Protection circuit for detachable operating unit used in audio device

A detachable operating unit for connection with a main unit comprising:

• at least two power supply units for applying operating voltages to said detachable operating unit, said power supply units generating two different voltages, respectively;
• at least two power supply terminals for engaging said detachable operating unit and said main unit, and for receiving two different voltages which are applied to said power supply units, respectively;
• a ground terminal for engaging the detachable operating unit and said main unit;
• and
• a protective circuit for applying a predetermined reverse voltage in response to a differential voltage between said two voltages of said power supply units, said protective circuit comprising a single diode having a negative end connected to one of said power supply terminals that receives a lower voltage and a positive end connected to said ground terminal, said diode forming a non-conductive circuit when said ground terminal engages the main unit, said diode forming a conductive circuit when said ground terminal is not engaged with the main unit and said power supply terminals are engaged with the main unit, said diode forming a conductive circuit to limit a reverse voltage across the main unit and protect the main unit from a larger reverse voltage which would otherwise be produced across the main unit by the two operating voltages when the ground is disconnected.