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.
You are fortunate to have found the cause!

I had a similar problem with Belles 150A Reference amps. Transformer hum that came and went at different times of the day and on different days. Drove me nuts.

There was no pattern. I tried everything including turning off every AC circuit in the house except the for stereo system. I also unplugged every other non-stereo device on the circuit. Still, the hum came and went. It's likely that there was some device somewhere in the house that was causing the transformer hum, but I never discovered what it was, and eventually I sold the Belles amps and replaced them with Pass Labs amps.

Silent as a graveyard are the Pass Labs amps.
Compelling and informative story Hoff48. Sorry audio is a source of friction in your marriage, bummer. I'm getting ready to plug in my first amp in years today or tomorrow. Sure hope it's great and now I'm thinking I hope it's

You might try installing a small isolation transformer on the line side of the low voltage lighting power supply.

The isolation transformer should block the DC offset voltage from passing from the secondary winding to the primary winding, of the isolation transformer. Thus keeping the DC offset voltage off the mains.

If you try the fix post back your results.
Loved the story, it's a lesson to be learned.
Thank you...
Thanks very much for the post. I'm chasing a hum problem myself and your experience will help.
Glad for the happy ending. I can well imagine your relief at discovering the cause!
Foster_9, Let us know about the amplifier. I always had an amp but recently went integrated and not looking back. The upper end integrated amps work just as good.
I wish I had problems that were resolved as beautifully as that. Unfortunately, mine require medication...(for the head)
Great troubleshooting on what is a real b**ch of a problem;
any chance you saw anything with the meter/measurements when those lights were on and then off;I was just wondering if the meter showed a different reading.
Hey all,
Thanks for all your replies. One thing I want to pass on is that the problem with DC induced hum is no indication of the quality of the toroidal transformer. Since I have isolated the problem, the Belles mono-blocks are dead quiet and my old trusty 150A Reference is very quiet, albeit I can hear a slight hum when I put my ear about 6 inches away. Since I don't listen to music with my ear on my amp, it's not a problem.

This is from the Bryston website:
"DC Offset
All power transformers work with AC. DC (direct current) cannot make the flux changes required to sustain a transferable magnetic field. Under normal conditions DC does not exist on power lines. However, because of nasty things like cheap lighting dimmer switches, sometimes a small DC component can occur on an AC line. Because toroidal cores are so efficient, they sustain magnetic flux easily. DC offset on an AC line will eventually cause the toroidal core to go into saturation by continually driving the core into one direction. This sounds bad both literally and figuratively. But, it’s rare and there are easy solutions."

Another thing I read is that the larger the transformer (the higher the VA rating), the more susceptible to DC induced hum. I guess that's why my mono-blocks were so much louder than the 150A Ref, and probably why an old low wattage Adcom I have laying around isn't affected as much. Hey, I've plugged just about everything I have to check out this problem.

Foster 9,
After 29 years, you have to have something to argue about. I guess if it weren't for my audio idiosyncricies, I'd be the perfect husband. (LOL)

I'm looking into an isolation transformer for that circuit. Something I can put safely in the attic. One thing I have been researching are the magnetic transformers for 12/24V vs. the cheaper electronic transformer. The kitchen lights I was having problems with have the cheap electronic transformer built into in them.

Since the lights are on a different circuit, there would no difference in the AC voltage readings. Even if I were to take AC V readings of that light circuit, I seriously doubt there would be much change off or on, since those lights have such a minimal amperage draw. Measuring AC and DC at the same time is beyond the abilities of my tester. I think you would need a pretty sophisticated piece of equipment to be able to measure DC V leakage into an AC circuit.
Heff48, DC offset caused by light dimmers, ceiling fan speed controllers, etc. is a well known cause of transformer hum. Its one of the first things you look for as a possible cause when you have such hum problems. Some amp manufacturers like Bryston and Classé include DC blocking circuitry in their amps.

I also have some sporadic hum, and have partially solved it with cheaters (on Conrad Johnson preamp) and keeping noisey lights off. As a builder, perhaps you can advise me.

I live in a modern condo but are stuck with 15 amp outlets. I am going to try to install a 20 amp home run this summer (at great cost unfortunately) for my audio gear. Is there any kind of isolation transformer I could install to keep stray DC and other bad stuff out of my new circuit?

Prevention is better than cure. Avoid using electrical devices that are sources of DC offset in the first place. However, you have no control over what your neighbor on the same power company transformer does, and their use of such devices could affect you. In such a case, the PS Audio Humbuster or Channel Islands Audio XDC-2 filter may be a solution.

