Hi Ramtubes,
The voltage of the amp is confirmed to be +/-80V by the service schematic of the amp.
80/1.414=56.6V RMS
56,6^2=3200
3200/1.6=2000W
I have two excerpts for you, the autoformer photo and schematic with voltages, but don't see how to upload images.
1.6 Ohms is the Z of the so called primary with load on the so called secondary.
Z of an transformer is calculated through Le, and Le is dependant on core crossection, lenght of magnetic lines through the core and core material permeability. Double C has less losses, and toroidal has evem less than double C. Also they have different path lenghts and are typically made of different materials.
Anyway.
56.6 to the required 126.5V RMS is a ratio of 1:2.23, even within the generally recommended ratio for an autoformer of 1:3 in order to be feasible. For 2kW into 16 ohms 179V RMS are required, making of 1:3.16 turm ratio. Quite manageable.
Regarding the output resistance that should be 0.2 Ohms for a DF of 40 into 8 Ohm load, this seems to be the image of the output resistance of the voltage source with some losses or with very shallow NFB.
Regarding the NFB, it is a separate tap from the autoformer and is summed wirh the normal NFB in some unclear way, at least unclear from the schematic of the service manual.
And regarding what it is there for... I just saw your comment and am questioning why I participate in this conversation.
All amplyfying elements are non linear, be it tubes or transistors... NFB is anty non linearity in the first place.
Anyway, my investigation is journalistic and performed by a non engineering person. Although hard to find, online there is enough knowledge to permit designing and manufacturing of such device by almost completely ignorant designer.
My quest was to unveal the design considerations of the MC2K.
Undoubtedly they are using low voltage in the amp and the corresponding high audio grade components. The typical transistor is voltage limited to 70-100V, up to 100W and up to 10A. It is obvious that the hardest limitation is the voltage. With the capacitors again the voltage is the main limiting factor. 126V RMS is upwards of 180V on the supply rails. I went through a electronic parts supply catalogue, big electrolytes diversity ends around the 80-100V rating.
And now I reconsider about the photos. Anyone in this discussion should have seen both the schematic and the manual with the photos. I tell you I've seen that the rails are +/- 80V and the wire seems in the 1mm-ish region. Prove me wrong.
