Blown petal input buffer by active guitar electronics

Guitar Pedals
1

During switching between guitars with active electronics my petal suddenly lost the sound (LEDs were OK).
I checked and I found that at some tiny moment when you unplug the cable - there is +9V (and even +18V from my bass) on the signal lead. Both guitars I have, stock.

MCP6004 operational amp has shorted the signal to the ground internally.
Temporary fixed with another op amp.

I know that the problem is on my side, just warning you all that this might happen.
Don’t know how to avoid this atm.

2

Yes, the cable acts to power the guitar preamp by connecting ground. When the tip passes the “ring” terminal it will see the battery voltage but current limited because it’s going through the preamp power rails. This isn’t normally a problem with stomp boxes. Is the petal input not a/c coupled?

3

It has 10uF capacitor, but it didn’t help.
The capacitor is OK btw.