Some time ago, I decided to “activate” a few passive three-way loudspeaker boxes using the active crossover I had developed and my so-called high-end amplifiers. Each box was intended to have one crossover with three power amplifiers. The power supply for the amplifiers was the usual circuit consisting of a transformer, a rectifier, and smoothing capacitors. This actually worked quite well, but then I heard a noise.

The Problem

After assembly and testing, I unfortunately found that an audible hum could be heard from the speakers. In such cases, the wiring of the setup is usually checked first. Optimizing the wiring did bring some improvement, but the hum was persistent and still audible.

After an intensive search, it turned out that a rather unusual phenomenon was to blame: the cause of the hum was the pulsed charging currents of the electrolytic capacitors on the amplifier boards. Due to full-wave rectification and the relatively large 100 µF capacitors, narrow current pul...