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

Again, David, keep in mind that the QS’s damping factor is higher than it is for many and I’m pretty certain most high quality tube amps. Assuming the spec is accurate, for the 4 ohm tap a damping factor of 20 means an output impedance of 4/20 = 0.2 ohms, not too far from solid state territory.

Although many audiophiles believe differently, Ralph ( @atmasphere ) and some other technically knowledgeable members (e.g. @kijanki) have explained in past threads that no speaker in existence requires a damping factor greater than the mid to high double digits, and perhaps even lower. Perceived differences in bass response and control between damping factors of say 100 and 2000 are due to other differences in the designs of the amplifiers, IMO and theirs.

Best regards,
-- Al

" A high damping factor will provide very good control of the bass drivers."

Hmmmm.

For a quick summation of my thoughts, skip to the last two paragraphs. Apologies for getting fairly nerdy in between here and there.

In practice, any series resistance in between the amplifier and the woofer’s voice coil effectively ADDS TO the amplifier’s output impedance, and correspondingly reduces the damping factor.

Let’s run some numbers. Supposes our speaker has a nice 2.5 kHz second-order crossover, which calls for a 1 mH inductor in series with the woofer. Power handling requirements are easily met by an 18 gauge air-core inductor, which can handle 300 watts before saturation. The series resistance of this inductor is .51 ohms.

And let’s suppose we have an uber-amplifier with a damping factor of one zillion. Or one zillion zillion. Or one zillion to the zillionth power. It won’t matter.

After the signal passes through that inductor, our uber-amplfier’s amplifier’s effective damping factor is now about 17. And this is assuming only the one series inductor, and ignoring any other wiring.

So in most cases it really doesn’t matter how high the amplifier’s damping factor is. The series resistance in the crossover (and/or speaker wires) dominates.

Okay, but what about this "very good control of the bass drivers" that we’re apparently missing out on?

Well, turns out that it’s not nearly as dramatic as the wording implies. It all shows up as a change to the electrical damping of the woofer’s motor - the electrical system Q, or Qes.

Assuming a typical high-quality 8-ohm woofer in the example above, the series inductor effectively raises the woofer’s electrical Q by about 7%. So if the woofer’s electrical Q was .28, the series inductor effectively raises it to about .30. This could EASILY be an improvement!  We'll get more bass with a higher Qes, but the designer should take it into account by sizing and tuning the box based on our modified Qes of .30. And if he hasn’t, this difference can still be largely compensated for with a few handfuls of stuffing material.

I think amplifier marketing departments may have oversold the benefits of having a high damping factor.

Or to put it another way, in my opinion, super-high damping factors are, in most cases, of academic interest only. I certainly would not trade off anything that really matters in order to get a high damping factor.

Duke

Hi David- Here's the impedance and phase curves of the series crossover Carbon 7 SE Mk2 loudspeakers.  
http://fritzspeakers.com/Images/speakers/Carbon7mk2ScanDome.jpg

http://www.fritzspeakers.com/DreamWeaverFritzImpedancePage.asp

And a bunch of comparative measurements.

Fritz wrote:

" Hi David- Here’s the impedance and phase curves of the series crossover Carbon 7 SE Mk2 loudspeakers.
http://fritzspeakers.com/Images/speakers/Carbon7mk2ScanDome.jpg "

In case anyone doesn’t understand the implications of what they see at the link, that impedance curve is SUPERB!

I had never taken the time to look that deeply into Fritz’s speakers. I have heard them consistently sounding disgustingly magnificent at audio shows, to the extent that I have absolutely NO DESIRE to compete with him. And now looking at his impedance curve, I can see why: He’s cheating. He’s making life so easy for the amplifiers that they all simply love his speakers.

Tube amplifiers may well love his speakers the most: They will reward you with more output in the bass region. This is about as close to a "free lunch" as it gets.

Duke
competing in very different market segments from the one Fritz owns