"Joe from Kokomo" wrote in message
...

50 Hz if the hum -originates- in the audio chain or 100 Hz if in the power
supply (presuming they are using full-wave rectification, single phase
power supply).

Most anything over 1KW uses 3 phase power. Full wave rectification of 3
phase 50 Hz power would have hum at a 150 Hz rate, IIRC... but shouldn't
have much of it, since 3 phase power is much easier to filter.