This is what I did:
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Burn Diet-Pi onto a micro SD ( see this thread, load it into the Pi and let it expand. I used a monitor and mouse off the Pi, but people have used SSH (see above);
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install Alsa and ffmpeg packages in the Diet-Pi software menu;
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select HiFiBerry-DAC as the sound card ( tube HAT needs to be connected and on for the software to see it);
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To fix the GPIO 13 issue (see above). Firstly install the Python package for Diet-Pi with:
apt-get install python-rpi.gpio
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Then create, save and load the Python script(s) as per rovinggeckoe’s post above;
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install Roon Bridge
It should then show up as a zone in Roon.