What ohm to set amp

Yes , and you should if a speakers dips to 4.1

I went over to the Mac site and the amp is rated for 4&8 ohms. If you'd like detailed info or any info really, e=mail Chuck Hinton of Mac. He can answer most any question you have.

Thanks guys, I called a SF rep and he said I should run them at 8 ohm since they are 8 ohm speakers. I'll contact Chuck at Mac and see what he saids.

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.

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