Not exactly guitar related but I am thinking about straying from my tube guitar amp builds and trying a stereo tube amp build. I have an old RCA power amp from an old console stereo that I got from my dad before he passed and have been wanting to rebuild it and get it running again. I am going to build a separate preamp for it and would like to use an mp3 or cell phone as the music source and was just wondering if anyone had suggestions on the best way to wire the input. I have heard that it is a good idea to use an audio isolation transformer to protect the digital device from stray voltage that may be present at the grid of the input tube or maybe a capacitor to block potential dc voltage. I have been unable to find a clear description/schematic of input options on the web so I thought I would reach out to my fellow forum members for help.