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
erik_squires
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.

That’s not correct. Voltage will swing +-170V. Peak to peak voltage will be 340V. Any meter will say 120V since non-RMS meters are scaled to show RMS voltage (for the sinewave) in spite of measuring average voltage.

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.

Dimmer or laptop won’t do it - you need much stronger load for that. In addition dimmer or laptop supply will draw identical current from positive and negative sides.

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.

Most of linear power supplies draw current in narrow spikes of very high amplitude producing a lot of high frequency noise. Computer power supplies often use primitive SMPS that produces a lot of noise while dimmers chop sinewave also producing a lot of noise. Some of them have decent noise filter but many don’t. In addition they "slurp power" pretty much evenly.

Whoops, my math was wrong! :)

However, the problem is related to the line impedance. If it were perfect, then an uneven draw wouldn't affect anything.

I thought most SMPS were positive only draws? I must be confused then.

Best,

E
erik_squires +1
This brings interesting question - what causes DC on the AC line.  Is it uneven phase loading, VFD drives or anything else?

So a dedicated line from the panel will never cause hum? Can another line sharing the panel bleed noise into the dedicated line? How can we get a perfectly hum free power line? Can an aging grounding bar cause noise? Inquiring minds want to know... 
One issue then would be - what to do with SMPS, wallwarts or otherwise, that are now prevalent as part of an audio setup.
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I try to keep my wall warts outside of my power conditioner.

For critical things like my DAC I purchased outboard linear regulated supply built like a tank. :)

That's as fussy as I'm going to be.

Best,

E
^^ How about a power conditioner for SMPS and a separate power conditioner for everything else?  :)
Would bolting Sorbothane under the transformer of one of those Carver "Magnetic Field" amps stop the bad hum that comes from them sometimes? (I'm guessing No, because the vibration is internal to the transformer.)
@joeylawn36111

Well, kind of sort of. ;) The transformer will still vibrate, but the panels vibrating along with it definitely make it louder. :)

You can experiment with your hands. If you put your hand on a panel to dampen it, you'll hear it. Then that's definitely something to pursue.

Best,

E
Thanks, eric. Any idea how heat-resistant sorbothane is? 
There are also some cases when the transformer is actually silent, but the panels vibrate! Not sure I've come across anything like that in decades though. :)
No Idea. You may want to check out some of the panels they use for damping PC's.

Best,

E
SMPS have bad rap from crude computer applications.  Jeff Rowland uses extremely quiet SMPS (cleaner than linear power supplies).  Advantage of SMPS is that it can operate from AC and DC (tolerates presence of DC) while many operates from wide voltage range. In addition they are line and load regulated, while linear supplies in power amps are not.
Here is a good site for cabinet cooling, damping materials, etc.

http://www.frozencpu.com/

Best,

E



dweller 1,377 posts                                                           02-27-2017 6:46pm

So a dedicated line from the panel will never cause hum? Can another line sharing the panel bleed noise into the dedicated line? How can we get a perfectly hum free power line? Can an aging grounding bar cause noise? Inquiring minds want to know...
So a dedicated line from the panel will never cause hum?
Hum? As in a 60Hz ground loop hum heard from the speakers? Even a dedicated branch circuit if improperly installed or using the wrong type of wiring geometry for the branch circuit wiring can cause a ground loop hum. Especially if more than one dedicated branch circuit is used and the branch circuit wiring is long in length.

As for a mechanical vibration, hum, heard from a power transformer I doubt a dedicated branch circuit would directly be responsible for the hum. I can’t see how it could stop it, or prevent it.

All transformers hum to some degree. Some more than others. A few things that make them vibrate, hum, louder than normal are, too high of a mains voltage, being overloaded beyond their manufacture’s rating, in the case of an EI transformer loose laminations, or DC offset on the mains. I have heard lighting power transformers in an older office building hum loudly, more than normal, running hotter than they should, due to a heavy concentrated load of personal computers, that are fed from the power transformer. The power transformer may be only running at 80% of its full load KVA rating. So what is the cause? Harmonics. All them damn switch mode power supplies found in the PC computer found at every desk.

Toroidal transformers of a decent size, as found in power amps for example, are susceptible to mechanical hum caused by DC on the mains. Some manufactures that use a toroidal power transformer in their power amps may employ a DC blocker circuit inside the equipment on the AC lines before the power transformer.

Quote from Nelson Pass:
If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer.

The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise.

You can put a pair of back-to-back electrolytics in series
with the AC power line to block this, and it works fine.
Makes sure the current rating of the electrolytics is
high enough, and the they are joined at a like polarity,
such as + to +.
http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

Most common things found in the home that can cause DC on the mains.

Hair dryers. Especially the high wattage hair dryers that are set to the medium heat setting.

Electric space heaters that have a reduced wattage setting example, 1500/900 watt that are set on the lower wattage setting.

Cheap lighting dimmers. Here I think it depends on the connected load to the dimmer.

//

Can an aging grounding bar cause noise?
As in equipment ground or as in main system, "Grounding Electrode System" ground of the electrical service?

As for the safety equipment ground a loose or corroded connection can add a series resistance in the equipment ground. The resistance can cause a difference of potential to exist from one equipment ground to another. A good example is a difference of potential from a CATV provider's coax cable shield to the safety equipment ground at the mains wall receptacle. If the CATV is connected in any way to an audio system using the wall receptacle safety equipment ground there is a very good chance of a ground loop hum problem.

In most cases the problem is an improperly grounded CATV cable provider’s Coax cable shield. Or it could be a loose or corroded connection at the CATV grounding block. But it can also possibly be a loose or corroded branch circuit wiring safety equipment ground connection.
.