Anyone try this way to wire your loudspeakers?

Kirk, you beat me to it! And it's nice to see you posting again.

A minor point, though. Wouldn't the 8 ohm tap have 3 db more voltage gain than the 4 ohm tap, not 6 db? In other words a factor of 1.414, resulting in the output transistors seeing the same load when an 8 ohm load is connected to the 8 ohm tap, as when a 4 ohm load is connected to the 4 ohm tap, as when a 2 ohm load is connected to the 2 ohm tap (since the impedance transformation will be proportional to the square of the turns ratio, and hence to the square of the voltage ratio).

Beyond that, I second Kirk's statement that "if you like what you hear, that's great." And a good takeaway from the experience reported by the OP is that experimentation with how multiple output taps are utilized can pay off big-time.

A not so good takeaway, on the other hand, would be an expectation that a similar connection arrangement will necessarily, or even probably, be optimal in the case of other amplifiers and speakers (although it certainly MIGHT be). In contrast to most amps that provide multiple output taps (i.e., tube amps), the autoformer-based McIntosh designs have output impedances that are low enough to be essentially negligible, as they are with most solid state amplifiers. And the XR100 speakers are complex four-way affairs having 15 drivers per speaker, and who knows what kind of impedance vs. frequency characteristics (I couldn't find impedance curves for them). So the OP's system is considerably different in relevant respects than the majority of others.

To the OP, again, congratulations on some imaginative experimentation that has paid off. Enjoy.

Regards,
-- Al

Hi Bruce (Bifwynne),

In a word, "bad."

As was mentioned, the turns ratio of the transformer or autoformer (the number of turns on the secondary or output side divided by the number of turns on the primary side) can be presumed to be about 1.414 (which is the square root of 2) times as great on the 8 ohm tap as on the 4 ohm tap. Equivalently, the turns ratio of the 4 ohm tap can be presumed to be about 1/1.414 = 0.707 of what it is for the 8 ohm tap.

So if the speaker were connected between the 4 ohm and 8 ohm taps the turns ratio would be (1 - 0.707) = 0.293 of the ratio for the 8 ohm tap. Since impedance is transformed in proportion to the square of the turns ratio, from the perspective of the output tubes or transistors the optimal load impedance for a connection between the 4 and 8 ohm taps would be (0.293 squared) x 8 ohms = 0.69 ohms.

So if just about any real world speaker were connected between those terminals the transformer's step-down ratio would be much too great, the speaker would see too little voltage, and the amplifier would see a very mismatched load. Although I suppose it is conceivable that there may be some rare exceptions for which doing that would provide results that are somewhat reasonable, perhaps such as certain older Apogee speakers, and the Infinity Kappa 9, that have extremely low impedances.

Best regards,
-- Al

Thanks, Kirk. Excellent points, as usual.

Best,
-- Al