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In Reply to: RE: Why not just get a Atma-sphere amp... posted by airheadair on August 10, 2020 at 12:35:28
If you read Tubecad, Broskie, who doesn't shy away from making provocative generalizations, says the circlotron and the Futterman output stages are fundamentally the same. I personally cannot see how that could be true, but I'd love to see Ralph's take on the idea. My last pair of Futterman amplifiers were Prodigy amplifiers, out of Texas. They sounded excellent driving the Sound Lab M1s I owned at that time (early 90s). Arguably, the Sound Labs are "modern". However, they presented a booby trap for either type of OTL, there was a very low impedance dip at midrange frequencies, as low as 2 ohms at between 1kHz and 2kHz. Neither Futterman nor circlotron is happy with that load at those frequencies.
Follow Ups:
"If you read Tubecad, Broskie, who doesn't shy away from making provocative generalizations, says the circlotron and the Futterman output stages are fundamentally the same."
That's not quite what he says. His whole point in that article is that what really matters, when you analyse the circuits, is where the signal reference point is in the output stage. As he says, the signal ground reference point is different in the circlotron and in the totem pole, and so they behave differently. However, he also points out that with a small change in how the ground reference point is set (by taking the signal ground to be at the midpoint between the two speaker terminals rather than at one end), then the totem pole would become operationally equivalent to the circlotron. I think that is an uncontroversial point.
Perhaps I faded out before completely comprehending Broskie's thought on this subject. As an OTL devotee, I want to understand it, so I will search it out and re-read it.
But its not the case. You get a little bit lower output impedance with a Circlotron, and you certainly get to use a lot more intuitive circuit to drive it :)
But you're right, he likes to be provocative.
The Stax F83's have a strange impedance curve: about 4 ohms at both frequency extremes, about 160 ohms in the middle. I now use a subwoofer for the bass (crossing over at 100 HZ), so that part of the story is out of the picture. I guess the remaining high impedance makes up for the very low efficiency, making the NYAL OTL a good match.
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