What ohm to set amp

Kalbi .. , don't know much about the Mac or SF impedance curves. So I only add this comment because it might help focus your discussion with MAC tomorrow.

As I've come to better understand with my own gear, using the lower impedance taps on my tube amp enables me to drive my speakers with a lower output impedance source. Of course, my amp uses output transformers and negative feedback, but doesn't have built-in autoformers like some MACs.

Frankly, I am a little puzzled why one would want to use higher impedance taps if the price is higher output impedance. Most tube-heads may say use whatever taps sound better to you. That's fair advice ... but what sounds better to you might be acoustically colored playback especially if the amp's output regulation varies more widely with respect speaker impedance which varies as a function of frequency.

I've killed this issue a zillion times already and if I start it up again, I'll start a negative feedback war. Yes ... NF comes with a cost. Read Ralph's white paper on the Voltage and Power Paradigms. But understand that a tube amp that uses NF does not perform like a pure Power Paradigm source. It's kinda' a hybrid which is good in some respects, but also inserts TIM and odd-ordered harmonic distortion into the output -- not so good. But life, and amplifier design, is about compromises. You takes the good with the bad.

Cheers,

Bruce

Perhaps Ralph or Al will edit my glarff so it makes more sense.

Thanks Al. Glarff means an inchoate stream of consciousness. Going back to the OP's Q re the MAC SS amp, I am just guessing here, but perhaps the 4/8 ohm switch avoids power doubling at 4 ohms if the 4 ohm switch is used. Perhaps the amp is more stable operating in that power configuration??? Dunno.

Of course the trade off is that many speakers have roller coaster impedance curves as a function of frequency response. So, I further surmise that if for discussion purposes, the OP's SF speakers have an impedance rating of 12 or 16 ohms at the midrange/tweeter x-over point (just guessing), the MAC's ability to produce power at that frequency point might be considerably less (maybe 25% if the x-over impedance is 16 ohms) based on the Voltage Paradigm approach. I guess that's where NF comes into play to ramp up power output or else the speaker's acoustic output will make the listener dizzy.

Sorry Ralph, more glarff.

That's kinda what I figured. In a sense, the 4 ohm switch is protective.

So .... as you mentioned if the impedance switch is set to 8 ohms and the amp is pushing power into a 16 ohm load, the power output should be roughly one-half the amount which can be delivered at 8 ohms.

Al, does that permit the inference that a SS amp that operates as a Voltage Paradigm amp would be expected to use NF of some type (local or global) to regulate actual power output into loads which vary as a function of FR in order to maintain output power that corresponds to the signal presented at the amp's inputs??

Btw, sorry. Sometime I confuse you and Ralph. The last sentence of my previous post was directed to vous mon frer.

Bruce

Al, I am somewhat familiar with the Ayre being touted as a zero feedback amp. So if it doubles power if the load drops in half, and halves power if load doubles, how does the amp manage to regulate the amount of current being shown to the speakers so that the speaker's acoustic output is flat?

It seems to me that regardless of whether voltage remains constant or current remains constant, ultimately the SPL generated by a speaker is based on power (i.e., watts, or the product of volts and amps). In cases where a SS amp or tube amp uses NF to throttle either volts or current, one way or the other it seems logical that the amount of watts presented to the speaker's voice coil should in some way correspond -- or perhaps more accurately mimic -- the frequency and amplitude of the wave form presented to the amp's input.

I recall reading a member's post some months ago that expressed some doubt that the Ayre is able to dispense with all forms of feedback, albeit local or global. I believe the member may have expressed some puzzlement how such an amp could adjust to changing impedances as a function of FR and thereby maintain a flat acoustic presentation. Unless of course the speaker in question has a ruler flat impedance curve and zero phase angle across the entire frequency spectrum.

Thanks Al. Would you kindly clarify my understanding of the Voltage and Power Paradigm area just one more time.

At least with my tube gear, ARC publishes so-called output regulation stats. In the case of both the Ref 150 and VS-115, output regulation off the 8 ohm tap is +/- .8 db and probably 60% of that number off the 4 ohm tap. See Stereophile and Soundstage bench test reports.

Perhaps I am misinterpreting what the term "output regulation" means. So please help me here. Based on the explanations provided in the two magazines, I gather that output regulation relates to how closely a tube amplifier's voltage output will vary so that the amount of power (watts) presented to the speaker will correspond to the magnitude of the input signal, thus compensating for varying speaker impedances which change as a function of FR. In short, if impedance increases, voltage will also increase. This in turn will increase current. All of this is accomplished through NF.

As I understand the Voltage Paradigm, SS amps generally maintain constant voltage and increase or decrease power in response to speaker impedance changes. But here again, if a SS amp is asked to feed current into a high impedance segment of the speaker's FR spectrum, somehow the amp must increase its current output or else power (watts) will decrease. Here again, I would think that NF would throttle up or down as the case may be the amount of the amp's gain in order to compensate for these variations.

I surmise that since Voltage Paradigm amps naturally increase power in low impedance loads, it is easier for the SS amp to deliver current and ultimately power into the bass regions. In contrast, if presented with a high impedance segment of a speaker's FR spectrum, the amp will need to deliver much more current in order to "power" its way through the speaker's impedance mountain, and may choke. Again this is where NF comes into play ... I think.

If I am tracking so far, what I don't understand is how a so called zero NF amp (tube or SS) can properly respond to impedance values which change as a function of FR in order to maintain constant power through a speaker's FR spectrum. I would have guessed that some type of NF, either global or local would be needed. It would seem that the only way to avoid NF is to use a speaker that has extremely flat impedance curves and phase angles.

Btw, I was reading some of the tech data on the Ref 150. Can't say I understand it, but from what I gather, ARC has somehow directly coupled the power tubes to the output trannies in order to regulate power output. I don't know if this is just another term for local feedback, but that's as much as I understand. I recall ARC used the term partial cathode following, or something like that.

Sorry for getting back into the tech stuff again, but all of this touches on the OP's question and the area is confusing, albeit interesting, to this lay person.

Al, thanks again for the education. So to restate what you said above, "[t]he tighter the regulation, meaning the smaller the +/- number, the LESS the voltage will increase as load impedance increases, and the LESS the voltage will decrease as load impedance decreases."

"An ideal voltage paradigm amplifier will have a regulation of +/- 0.0000 db. Its output voltage will not change at all as load impedance varies. And, per Ohm's Law, for a given output voltage the amount of current drawn by the load will vary inversely with the impedance of the load. So since power is proportional to voltage x current, and equals voltage x current in the case of a purely resistive load, the power delivery of a voltage paradigm amplifier will NOT "correspond to the magnitude of the input signal."

And as to Ralph's comment about tonal balance problems, I suppose using a tube amp that has a high output impedance implicates the mismatch issue more so than using a tube amp that has a lower output impedance.

Last and final point: I think I'll keep my day job.

And Al .... thanks again for the education.

Bruce

Thanks so much Al. But as I said, I'll keep my day job. ;>')