Current amp vs Voltage amp

Bifwynne, by adding negative feedback to an amplifier you do indeed move the amp towards the Voltage Paradigm. This is because as you surmised the 'output impedance' is lowered. I paraphrased the term because it is misunderstood however, and that is where the clarification comes in. Unsound, you might want to pay attention to this.

Under the Voltage Paradigm, you have the term 'output impedance'. The term has a definition which is not intuitive. It refers to the amount of servo gain that the amplifier has which allows it to react to a load. It does not refer to the actual output impedance of the amp, as measured by any other field of endeavor in the world of electronics.

How can we know this? The answer is simple. If the output impedance were indeed lowered, the amp could drive a progressively lower and lower impedance. It might even make more power. But we see by adding feedback to an amplifier that the output power into lower impedances does not change.

IOW, what is happening is that the feedback gives the amp the ability to adapt to its load within certain limits by taking samples of its performance and using that as an error correction. The only way you can really get a lower output impedance is with bigger output transformers, more tubes or more transistors. The extra ability to drive a lower impedance does not come out of thin air or feedback- to do so would violate a fundamental rule of electronics known as Kirchoff's Law.

The difference between the Voltage and Power Paradigms has more to do with feedback then tubes/transistors.

Ralph, thanks for your characteristically erudite inputs. I think I understand this, and it seems to make sense to me.

At the same time, though, referring to the conventional and intuitive definition of "output impedance," isn't it true that the output impedance of a tube amp for its 4 ohm tap will be about one-half of its value for the 8 ohm tap? And if so, regardless of whether the amp uses feedback from its output or not, won't that reduced output impedance reduce the frequency response variations at the amplifier output/speaker input terminals that would result from the interaction of that output impedance with the impedance vs. frequency variations of the speaker? Albeit at the possible cost of compromising the performance of the amplifier itself, depending on how well the amp can handle the mismatch that may result. That often being a subjectively more significant consideration, as you pointed out in your second post.

Best regards,
-- Al

Why are there speakers designed with widely fluctuating impedances and steep phase angles in the first place? Why is there not more of an effort to build speakers with flatter impedance curves and gentler phase angles? Is the availablility of many high power solid state amplifiers to compensate for this their bail out safety net? Is there the belief these types of speakers sound better than those with easier load characteristics?
Regards,

Why are there speakers designed with widely fluctuating impedances and steep phase angles in the first place? Why is there not more of an effort to build speakers with flatter impedance curves and gentler phase angles?

IMO, it is because building a speaker with a flat impedance & phase curves requires some serious engineering & knowledge of physics. Not every speaker manuf has that; in fact, most do not. They cover this up with marketing hype - computer-aided design, CNC manuf, lustrous finish, exotic woods, Berrylium & Diamond drivers, blah, blah.
There are several trade-offs in manuf speakers such as on-axis & off-axis response, power handling, sensitivity, resonances, tuning the port, bass overhang, etc, etc. I am no speaker designer expert but I've spoken with a few manuf in the many years I've indulged in this hobby.
Most of these speaker manuf will make certain trade-offs that will yield wild/roller-coaster impedance & phase curves. With the wide availability of class-AB high power power amps, they figure that some amp on the market will drive their speaker. And, coupled with this is the reliance by the speaker manuf that many, many listeners will not know the difference & not know any better. A speaker that is distorting due to impedance & phase mis-management can be sold to these people as one having more "details" in the sound. Also, such speakers have a "wow" factor - hear a demo in the audio store or in-factory. The person is sold & buys the speaker. A little while later you read a "listener's fatigue" post on Audiogon from this person!
This is gradually changing & will force the speaker manuf to wise-up & not output junk designs but the process is slow.
That is why I hear: ARC used to voice their amps with Vandersteen, Avalon with Spectral & MIT cables, B&W voiced with Classe amps & so on. Deviate from this combination & you will be in sonic hell (as Bifwynne! :-)
I'm sure that there are other (better) reasons & I hope that some other more knowledgeable members chime in.

Bombaywalla,
I appreciate your perspective and input.It does seem the existence of high power SS amps can allow certain speaker faults to be some what masked by applying brute force as a remedy.
Regards,

Charles1dad -- Bombaywalla makes a fair point that many fine speakers have roller coaster impedance and phase angle plots. As I posted above, even the "mighty" Revel Ultima Salon 2's impedance plot is a bit of rock and roll.

OTOH, I think Bombaywalla's comment about me being in "sonic hell" may be a bit extreme. As I also posted above, the ARC Ref 150 and VS-115 amps were bench tested by different labs. Yet the results were consistent. As Ralph and Al explained, the use of NF reduces the output impedance of the amp, thereby making them more amenable to driving speakers with roller coaster impedance and phase angle plots. That is, somewhat SS-like.

Al also made a very important comment that even though the ARC amps FR plots were pretty flat at the speaker input interface, the most important stat is what comes out of the business end of the speaker. In the case of the S8s, they are rated, and have been bench tested to be very flat -- when driven by SS amps.

I surmise that by using the 4 ohm tap on my VS-115, which as Al thinks might reduce the output impedance of the VS-115, the S8's FR output might approach that achieved with a SS amp. Based on my subjective experience, since switching over to the 4 ohm taps, I think the S8s have a tighter bass response and are less forward as compared to using the 8 ohm taps.

BTW, I seem to recall that ARC uses Wilsons to voice their amps.

Bottom line: if I were to switch out my speakers to the Revels, I would probably also switch to a high current/high power SS amp, as recommended by Revel. At least Revel was honest enough to tell the consumer the type of amp that should be used.

P.S. Bombaywalla -- I think it's unfair to imply that the S8s are junk. I have no intention of starting a speaker war, but suffice to say, I think if properly tweaked and set up, they do a very credible job.