We fly a mixed fleet of old 212s and 412s. The 212s do use the old low impedance systems. You can either shell out a lot of cash for a David Clark head set with a high/low switch, or buy a separate low impedance mike from CommInnovations. Some of our guys have wired two booms together and then connect which ever one they need to use. The other alternative is to buy a low impedance box (also from CommInnovations), into which you simply plug your HI head set.