Hum very often comes from ground loops, in which case an isolation
transformer in the audio line fixes it. Radio Shack 270-054A
about $15.

It will come in handy for other things, even if it doesn't fix this.

I need them all over, between the radios and the computer.

Batteries are not very efficient at 12v; my own wall wart solution
is a switching power supply in the basement, powering about 30
12v devices each of which came with a wall wart. If you go this
route, fuse every line at below its capacity (eg. 7a for the
main 12 line, and 1/4 and 1/2 and up amps on each individual device's
line from that). Use fast blow because slow blow gets quite hot.

Normally the internal resistance of a wall wart prevents starting
fires in the power cord, but a big 12v supply is fully capable of
turning a short into a fire without overloading itself.

--
Ron Hardin


On the internet, nobody knows you're a jerk.