Speaker Impedance

Discussion in 'Speakers & Subwoofers' started by John-Miles, Sep 24, 2003.

  1. John-Miles

    John-Miles Screenwriter

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    So what effect is there if you have a 4 6 or 8 ohm speaker?

    I know this is a very general question, but feel free to get as technical as you wish.
     
  2. PaulT

    PaulT Supporting Actor

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    To simplify it the best way I could would be to use Ohms law:

    R = V / I, where
    R = Resistance, V=Voltage, I= Current

    A speaker is not in fact a pure Resistance (it is really an Inductor with some capacitance) but in this simple explaination it could be thought of as a Resistor.

    If V is the output Voltage of the Amp (say for an example a maximum of 24V), and I is the Current drawn by the Speaker, then you can see that lowering the Speakers Impedence (or what we are calling Resistance) will draw more Current from the Amplifier:

    8 Ohms = 24V / 3A
    6 Ohms = 24V / 4A
    4 Ohms = 24V / 6A

    As a general statement, then, lower Impedence speakers will require amplifiers that can supply higher currents, or will overdrive a lower current amp quicker than higher Impedence speakers.
     
  3. Bob McElfresh

    Bob McElfresh Producer

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    In truth - your speakers are only 8 ohms Nominal.

    They actually change their impedence with the frequency. Low frequency sounds approach 1-2 ohms, higher frequency sounds go up to about ... 30 ohms.

    When there is a LOT of low frequency sounds on a system with full-range speakers, the receiver has to push a lot of current which generates HEAT. When "The Matrix" came out, a lot of receivers went into "PROTECT" mode during the lobby shoot-out scene simply because it tried to produce several minutes of very low frequency sound. The receivers shut themselves off because they got too hot.

    This is one of the reasons we have self-powered subwoofers and not self-powered mid/tweeters: it solves a LOT of problems to have a dedicated 300-2,500 watt power supply just to drive the woofer.

    Even a modest-grade receiver can drive 6/4 ohm speakers. But you have to be careful to not crank the volume way up, and it often helps to have a self-powered sub and define the speakers as SMALL so the receiver does not even try to send the power-hungry signals to the speakers.
     
  4. John-Miles

    John-Miles Screenwriter

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    So what is the difference between nominal 8 ohm speakers (most of them) and PSB speakers which are nominal 6 ohms. is there a significant margin here? will i be getting allot less out of my amp by having 6 ohm speakers as opposed to 8 hom speakers?
     
  5. Bob McElfresh

    Bob McElfresh Producer

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  6. John Garcia

    John Garcia Executive Producer

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  7. John-Miles

    John-Miles Screenwriter

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    Ok I have done soem more looking into this and as far as I can tell Bob is completly correct that you get higher currents, HOWEVER you get less power dissipated across the speakers.

    So which is more desirable?

    or does the 4/6 ohm switch on a receiver lower the internal resistance of the receiver to compensate for the lower resistance of the speaker?
     
  8. Chu Gai

    Chu Gai Lead Actor

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    No, it generally reduces the voltage that goes to the output stages by about 1/4. Using it with say 8 ohm speakers will lower the maximum power going to your speakers by a little. As a result, the receiver runs a bit cooler.
     
  9. Shane Martin

    Shane Martin Producer

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    Please don't make the mistake of buying speakers based on the ohm rating. It is a consideration on how you plan on driving them but buy what you prefer sound wise and then deal with how you plan on powering them.
     
  10. Chu Gai

    Chu Gai Lead Actor

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    good advice.
     

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