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Thanks Al, Bombaywalla and Ralph. Obviously not being a EE tech, the only thing the joule power supply stat empirically and intuitively meant to me was that it helped the amp "muscle" its way through some tough speaker loads. But I didn't understand the current aspect.

My speakers present a 4 ohm impedance load and some highly capacitive phase angles in the bass region. Based on Bombaywalla's explanation, I gather that my amp not only performs "somewhat SS-like" because it has a somewhat "low'ish" output impedance, but the 10 amp and 2.92 second estimated stats (or perhaps less??) enables the amp to actually do what a real SS can do -- deliver some serious current into a speaker that presents a low impedance load and capacitive phase angles at low frequencies. Well ... at least to some degree. Cool!

Btw, ARC's new Ref 250 and Ref 750 mono amps have advertised power supplies of 900 and 1300 joules, respectively. And that's per channel.

Now I understand why it has been reported that a number of folks who own those amps are quoted as having cried out, "it's alive ... it's alive....," when cranking some serious volume. LOL :) Just kidding!

Best,

Bruce

Al, Ralph and Bombaywalla -- I'm still confused. 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.

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.

Presumably, current output may also change as a function of voltage change, but power (watts) is power (watts), i.e., V x A. I'll re-read the article again. Here's s the URL link to JA's Ref 150 report (Graph 1 in particular):

http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements

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. If I am tracking here, 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.

I hate to even broach the subject of damping factor in light of this discussion.

Thanks for the clarifications.

BIF

P.S. 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?? If so, then even saying a speaker was voiced to be driven by a SS amp is a bit of a misnomer. If the SS amp used as the reference source by the speaker designer wasn't a true constant voltage source, then one will never be certain of which SS amp to match up with the speaker of choice.

Guess one has to just go with what sounds good. Huh, ... we're back to where we started. If it sounds good (to you, or me), then it is good. LOL :)

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