Can someone explain to me IMPEDENCE ?

Discussion in 'Archived Threads 2001-2004' started by Jim Carr, Jan 4, 2002.

  1. Jim Carr

    Jim Carr Stunt Coordinator

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    Keep in mind I'm a HT newb [​IMG]
    Learning as fast as I can.. but there is alot to take in! lol. I don't understand impedence.. can someone explain it using the smallest words possible? Ok maybe not the smallest words.. but keep it simple for a novice like myself can understand [​IMG]
    thanks alot!
    Jim
     
  2. Tom D

    Tom D Stunt Coordinator

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    in simplest terms, impedence = resistance. Typically used when stating speaker impedence (resistance). The average speaker is rated at 8 ohms for example, now this is an average rating, because the impedence changes with frequency or the load that you are trying to reproduce. The lower the resistance the closer you are to a short cct. So in terms of amplifiers, the power ratings are averaged to 8 ohm loads, but the important number to look at is the 4 ohm loadpower rating because that gives you a good idea about the power supply. If you half the load, you should be able to double the power. If you familiarize yourself with ohms law then it becomes a little clearer on what the specs are trying to tell you.
    Ohms Law: Voltage = Resistance x Current, Wattage = Voltage x Current
    In a nut shell, hope this helps.[​IMG]
     
  3. ThomasW

    ThomasW Cinematographer

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  4. John Miles

    John Miles Stunt Coordinator

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    Briefly, impedance (Z) is resistance to the flow of an AC current. It is measured in ohms, just like conventional DC resistance.

    A pure resistor has the same effect on current of any frequency, from DC on up. But in the real world, there's no such thing as a "pure" resistance. Speakers, in particular, are a complex combination of resistance, capacitance, and inductance. The latter two properties mean that a given value of impedance is dependent on the frequency at which you measure it -- something that's not true of DC resistance.

    A given impedance value always has both resistive (power-consuming) and reactive (phase-shifting, but not power-consuming) components. It's a vector (R+jX) quantity, where R is the plain old ordinary DC resistance component and X is either capacitive (Xc) or inductive (Xl).

    As if all this weren't complicated enough, it's not even possible to state the precise impedance of a component such as a loudspeaker, because it will vary dramatically depending on the frequencies and amplitude levels the speaker is reproducing at any moment. Because the figures you generally see in print ("8 ohms" and "4 ohms") are derived under particular conditions involving a pure test tone at a standardized frequency, all they really tell you about the real-life characteristics of different speakers is that one has roughly half the impedance of the other.

    Any good-quality power amplifier should have no problems driving either 8- or 4-ohm speakers, regardless of whether it has a speaker impedance switch. It takes less voltage (but more current) to get the same sound pressure level from a 4-ohm speaker, all other things being equal, but because speakers are strongly dynamic loads to begin with, any amp that can't safely drive a 4-ohm speaker is a hopeless POS anyway.

    In terms of sound quality, a vacuum-tube amp with a transformer output is likely to be more particular about working into its specified speaker impedance than a solid-state amp that acts more like a "stiff" voltage source. At the end of the day, though, as long as your speakers sound good with your amp and you don't hear any distortion or smell any smoke, don't sweat the impedance thing too much.
     

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