EI transformer rattling the metal enclosure

[Khron]

Background:

https://khronscave.blogspot.com/2017/11/25-behringer-ada8000-ultragain-digital.html

Unfortunately, it seems i hadn't tested the whole thing with the top of the case reinstalled.
The unit's been sitting on a shelf since then, but today i wanted to re-test it to make sure it's usable.

But as soon as i flicked the power on, case-buzz-city 

All is quiet when the top case is off. Is the whole setup creating some sort of shorted-turn type deal?

Or any idea what's going on, at least? If not even some sort of solution for that (short of "just use it without the top case")...

 

[JohnRoberts]

The cover can distort the magnetic field.

Transformers vibrate and to keep them quiet laminations are often lacquered? Caveat I am not the transformer expert here.

JR

 

[analogguru]

Or any idea what's going on, at least? If not even some sort of solution for that (short of "just use it without the top case")...

transformers produce a magnetic field.  When the magnetic field is strong (e.g. cheap transformers) it will excite the top cover metal sheet to oscillate (like a speaker membrane).  That is the buzz you are hearing.  (Old telephone headphones with 2000 Ohm impedance used this principle).  Forget all these Akg-, Sennheiser-, Sony-, Beats-Headphones - here is your chance  :
http://oldheadphones.com/crystal/phones/phones.htm

Magnetic Headphones:
These were the first headphones ever built and were popular for many years although modern headphones work better and have replaced them.  Magnetic headphones are constructed by winding many turns of fine wire to form a coil around an iron core that includes a permanent magnet. A thin iron plate which makes the sound is held in place by the magnetic field. The center portion of the thin plate is pulled by the magnetism. As an audio (i.e. AC) current is passed through the coil the magnetic field increases or decreases in response. The variation in the magnetic field modulates the force of attraction of the iron plate which then moves slightly thus making a sound. Magnetic headphones generally have an impedance in the 2,000 to 20,000 ohm range.

http://www.kennethkuhn.com/students/crystal_radios/headphones.pdf

If the top cover is screwed it will be a it better, but the solution is to put a piece of harder foam between the transformer and the top cover (think of a damper on drums).  The top cover can't move anymore and the buzz will disapear.

 

[Mr. Dave]

I had this happen with a compressor I was working on, the case and cover were both steel. I made a new top from aluminum and the buzz almost completely disappeared.

Dave