Has anyone figured out what the purpose of the TL431 is in this circuit? You can certainly leave it out and adjust the other resistors to get a working regulator with pass transistor, no?

circuit can work without TL431. the only difference is that with TL431, current through potentiometer can be significantly lower which seem to be the goal here. potentiometers last longer when wiper current is really small.

 

several posts called the circuit low dropout regulator. this is not it. it is a linear voltage regulator but not an LDO

 

I believe that member wireless is correct: there must be different product clones, and some of them are cloned incorrectly. Leading to odd behavior.

 

It's possible that the Weiliang/Breeze Audio version is patient zero, they're a known manufacturer of audio gear who are known to use decent-quality components (but see below), and in this case either they've got some really good fakes of known brands or it's mostly the real thing:

Onsemi NJW0302G
Onsemi LM317T
HJC MKP-X2
Nichicon PM(M) 2200uF 50V, but 16x26mm not 18x40mm.
RIFA PHE 100nF
Baoter 3296 trimpot
MIC HER 60x, can't read the final digit but I'm guessing a 601 or 602, in an R-6 package.
Chemi-Con KY 220uF 63V, 10x25.

The one problem is the Nichicon PMs for which the physical dimensions are wrong, but except for that irregularity I'd have picked them as the real deal, and the rest are either very, very good fakes or the real thing. For the Nichicons I'm not overly motivated to pull it to pieces to carry out capacitance/ESR measurements on one given that it appears to perform as advertised whatever the caps are, so I'll tag them as "alleged Nichicon".

For the "see below" note above, while Weiliang has a record of using decent-quality components it doesn't match their record of using them properly. OTOH the one being reviewed there could be another clone, in my case I contacted the store owners and asked a few questions which indicated that they were most likely the original manufacturers and not resellers/dropshippers/etc.

 

Another comment based on an explanation in another forum, I was a bit surprised to see HER 60xs in there and thought it was just gold-plating but from the explanation there:

The biggest problem is this instance on the use of slow rectifiers, many, notables are the 1N 400x/1N540x, they are hardly fast enough for 50/60 Hz. Not being able to switch fast enough, reverse current builds until the junction effectively "snaps" shut causing the RFI.

their use is actually justified. So at least one version of this circuit was designed by someone who knew what they were doing, it's just a case of finding that version and not possibly-incorrect clones.