. . . . I've also noticed rolled off highs on the McIntosh solid state amps that use an output transformer. They call it an autoformer I'm not sure what the distinction is?. . . .
I'm surprised you hear a roll-off on Mac solid state gear. They are typically down 3dB at 100 KHz and completely flat well beyond 20 KHz. That also goes for their tube amps.
Look up inductance and transformers on Wikipedia or some such. Inductors pass DC and low-frequency but block high frequencies (capacitors do the opposite). Transformers are coupled inductors; two or more inductors that work together. There are transformers that work into the microwave range, but audio transformers tend to be large to handle the power, and have lots of windings to convert from the high-impedance tube plates to low-impedance speakers. That all reduces their bandwidth. An autoformer is a tapped transformer that is often configured as a single continuous winding instead of isolated coupled coils. It is also used to provide impedance transformation.
Myles-I can’t understand why a bigger power amp (and I’m assuming you mean higher power) would have a higher bandwidth than a lower powered tube amp. I’m thinking that a high-power high-bandwidth output transformer would be much harder to wind than a low power high-bandwidth output transformer. I believe that it was David Manley that talked about the “magnetic window” that output transformers have to be designed to fit in and wound in. In reality, the properties for an output transformer designed to reproduce deep bass are different than an output transformer designed to reproduce the upper frequencies and thus the “window” so you don’t throw one or the other out of it with a conventional output transformer. We have ample proof backed up by measurements that very wide-band output transformers do exist.
Getting back to the OP’s question, it was always my understanding that the reason high frequencies of tube amps aren’t ruler flat is the output impedance of tube amps is high and interacts with the impedance of loudspeakers which depending on the impedance curve of the loudspeaker can cause deviations in the FR which can be heard.
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