The audio system is plugged into a general lighting circuit. With all lights in that circuit off, the buzz persists. So a dimmer switch is assumingly ruled out.
I unplugged (one by one) all of the audio/video components plugged into that circuit. The buzzed ceased only when the preamp & the power amp were unplugged.
I unplugged (one by one) all of the audio/video components plugged into that circuit. The buzzed ceased only when the preamp & the power amp were unplugged.