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Re: 8 Ohm - 4 Ohm Speaker question

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  • gabevee
    ... Ah! Thanks for the correction. I was thinking in terms of the resistor being part of the load. Yes, the back emf damping (from the speaker) ought to see
    Message 1 of 40 , Jul 2, 2012
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      --- In Electronics_101@yahoogroups.com, lists <Stuartlists@...> wrote:
      >
      > In article <jssba4+bkte@...>,
      > gabevee <gabevee@...> wrote:
      > > I thought damping factor was the result of output impedance of the
      > > amplifier and the NFB loop? In modern direct coupled amps, damping
      > > factor is not really an issue as it is usually in the hundreds.
      >
      > Correct.
      >
      > It's actually the ratio between the amplifier output impedance and the
      > loudspeaker impedance and governs how well the amplifier controls the
      > loudspeaker.
      >
      > Unfortunately by adding a 4 ohm resistor you are effectively making the
      > amplifier output impedance, as seen by the speaker, 4 ohms
      >

      Ah! Thanks for the correction. I was thinking in terms of the resistor being part of the load. Yes, the back emf damping (from the speaker) ought to see close to zero, but will instead see the 4 ohms, making the effective damping ratio 1.

      Thanks!
      Gabe
    • 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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