Looking for advice on mechanical hum

[Ryan Berry]

Sorry I caught this one late, but glad you got it sorted.   We've ran into a few instances where someone would say a transformer was humming but could not replicate it at Ayre.  The problem is that most companies have a lot more ambient noise between lighting, equipment, people, etc. than you do in your house, which can easily mask the sound.  I've had to stay after everyone else has left on a few occasions to shut everything else in the building off to hear what the customer was talking about.

 

Almost every transformer has a little variance and it in some cases can cause otherwise unexplainable audible hum.  Geometry of the transformer (particularly with EI core bobbins) is one of the biggest contributors to how quietly it will perform, though there's other factors as well.  We've tried various experiments including having the wires individually coated in epoxy as they were wrapped to try and get the noise down, but in the end the only real solution was to get the shape right for the current we needed.  Even then, you run into one every so often that doesn't behave as well as the others and there's not much you can do except replace it.  Of course, as  you noted, the transformer is still "within tolerance", so figuring out what to do with them is the next problem.