My Threshold FET 10-Pe (1994) phono preamp has ferrite beads on both inputs, hot and ground wires. I've followed that build technique on the 6 phono preamps I've built. I also use ferrite beads on torroid mains.
Hi cvanc,
Can you post details, like the cable lengths, if they are shielded or not, and the phono amplifier’s input capacitance, etc? I guess I’m wondering how you determined it was cellular and not another source?
Anyway, these might help diagnose the cause.