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Re: 8 Ohm - 4 Ohm Speaker question

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  • AlienRelics
    ... There is also the fact than an 8 ohm speaker is not really 8 ohms. It is more of an average, as it changes depending on frequency. I have 1kW dummy loads
    Message 1 of 40 , Jul 2, 2012
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      --- In Electronics_101@yahoogroups.com, "gabevee" <gabevee@...> wrote:
      >
      >
      > --- In Electronics_101@yahoogroups.com, "Frank P" <qz9090@> wrote:
      > >
      > > Gabe,
      > > If you use 3 speakers as you suggested and have 6 ohms resistant, doesn't that have the potential of drawing too much power from the amplifier (resulting in damaging the amp)?
      > >
      > > With the three speaker configuration you would need a 2 ohm resistor, wouldn't you?
      > >
      > > My question would be what size resistor?
      > >
      >
      > NO.
      >
      > As I mentioned before, most amps work from 4 up to 16 ohms. 6 ohms is nominal, actually. Many high end speakers are 6 ohms to accommodate both transistor amps and tube amps with 4 or 8 ohm outputs.
      >
      > Gabe
      >

      There is also the fact than an 8 ohm speaker is not really 8 ohms. It is more of an average, as it changes depending on frequency.

      I have 1kW dummy loads made from heating elements. They are 5.7 ohms and work just fine. The impedance is flat up to something like a MHz or so.

      Steve Greenfield AE7HD
    • gabevee
      ... My contention about the 4 ohm resistor was not about sound quality. but about power waste. Yeah, 3dB is not perceptible but being able to provide another
      Message 40 of 40 , Jul 5, 2012
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        --- In Electronics_101@yahoogroups.com, John Popelish <jpopelish@...> wrote:
        >
        > On 07/04/2012 04:27 PM, Goran Finnberg wrote:
        > > Gabe:
        > >
        > >>> We were talking about damping. The amplifier has some output
        > >>> impedance that damps the electromechanical transducer
        > >>> impedance, but the DC resistance of the speaker voice coil
        > >>> is effectively part of the amplifier output impedance, as
        > >>> far as that damping process goes.
        > >>
        > >> How? The output impedance is only less than some 0.1 ohms.
        > >> The speaker's resistance is far greater. Do you perhaps
        > >> mean that the effective *damping* is determined by the
        > >> speaker impedance inasmuch as that is what the equation
        > >> dictates, as opposed to the speaker resistance being part
        > >> of the output *impedance*?
        > >
        > > http://tinyurl.com/cqdxz7z
        > > <moderator checked link>
        >
        > Thanks for this well written expose of the dam[ping factor
        > debate. It demonstrates, very simply, how damping has very
        > little to do with amplifier output impedance (unless you
        > make one with negative output resistance). The voice coil
        > resistance dominates damping for almost any modern
        > amplifier. And this is why adding a 4 ohm series resistor
        > to a 4 ohm speaker to limit the peak current drawn from an
        > amplifier rated for an 8 ohm speaker won't make the speaker
        > sound terrible, because of no damping. The 4 ohm speaker
        > didn't experience much damping, to start with.
        >

        My contention about the 4 ohm resistor was not about sound quality. but about power waste. Yeah, 3dB is not perceptible but being able to provide another speaker with that 25 watts is.

        As for the internal resistance of the amp not being an important factor, that's odd, since page two paragraph one calls amplifier output impedance "the most important factor". If it wasn't it would not be a big part of the equation. DF = Rload/Rsource. But the article is saying both the internal resistance plus impedance of the speaker needs to be included for that numerator. Also big numbers mean nothing above a certain point fro speaker control and sound quality.

        As for sound quality, there are always points of diminishing returns. Damping factor like THD to the thousandth and frequency response into the hundreds of thousands of hertz are tools of the advertizing trade which are always used to lure the unawares.

        An amp of only 30-18kHz at +/- 0.3dB and a THD of 0.2% with a damping factor of 15 can kick butt!

        This article really gets into the dirt about damping - http://www.trueaudio.com/post_013.htm.

        It shows how a damping factor of 3000 can disappear when all the actual resistances are included.

        If an amp has a Rsource of 0.0025 and plugged into the equation with a load of 7.5 (the four ohm speaker), then it has a DF of 3000. However, with a wire having 0.25 ohms added to that 0.0025, then one has 0.2525. That reduces the DF to 29.7! Fascinating stuff! LOL!

        Gabe
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