I actually tried to speak very generically about some of some of the challenges associated with designing a successful OTL amplifier . . . which seemed to be somewhat relevant to the original poster's question. I was NOT referring to disadvantages with Atma-Sphere in particular. NONE of these are necessarily of any particular disadvantage to the end user of a competently-designed amplifier, but they ARE obstacles for the circuit designer.
I personally have a little experience with the Futtermans, and am reasonably familiar with the topology-usually-refered-to-as-"Circlotron" (which isn't necessarily/originally an OTL) . . . and these circuits REALLY aren't all that different from each other -- they're all variations on "push-pull". An excellent analogy would be the QSC solid-state amp vs. the conventional topology - it's disorienting to look at the schematic, but all of the same elements are still there, doing the same things. I share Ralph's view of the "Circlotron" as being the more elegant arrangement, mainly because of the equal-amplitude drive voltages. And I'd speculate that we might agree that the capacitor-coupled "totem-pole" arrangement as being the most problematic.
Engineering is very much an "in-spite-of"/"because-of" kind of discipline . . . and IMO it's the ability to keep this in balance that defines whether or not a design is ultimately successful. Atma-Sphere's longevity as a company is a strong testment to their product being hugely, vastly improved over the NYAL (Futterman) amplifier, and they get good reviews for sound quality in-spite-of/because-of (choose one) the fact that it's OTL, tube, low-feedback, class-a, high-output-impedance, etc. etc. etc.
I personally have a little experience with the Futtermans, and am reasonably familiar with the topology-usually-refered-to-as-"Circlotron" (which isn't necessarily/originally an OTL) . . . and these circuits REALLY aren't all that different from each other -- they're all variations on "push-pull". An excellent analogy would be the QSC solid-state amp vs. the conventional topology - it's disorienting to look at the schematic, but all of the same elements are still there, doing the same things. I share Ralph's view of the "Circlotron" as being the more elegant arrangement, mainly because of the equal-amplitude drive voltages. And I'd speculate that we might agree that the capacitor-coupled "totem-pole" arrangement as being the most problematic.
Engineering is very much an "in-spite-of"/"because-of" kind of discipline . . . and IMO it's the ability to keep this in balance that defines whether or not a design is ultimately successful. Atma-Sphere's longevity as a company is a strong testment to their product being hugely, vastly improved over the NYAL (Futterman) amplifier, and they get good reviews for sound quality in-spite-of/because-of (choose one) the fact that it's OTL, tube, low-feedback, class-a, high-output-impedance, etc. etc. etc.