry too much.
     
  12. Jerome Grate

    Jerome Grate Screenwriter

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    I've been running my Platinum Audio speakers all around 6ohms through the HK 500 with out an issue.
     
  13. John Garcia

    John Garcia Executive Producer

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    I'm running three 4 Ohm (center and surrounds) speakers on my Marantz 8300 which is rated for 6 Ohms minimum, and it works, but the surrounds generally aren't working too hard, so the receiver can handle them without issue. My mains are handled by monoblocks.

    Here's what it comes down to: If you take two similar speakers with the same sensitivity, "A" being 6 Ohms and "B" being 8 Ohms and drive them to the same measured SPL, speaker A will draw more current than speaker B to achieve that same SPL.
     

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