Autoformer vs Speaker impedance Curve

You may want to ask McIntosh or a big McIntosh dealer like Audio Classics about the function of their autoformers.

An autoformers is just a transformer where the primary and secondary share a terminal. It doesn't make them behave radically different than a normal transformer. Pretty much everything that applies to trasformers is true for autoformers. 

Thanks all!

I’ve looked at Mac’s Autoformer benefit explanations. My takeaway: better matching of amp to speaker impedance. Ie, 4 ohm terminals for 4 ohm speakers, 8 ohm terminals for 8 ohm speakers, etc. The catch is these are “nominal” loads. 

What isn’t addressed is, what happens “electrically and sonically” with Autoformers when 8 ohm speakers, like 802D3s, dip down to 3 ohms or exceed 20 ohms???  

With typical output transformers (common with tube amps)  speakers with wild impedance swings (and phase) present a challenging load to the amp and fidelity can suffer.

Can the same be said of autoformers? Or, are these electrically different enough from transformers as to not present the same issues?

Can any of you electrical engineers or techs explain the relationship between autoformers and large speaker impedance swings? 

Large impedance swings and severe phase angles do exactly what you’d expect; they produce dips and humps in response. McIntosh is about the only solid state manufacturer to use output coupling transformers. If you’re driving benign, modestly reactive loads, all is peachy. But you throw some significantly reactive speakers at it, not so much. The autoformers in a SS Mc mainly pad the output devices from the messy business of truly gripping the speakers for the sake of letting the gain devices behave nicely. Not the best approach in my opinion and many others. Tubes are a bit different in that the transformers are converting fairly high voltage into current to drive a speaker. Mc autoformers aren’t doing the same job since transistors operate at voltages and currents loudspeakers are quite happy with. My Focals are directly connected to the drain pins of the amp’s output MOSFETs. It works well.