Midi through USB cable - problem fast ticking noise rd 700gx

[PhilipVDH]

I'm since recently the owner of a rd-700gx. Besides using it as a stage piano, I was also hoping to use it as a master keyboard in connection with a virtual synth on my iPad mini.

Functionally this works perfectly. The piano is connected to the iPad through a USB cable that is hooked up to a lightning camera connection kit and I'm perfectly capable of playing the synth.

But as the sound generated by the synth doesn't go back to the piano, I have to use the headphone connection from the iPad to connect to my amplifier. By itself this also works fine.

But there is a strong very fast ticking noise that makes the entire setup unusable. After some experimenting, I discovered that this ticking noise is produced as soon as I plug in the USB cable between the piano and the iPad, even when the piano is powered off. Without the USB cable, I can play on the virtual keyboard on the iPad and this works perfect without any disturbing noise at all. But as soon as I plug in the USB connection into the piano, the ticking noise begins. I have this when the iPad is plugged into my amplifier, but also when I use headphones, so it's not related to the amplifier. It almost certainly has to do with the USB cable ?

Has anyone else experienced something similar, and is there a way to solve this ? Looking forward to your ideas.