Toroidal transformer hum

[adauphin]

I trust this is in the correct forum, I'm looking to hopefully find the root cause of my power supply hum in my amp. I don't want to call out any vendors to avoid any negative feedback as I don't believe it's the amps fault.  I have been hearing a very faint hum in my toroidal transformer that's well damped. There's a chance I have some DC in my line, but not sure how to verify.

The amp is on a 20A circuit by itself, however it does this in other outlets. The amp is wired from the main panel, however if I shut the 50A feed to my sub panel, the noise stops, even if I turn it back on again. The amp will also hum in a varying pattern if I turn on my Plasma TV which sits about 12" above the amp....the amp is not connected to the sub panel.

The neutral and ground are separate in the sub panel. If I shut off every breaker in the sub panel, the noise remains, only if I kill the feed will it stop. The transformer will either have a very, very faint hum, or it will be slightly loud, when it's loud I can stop it by turning off the breaker for the sub panel,

Also, when the hum is quieter, if I kill all the breakers in the.. main panel except for the 20A circuit for the amp, the hum is still audible.

Sometimes there is no noise, so just trying what could cause the transformer to hum.

Outlets are good, new wire, power cords are good, what's really odd is when the plasma is on, it hums in waves, comes and goes.\
All thoughts and crititicism appreciated.

[thermistor-guy]

If you are getting acoustic emissions from the transformer, then something is moving (to vibrate the air).

The transformer's windings might be moving. Or, the whole transformer might be rocking slightly on its mounting.

[Armadillo]

Just make sure your outlet is dedicated. Don't plug laptop, computers etc... onto same outlet branch circuit.
Hope you know what I mean.

[oldway]

Check if there is no open circuit diode in the bridge rectifier of your amp.

[james_s]

Baffles me how having the breaker to the sub panel on would have any effect. Especially with all of the breakers in that sub panel off, there's a small amount of capacitive loading from the wire run but that shouldn't be enough to affect anything.

IMHO if noise and minor fluctuations on the AC line have any effect on the operation of an amplifier then there is something wrong with the amplifier.

[adauphin]

A little more info on the home wiring.

The sub panel feeds the laundry room, along with a few other areas. The laundry room has 4 florescent light fixtures, maybe the ballast is bad or wired backwards. The sub also feeds the microwave and the refrigerator.

I have two ceiling fans upstairs, not on the sub panel, that have remote control dimmer switches...so the control box is in the canopy and is always wired hot...no physical switch to shut off.

I also have a CL dimmer in the dining room, as well as two slide-dimmer control switches for table lamps.

I have what appears to be an old doorbell transformer in my attic, however it's not being used albeit still wired.

I came home tonight and the amp is quiet, last night I came home and it was a very low hum but went away when I cycled my sub panel breaker.

My daughter was later watching the TV, and I could only barely hear a hum....had to press my ear against the case and even then you had to know what you were listening for.

Sometimes the amp is dead silent, wouldn't that point to something outside the amp?

[Armadillo]

Your circuit branch to looped socket outlets which is termed for portable equipment won't be mixing with fixed equipment or lightings, I suppose.
Don't say you wired your home wirings yourself. ?? 

[vealmike]

A little more info on the home wiring.

The sub panel feeds the laundry room, along with a few other areas. The laundry room has 4 florescent light fixtures, maybe the ballast is bad or wired backwards. The sub also feeds the microwave and the refrigerator.

I have two ceiling fans upstairs, not on the sub panel, that have remote control dimmer switches...so the control box is in the canopy and is always wired hot...no physical switch to shut off.

I also have a CL dimmer in the dining room, as well as two slide-dimmer control switches for table lamps.

I have what appears to be an old doorbell transformer in my attic, however it's not being used albeit still wired.

I came home tonight and the amp is quiet, last night I came home and it was a very low hum but went away when I cycled my sub panel breaker.

My daughter was later watching the TV, and I could only barely hear a hum....had to press my ear against the case and even then you had to know what you were listening for.

Sometimes the amp is dead silent, wouldn't that point to something outside the amp?

I assume we're talking about mechanical hum from the transformer, not hum on the speaker output. If so, I think you're over thinking things. It is typical for a humming transformer to hum intermittently.

EI transformers are renowned for humming, if the laminations in the core aren't held together tightly they vibrate against each other and you hear a hum. An EI is constructed from punched laminations. Because the EI cores are punched, the edges deform and the laminations aren't flat, making it impossible to pack them together tightly.

Toroid cores for mains are still constructed from layers.  But the toroid core is made from a single strip of material wound in a helix. Because the edges aren't punched, the "laminations" can be more tightly packed resulting in less hum.

This is not the only reason for the hum difference between the two cores.

A toroid can hum if the core isn't tightly packed.

Your options are:
 * Replace the toroid.
 * Provide mechanical isolation.

You may not be hearing the hum from the toroid directly. It is likely that the enclosure is acting as a sounding board. The toroid is exciting the enclosure and the enclosure is moving the air around it to create the hum. Just as you don't hear the coil moving in a speaker, the coil excites the speaker cone and the cone moves the air.

