For a good result you will need a MiniDisc Walkman. You can use the MZ-R30 or 35 or 55. Connect the external microphone to the "Mic In" of the MZ-RXX . Using a stereo-jack audio cable, connect the "Line out" of the MiniDisc to the "Line In" of the PCG-505. This way the MiniDisc acts as an amplifier. The quality is excellent.
Remark: the PCG-505 models do not have a "Microphone In".