What can you do if the mains transformer hums?

The hum of mains transformers can have very different causes. In principle, it must be noted that every mains transformer, regardless of construction type, hums on the 50 Hz AC mains; however, in correctly built transformers for audio applications, this should not be disturbing. Assuming correctly designed mains transformers, there are several factors that can influence this hum:

The hum excites other components (resonance) → remedy: damp the vibrating component

The transformer hum is amplified by the chassis (resonance) → remedy: mount the mains transformer on rubber buffers

The transformer hum is caused by very high idle current → remedy: reduce the bias / idle current

High DC component in the 50 Hz mains supply → remedy: DC blocker / DC filter

Transformer also becomes very hot due to high load → remedy: use a stronger transformer

The problem of hum caused by DC in the mains is found mainly in urban areas or areas with a high industrial share, especially in connection with toroidal transformers (e.g. custom-made BTB transformer) or strip-wound transformers (Lundahl).

Tip: Subsequent potting of mains transformers is not recommended, as it can reduce power output, and in a finished product the impregnating varnish can never penetrate the inside in the same way as in a product that was designed for impregnation from the start.