I think I've figured it out: pre-CORE it was probably a topology like the one from the Audio-Technica (which would be later than DPA doing this).
Why do I say this? I've found , years before 2017 (when CORE was introduced), listing the output impedance as 30-40 ohms (JFET topologies have output impedances of 1000 ohms or more).
So this was at least an emitter follower even then, and 33K makes sense as a value for R1!
In this topology, VDS is "regulated" by the VBE of Q1, which means that for large input SPL generating e.g. ~2V pk-pk at Q1 base, it will still vary by 40-50mV.
In what I suspect is the CORE topology, VDS is regulated by feedback from the error amplifier and under the same input conditions will vary by 4-5mV, extending the linear range and improving the max SPL at 1% THD spec.
Are you in the USA? ?I'd be happy to mail that capsule to you for further inspection.
Thanks for the kind offer, I live in Europe and I've already ordered one from the UK a couple of days ago, listed on eBay as defective with "thin tinny sound" for the equivalent of $38, shipping included. I don't know if it's CORE or pre-CORE, I guess I'll find out when I do the teardown.
Like I said, I own a 4080 CORE bought new from Thomann a couple of years ago, but I won't go past measuring the U(I) curve in my investigations on that one.
