That "tube sound" and power ratings

We all know there are plenty of 8 ohm speakers. 16 ohms is a little harder to find. Some examples:

Classic Audio Loudspeakers
Audiokinesis (several models)
High Emotion Audio (Bella Twin- one of the best monitors I've heard)
Quad ESL57 and ESL 63
Lowther, PHY and other full-range drivers
Coincident Technology (not always 16 ohms, but definitely higher impedance)
Rogers LS35A
many vintage horns- Altec, JBL, EV, etc.
ZU Audio has made 32 ohm versions of some of their speakers
Some Pipedreams are set up as higher impedance

Its not important in a 16 ohm load that the impedance curve be ruler flat. In fact that is true of 8-ohm speakers as well. What does seem to be important is that the impedance variation does not fall below about 1/2 the nominal impedance, and when so doing is at a crossover point. ESLs seem to be an exception.

An example of a speaker that is difficult for a tube amplifier is the B&W 802D. This speaker is rated nominally 8 ohms but employs dual woofers, which are both 8 ohms and in parallel, so really this speaker should be rated at 4 ohms since the most power made by the amp will be seeing this load. If you see dual woofers in a loudspeaker, it is really good idea to inquire with the manufacturer about the impedance at the woofer frequency! If it is 4 ohms I would ask if its possible to make a version where the woofers are in series rather than in parallel. If you get a lot of pushback on this topic, move on!

Something that is really important when matching a tube amplifier to a loudspeaker is to understand that it can't double power as impedance is cut in half. Where this gets you in trouble with speaker designers is that many of them use the sensitivity specification when rating their speakers. Sensitivity is 2.83 volts into the speaker and measured at 1 meter. If the speaker is 8 ohms, this is one watt. But if the speaker is 4 ohms, this is 2 watts- IOW the speaker is actually 3 db less **efficient** than it would seem to appear.

Where the trouble spot is: many designers don't really see a difference between sensitivity and efficiency- so they think that by paralleling a driver, the efficiency of the speaker is increased by 3 db! IME when you try to correct them on this issue, often you get an angry response.

Now it happens that if you parallel a pair of woofers or put them in series, they will act exactly the same in the cabinet designed for them, the only difference is the values used in the crossover. I am going to use Kirchoff's Law here to make the point- if you have a single woofer and put one watt into it, it might make 92 db at one meter. If you have two of the same woofers in either parallel or in series and put 1 watt into the array, the result will be the each driver will have 1/2 watt on them- and the output will be exactly the same 92 db. If the efficiency were to actually go up, it would violate Kirchoff's Law, and because that is impossible (just like its impossible to violate Ohm's Law) it doesn't happen.

I can't tell you how hard it can be to get this fact across to speaker designers- its enough of a problem that I consider it an embarrassment (due to lack of engineering knowledge) when I encounter it. When you run into this, its guaranteed that the designer is using a transistor amplifier for design and testing and really does not understand the relationship between the amp and the speaker.

Now if you know all this above, and you know that higher impedances are good for tube amps as they will make less distortion and more power, then you are well ahead of many speaker designers and you now have a better chance of extracting all the performance out of the tube amp that otherwise is left on the table. Its your dollars in that amp- why not use it to best effect??

Atmasphere - you have been most helpful. I won't sum up the list of your contributions quite yet though. Am hoping you're not done here ...

Great points by Ralph, as usual, with which I agree.

09-25-14: Charles1dad
Ralph, there's something that doesn't make sense concerning your comments about speaker designers. These designers have to have at the very least a rudimentary understanding of science and engineering. Something as basic as Kirchoff's law has to be easily comprehended and applied. They have the equipment available to confirm that concept and its effect. I just don't understand why you would be met with such resistance to something so plain and fundamental. How do these designers counter your point/position?

Charles, pending Ralph's answer, as I see it the key to reconciling that is the fact that the majority of speakers are designed with solid state amplification in mind. And with a few unusual exceptions, nearly all solid state amps (in contrast to tube amps) are designed to provide an output voltage which does not vary significantly as a function of load impedance, as long as the amp is operated within the limits of its voltage, current, power, and thermal capabilities.

Therefore, as a consequence of Ohm's Law and the definition of power, while of course not all solid state amps will double their rated MAXIMUM power capabilities into halved load impedances, nearly all of them WILL double the amount of power that is delivered into a halved load impedance (albeit perhaps with some increase in distortion), as long as they are operated within those maximum limits.

So when Ralph says:

If you have a single woofer and put one watt into it, it might make 92 db at one meter. If you have two of the same woofers in either parallel or in series and put 1 watt into the array, the result will be the each driver will have 1/2 watt on them- and the output will be exactly the same 92 db.

... Note that he refers to an input of 1 watt.

If he had referred to an input of a given number of volts, say 2.83 volts (which corresponds to 1 watt into 8 ohms, as he indicated), applying that voltage to two identical woofers in parallel (which together have half the impedance of a single woofer) would cause twice as much power to be delivered as if the same voltage were applied to only one woofer. Resulting (everything else being equal) in a 3 db increase in SPL. And conversely, applying that voltage across a combination of two identical woofers in series (which together have twice the impedance of a single woofer), would result in half as much power being delivered as if the same voltage were applied to only one woofer, resulting in a 3 db reduction in SPL.

Basically, the speaker designers Ralph referred to are thinking in voltage terms, rather than power terms. Which is arguably reasonable in the context of most solid state amplifiers, but can be very misleading when it comes to tube amps.

ZD, thanks for the nice words.

Best regards,
-- Al

Hello Al,
Yes I understand the premise that most speakers are designed with the intention of using transistor amplifiers. It seems what Ralph was explaining is their(designers) reluctance (or even rejection) to "recognized" his point as it pertains to tube amplifier -speaker interactions.
Charles,