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What Makes SET Amps Tick?

In a past life, I owned many tube amps. Almost all of them were single-ended, and almost all of those were DH-SET: 45, 50, 2A3, 300B, and - especially - the big transmitter triodes (211, 845, 805).

I largely stepped away from the hobby, and was very happy listening to my old, restored Quad 63/988 ESLs with a Schitt Yggy DAC, Schiit preamp, biamping with an active crossover to hefty subs, and a Purifi-based amp powering the Quads.
This lasted several years.

I think I fell for the left-vs.-right-brained listening mode tricks again because, on a whim, I tried an $850 300B amp from Amazon in place of the Purifi and have had a few of those "I'm hearing everything new again" sessions.

It's not "I like euphonic distortion." It's not "I like soft clipping." The reason I know is that what is wowing me is the low-level stuff: The extra texture I am hearing in all kinds of things (it's not "uniform" so it can't be some intrinsic coloration) and the extra nuance and detail. This goes all the way from Depeche Mode to Patricia Barber to Freddie Hubbard.

(I learned these things about SET amps long ago, but have learned them anew.)
HERE IS THE PURPOSE OF THIS POST: I am not looking for any kind of validation of this experience (subjective as it is), or anything of the sort. The question on mind now is *what is responsible for this*?

The Purifi amp is perfect in every way that can be measured. I am entirely aware that measurements are limited - we don't measure amplifiers playing *music* - but, still, an amp that has THD and IMD distortion lower than the threshold of the measuring equipment, at all power levels (a first), is, at least, a very good performer.
I am focusing on two things now: No splitting of the waveform, and transformer coupling to the speaker. Is the secret one or both of these things?

From what I know of digital amps, it seems possible that crossover distortion is nonexistent. But I'm not sure. And what I am hearing now from this 300B tube indicates otherwise, frankly, because I don't know how else to explain this, except that the SE amp is preserving tiny signal levels better.

Or is it that output transformer? I read something, once, about the transformer insulting the amplifier from the back-EMF of the speaker having very good effects. I don't know - and I don't know how that applies to an electrostatic, which is a very different type of load.

It's often pointed out that an audiophile's power amplifier is the very last stop for a signal that has been through countless steps of manipulation and amplification; how can this last step somehow be so "special," have such a large effect on the actual sound?

I guess the answer is twofold:

- It's by far the largest magnitude of amplification
- It involves interaction with the transducer

I am looking for people with greater knowledge of audio engineering than I to chime in here. Describe what I am hearing.

(Just don't tell me that I'm *not* hearing greater low-level nuance, because reality is reality.)

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Topic - What Makes SET Amps Tick? - PaulF70 18:19:22 05/16/25 (101)
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