I need to isolate the dc input from the dc current inside the circuit (both 9v), in the same way as a 1:1 transformer isolates the AC current inside the circuit from the input AC. I already found out that there are also 1:1 DC-DC converters, but do those converters also really isolate the input from the output? And is this also the best (or only) solution for this? I need this to prevent ground-loops.
edit: Thanks for your comments so far, I've corrected some things in my original question, and below I'll try to explain my problem as well as possible below:
I'm building an effects pedal for my guitar. Effects are usually placed in 2 different locations in the signal chain (before the amp, and between the pre and power amp(effects loop)). If you use the same power supply for pedals in front of the amp and in the effects loop, you can create ground-loops (effect1->audiocable->pre-amp->audiocable->effect2->powercord->effect1), so usually you use 2 different power supplies for this. However, for some reason I need an effect which has parts of it in front of the pre amp, and parts of it between the pre and the power amp*. As a result, I need to find a solution to break the ground-loop inside the effects pedal.
*In theory I could use multiple pedals for this, but then I need to do some crazy tap-dancing on stage to switch multiple pedals at the same time. So this is not a possible solution.
edit2: I forgot to mention, but the only thing still physically connecting both parts of the effect inside the pedal is the power supply. Of course I could use 2 power supplies for one effects pedal, and that would solve my problem, but I think that's an ugly solution.