Kosst_Amojan 12-14-2017
It's called impedance matching. The closer a load matches the impedance
of the source, the more efficient the energy transfer. Generally you
want the load to be at least twice the impedance of the source to avoid
odd coupling effects. The ratio of impedance difference also defines the
damping factor which can significantly effect speaker behavior.
Kosst & Miketuason,
The following statement appears in the manual for the McIntosh MC601 that is listed in Mike's system description. I believe similar statements appear in the documentation of other McIntosh solid state amplifiers which use autoformers. And this is what I was alluding to in my previous post, although I referred to performance benefits rather than benefits to long-term reliability. (Kudos to Mattmiller for citing both benefits):
All solid state power amplifier output circuits work best into what is called an optimum load. This optimum load may vary considerably from what a loudspeaker requires. In the case of more than one loudspeaker connected in parallel, the load to the power amplifier may drop to two ohms or even less. A power amplifier connected to a load that is lower than optimum, causes more output current to flow, which results in extra heat being generated in the power output stage. This increase in temperature will result in a reduced life expectancy for the amplifier.
The special Balanced Winding Autoformer creates an ideal match between the power amplifier output stage and the loudspeaker.
Also, regarding the comment about damping factor, the MC601 has a specified damping factor of "greater than 40," which presumably means effective output impedances of the 8, 4, and 2 ohm taps of approximately 0.2 ohms, 0.1 ohms, and 0.05 ohms respectively, all of which of course are very small fractions of the impedance of most speakers. Which in turn reinforces the notion that the multiple taps are provided mainly to benefit the amplifier's output stage, as opposed to optimizing interactions with the speaker, such as impedance interactions, bass damping, and energy transfer.
Regards,
-- Al