To frame how circuit simplicity may hide complexity and deliberate voicing chicanery, consider 2picoDumbs' below example. It's of a 2nd harmonic wave in red above the blue fundamental which for the F7 occurs at -65dB whilst the 3rd harmonic sits at -75dB and the 4th and 5th well below 100dB [right]. It's not simply about the amount of residual THD, its distribution and how/whether those relationships are fixed or variable with amplitude. It includes the phase relationship of the harmonic to the fundamental (green lines). The left graph seems to have the harmonic lined up in nearly perfect phase, the right graph in anti phase. Or do they when the fundamental's opposite phase inverts said relationship? Is it just academic curiosity? Or do these sound different because the fundamental's expansion/contraction happens on the outgoing vs. negative driver stroke? In one of his talks, Nelson mentions how they do not sound alike. Also, a circuit of fewer parts tells us nothing yet about what specific behavioural aspects its few parts were picked for. Stringent matching requires big parts inventories. Only those can whittle down to the desired values and end up with the strict tolerances required to eliminate common compensatory or control parts like trim pots. Like saying more with less usually does, one must start with a lot to end up with a little. Not knowing Nelson's selection parameters should make it virtually impossible for the toothless shark cloners to end up with the same sound even if they otherwise copied his circuit. That would be an example for how a master may obscure many design considerations inside a schematic's minimalism. It's also our cue to admit that this gets into areas relevant only to those with the required higher understanding. We plead the fifth—nearly absentee harmonic—and focus on what even tech vegetables can do: listen. For those still frisky for theory, here's a link to a 1957 article on positive current feedback.

from this page of the DIYAudio F7 thread

Here now are Dawid's inside photos.

It's hard to imagine a simpler push/pull circuit that actually works.

180'000µF of capacitance around a thermostat make up the power supply board.

As per usual with FirstWatt amps, it's all fitted again into the same casing with the same signature heat sink profile. Perhaps in a nod at the F5 whose circuit it simplifies, the face plate is the same silver whereas the F6's had been black. With its strategic mix of negative and positive feedback, output impedance is 0.08Ω to net the specified damping factor of 100 into 8Ω whilst feedback is around 10dB depending on load. That means more control over the type of impedance fluctuations and phase angles which are so typical for the ubiquitous ported speakers. Relative to our photos giving away too much, "I always assume that 6moons will photograph the interior. That's why I hide anything I don't want you to see. None of the circuits proposed match the F7 except to the extent that I have described it. Also keep in mind that the notion of positive feedback is reasonably old. Even I have played with it in the past. It is a general concept and it so happens that I have found a cute reason and way to apply it. Of course I could have kept my mouth shut and just released product. But it is too cool to not share and I knew that it would create the usual wave of speculation. So you have, mostly thanks to lhquam, a selection of potential topologies, even some nice equations for them. Probably more than you asked for. This being DIY, I suggest that you pick one and build an amplifier." Relative to the tango between positive and negative feedback, "there is an equilibrium set of values where the load is cancelled out of the equation." Or as Ihquam put it who generated sundry circuit variations, "it makes the output insensitive to the load resistance, i.e. infinite damping factor."

A week or so later, Dawid checked in. "This is a seriously friggin' good amplifier, Srajan. I don't think you will get anything valuable and new from my impressions yet after several days and a 'triangulation, decomposition and reconstruction' phase, I finally had to tell someone. I am utterly amazed by what Nelson did here. With my Boenicke W5, it's a jaw-dropping experience. I'm not hugely familiar with tube amps but at least now to some small degree, I understand why you switched. For me it's a mind-opening experience. It's good to finally witness what you wrote about for so many years. Yet what strikes me the most is that in the F7's case, it's not merely about blunt warmth and fuzziness that often come with it. To my ears this amp has huge doses of smoothness, delicacy, saturation and charm yet still is able to maintain an exceptionally informative character and wide bandwidth. Very refined and polished, it's a spectacular pleasure machine. But I'm guessing all these aspects are Nelson's standard features." Our FirstWatt rookie definitely had his nookie and was deep in the afterglow of it. Once he returned to his senses, I was mighty curious just how he would break down his experience into audiophile jargon. After all, "seriously friggin' good" is what we all want from and for our components yet from a review—sniff—we expect a lot more. To create a useful back drop for Dawid's commentary, we now turn to Nelson Pass to help us avoid misleading purely theoretical assumptions.