Amplifier Hum - A Problem Solved


I thought I would post this hoping it may be helpful to someone troubleshooting amp hum or buzz.

I have a Belles 150A Reference amp. It always had a little hum from the transformer, but nothing real loud or what I thought was out of the ordinary. Funny thing is, the hum was always loudest either early in the morning or late at night; times when you would think the grid was cleanest. The hum was never sent to the speakers.
I do have three dedicated 30A circuits. It didn't seem to matter which circuit
or which combination of equipment was plugged in where; still that same hum and most noticeable at those certain times of the day.

About a week ago, I received my new pair of Belles MB-200 mono-blocks. I cleaned up my rack, created some room, and by 8 o'clock had those babies plugged in. No hum. Glorious. By 10 o'clock the hum was back and 5 times louder than my other amp. WTF! I started plugging into different circuits, tried unplugging everything but the amps. HUM. The hum on the mono-blocks was much louder than on the 150A-they have a much larger transformer. It was getting late, so I turned them off and went to bed. Early the next morning, I turned them on - no hum. Later that morning I called my electrician, explained my problem, and asked him if there was any way that noise could get into those circuits or if the grid could cause that much noise. He didn't think so. (We live in a rural area with no industry and always had, what I thought, was pretty clean power.) He said voltage fluctuations can cause torroidal transformers to hum and asked me to take some readings on my voltage at different times of the day. I have 123V morning, noon and night.

I talked to a number of other people about the problem. Most said power conditioning. I knew I had "dirty power", but I just had a feeling that this problem was not going to be solved by a power conditioner.

This past Friday night I had the house to myself, so I was going to do an extended session. About 7 o'clock I turned everything on and by 8 I was cranking. My system had never sounded so good. Of course, there was no hum. I listened to well past midninght. I was excited. The gremlins had left. Or so I thought.

I awoke Saturday morning and immediately went to my listening room. No hum.
About 9 o'clock that night, I sat down to see if I could get a repeat performance of the night before. No such luck. The Gremlins were back and they had brought friends. The hum was louder then ever. I could hear the "swarm of bees" from 15' away. My wife was on the computer, so I had to do my searches on my iPod touch. I went to the Audigon forums and did a search for "transformer hum" and "amplifier hum". I learned a few things. Torroidal transformer hum can be caused by voltage fluctuations (which I do not have) and stray DC current. Stray DC can pollute the whole panel; it is not circuit specific. So I wandered around the house thinking DC current? DC current? I don't think I have any DC fixtures or appliances in my house.
I walked into the kitchen and there it was staring me in the face-the soft illumination of under-cabinet lights. I rushed to the switch, turned them off and ran back to my listening room. NO HUM. I ran back to the kitchen, switched the lights back on, ran to my room. HUM. Laughing hysterically, I ran repeatedly between the rooms switching the under-cabinet lights on and off (Hum, No Hum). Then I thought, why is the hum so much louder tonight than other nights? So I stared at those damn lights until I noticed that they were dimmer than usual. I checked the individual switches on each light and sure enough my wife had switched them all to low (must have been that evening). So now I'm running back and forth switching those lights on-high, switching on-low, switching off (Hum, Loud Hum, No Hum).

So there you have it. Something as innocuous as under-cabinet lights can cause real problems. The light is so subtle and pleasant that I just overlooked them as the "Cause". My wife always turns them on later in the evening, and since I'm an early riser I'll turn them on early in the morning, hence my amp hum at those times of the day I mentioned above. The lights in question are the 12V xenon under-cabinet lights with an electronic transformer. I am a custom home builder and I have unwittingly polluted a lot electrical panels. These types of lights are extremely popular. We audiophools are few and far between, therefore I have not had any feedback from clients with similar problems.

I want to do some really neat 12V or 24V lighting in my listening room, but now I am having second thoughts. I have read that the more expensive magnetic transformers do not have the problem of stray DC current "back charging" the whole panel. Any thoughts on this?

My audio habits have been a source of friction between my wife of 29 years and me. Now I have to add one more to the list: "Honey, I'm going to listen to music for a little while. Could you leave the kitchen lights off?"

I hope this helps someone down the line.
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@steakster 

I should have been more thorough with my explanation. I have more info below. 

There could be several possibilities for the hum.


1) The sub amp itself might need to be grounded to an external ground. This could be especially true if the sub amp is plugged into a different circuit than the rest of the rig. This would cause a ground differential.
The humming is occurring even when I don’t have any components hooked up to the sub and I unplug every socket in the house. I’m renting so I don’t have the luxury of removing the dimmer switches the owner installed. I cracked open the sub yesterday and the hum definitely is coming from the power transformer.


2) If you’re running RCA’s from the preamp to the sub amp, humming might occur if another component patched into the preamp is not grounded correctly.
The humming bleeds from the power transformer to the sub cone as soon as I plug in an rca with nothing attached at the other end. 

I just remembered that that I made some grounding check rca tools when I made my bottle head crack. I should plug that into the sub. 

When end I plug the rca into my pre amp the humming just stays the same (quiet hum in the transformer loud hum in the speaker cone). 


3) If the preamp is plugged into a power strip or conditioner that has other components plugged in, then humming might occur if one of those components is not grounded correctly.
I’ve tried every combination of plugging and unplugging my gear. It was working fine at my old house before I moved into the new house one month ago. I had the problem as soon as I plugged it into the new house. 


4) Bad capacitors in the sub amp.
 
I checked all the caps when I opened the sub. They were all fine. I have pictures if you would like to see. 


5) Loose tie-down bolt clamping down the transformer in the sub amp. 
It’s a class AB amp Rel T7. The power transformer isn’t toroidal, I want to call it a bell transformer for some reason. 

I will check the tie downs. If it’s not that I was hoping I could find something fir under $200 to fix the hum. 

To test #2 & #3, just unplug one component at a time. If the problem is a poorly grounded component, the hum should go away. 
To test #5, unplug the sub amp and take off the cover. Tighten any nuts that hold the transformer down. This primarily for older amps.
Thank you for all the advice. I’m going to check the transformer tie downs. 
@fsonicsmith 

That chord is 3x more expensive than my Rel T7. Any fix that costs more than $200 will probably result in me buying a Class D sub. My rel T7 is class AB and all my other amps and my wife’s amp are Class D. They all work fine in the new house (built in 1950) my old house was built in 2010 and had no problems with my class D amps or my class AB rel T7. I’ve thought about buying an online UPS or a power isolation transformer, but at those prices I might as well sink $700 into a Sealed class D sub and put my sub on the bench for 11 months until my lease ends and give my wife the Class D sub for her system when we move again. 

I also Have a bottlehead crack tube amp. When I used this at my old house it had no issues but the transformer would hum when I brought it into work. I should plug this in at my house and see if it has a hum. 

Thank you for the suggestion on the chord though. 

Hi , I have a new monolith 7x and I experience a ver loud hum from her when my dishwasher is running or my wife is blow drying her hair when either of these things are not happening it is extremely quiet, please how do I fix this