The 220 uF cap and the lower 1N4148 diode form a negative "DC restorer" (q.v.). Regardless of the DC level on the positive terminal of the capacitor, the negative swings at the positive terminal will pass through it and the diode establishes the baseline as near ground. (The diode keeps the negative end of the capacitor from going substantially positive with respect to ground, and the charge in the capacitor will hold the voltage across the capacitor somewhat constant.)