[sdiy] Barron ringmod

[bbob]

On a page about Bebe & Louis Barron, Barry Schrader includes what looks
like a notebook page w/ an odd ring-mod schematic:

http://www.barryschrader.com/blog/bebe_barron_1925_2008
http://barryschrader.com.hostbaby.com/img/Bebe_Barron/Barron_Circuit.jpg

Any ideas what's going on w/ the zeners in the middle of the ring? and the
following opamp?
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