8 Ohm - 4 Ohm Speaker question
- I have a Sony AMP/Tuner that specifies 8 Ohm Impedance Speakers at the rear hook up and several sets of 4 Ohm speakers.
Question: How can I use the 4 Ohm speakers with this 8 Ohm Amp - Or, can I?
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- --- In Electronics_101@yahoogroups.com, John Popelish <jpopelish@...> wrote:
>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.
> 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.
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!