Try putting some rubber mat between the toroid and the enclosure. If the enclosure is your design, can you stiffen the enclosure to stop the surface that the toroid is mounted on from resonating?

[jaycee]

Sometimes the best thing to do is "suck it and see". A DC trap is fairly easily constructed from 2x10,000uF 25V capacitors connected in series, and a 35A diode bridge with the +/- terminals bridged:

[drussell]

That would be truly bizarre if it isn't just some kind of random chance.

Put an oscilloscope on the mains at the amplifier and watch how it changes as you add/remove the other loads in the house.  The amplifier's transformer will obviously make more noise when there are wacky harmonics on the line rather than a pure sinusoidal wave and you'll be able to see what is actually going on when you look at the scope.

You're sure it isn't just random line noise variations?  It really does repeatedly, verifiably stop making noise right when you open the subpanel breaker?  That really would be odd...  I'd really love to see the scope traces on THAT one. 

[Gyro]

Put an oscilloscope on the mains at the amplifier and watch how it changes as you add/remove the other loads in the house.
...
That really would be odd...  I'd really love to see the scope traces on THAT one. 

That would be a VERY STUPID thing to do and VERY IRRESPONSIBLE to encourage someone else to do!  Especially when they might not have the experience to ignore your advice! You risk, at best, destroying their equipment, or worst case, killing them!

NEVER connect a scope directly to the mains, it is NEVER safe to do. There are enough threads on this forum giving that message. 


To the OP:

The only way to even contemplate scoping the mains is to use an apporpriately CAT rated high voltage differential probe! Don't even contemplate floating the scope.


https://youtu.be/xaELqAo4kkQ

[Simon]

That would be truly bizarre if it isn't just some kind of random chance.

Put an oscilloscope on the mains at the amplifier and watch how it changes as you add/remove the other loads in the house.  The amplifier's transformer will obviously make more noise when there are wacky harmonics on the line rather than a pure sinusoidal wave and you'll be able to see what is actually going on when you look at the scope.

You're sure it isn't just random line noise variations?  It really does repeatedly, verifiably stop making noise right when you open the subpanel breaker?  That really would be odd...  I'd really love to see the scope traces on THAT one. 

You don't simply put an oscilloscope across the mains. The ground clip is ground not a floating input and it is connected to the entire chassis.

I only do this if i really have to and I do it with utmost caution. If you do need to scope the mains the safest thing is to use 2 probes and use the math subtraction function to see the mains, this way you are using the two more or less isolated inputs (not chassis connected and with a 1M impedance to ground/chassis) to probe the mains. Ideally remove the ground clips just to make sure no accidental contact happens and ensure that the earth pin of the mains plug IS connected to the chassis and that the electrical supply has a working mains.

I am not a mains expert, I've not killed myself yet but remember that mains kills.

[Kleinstein]

Usually toroid transformers are relatively quiet, compared to conventional transformers. However there is a little residual hum from magnetostriction. With a not so good mechanical construction this may be audible.

DC voltage in the grid could be a possible cause. Already a rather small DC voltage can drive toroid transformers to get close to saturation on one side and this way cause high current spikes that might cause significantly more hum.

For a test one could check the amplifier powered through an isolation transformer, if available. Another possible test would be series resistor or similar: it would reduce DC current (it does not take that much resistance compared to the DC resistance of the transformer) and in addition limit current peaks and would thus reduce the hum by quite a bit. The DC stop with diodes / caps would be another option for a test. If you have a current measuring probe (e.g. current sensing transformer) , one could look at the mains current flow.

Some audio amps with very large filter caps can also cause hum from large current spikes - so it might be the amplifier itself. A broken / lose diode in the rectifier could make the supply draw DC current and thus cause DC in the grid.

[Richard Head]

I suspect the core is running very close to saturation and the magnetising current is spiking at the end of the mains cycle. An easy way to check is to run the amp off a variac and lower the supply voltage to the amp by 5-10%. If impending core saturation is the cause then lowering the mains voltage slightly should cause the hum to dissapear completely.

[schmitt trigger]

I believe Richard's answer is the correct one.
Once that you go over the saturation curve, magnetization current increases very rapidly.

And as he explains, a very simple experiment involving a Variac will tell you if this is the case.

[Armadillo]

I believe Richard's answer is the correct one.
Once that you go over the saturation curve, magnetization current increases very rapidly.

And as he explains, a very simple experiment involving a Variac will tell you if this is the case.

Those hums are apparently audible even when the amplifier is in the idle state example the magnetising current is at the "lowest".
But, the correct term to use is "The walking flux" which clearly depicts the problems more correctly.

Those audiopiles talking about panels and feeders know what they are doing.
 

[Gyro]

Approaching saturation can certainly make them hum. There's a large torroidal in my Instek bench PSU and I suddenly hear it start humming at certain times of the day (usually early afternoon). I suspect it's a combination of probably originally being designed for 60Hz and the high mains voltage level around here.

[EHT]

So on the topic of (mains) transformers physically buzzing, is there a useful way of preventing it (maybe silicone sealant?) or is this futile? I have two devices with mains transformers which buzz annoyingly. This is even under zero-load, and both items are functioning correctly. I'm talking about physical buzzing, not electrical interference.

