Increase total Speaker Impedance Question

Hi Bill and Almarg,

Thanks again for your detailed explanation. I understand the power loss in this setup. However, I am still having difficulty grasping why the sound quality would also take a turn for the worse. Will the frequencies be improperly passed or cut off by the resistor? If not, then, all frequencies will still be passed to the speakers, but at lower power, and that shouldn't affect the sound at all theoretically isn't it?

Thanks!!

I did make an error in my initial calculations above resulting in the discrepancy, and Al's math is of course correct. I believe that the primary difference in sound quality will be due to a loss of damping factor from the amp when driving the additional series resistance. The only way to really know how significant the difference will be is to try it with your specific setup. I can't predict it beyond that with any real degree of confidence.

Angelgz2, see this Wikipedia writeup on the voltage divider effect. In Figure 1 consider Vin to be the voltage the amplifier (or receiver) is "trying" to output to the speakers at any given instant. Consider Z1 to be the sum of the amplifier's output impedance and the 3 ohm series resistance. For nearly all solid state amplifiers the output impedance will be negligible, and so Z1 will be essentially 3 ohms. Consider Z2 to be the combined impedance of the paralleled speakers (nominally 1.6 ohms), and Vout to be the voltage applied to the speakers.

As you will see in the writeup, the voltage applied to the speakers will be proportional to Z2/(Z1 + Z2). Since Z2, the combined impedance of the paralleled speakers, will inevitably vary significantly as a function of frequency (to a greater or lesser extent depending on the particular speakers), Vout will vary significantly as a function of frequency. The result will be significant frequency response irregularities.

That will be similar to what happens, as we discussed earlier, when two non-identical speakers are connected in series, except that it will probably not occur to as severe a degree in this situation since Z1 is constant.

Also, whatever damping factor the amplifier may provide will be reduced to the rough vicinity of 4/3 = 1.33 for 4 ohm speakers that may be included in the combination of speakers, or 8/3 = 2.67 for 8 ohm speakers that may be included in the combination of speakers. (The reason I'm saying "the rough vicinity of" is that I'm ignoring the damping effect of each speaker on the other speakers). Those are very low damping factors. Chances are the result will be loose and flabby bass, to a greater or lesser degree depending on how critical woofer damping is for the particular speakers.

Finally, to the extent that woofer damping is degraded by the presence of the resistor it seems conceivable that "back emf" generated by the woofer of each speaker in the parallel combination, instead of being effectively damped by the amplifier's low output impedance, could to some degree enter the other speakers and be reproduced as sound, thereby adding distortion to the sound each speaker should be reproducing.

Regards,
-- Al

Thanks again Bill and Al,

So in short, there's no way to rectify my current situation with a resistor. Is there possible to redesign a 4 way crossover so that the speakers can be wired as a series or series-parallel? Or, as a last resort, to use separate amplifiers for each channel.....

So in short, there's no way to rectify my current situation with a resistor.

No, we haven't said that. Despite all the potential issues we've cited, using the resistor with the paralleled speakers could conceivably still result in acceptable sonics, and as Bill indicated it does seem to be worth trying.

Is there possible to redesign a 4 way crossover so that the speakers can be wired as a series or series-parallel?

Not sure exactly what this means, but I think you had indicated that all of the speakers that would be used in each channel were different designs, so connecting any two of them in series is unlikely to result in good sonics.

Or, as a last resort, to use separate amplifiers for each channel.....

If you mean using separate amplifiers for each speaker, or perhaps for each parallel combination of two speakers, yes, you could of course do that, as Czarivey indicated earlier. You'd have to make sure that whatever is driving the multiple amplifiers, such as a preamp, or pre-out connections of the receiver if it provides them, would have a low enough output impedance to be able to properly drive the combined (paralleled) input impedances of the amps. Although with most solid state preamps that is unlikely to be a problem.

Regards,
-- Al