What you are doing is called floating a line. There is a lot of misunderstanding about this. It is very easy to create a ground loop.
If you bought your adapters I suspect they are wired like this.
But since the RCA end is the source they need to be wired like this.
Now you might think that this would make no difference.
However only pin one is a ground. Pin 3 is not really a ground and not connected to ground in your amp. Both pins one an two are actually isolated from ground, the pin two carrying the positive side of the wave form, and pin 3 the negative half of the wave form.
So in order to make it work you have to make pin 3 a ground also. Where you do this matters.
This connection needs to have minimal resistance between pin 1 and two at the input to the amp.
If you wire it like the top diagram, then you will very likely create a ground loop because the screen (black) and the blue cable are different and will have small but differing resistances. This is the essence of a ground loop.
The first job is to connect the proamp correctly.
This may not solve it as many pro amps are notorious for internal ground loops when connected unbalanced. Then you will have to use a cheater plug to lift the ground. If you are worried about electric shock then you need to bond all your grounded units to ONE ground with 5 gauge copper wire.
