Connecting Your Sound Card to the Phone Patch Plug

Problem: The soundcard software program that I occasional run needs USB, it is MT63 by IZ8BLY, unfortunately my old MP does not have the CPU upgrade that allows for the packet mode USB but only LSB. For the record I use the packet input for all other soundcard modes and they work great without any hum, however if I use the phone patch input on the back of the MP I can hear some hum. I am using isolation transformers for all inputs to and from the MP to the computer. Question? Does anyone know if there is a load requirement for the phone patch input. I am using a Radio Shack 1:1 isolation transformers, any suggestions? Should I isolate the transformer with a cap? Any parallel resistor network required? Ron W4LDE

Solution: I just hooked mine up for a contest voice recorder to use with my Writelog contest logging recorder. Here is what I ended up with. 100 ohm resistor across the output of the sound card. Then 10K ohm resistor in each leg. Then a Radio Shack 1:1 transformer. Then a 470 ohm resistor across the output of the transformer and onto the phone patch input of the radio. Built it on a piece of Radio Shack perf board. No hum. I do have a little RF getting into things when I run 1500 watts. Have not trouble shot it yet though. Jim W7RY