The power amplifier works on the basic principle of converting the DC power drawn from the power supply into an AC voltage signal delivered to the load. Although the amplification is high the efficiency of the conversion from the DC power supply input to the AC voltage signal output is usually poor.
Unfortunately we are not quite done yet. I mentioned the concept of a ground loop earlier. This is a particularly insidious problem that can easily ruin a good project. In very general terms it is formed whenever there are multiple signal ground paths to the same termination. I can be internal or external to the piece of equipment. The most frequent result is a hum that either will not go away or happens only when something is connected to the piece of equipment. I need now to provide a few brief words about hum. If the hum is at the same frequency as the AC mains (either 50 or 60 Hz typically) then it is likely from interconnections external to the equipment or poor shielding internal to the equipment. If the hum is at twice the mains frequency then it nearly always because of inadequate power supply filtering. Ground loops are usually at the mains frequency. So if you encounter one, then you must search for the alternate ground paths that relate to the signal chain. If it external (occurs only when the equipment is attached to an external item), then check for things like phonograph grounding at the phonograph end. As an example I have seen is when one terminal of a cartridge connected to the ground in the tone arm (OK and fairly common) and a separate ground from the same tone arm (not OK) is provided for connection to the amplifier chassis ground (this is not to be confused to the situation when there is a separate ground wire from the phonograph chassis that has no connection to the ground in the cartridge). Since both the signal ground and tone arm ground are connected at the phonograph they will form a ground loop (between the shields and ground wire) when connected to the amplifier. The solution in such cases is to separate the grounds at the phonograph. An internal example was mentioned earlier when both ends of internal shielded cables are joined in two different places (at the input jacks and volume control). The irony of the situation and part of the insidious nature of ground loops is that they can on occasion be benign and not cause hum. They can later show up when a new piece of equipment is attached to the system. In all cases however, they have the same fundamental cause, alternate paths for the signal return.
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