Besides noise, which we dealt with in the previous two installments, hum can also seriously spoil our listening pleasure. All the more reason to focus on hum in the theoretical part of this installment. In the practical section, we once again build a preamplifier — but this time one with a balanced input. In addition, an isolation amplifier is discussed as an extra.

Theory 3

Hum
Hum always occurs when the 50-Hz mains frequency, or twice that, 100 Hz, somehow finds its way into the audio signal. This can be the result of inductive or capacitive injection, hum voltage on ground connections, chaining multiple devices together, or a poorly smoothed supply voltage in the humming circuit. This last cause can easily be prevented by keeping the supply voltage constant with simple regulator ICs, for example, a 7815 and a 7915.

Hum Caused by Inductive Injection
Inductive, magnetic injection is usually caused directly by the transformer, for example, when signal-carrying wires or PC...