What ohm to set amp

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??

Usually voltage paradigm characteristics (i.e., maintaining constant voltage, not power, into varying load impedances) go hand in hand with the use of some amount of feedback. But that is not always the case. For example many and perhaps all of the Ayre amplifiers use zero feedback, yet have output impedances of a small fraction of an ohm. As a consequence of that low output impedance they will deliver essentially the same voltage and very close to twice as much power into 4 ohms as into 8 ohms when operated within their maximum power capabilities. And, in addition, those maximum power capabilities double into 4 ohms relative to 8 ohms.

Best,
-- Al

Thank you everyone for taking the time to research and post. The SF speakers I have are the Liuto towers and the matching center the smart. I talked to a rep at Mac, and he told me doesn't matter if I have it set at 4 or 8 ohm, as the amp would switch on its own when needed. But he also asked if the amp was running hot at the 8 ohm setting, since this is my first Mac, I didn't know how hot it is suppose to run. I'm wondering why would it matter if it switches on its own.

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.

Kalbi, I couldn't find a complete impedance curve for the Liuto, but I did find the following comment in this review:

... a nominal impedance of 6 ohms would be more representative than the quoted 8 ohms.

Measured data was presented indicating a minimum impedance of 4.5 ohms at 124 Hz, a current-hungry phase angle of -54 degrees at 68 Hz, and an "equivalent peak dissipation resistance" of 2.2 ohms.

And the measurements in this review indicate a minimum impedance of 4.1 ohms at 100 Hz, at a phase angle of -23 degrees.

Given that those impedance minima occur in the bass region, where lots of energy is typically required, and given also that the manual for the amplifier specifies that the 4 ohm setting should be used for a 6 ohm speaker, it all seems to add up to the 4 ohm setting being best.

My guesses are that the automatic switching mechanism that was referred to may involve circuitry whose primary function is protection, and that its operation may not be sonically seamless.

Bruce, regarding your question consider the case of a small two-way speaker. The combination of small woofer size and small cabinet size will result in low efficiency in the bass region. It is not uncommon for that kind of speaker to have an impedance of close to 4 ohms in the bass region, and close to 8 ohms at higher frequencies. Generally that kind of speaker will conform to the voltage paradigm, and will be intended for use with a solid state amplifier. The near zero output impedance of the amplifier will result, for a given input signal level, in twice as much current and power being delivered at low frequencies as at high frequencies. That doubling of power at low frequencies will compensate for the low efficiency of the speaker at low frequencies, presumably resulting in a flat overall frequency response. If the amplifier were to deliver the same number of watts to that speaker at high and low frequencies, for a given input level, the response would be decidedly bass-shy.

Best,
-- Al

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.