90db 4 ohm with tubes?

Bifwynne,
A good friend of mine recently bought a pair of the Rockport Altair speakers. They are a nominal 4 ohm load but are driven effortlessly by tube amplifiers and sound wonderful and very natural. For this reason I'll assume they must have a relatively smooth impedance curve and thus aren't restricted to high current SS amplifiers in order to be driven. I wish other designers would follow Andy Payor's example. If it's going to be a lower nominal lmpedance, strive to keep it reasonably flat.
Regards,

I think you will still have issues with frequency response variations due to impedance variations, if you run a large solid state amp. If you run a speaker with a nominal 8 ohm impedance that rises to 16 Ohms, and drops to 4 ohms at times, I'm guessing that may cause up to a 6 db variation in levels at different frequencies, with an amp that doubles its power every time the impedance is halved.

For example, if an 8 Ohm speaker rises to 16 ohms at roughly 12 kHz, the amp (large solid state) will only put out half of the power at the time, causing about a 3 db drop in sound level, at that frequency and time. If the load drops to 4 ohms at 1 kHz, the power will double at that time, giving it roughly a 3 db level increase during that drop to 4 ohms, due to the amp doubling its power at that moment. That's why I'm guessing up to a 6 db variation in this case, if the amp doubles its power that way. Even more of a change in frequency response variations yet, if the speaker impedance varies more than what I just mentioned.

So a potent solid state amp that doubles its power when the impedance is halved, may be less neutral sounding then an amp that can't double its power. I can see this happening. So basically, its still about how each amp reacts to certain loads demanded by various speakers, solid state, or tube.

With my other post, I'm thinking what an amp may do when it just has to power that certain frequency (or frequency range), that has a high or low impedance variation, in comparison the frequencies that are more linear as far as impedance goes. It may be more noticeable during a solo performance too. When the amp has to power other frequencies at the same time, the power to that frequency with a different impedance can be more stable is what I'm thinking.

That speaker mentioned above with the 28 ohm peak, who does know how it will sound when something, maybe even more-so, a solo performance that may have a lot of music at the frequency itself a lot of the time. It may do okay with one amp, have problems on the next. That's one major thing I like when they do reviews, is the part of measuring something for possible problems to be aware of.

Bruce, thanks very much for your comments. Charles and Hifitime as well. Here are my comments on some of what has been said:

06-10-13: Bifwynne
The ability of a tube amp to drive a speaker with wide impedance fluctuations with tight output regulation is a function of the amp's output impedance rather than the muscle of the power or output trannies.

Yes. Of course, if the impedance of the speaker reaches low values at some frequencies muscle, good trannies, and good power supplies are ALSO likely to be necessary.

06-10-13: Bifwynne
I think Al might concur with the surmise that if an amp's output impedance is higher, say 5.5 ohms, its output regulation would be much higher in dbs as a function of the speaker's impedance variations.

Yes, absolutely.

06-10-13: Bifwynne
It is my anecdotal understanding that there are a few speakers that have relatively flat impedance curves that would make them tube friendly.

Certainly. My Daedalus speakers are one example. I believe that the Coincident speakers, one of which Charles uses, are another. I'm sure many other examples could be cited.

I would add that widely varying impedance does not NECESSARILY mean that the speaker won't work well with an amp having high output impedance. For example, electrostatic speakers commonly have very high impedances at low frequencies, which descend to very low values in the upper treble region. Check out this curve for the classic Quad ESL, which, like many electrostatics, works well with tubes. Or, among dynamic speakers, check out this curve for the Harbeth M40.1, which a lot of people use with tubes with excellent results.

As Ralph (Atmasphere) has said, and this also addresses the point in Hifitime's first post above, it depends on the intentions of the designer, and tonal balance problems usually result when the amp and speaker are not of the same paradigm.

And in the case of a speaker having an impedance that is near 4 ohms across much of the spectrum, but rises to 28 ohms in the upper mid-range, it would seem to be a good bet that it was not designed to sound its best with an amplifier whose output impedance is even higher than that of most tube amps.

Regards,
-- Al

Al,

Just a quick follow-up to your last post. Copied below is an excerpt from the ARC web site that specifies the VS-115's output regulation:

"OUTPUT REGULATION: Approximately 1.2dB 8 ohm load to open circuit (Damping factor approximately 8)."

I assume that the reference to "open circuit" permits the inference that the amp can manage wide impedance fluctations ranging from 4 ohms to 28 ohms within a relatively narrow band, +/- 1.2 db. As I said before, when Stereophile and Soundstage tested the output regulation of the Ref 150 and VS-115 using a simulated speaker load having wide impedance variations, the output regulation results were quite similar.

I suspect that even a solid state amp that uses NF will likely compensate for speaker impedance varations too. Of course, as you said, the designer's intentions count for a lot.

As a layperson, I would describe ARC's output regulation control as not perfect; but just adding a little "flavor" to the acoustic presentation. I surmise, a room's sonic ambience will add much more "flavor" to the acoustic presentation than a 1.2db output fluctation. Just a guess.

BIF