What ohm to set amp

Thanks Bruce; I've learned a new word today :-)

As I indicated, the MC205, as well as the MC solid state amplifiers that use autoformers (which the MC205 does not use), have negligibly small output impedances. The MC205 is spec'd as having a damping factor of 140, corresponding to an output impedance of 8/140 = 0.06 ohms. So it will act as a voltage source with respect to the interaction of its output impedance with the speaker's impedance vs. frequency variations.

Which means that for a given signal level at the input to the amplifier it will output twice as much power at frequencies for which the speaker's impedance is 4 ohms as it would deliver at frequencies for which the speaker's impedance is 8 ohms, and half as much power into 16 ohms as into 8 ohms.

PROVIDED, however, that the maximum limits of its power capability are not exceeded. What will NOT double into 4 ohms, compared to 8 ohms, is the MAXIMUM amount of power it can put out. And I suspect that if the rear panel switch were set to 8 ohms and the impedance of the speaker is close to 4 ohms in most of the bass region, its MAXIMUM power capability into that 4 ohms would be significantly LESS than the 200W it could deliver into 8 ohms at that setting.

Basically, what the switch is apparently doing is optimizing internal voltage, current, and thermal operating conditions for whichever of the two load impedances is selected, such that a maximum of 200W can be delivered into either impedance.

Best,
-- Al

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

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.