Relate sensitivity/impedance to speaker efficiency

There are a couple of other things to consider, in addition to the points that Al makes. some are here:

http://www.atma-sphere.com/papers/paradigm_paper2.html

As I understand it though, both speakers are friendly to tubes. However impedance plays a couple of other roles. Output 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.

While 4 db may not seem to be much, that is more than doubling the amplifier power to make the same sound pressure. That can be a significant cost! So it should not be ignored.

Thanks for the additional info. Its all quite interesting I think.

And I'm still waiting for a response from Acoustic Zen to verify whether their sensitivity rating of 89 for the Adagio floorstanders is for an input of 1W or an input of 2.83 volts.

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.

Now lets look at it as if both are under the Voltage Paradigm, (sensitivity, 2.83V 1 meter). Since the Zen is 6-8 ohms, and if the other speaker is 4, in this particular case the difference in sensitivity is about 3-4 db, which might make things easier to live with. But you should note that the reason the sensitivity spec exists is to accommodate amplifiers that can double power as the speaker impedance is cut if half. Tubes won't do that! Tube power is also expensive and while 3 db does not seem like a lot, it would be a mistake to think that you won't hear it.

It does seem to be "common knowledge" that 10 db sounds twice as loud.
However, what does that mean? Try listening to some music at a normal volume. Note the setting (this assumes you have a db scale to refer to).
then close your eyes, press and hold the Volume Up button until the sound is twice as loud to you. Then open your eyes & see how many db it took. It sure isn't 10 db for me.

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"?