Diode D2 has been reversed so that the polarity of the voltage across C2 will be as shown in the diagram. When the carrier is unmodulated, the voltages across CI and C2 are equal and additive The audio output is taken across C2(or R2 of Capacitor C6 is a large electrolytic capacitor, lt charges to this voltage and, owing to the long time constant of C6, R1, and R2, the total voltage across it remains virtually constant at all times. In fact it just acts as a power supply or a battery. The important thing to note is that it keeps the total voltage of C1 t C at a constant value.