"Graeme, any reason why you changed C4 from 1uf to 4.7uf?"
Hi Jerry, yeah the reason I originally changed C4 is because I had a bunch of 4.7s and no 1uf on hand when I was testing this circuit out. I poked the 4.7uf in there as a stop gap in the early days and found no audible difference so it's stayed that way.
"These were draw mistakes, C4 and R4 drain resistor, hopefully corrected to original schematics."
Nope, C4 wasn't a mistake, it was a choice for the reason above. BUT, I'd encourage anyone who wants to change it out with the original value to give it a go. It's no extra effort, or confusion, and I'm always super-happy to hear about what other people have noticed and heard. Please post up your observations if you have any.
I'm not sure what you mean about R4 though. That hasn't changed from the Neumann schematic. Could you flesh out your thoughts on R4 for me, please?