When multiple 9v guitar effect pedals are connected with a common ground, extra noise is introduced into the signal path. Apparently a ground loop is formed in this instance, when the pedal's power supply has more than one path to ground. Isolated 9v power supplies are over $150. Would it attenuate noise to install diodes along the ground wire return path, disallowing backfeeding of grounds?

 

Keep All grounds to one Star point to eliminate Ground Loops.

 

Why would the isolated power supplies available be so expensive then? That would be a serious hoax. Perhaps they're made with individual transformers for each 9v output. I don't know if that's necessary for quiet operation.