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On Wednesday, September 12, 2018 at 2:36:07 AM UTC-4, wrote:
I notice this circuit uses only one ground, rather than different ground and
chassis earth. This may come to bite you if you're trying to get noise down.

Cool. I wondered about grounding and have drawn the power supply ground separately to the "everything else" ground in KiCAD. I suspect that's not what you're suggesting though. How would you configure the ground(s), and where would you advise connecting them together?
Thanks!

the most important ground issues involve signals that enter and exit

1) the + and - leads from the wall wart should go directly to the 470 uf cap so that large ripple currents do not flow through any grounds. I would also use the one point to tie the circuit to chassis ground.

2) the pin 1 connections for the XLRs should go directly to the chassis so that any large ground currents do not flow through the circuit grounds. You can google "pin 1 problem" to learn more about this.

enjoy

mark