Grounding is a catch-all label for a number of different topics, which IMHO is confusing. The topics are:
- Electrical safety;
- EMI protection;
- Current routing (especially power currents, e.g. power supply or speaker connection);
- Correct signal reference points (a.k.a. "signal grounds").
The previous two posts were about electrical safety and EMI protection.
Current routing issues arise because the current from one wire can inductively couple into another. The stronger the current and the higher the frequency, the better is the coupling. In particular, Class B (or AB) power amplifier supply currents essentially are the rectified output (loudspeaker) current of the amplifier, rich in harmonics, which will happily inductively couple into other signals.
The way to deal with it is to make sure the wires or PCB traces carrying any current are as close as possible to the wires/traces carrying the corresponding return current. This is effective for both power currents that emit noise and for small signal wires that receive the noise. Twist power supply wires with power supply return wires. Twist together loudspeaker wires. Twist together mains wires and power transformer leads. Route PCB traces close to each other. For small signals, always route a signal and its reference ("ground") together. Twist signal wires.
This is how our LM3886 PCBs are routed. Note they use 4-wire connection to power supply (+V, +PG, -V, -PG) to simplify such routing on a compact PCB. Using one only PG connection instead of two will increase the coupling of power supply currents into input signal and will increase distortion.
Sometimes, power supply currents are routed using copper polygons on the PCB, which provide lower impedance. However, polygons do not allow control over where the current flows, and hence can increase inductive coupling of those currents.