Audible Transformer Hum or Noise

[Gepetto]

In tearing down a recent PL700B for rebuild, I took notice that the transformer coil to lamination stack was loose and I could physically move the coil on the stack a small amount (~1/32") back and forth. I don't know if it was always that way or has become that way over many years due to stress relief of the winding coils and coil form.

At any rate, this looseness can be a source of audible noise because the coil will definitely vibrate on the lamination stack unless it is securely bonded to the stack. Epoxy can be used for this if you experience a similar situation. This is not obvious because one side has the black exterior end bell on it and the other side has the die cast capacitor mounting bracket on it. The coil is inside these 2 mechanical covers. A picture is below of the naked transformer.

No matter how well the coils are bonded to the lamination stack some level of hum will be present due to magnetostriction of the lamination steel itself. It microscopically grows and shrinks in size with the alternating magnetic field producing noise. This is a property of the magnetic steel and there is little you can do about this.

For those of you who have searched for the source of your audible hum, this looseness may be part of the culprit.

 

[laatsch55]

Damn, you're deep into that one eh??

 

[Gepetto]

Like digging wells Lee...

 

[laatsch55]

Yep, cept you can see what you're doing...

 

[orange]

In tearing down a recent PL700B for rebuild, I took notice that the transformer coil to lamination stack was loose and I could physically move the coil on the stack a small amount (~1/32") back and forth. I don't know if it was always that way or has become that way over many years due to stress relief of the winding coils and coil form.

At any rate, this looseness can be a source of audible noise because the coil will definitely vibrate on the lamination stack unless it is securely bonded to the stack. Epoxy can be used for this if you experience a similar situation. This is not obvious because one side has the black exterior end bell on it and the other side has the die cast capacitor mounting bracket on it. The coil is inside these 2 mechanical covers. A picture is below of the naked transformer.

No matter how well the coils are bonded to the lamination stack some level of hum will be present due to magnetostriction of the lamination steel itself. It microscopically grows and shrinks in size with the alternating magnetic field producing noise. This is a property of the magnetic steel and there is little you can do about this.

For those of you who have searched for the source of your audible hum, this looseness may be part of the culprit.

View attachment 38254 View attachment 38255

I had all THREE of my Advent 201s do this, the transformers literally SANG. I did not know what to do about it then so off they went.

 

[Northwinds]

Carver mag coils are known for this also, I discussed magnetorestriction a few years ago when I had a Carver 1.5T

Can the whole kit and caboodle can be wax potted like humbucker pickups as an alternative

 

[Gepetto]

Carver mag coils are known for this also, I discussed magnetorestriction a few years ago when I had a Carver 1.5T

Can the whole kit and caboodle can be wax potted like humbucker pickups as an alternative

They are vacuum impregnated with varnish but that breaks down over the course of 40 years...

 

[WOPL Sniffer]

I know one of the guys was talking last year of wanting to disassemble his transformer piece by piece and re-varnish it and put it all back together.... OMG!!!

 

[wattsabundant]

My first real job was at a transformer shop. I learned far more about transformers than I wanted to know and despised the job. It was a common practice to shim the coil with a wooden wedge. Then the transformer was dipped in a resin and baked in an oven to cure it. It would be possible to use a door shim on all sides , to keep the coli centered. This is desired to make sure the steel doesn't cut into the coil. Of course the wedge would have to be epoxied as Joe points out.

I'm guessing most consumer transformers do not use the vacuum process as it is laborious. Incidentally transformer temperature rise is usually lower after the impregnation process.