While output ballast resistors in the basic bridge/parallel circuit work well to keep separately biased IC outputs from fighting each other, the addition of servo circuits will minimize
output offset
voltages that cause output voltage inequalities.
Different output offset voltages cause a constant current to flow between outputs that increases IC power dissipation. By minimizing output offset voltages, all of the ICs will run cooler, expanding the IC’s long term reliability and output power capability without activating sensitive protection circuits.
Typically, offset
voltages are compensated for by using input and output coupling capacitors. Power amplifiers used in a single-supply configuration, utilize large value, large size electrolytic or polypropylene capacitors. This is because the load impedance is 4Ω or 8Ω and the RC combination creates a highpass filter that can rolloff audio frequencies.
Since these output coupling capacitors have nonlinearities and are quite large, many designers choose to employ split
power supplies. While split power supplies don’t use these capacitors, a DC blocking capacitor is needed somewhere in the circuit to protect speakers. This capacitor is typically, Ci1, Ci2, Ci3, and Ci4.