What ohm to set amp

I took a look at the literature on the MC205 at the McIntosh site. It is a solid state amplifier that has neither output transformers nor autoformers nor multiple output taps. It has a rear panel switch for selection of either 4 or 8 ohm speaker impedances. Apparently the function of the switch is to select higher internal voltage rails (the DC power supply voltages that are provided to the output stage) when an 8 ohm speaker is connected, compared to when a 4 ohm speaker is connected. The manual recommends using the 4 ohm setting if a 6 ohm speaker is being used. The power rating of the amplifier is 200 watts into both 4 and 8 ohms, assuming that the load impedance corresponds to the switch setting.

Kalbi, what specific models are your Sonus Faber speakers? Knowing that would hopefully allow us to look up their impedance curves. In any event, if the dips into the 4.1 to 4.7 ohm range occur in the bass region, where a lot of energy is typically required, it seems clear that you SHOULD use the 4 ohm setting, as Schubert indicated. And likewise for the 6.2 ohm center speaker, although the selection is presumably less critical with respect to that impedance, and in any event the amplifier just provides one overall selection between 4 and 8 ohms, not independent selections for each of its channels.

Bruce (Bifwynne), I don't know what a glarff is, but I'll comment on it anyway :-). Both this amplifier and the Mc solid state amplifiers that have autoformers have output impedances that, as with most solid state amplifiers, are a tiny fraction of an ohm regardless of tap or setting. So they will act like typical solid state amplifiers with respect to the interaction of their output impedance with the impedance vs. frequency variation of the speaker.

As to why using a tap on a tube amplifier that is intended for an 8 ohm load can sometimes be preferable to using a tap intended for a 4 ohm load, as you realize some speakers conform to what Ralph refers to as the power paradigm, and are designed to sound their best when used with an amplifier having relatively high output impedance. Also, a mismatch between the actual load impedance and the impedance a given amplifier output tap is designed to drive can, if severe enough, degrade the sonic performance of the amplifier, as Ralph indicated above.

Best regards,
-- Al

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