What ohm to set amp

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.

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.