Background:
SO2R requires that two radios be connected to the same ground due to the RF outputs, etc. When the DXD is in use, the headphone grounds will also be connected together. Some people have experienced audio hum in either the headphones or the mic, even though the mic ground is not connected to the DXD chassis.
Solution:
The problem can usually be corrected by connecting the radios together with a heavy ground wire or braid. When this fails, it may be necessary to isolate the headphone grounds on one or even both of the radios. Using audio transformers to isolate both the main and the sub receiver audio will do this. The schematic below shows a typical hook up using a pair of 600 ohm audio isolation transformers. The parts are assembled into a small plastic box as shown in the photo.
