Quote Originally Posted by Barry View Post
In times past electronic gear used to be fitted with mains voltage selectors which selected the appropriate tapping on the transformer primary. This was because the transformers used were designed deliberately to be run close to saturation to improve regulation. If extra turns were wound on the primary to increase the saturation margin, the increased resistance would result in poorer regulation.

When a transformer saturates the inductance falls and the primary current rises. Saturation can be caused either by running the primary on too high a mains voltage or by having some DC on the mains (which is effectively is similar, but only occuring each cycle, rather than every half cycle). It only takes ~100mV of DC offset to cause toroidal transformers to buzz (owing to a lack of air gap in the core, toroidals 'hard limit' when saturated).

Early power supplies were unsophisticated, so it was important not to compromise regulation, but modern regulated power supplies can be supplied with a transformer that has a much larger saturation margin, and hence no need for a mains voltage selector.

However regardless of the margin of saturation, if the laminations are loose they will buzz, simply due to the AC nature of the mains supply.
Excellent!