Relate sensitivity/impedance to speaker efficiency

Meiwan, see if this helps:
at 8 ohms on 87db speakers, at 1 meter in front of them
1 watt = 87db
2 watts = 90db
4 watts = 93db
8 watts = 96db
16 watts = 99db
32 watts = 102db
64 watts = 105db
128 watts = 108db
It takes double the wattage to gain 3db.
You have to gain 10db to hear a "perceived" doubling of volume. No one is going to be able to tell the difference between an 87db speaker and a 90db speaker unless your amp is only 3 watts(2A3 tube amp).
However, impedance is another issue. Most solid state amps will double power output with halving of impedance. Some only give a 50% or so gain. Most tube amps will not give any increase in output.

For Nnck-are we talking stereo or theater. Almost any 2 channel amp will handle 4 ohms. Many theater receivers have issues driving five 4 ohm speakers. Be careful there. Do the math when looking but most theaters require 110db to 112db continuous output for THX or Ultra THX.

Elevick-
I am mainly concerned with 2-channel stereo right now. And I am mainly looking for help in comparing speakers relative to one another with respect to how well they might perform with tube amplification.

Specifically, I would love to hear what anyone has to say about Dali Helcion 800 vs Acoustic Zen Adagio given the info on sensitivity and impedance charts I provided in my post previous to yours. Thanks again everyone.

The Dali Helicon 800 is a 4 ohm speaker with a listed sensitivity of 89.5. I see the units shown on the Dali website are 2.83 V/1 m. According to what Almarg is saying this would correspond to a sensitivity of 86.5 for a 1w input - is that right?

Yes, assuming the specs are accurate (which is not always the case).

If the 1w / 1m unit is true for the Adagio, then according to what Almarg is saying, to directly compare these speakers you would have to say the sensitivity of the Dali is 86.5 vs 89 for the Adagio for a 1w input.

Yes, assuming the specs are accurate.

The impedance curve of the Adagio is fairly flat as seen here in the 6moon review.

Looks about as good as it gets, in terms of flatness and avoidance of severe capacitive phase angles. I would expect it to work beautifully on the 4 ohm tap of most medium powered tube amps. Perhaps even with an OTL (output transformerless) amp, with the flatness of the curve possibly overshadowing the low nominal impedance. I would definitely expect it to work well with an OTL with a pair of Zero autoformers connected between amp and speakers.

The impedance curves of the Dali 400 don't look particularly worrisome either, but I would not assume they are similar to those of the 800, as the driver complements are quite different. You may want to search the discussion threads and system descriptions here for the 800, and see what amps people are using with it.

With respect to how much power you would need, obviously that depends on listening distance, room size, volume level, and on the dynamic range of the music you listen to (the difference in volume between the loudest and softest notes, classical symphonic music being one of the most challenging kinds in that respect). For most people, I would expect 50W to 100W to be comfortably adequate with the Adagios, assuming 89db/1W/1m, and perhaps 80W or more for the Dali's. Those numbers can vary widely, though, depending on the variables I listed, since as was mentioned by the others a factor of 10 difference in power (which is 10db) corresponds roughly to just a factor of 2 difference in subjectively perceived volume.

Regards,
-- Al

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.