[cdev]

Are you talking about a conventional, non-toroid power transformer?

There are several things that can be done there, but they are non-trivial fixes, so only worth it when the equipment is valuable.

Most transformers were encapsulated with varnish, which tends to dry out over time. (decades of time)

After a few decades the hum comes back. This varnishing can then be redone using a vacuum to pull it deep into the transformers core, then the varnish needs to be dried out in a oven but not at such a high temperature.

Shops that repair and rewind electric motors can often do it for you.

Also, wedge-like wooden shims are commonly used, they are pushed into the transformer to reduce the buzzing by physically preventing the coils movement.

So on the topic of (mains) transformers physically buzzing, is there a useful way of preventing it (maybe silicone sealant?) or is this futile? I have two devices with mains transformers which buzz annoyingly. This is even under zero-load, and both items are functioning correctly. I'm talking about physical buzzing, not electrical interference.

[thermistor-guy]

Humming can occur due to magnetostriction in the transformer core:

https://en.wikipedia.org/wiki/Magnetostriction
http://www.mgmtransformer.com/faqs/why-do-transformers-hum/

So in principle, a sound-deadening sealant (with appropriate thermal and electrical properties) and a sound-deadening mounting should help.

[Armadillo]

Humming can occur due to magnetostriction in the transformer core:

https://en.wikipedia.org/wiki/Magnetostriction
http://www.mgmtransformer.com/faqs/why-do-transformers-hum/

So in principle, a sound-deadening sealant (with appropriate thermal and electrical properties) and a sound-deadening mounting should help.

That would suggest, tantamount to an excuse, of an inherent material attribute, but nobody is going to buy a audio amplifier or sound system with such inherent hums in it.
If the hum can be cause to appear and made to disappear [inaudible] by not even touching the transformer, then the key is to find and discuss the element that causes these excitation because in reality,

"Everything in the universe vibrates [Albert Einstein]".

Would that be a better universal "Excuse"?
 

[jh15]

Simple,

It doesn't know the words, so it hums.

[adauphin]

Thanks for all the informative replies Sorry I'm not quoting everyone but will try to touch on everything.

Yes, the hum is mechanical, and at most times only audible with your ear close to the case or touching the case. However at times I can hear it from a foot or two away. There is dampening material between the transformer and the case, I guess until now I didn't realize that all transformers did hum, but the sub panel being somehow directly correlated is odd.

I would like to try a DC trap just to see how it may affect the unit.

With regards to scoping the mains....I would like to try that. I do appreciate the concerns with performing said task as I'm not well versed in scope functions and procedures...I surely won't try that unless I know exactly what I'm doing.

I did pick up a Tek 465B a couple years back just to learn on it, that way if it ended up being damaged it wouldn't be a huge loss. Not that I bought it solely to measure mains, but would be cool to see how mine looked on a scope. I would love a good sub $1K scope just not sure where to look but we can address that later. Great video BTW, I did see that guy in a few other videos but haven't seen that one....many thanks.

The ideas of saturation make sense, I would like to see how it responds on 110-115V compared to my 121V.

If the transformer stays with a very, very slight hum, I'm fine with it, but it's the fluctuations that have me concerned, quite a few times being audible from a couple feet.

[adauphin]

I went down to check the status of my lively torodial, as of this morning it's only audible if you press your ear against the case and you know what you're listening for...almost imperceivable. That's how I understand they are supposed to operate but given my case it will probably change throughout the day.

I'm going to try to track down any loose wires in some outlets make sure everything's wired correctly. Something possibly could be coming back on the neutral at times I don't know if or what it could be.

[cdev]

Simply put some damping material in contact with the thing..

Any material that is non-magnetic and non-flammable/melt-able at normal temperatures.
 so that the energy is used up in it.
Also, how the toroid is mounted would make a difference.

[Armadillo]

Simply put some damping material in contact with the thing..

Any material that is non-magnetic and non-flammable/melt-able at normal temperatures.
 so that the energy is used up in it.
Also, how the toroid is mounted would make a difference.

Don't even waste your time listening to it.      

[adauphin]

I wanted to update this issue as I heard the hum again a few times over the last few weeks. Each time I opened the sub panel breaker and the hum stopped.

Tonight it hummed lightly again so I shut down one breaker at a time in the sub panel. Last time I did this I don't remember anything happening, but this time it was different.

When I opened the breaker for the microwave, the humming stopped, was dead silent. The microwave was not in use, only the clock and the two incandescent surface lights.

I closed the breaker again and the humming was still gone.

What can be in the microwave that could possibly allow DC into the mains after a period of time?

Anything else come to mind?

[Richard Head]

Armadillo
Magnetising current is load independant.
But it is very much Volt.Second dependant.

[Armadillo]

Armadillo
Magnetising current is load independant.
But it is very much Volt.Second dependant.

Are you responding to me?
Nevertheless, from what you say, then forget about transformer VA rating.
Just apply enough V for enough S Not in the real world.

Edit: Thanks for pointing out. In my previous response, i should use load current instead of Im.