dfuruta said:yes, that's what i ended up with. final schematic is attached to this post. i've been using it for the last couple months and it does what it should. my pcb layout is more cramped than it should be for optimal usability.
rotary switches are from grayhill & toggles from e-switch. xlrs are neutrik d series, and bnc connectors are from molex.
the limits are set by the qa400's unbalanced connections; it wouldn't hurt if the interface had a balanced input from the qa400, and maybe some sort of ground-canceling output. for now, this works ok. eventually i might have another go at it.
What do you use as FB1 and FB2? If you have a part list, would you mind share it?