Tony L
Administrator
The two problems with SET amplification are the exponential rise in harmonic distortion as you turn up the volume and the high output-impedance which makes it very sensitivy to load (combined amp/speaker frequency response depends on the speaker impedance characteristics).
I wouldn't be surprised if your Unison is already putting out 1% THD at 10W.
As I see it, SETs should ideally be used band-passed in active configuration with high sensitivity speakers ≥100dB.
But because the harmonic distortion is low-order it sounds nice to some people. Of course it will be accompanied by high levels of IMD and that is not as pleasent.
The thing with all of this is to view it as a whole. No amplifier exists without a speaker, and I’d put money on a good SET with really good (i.e. expensive!) horn speakers having lower overall distortion than almost all conventional AB amps into conventional box speakers as horns are just so much more efficient and lower in distortion. Typical ported 6-8” bass mid drivers etc are very far from distortion free. The big picture is all that matters in audio, it is all about synergy. To my mind the main thing wrong with SETs & horns is they are obscenely expensive to do well. I don’t care in the slightest about a level of distortion in am amplifier that only ever shows on test gear unless you are trying to thrash it into an entirely unsuitable loudspeaker. It’s just irrelevant.