Been reading the S300 thread with interest, as the issue of transformer hum has cropped up. [For those who don't suffer from it, and don't want to start hearing it turn away right now :) ] It would be useful to understand reasons/cures for hum a little more, as most dealers and manufacturers just end up saying 'learn to live with it' From my v. limited knowledge, there is hum: - caused by ground loops, due to earthing, heard thru the speakers; - and hum which is mechanical from the transformer itself. Why should transformers hum for this reason. Isn't it just poor design? Esp as some amps hum and others don't. Is the hum due the windings/laminations rattling. It seems toroidal transf. are more sensitive to hum, esp if there is dc leakage into the transformer (not that I understand why). Or is it something also to do with the frequency of the ac current, and if this varies too much, the transformer may hum. Intermitent hum seems weird too. Is this just random, or induced by variations in the ac? Anyhow, would be better to hear from more knowledgeable folks cheers