If you gave the earlier time line graphic a closer look, you'll have noticed how the non-NFB designation cropped up consistently since 1993. That's shorthand for zero negative feedback. Feedback is a nearly ubiquitous correction technique to improve a circuit's linearity. Feedback can be applied nested/local and/or global. It loops back anti-phase signal to an earlier point in the circuit to compare the two and reduce distortion. As such, feedback also eats into overall gain. It's why feedback is measured in decibels. The more feedback is used, the higher the circuit's gain must be to compensate. This can lead to additional gain stages, hence a more complex circuit over what a zero feedback layout would need to generate the same amplification factor. That can create so-called higher-order THD which is very disharmonious and sonically detrimental at even subliminal doses even if otherwise measuring impressive for lower-order distortion like 2nd and 3rd.

This led US amplifier legend John Curl to state that the most important thing to get right in transistor circuits is complete eradication of 7th-order harmonic distortion. In the more exotic high-end, gain circuits without feedback are more popular though examples of the complete opposite exist as well. Where Ayre Acoustics shun feedback, Bruno Putzeys of Hypex, Ncore and Mola Mola for example celebrates a lot of it. So-called operational amplifiers or op-amps for short—gain circuits on an integrated chip or IC—wouldn't even work without it. Likewise ultra-low amplifier output impedance.

Whilst opinions on feedback and harmonic distortion spectra diverge, it's fair to say that in the mass market and transitional high-end sectors where Soulnote operate, zero feedback circuits are more rare if not entirely unknown. That's because it's harder to design a circuit that doesn't need negative feedback correction in the first place. If you remember the spec wars of the last century, Japanese receivers from the big corporations one-upped each other on more and more zeroes behind the decimal point for various forms of distortion. That this often didn't correlate with better sound but perhaps the opposite wasn't lost on the SET renaissance. Their low-power tube amps measuring poorly often sounded vastly better and cited absence of feedback as a major contributor. In that context, Soulnote's unique selling proposition are no-feedback transistor circuits in the upper mass-market sector. As they believe, this means sonic quality not to be had elsewhere at their price points.
For today's assignment however, it's academic. Late into the review, I was informed that actually, "the SD300 contains no non-NFB technology". A double no makes for a yes. Ergo, this machine does include negative feedback. No feedback is reserved "for the 700 Series models".