Make a trasnformer hum 101

So this comes up repeatedly and I wanted to share this with you.

Sometimes perfectly good transformers mechanically hum. Often this is caused by having DC on the line. This is kind of a weird concept so let me take it step by step.

Normally a good AC line has a neutral which is essentially at ground potential, and a "hot" which we say is 120 VAC. That is, it has a sinusoidal waveform which repeats at 60 Hz (for America) or 60 times/second. The magnitude of the complete swing is measured in RMS. It’s an average which allows us to easily determine power dissipation. What this really means is that the voltage "swings" from about +85V to -85V. This is also called Peak to Peak or Vpp. An RMS measuring multi-meter will correctly say 120VAC, but if you look on an oscilloscope you will actually see the very top and very bottom of the waveform at around +- 85 V.

It is possible to have DC, AC or both on a conductor. In an incoming power line however it is NOT desirable to have anything but AC. What’s odd is that stupid little power supplies and dimmer switches can actually take an otherwise fine power line and distort it by adding DC to it.

This happens for a couple of reasons

1. Some amount of impedance/resistance on the wire to the transformer, and through the transformers.
   
2. A power supply that ONLY takes from one or the other voltage swing.

Consider a dimmer or laptop power supply that only takes current during the positive cycle. With a little resistance on the line, this pulls down the maximum positive voltage. Say from +85Vpk to +83Vpk, but the negative swing is still -85Vpk. Congrats, you’ve just added -2Vdc to your waveform. To make matters worse, these supplies may take current only for very short periods of time, causing the waveform to distort far from sinusoidal, and adding harmonics, or high frequency noise, to the incoming power where none existed before.

It is this DC voltage that will make otherwise fine functioning transformers develop a mechanical hum in the laminations.

The point I wanted to make is that these power supplies don’t really inject noise, so much as slurp power unevenly, causing a host of possible audible or visible effects.

Everything I’ve written here is scientifically verifiable with models, measurements and test gear, however it does not automatically justify spending $50k on power conditioners. How much of a problem you have and how much it’s worth to solve is entirely up to the music/movie lover.

I hope this is helpful,


E