some causes are:
Loose or incorrect transformer mounting.
DC voltage on the line. Does the hum vary at different times of the day? Do you hear it through the speakers?
Here's info from people who want to sell you a power conditioner to fix hum: Finding and Fixing Hum
.
Loose or incorrect transformer mounting.
DC voltage on the line. Does the hum vary at different times of the day? Do you hear it through the speakers?
Here's info from people who want to sell you a power conditioner to fix hum: Finding and Fixing Hum
.