So-called ground loops can cause a hum when, for example, the PC is connected to the stereo system. A ground loop is caused by multiple ground or earth connections, where compensating currents choose the shortest path through the audio cable. The solution is a sheath current filter that separates the grounds from each other.

A transformer from audio equipment was found in the tinkering box. Unfortunately there is no measurement data available. Also no sound quality miracles are to be expected from this tinkering. Nevertheless the construction is suitable for most of the smaller problems. In professional audio technology, there are higher quality transformers as they are also found in DI boxes. Alternatively, capacitive coupling of the audio line may solve hum problems.