Relate sensitivity/impedance to speaker efficiency

Atmasphere: I use an ARC VS 115 tube amp in my rig. Its rated output is about 115 watts driven into an 8 ohm load. My speakers are nominally rated at 8 ohms, but as mentioned above, their impedance curve drops in the lower frequencies and their impedance is more capacitive in the low frequencies. The amp/speaker combo subjectively sounds ok to me and plays quite loud, but maybe I'm not listening critically enough.

You mention that "[o]utput transformers in tube amps tend to be more efficient when driving higher impedances. Certainly speaker cables become less critical as the impedance of the speaker is increased. The result of this is that (all other things being equal) as you raise the speaker impedance it will appear to become smoother and more detailed, regardless of the amplifier used."

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). I think I'm using Kimber speaker cables -- don't remeber which type. I also run my speaker cables off the 8 ohm taps. Do I gain anything by trying the 4 ohm tap?

Oh . . . one more factor to consider. ARC is replacing the SED "Winged C" 6550C power tube with the new Tung Sol KT-120 tube. I understand that the KT-120 tube will increase the power output of the VS 115 by about 10%. I also understand that the new KT-120 tube sounds better than the SED 6550C tube. Will the use of the new KT-120 tube change the impedance/capacitance analysis in any way? That is will my speakers sound "smoother and more detailed"?

Nnck, First lets look at the two options as if they are both on the Power Paradigm (efficiency, 1 watt 1 meter).

Under this model the Acoustic Zen is 90/91db. The other speaker is 84/85db if it is also 4 ohms. If you want a tube amp the latter is going to be hard to live with unless you are near-field.

I lost you just a bit here. Seem like under the Power Paradigm, if the Adagio is listed as 89dB efficient, despite it being a 6ohm speaker, it's efficiency is still just 89dB, not 90/91 (that is, if the efficiency was measured in units of 1w / 1m, it wouldnt matter that the impedance is 6 ohms, and not 8).

Is still havent heard back from Acoustic Zen, but the Dali Helicon 800 is very clearly labeled on the manufacturers website with a sensitivity rating of 89.5dB at 2.83 V/1 m. Which means it must be 86.5dB at 1w / 1m (since it is a 4 ohm speaker).

Acoustic Zen lists the Adagio with an Avg. Efficiency (not 'Sensitivity' if that is at all meaningful) of 89dB SPL @ 1 meter. 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). But if the measurement was at 1 w / 1 m (as some websites note), then the efficiency is simply 89dB, as stated.

Of course, I am new to this. So I dont know if I have it all right.

Still hoping for some explanation of the comments I made directly above.

But I did hear back from Acoustic Zen this afternoon and they verified that the efficiency of the Adagio floorstanding loudspeaker is about 89dB and the measurement is for 1w / 1m.

So it looks like I am comparing 2 speakers: the Adagio (6 ohm speaker) is 89 dB at 1w / 1m vs. the Dali Helicon (4 ohm speaker) which is 89.5dB but at 2.83 volts / 1m.

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