The Audio-Technica is very much in force (anticipated expiration 2038).
However (with the disclaimer that this is not legal advice and I'm not a legal professional), here is some color:
- AT patent is filed in 2017, DPA already had lav mics on the market using this topology
- the one independent claim of the AT patent requires:
- "a constant current diode configured to supply a current to the FET" (not the case for how DPA mics are biased)
- a second BJT, "a collector grounding second transistor" (Q2 in the diagrams)
- the next claims are dependent and do not remove the two requirements I mentioned (that would not be proper form for a dependent claim anyway).
In my unqualified opinion, even if a company would build a lavalier microphone with the JFET-PNP topology today, it would not infringe on the AT patent (and DPA never patented this topology, or the CORE preamp for that matter).
Again, take this with a grain of salt, the closest I've ever been to this stuff is being an inventor on one US patent, but a very good patent attorney, paid for by my employer, wrote that patent based on my technical docs and explanations.
