How to accurately gauge speaker sensitivity to match with tube amp?

I think you may be better off asking people which amp they use with their Focal 936s or Focal speakers in general.  There may already be threads on this topic that you can consult.

Try changing the output taps on the amp and see if that helps!

Your advice can help me eliminate candidate speakers that would pose similar challenges to my amp as these Focals have.

Yes, David (@hilde45), as you realize and as I have said in a number of past threads that is precisely the main potential usefulness of specs and measurements. **Ruling out** candidates that are likely to be mismatches with either user requirements (e.g., desired peak volume capability) or with other parts of the system, thereby narrowing the field of candidates as well as reducing the likelihood of expensive mistakes.

Your Adcom and QS amps have similar power ratings, so it seems that for your purposes speaker sensitivity is not an issue, as long as it is in the area of 90 db or so or higher. If you already haven’t, though, do make sure that the brief dynamic peaks of recordings having wide dynamic range (i.e., large differences in volume between the loudest notes and the softest notes, such as in the case of many classical symphonic recordings) are reproduced cleanly, without noticeable distortion.

What should I notice in those curves? Just that they don’t dip down as you described -- down to... 2 ohms? For how far across the mid bass?

Erik has given you excellent advice re impedance curves, and as he alluded to it’s hard to provide a more specific answer that would have general applicability. But I agree that in this specific case the 936’s impedance curve raises a lot of concern about how optimal it would be for use with tube amps. Although the relatively high damping factor/low output impedance of your QS (for a tube amp) will tend to lessen the degree of adverse impedance interactions with this speaker than would be the case with many other tube amps.

The suggestion of researching what kinds of amplification others have used with specific speakers is of course also a good one, as I’m sure you realize, as is the suggestion of changing the output tap that is being used. Especially if you have been using the 8 ohm tap, which perhaps you have been doing since the speaker has an 8 ohm nominal rating.

On another note altogether:

... as Roseanne Rosanna Danna used to say, never mind.

That was actually said by Gilda Radner’s Emily Litella character. Her Roseanne Roseannadanna character’s famous line (and also the title of a book Gilda wrote, which I believe dealt extensively with her battle with cancer) was of course "it’s always something."

Regards,
-- Al

The Focal isn't crazy low for tubes but it will give them a hard time. You should use the 4 ohm tap with this speaker.
Generally speaking though if you want to get the best out of a tube amp then the speaker should not go much below 8 ohms especially in the bass region! If the amp has output transformers (and most do) its common for the output transformer to lose bandwidth by as much as an octave on the bottom when using the 4 ohm tap (and that's even when its properly loaded with a 4 ohm load). This can cause the amp to have additional phase shift and that is interpreted by the ear as a loss of impact.


So I see two ways out of this if the 4 ohm taps don't do it for you and you want to keep the amp. Either get a set of ZEROs (www.zeroimpedance.com) and see if they can help you out (by allowing you to use a higher tap on the amplifiers) or get a set of speakers that are an easier load.


Its really not a good idea to make an amp work hard for a living. They make a lot less distortion if they have it easy, and that translates directly to 'sounding more real'.

@erik_squires 

 The more variation, the more the impedance curve takes command of the frequency response….

…but look how broad the under 4 Ohms is. That’s what I’d listen for. For the most neutral presentation from these speakers you are going to want a beefy solid state amp.

Great! Very helpful. FWIW, I only looked at the curve after I knew I was hearing something and wanted to understand why. I had seen such graphed curves before, but now they are connecting with my listening experience; they have meaning, now.

@miller

The only measurement you need is sensitivity greater than 90. Then from there you pay attention to listening impressions. Then when you find people describing a sound that matches what you like look for people who are hearing that sound with tube amps…..But on the other hand, maybe you don't care about such things as musical involvement.

LOL! If I didn't care about musical involvement, I'd be a fool. I hope I've not given off that impression with my question. I'm not a specs fanboy by a longshot, and your suggestion is most appreciated. People's taste (and hearing) can vary a lot, and so sometimes that's an X factor which is hard for me to glimpse in the many posts mentioning speakers in which I might be interested. Another tool (like the sensitivity number) was what I was seeking here, but I'll remain open to the possibility that you're right — that more than just sensitivity measurement is worthless and I should read up on listening impressions almost exclusively. (And, of course, all of this is not replacing listening, but getting whittled down the number of things to which I might go to the trouble of auditioning.)

@yogiboy  Thanks— good suggestion about the taps. They're currently on the 4 ohm taps, though.

@almarg That's right — I'm trying to use specs to help delimit a rather wide range of choice. The power of the amps are similar, but I know their damping factors are very different (20 vs. 100, at least). I will look into what other people's anecdotal reports say, too. I never ruled that out, but the metric of speaker sensitivity has been very helpful in narrowing down my choices and that's why I'm looking into related metrics, now.

The dynamic jumps in symphonic music (I spent a day just listening to symphonies) were not distorting with the tubes.