I just built a clone of an LPB-1 pedal, as my first ever guitar pedal project. It works great between my guitar and amp with no other pedals in the chain. When it is placed in a chain with pedals before or after, though, it doesn't work properly. When it shares a power supply with other pedals, it completely blocks any signal from passing through. With its own power supply, though, it works, but also generates a loud 100hz hum. I don't know too much about electronics but I'm confused as to why the pedal would work by itself, but not in series with other effects. What could be causing this to happen?
I am powering the pedal using a standard 9v power supply. I also added a DPDT footswitch, wired as a true bypass switch connected to the input and output jacks.