Power line hum is a 60/120Hz hum coming from the speakers. It can have several causes. From poorly designed power supply to bad amplifier layout. The first cause can be fixed by sufficient filtering and proper layout. The second cause is more diverse. Most common source is magnetic coupling between power transformer, output transformer, input tubes and circuits. Another cause could stem from eddy currents in the chassis. An eddy (circular) current is a local electric current induced in metal by the AC magnetic field. Steel, is a ferromagnetic material but a poor conductor. If the chassis is made out of steel and is used as the main ground conductor, eddy currents mix with ground return (signal) currents. This results in hum and noise. The solution here is star grounding (see below) and proper transformer (shielding) placement.

Fig.1

The microphone and the loudspeaker share a common line between node N1 and N2. A small signal voltage is generated due the loudspeaker relatively high current and some resistance in the line (wire). The voltage equals i * Rl. The sensitive microphone input will pick-up this unwanted signal and amplify it further in the amplifier stages. In the worst case, there could be a positive feedback, resulting in spontaneous oscillations. Damage to speakers and/or amplifier may occur. At best, feedback signal will modify the overall frequency response of the audio equipment.
Fig.2 shows a correct common ground wiring of the same device.

Fig.2

Fig.3

If there are to many points to be grounded, we may compromise and supply only a separate ground connection for particularly long ground leads and/or carry relatively large currents. Other points can be connected to a common busbar. A busbar is a piece of bare copper wire, which runs elevated trough the under-chassis of an audio device and is terminated at the center ground connection. In tube equipment, center terminals of the tube sockets provide a convenient support for it. All low current components to be grounded are connected to the busbar. Utilizing a busbar is shown in Fig.4

Fig.4

One external connection to power line Ground/Earth must be made, where required by code. When two or more units are interconnected, the chassis of each unit should be daisy chain grounded. Connection to power-line ground should not be duplicated. When more than one unit has a three-prong power-plug, the audio patch-cable shield must have only one (side) ground connection. The shield on the other end of the cable must not be grounded.

• Place transformers in such a way, as to not cause magnetic coupling. Use shields.
• Keep power supplies components close together to avoid long wire leads.
• Do not use the chassis as the ground conductor. Therefore make only one ground connection to the chassis.
• Do not duplicate ground connections with main power line. If that is not possible, connect all units into same power outlet.
• Use shielded wire for audio inputs. Shield should be grounded at one end only.
• Keep high current carrying wires away from sensitive input wires (circuits).
• Rout all AC carrying wires away from sensitive circuits. Keep them close to the chassis. Twist AC filament wires together.