Originally Posted by
RothwellAudio
That sound file doesn't sound like transformer buzz at all to me - it sounds like humming coming from the speakers. A hum in the signal can have many causes but in this instance I suspect power supply ripple. I could be wrong about that though.
Of course, hearing the humming in isolation doesn't give any indication of the signal-to-noise ratio. That hum could be at a very low level or a very high level, there's no way to tell. Also, no indication of whether or not the hum goes up/down with the volume control.
Anyway, it doesn't sound like transformer buzz and isn't caused by DC on the mains.