Relate sensitivity/impedance to speaker efficiency

Nnck, IMO everything in your two posts immediately above is correct, with the very minor exception (which is also irrelevant, based on the response from Acoustic Zen) that in the statement "If that is a measurement at 2.83 volts / 1m, that would mean a sensitivity of 87.5dB (since it is a 6 ohm speaker)" the figure "87.5" should be "87.75." The interpolation between 8 ohms and 4 ohms is not a linear function, since the dB scale is logarithmic, and impedance factors in as a reciprocal.

Best regards,
-- Al

02-11-11: Bifwynne
My question is whether I should think about raising the impedance load in my speaker circuit, perhaps by trying "high(er) impedance" speaker cables (if such things exist).

No, there are many reasons why that is not done and should not be done. For starters:

1)A lot of the amplifier's power capability would be converted into heat in the cables, instead of powering the speakers.
2)Tonal imbalances would result, due to interaction of wire resistance with variations of speaker impedance as a function of frequency.
3)Woofer damping would be severely degraded.

You may be thinking of something called "characteristic impedance," which is not the same thing as "impedance," and which does commonly have fairly high values (sometimes approaching 100 ohms). "Characteristic impedance" is, misleadingly, sometimes referred to as "impedance" for short. "Characteristic impedance" is a different subject altogether, which is primarily relevant at rf frequencies, and does not directly relate to your question.

Do I gain anything by trying the 4 ohm tap?

The only way to tell for sure is to try it. Given that your speakers have a lower impedance in the bass region than at higher frequencies, you may find that the bass is both tighter and stronger (relative to higher frequencies) on the 4 ohm tap. The maximum amount of power that the amp can deliver on the 4 ohm tap, though, will be less compared to what it can deliver into the SAME speakers on the 8 ohm tap.

Will the use of the new KT-120 tube change the impedance/capacitance analysis in any way?

Don't know.

Best regards,
-- Al

Thanks Al and Atmasphere: I experimented with the 4 Ohm taps. Although I can not offer a technical explanation, I can only say that my speakers sounded terrible across the board when played off the 4 Ohm taps: way less efficient, rolled off treble and muddy bass. By contrast, everything was great on the 8 Ohm taps. As I mentioned above, the speakers are nominally rated at 8 Ohms, so I guess that's the way the manufacturer intended them to be played.

I plan to switch out the 6550C power tubes for the KT 120 tubes in a month or two. There's an outfit that breaks the tubes in for 72 hours and matches the tubes using three criteria. The cost is less than half of what ARC charges. I may try them out. When I do, I'll report back. Hifigeek1 is an ARC buff, so he may be interested. Thanks again.

Hi Al, I was involved in a rather ambitious speaker cable project about 25 years ago, wherein we used a time delay reflectometer to analyze a variety of cable geometries. What we found is that characteristic impedance does indeed play a role (not nearly so important as it does at RF frequencies though) in the performance of the cable.

If the load is highly reactive, then the characteristic impedance can be important if the amplifier is otherwise unstable with the back EMF. Just a side note- obviously I am way OT here.

Nnck, my apologies- you were correct and it was me that was getting confused with all the conjecture. But as it has turned out, the less efficient speaker is on the Voltage Paradigm and the more efficient one is on the Power Paradigm, so now we find that it is true that one is about 4 db more efficient than the other, despite the sensitivity of them being almost the same.

Thanks, Ralph. Yes I recall discussing those experiments with you and others in a thread here about a year or so ago. And it certainly does seem conceivable that characteristic impedance could affect the sonic performance of a speaker cable in some systems, due to the effect you mentioned, as well as because of its correlation with inductance, and perhaps because of its relation to antenna and vswr effects.

As you realize, though, my point was simply that it is not relevant to Bif's question about easing the load on the amplifier, in the manner that going to a higher impedance speaker would ease the load.

I'll add, also, that to the extent that characteristic impedance may have audible consequences in some systems, both the anecdotal indications and some technical considerations would seem to suggest that in general lower is better. The technical considerations being the correlation between low characteristic impedance and low inductance, and possibly (although I am dubious) the impedance match with the speaker at high frequencies.

Best regards,
-- Al