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Hi Bruce,

Starting with your last question first:

If I am finally starting to get it, does that mean that not all SS amps are true constant voltage sources if power doesn't double down if impedance halves, and the opposite being the case if impedance doubles??

An important distinction needs to be kept in mind between the amp's MAXIMUM power ratings doubling down, and the behavior it will have when operated within those maximum limits. All or nearly all solid state amps having feedback WILL double the amount of power delivered into a halved impedance as long as they are operated within those maximum power limits. (In saying that, I'm oversimplifying a bit by putting aside the effects of phase angle variations). Although, of course, in doing so their distortion performance may suffer.

Are you saying that a tube amp that uses negative feedback is able to adjust the amp's voltage output to compensate for impedance swings in such a way that power (i.e., watts, or volts x amps) approximates the amount of power (watts) a true constant voltage source SS amp would make at a given reference impedance and frequency, assuming that the SS amp was operating within its specs.

Feedback would cause the tube amp's output voltage to vary LESS as a function of load impedance variation than would otherwise be the case. In that respect, its behavior would come closer to the behavior of a solid state amp than would otherwise be the case.

If that is correct, then how should I interpret John Atkinson's bench specs of the Ref 150 when he says that output (???) regulation is +/- .4 db off the 4 ohm taps (output impedance being .55 ohms) and +/- .8 db off the 8 ohm taps (output impedance being about 1.1 ohms)??? Is he speaking about output voltage or output watts?? Based on Ralph's explanation, if JA is referring to watts, then presumably voltage must be swinging all over the place to match the power output of a voltage source amp.

As I see it the only conflict between Ralph's point and JA's statements would concern JA's use of the term "output impedance." Ralph's point is that that term is misleading with respect to what is really going on inside the amplifier, but, as Ralph indicated in his last response, when considering the amplifier/speaker interactions that result from varying speaker impedance there is no conflict.

The basic point is that the feedback that is designed into your amp REDUCES the amount by which the output voltage of your amp varies as a function of load impedance variation, thereby changing its behavior in the direction of being more solid state-like. An ideal voltage source would have regulation of +/- 0 db. An amp that is similar to yours but uses no feedback would have a regulation number that is significantly greater than +/- 0.8 db.

I also assume that an amp's output impedance number may have much to do with the number of turns on the output trannies' secondary windings. But here again, I assume negative feedback is also at play, further reducing measured output impedance.

True, if we define "output impedance" in the sense JA (and others) use that term.

I surmise the "constant voltage source" paradigm breaks down with tube amps because in the end, as Ralph has said many times, a speaker's SPL is a function of the power (in watts) being pushed into the speaker circuits at a given frequency. So, the NF servo circuit is telling the amp to do "ramming speed" (ala Ben Hur) with **voltage** in those cases where speaker impedance is low, and the opposite when speaker impedance is higher.

It doesn't break down, it's just that even with feedback your amp doesn't hold output voltage AS CONSTANT as nearly all solid state amps would (if operated within their limits), under the same conditions of load impedance variation. Therefore while your particular tube amp, operated within its limits, would increase power delivery as load impedance decreases, it would not do so to the same extent that a solid state amp would. It's a matter of degree.

Best,
-- Al

Thanks Al. So what is JA measuring in the first graph?? Is it volts or watts?

Hi Bruce,

I would think that he is measuring voltage, since it would be more practical to do that, and since the majority of speakers conform to the voltage paradigm.

BTW, this article may be of interest. It describes the standard simulated loudspeaker load he uses for those measurements.

Best,
-- Al

That there's a "black box" aspect of synergy or a witch's brew of "X" factors going on in an amp that come together (or not) when driving a particular speaker.

To this point, I seem to recall that a while back Al may have posted something in another OP to the effect that the theory may help one identify a amp/speaker combo that MAY (??) work well and conversely MAY (??) not work well. But, and this is a big BUTT (sic), one will not know for sure until he/she tries out the particular amp/speaker combo of interest.

There really isn't a 'witch's brew' unless the designer of the amp really doesn't know what he's doing :)

To the latter question, because of the confusion that exists in the industry, you still have to try things out.

Al, it sounds to me like you got what I was trying to convey. I am wondering if there can be a better term than 'output impedance'...

Thanks again Al. Well ... I'm back from where I started. It seems that one reason, among many others, that my Ref 150 tube amp does a nice job driving my S8s is because the 1040 joule power supply acts "somewhat" like a monster-current SS in its ability to deliver some serious juice when needed. One second, two or three, dunno. It just seems to work.

And Al, after reading the thread running about tube amps and ESLs, if you think I'm gonna climb onto the speaker "merry-go-round" -- forget it. As a practical matter, there are too few B&M stores in my area that would permit meaningful audition opportunities on like gear. In addition, after having read some reviews about so-called super "top-o-the-line" speakers that have stats that look less stellar than the stats for my speakers, I'm just not willing to drink the Kool-Aid.

Thanks as always.

Best

Bruce