Why do transformers hum?

Transformers hum because their laminations and windings rattle mechanically.  This is best known as mechanical hum.  This can be caused by several things, including poor construction and poor AC power conditions or both.

If your equipment has a noisy transformer, you’ve probably also noticed that it’s intensity varies depending on the time of day, sometimes even the time of month. The reason it varies is due, in large part, to the quality of the AC line voltage and how much DC is on it.

Why do transformers hum?
The short and simple answer is that many times, transformers hum because of an effect known as ‘lamination rattle’ or ‘winding rattle’ caused by DC voltage on the line.

‘Lam’ or ‘winding’ rattle occurs in all transformers to some degree, that degree being related to the quality of the transformer and the quality of the line voltage.

One of the problems we find on the AC line is when there’s an unwanted DC component.  DC (like battery voltage) leaks into most AC lines but its level is low enough not to matter.  In some cases, the DC level is high enough to wreak havoc with a power transformer.  When there’s DC on the line, it creates an asymmetrical magnetic field in the transformer which causes greater vibrations of the laminations and windings.  The laminations are ‘pushed’ together in one direction because of the DC.

To reduce these noises, transformer manufacturer have several tricks up their sleeves: they can varnish, or use super glue to stick the laminations together so they rattle less, and they can make bigger transformers that don’t have to work so hard, even in the presence of DC. The harder a transformer has to work, the more stress and strain is placed on the laminations.

But these measures don’t entirely solve the problem because you need to do that at the source of the problem, the DC on the line.

Why this is bad ?
When a transformer hums, it is actually physically vibrating or shaking inside of the chassis. This, in turn, shakes and vibrates everything else inside the chassis.

Many components in the chassis are sensitive to vibrations, including tubes and capacitors. In an even moderate case, this vibration can effect sound and picture quality as many of these internal components are microphonic and reproduce the humming into the audio or video signal.

Certainly the worst problem is the humming noise your equipment makes.  It’s downright irritating, especially if you’re sitting in close proximity to the humming equipment.

So, what can we do?
It’s best to fix the problem at the source.  Regardless of how well a transformer is made it’s best to keep the DC out of it.  Transformers with DC on them have core saturation problems, some amount of mechanical noise and lowered efficiency.

The Humbuster III lowers DC on the line and in most cases eliminates hum in transformers and, at the same time, helps the system to sound better.

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