I made a SMD version of the Protector PCB for the CFA3, and after testing it this weekend I came up with some ideas for changes
#1 - add blocking diodes in series with the power supply inputs. first thing I did was fry the V+ regulator by hooking up power backwards. other protection diodes for the regulators could be added, but I think they're less important
#2 - add minimum load resistors for the regulators (R25, R26). I've previously noticed some 3-pin regulators can have an unstable output voltage if their load is too low. the resistors are so each regulator has at least a 5mA load
#3 - option of getting DC voltage reference from the +/-12 regulators or the external supply. In this case, the +/-12 regulators could have as much as a 0.5V imbalance which requires hand selecting resistors to set the voltage reference (the voltage across R8 + RV1). if powering the protector with LT1021 referenced GRLVs, the voltage is going to be within millivolts for both + and - references. You would install either R1 or R9, and R2 or R10. the PCB could also be designed to allow switching between the two for two different range options.
one feature of using the GRLV / higher voltage supplies for the reference is if those supplies were to become damaged and start outputting a different voltage, the reference will probably be thrown off more than enough to permanently disable the output jacks until the problem is found/fixed.
#4 - added RV1 pot, the purpose is to adjust the range of the protection if it is too sensitive or not enough
#5 - added dual indicator LEDs, for "Good" (D9/white) and "Protection On" (D10/orange). Not a bi-color LED, but 2 separate LEDs next to each other. The white "good" LED will be lit but very dim when the Protect LED is on, and the orange will overpower it.
#6 - changed relay from 12 to 24V for 1/2 the current and to use more equal current from the +/-12V regulators
#7 - added 4.4mm Pentaconn jack short protection. The 4.4mm jack has a switch in the back that only opens when the plug has been fully inserted. The relay keeps the L- and R- pins disconnected, which is every other pin on the jack, until the plug has been fully inserted + a .5 second delay, and then instant release of relay when the plug starts to be removed
SMD version & in CFA3