I'm creating a PCB for a client for a guitar amp and fuzz, see attached circuit diagram. It should be possible to power it from a 9 V battery and from an external power supply. Note that it uses -9 V and 0 V because of the PNP transistors.
Here is a full example of a very similar fuzz.
For reverse voltage protection I added some diodes, but how can I protect it, if the power supply is gavanically coupled to the GND of the audio input stage through the output connector? The usual power supply would have the same GND as the audio device. When plugged in the wrong way, the -9 V of the device would be GND, and the GND of the device would be +9 V. This would then short circuit to the GND of the audio device.
For protecting the device and for decoupling the battery and external power supply input, I added 2 diodes. For gavanically coupled GNDs, I added the fuse, but the client doesn't like this, because the usual musician might have problems replacing it. Is there a better way to protect this path without a fuse?