MC352 into varying-load speaker?

Sean, confused. Your first post in this thread seemed to imply that autoformers could work. But the blasphemy comment is pretty strong in the opposite direction.

Not knowing where to begin picking your brain, perhaps you could summarize the idea, if it's not asking too much. Essentially, the question would be: "Do autoformers hurt, help or are neutral to sonic quality under most/average circumstances, and in what circumstances do they veer from this stated path"?

Kck: Autoformers act as a buffer. If you have an anemic amp and a speaker system that presents a wild and highly reactive impedance as a load, that "buffer" could be a good thing. Otherwise, that "buffer" acts as a sonic veil between the amp and speaker interface, lowering the sound quality. Using an autoformer with a good amplifier would be equivalent to running hundreds of feet of small gauge highly inductive speaker cable. Would you ever consider doing this under normal circumstances??? Sean
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The autoformer doesn't work like Sean thinks it does.. Mcintosh output transformers are bifilar wound which is a complex technique used to nearly eliminate the inductance associated with the wire. We use it in the lab for special EMI filters (different core setup though). The coupling is just about perfect with a larger bandwidth than the output transistors. Just like tube amps - only better because not all manufactuers have the ability to bifilar wind their trannies.

Aball: Has Mac always used a bifilar design or is that what they've gone to on more recent models? While i saw the test results / bandwidth on one of their more recent models, and it looked pretty impressive for an output transformer coupled model, i don't remember the older models being anywhere near as linear over that wide of a bandspread. As such, either their transformers have changed or the designs of their amps have improved with age.

Either way, the transformer still acts as a buffer. Rather than directly dealing with the input of the speaker and the reflected EMF that it would present, the output stage sees the transformer first with the speaker as a secondary load. As i mentioned, this increases the stability of the amp as it always sees a somewhat consistent load. That "consistent load" is also what gives these amps their "house sound" and electrical characteristics. If you like them and it will do what you want, buy it and enjoy it. Sean
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Yes, all autoformer McIntoshes use bifilar winding (trifilar on some amps). McIntosh patented it in 1947 for their "unity-coupled" circuit, originally for tube amps of course but then they applied it to solid state in the early 60s and have ever since. I have performance data for a MC2255 and the bandwidth is nearly as wide as today's models.

I have tried Mc amps with autoformers and without, concluding that there is actually little sonic difference between them (when comparing appropriate models). The autoformer mainly acts as a stabilizer, as you said. I currently use a direct-coupled McIntosh amp myself. I like them either way. Arthur