a hum filter for a 9vdc wall wart

[steve-kc7byp]

I have a Technosonic 900mhz cordless headphone set that I got at a yard sale. The cordless setup works works great on batterys. The base will also work off a 9vdc plus tip wall wart. I found a 9vdc plus tip wall wart in my junk box. It is the only 9vdc wall wart I have (period right now) with the right plug on it to fit the power socket on the base transmitter. The cordless headphones work on the wall wart. But there is a very loud hum there. That isn't there when the base is running on batterys. I think this wall wart is only half rectifying the a/c. I am a fixer, I can fix things but designing a circuit is a bit out of my league. I need some quick, smiple ac filter that I can build from parts in my junk pile to put inline on that 9vdc wall wart. I also have some old laptop style switching power packs that put out 12vdc. But I don't know the current draw of the cordless transmitter to compute the ohms and resistor size to drop the 12vdc to 9vdc. I could use some help on this proj please.

Steve