I have a similar humm problem from an unknown source. I measured the DC voltage at 29mv DC on the 220vAC/50hz line, but 59mv DC after the toroidal step-down amp. Is this enough DC voltage to create a hum in my amp/speakers?
Is this enough DC voltage to create a hum in my amp/speakers?
04-12-10: Danhirth


If you are hearing a hum through the speakers, jmho, the hum is not caused by DC offset voltage on the ac mains. The hum is more than likely a ground loop hum. The hum could also be caused by a bad electrolytic capacitor in the power supply of the amp or preamp.

You need to isolate the problem piece of equipment that is causing the hum. Start from the amp and add equipment upstream one at a time until the hum is heard.
If the hum is heard through the speakers with only the amp, no ics connected, then the problem is the amp. Could be a dried out electrolytic cap in the power supply
Danhirth, I'm curious, what setup did you use to obtain your DC offset readings ? IMO 59mV is kinda low to cause hum problems.
I just read this post, AFTER I went through exactly the same process and discovering the exact same cause. I cannot for the life of me imagine how these lights a) make the power dirty, and b) how that dirty power makies the transformer in my Emotiva XPA-5 emit what sounds like a 60 Hz. mechanical hum. Firckin' amazing.

Now- I wonder if there's a way to filter so the lights won't interfere...

Contact Agon member Gbart he may be able to help you with a custom made DC Blocker.

Dirty power is only going to get worse due to the elimination of the incandescent light bulb. CFL and LED lightning puts all kinds of nasty things back out on the mains. Bad harmonics as well as DC offset.
I wish this was the answer to my similar issue. Maybe I need to flip their breakers too!
Hoff - your post was both entertaining and illuminating! I, too, have that mysterious, transient hum -- and now I know what to look for. Thanks!

I have found the source of my hum issues. The remote ceiling fan speed/light dimmer switches.

I would like to add this post by Wayner, a member of Audio Circle. This was apparently the cause of the OPs hum/buzz problem.

Re: Hum whenever preamp and amp put in system...Help!
« Reply #25 on: 16 Jul 2014, 02:45 pm »

"I have an FM tuner, that when turned on, will make my systems hum, because the transformer in the FM tuner is poorly designed/poorly shielded. I too, was fighting a hum issue, thinking it was caused by dirty AC or poor interconnects or something else related to a ground loop.

The answer was distance. Moving the FM tuner away from the amp and preamp solved the problem. The tuner's transformer was creating a larger then normal magnetic field, and that field penetrated my preamp and induced hum. This may be the cause of the hum for you. Perhaps your amplifier's transformer is affecting your preamp, or your preamp's transformer is affecting your power amp.

My suggestion is to separate the 2 much farther then you have them now, even if that means using a 2 meter (or longer) interconnect to accomplish this. I have a semi-high degree of confidence that your hum is caused by EMF."
End of quote.

OPs later response,

Re: Hum whenever preamp and amp put in system...Help!
« Reply #26 on: 16 Jul 2014, 09:38 pm »Wayner,

"You are my hero! After several years of this quagmire, it has been solved. Moving my amp as far away from the preamp, and as far away from the outlets and electric panel has solved the issue! This has escaped 3 electricians, countless audiophiles, and tens of hours with me getting increasingly frustrated. THANK YOU!!!!!!!!!!!!!!!!! When surveying the landscape I realize that there is essentially an equilateral triangle with the 2 base corners representing my 2 electrical panels in the home and the apex represents a confluence of 3 separate circuits converging at that point. Whether or not moving the components out of that is the reason for solution, I'm thoroughly relieved. Thank you all for your help! Special thanks to Wayner of course but again also for Dave E, Tom S, Frank V A, Roger M, and everyone else that I've not had the pleasure of meeting in person or via telephone conversations. I'm indebted to you all."
Hum Busters used to be relatively common gadgets. They block DC, which is the usual cause of buzzing transformers. I have a Channel Island XDC2, which totally solves this problem:

Unfortunately, its been discontinued. I'd encourage anyone with this problem to locate this device or a comparable one.
Dedicated lines for your audio equipment would eliminate line noise from lights and other appliances.

Another annoying source of hum is from cable T.V. outlets used with home theater equipment (RFI). I have been able to eliminate RFI from my TV cable by switching to Verizon FIOSS. The signal is carried by light waves. Evidently, RFI is not an issue with FIOSS. The slight, ever present hum that I had been trying to kill for years, but could only suppress, completely disappeared when I switched to FIOSS. My system is dead quiet now. Music and movie sound emerge from a silent black background.

I think using dedicated lines and eliminating standard TV cable is more effective in eliminating line noise and RFI than filters and conditioners.
If the source of the DC offset is outside of your home, as it often is, there will be no benefit from using dedicated lines or any other change made within your home. A DC filter (not a conditioner) is the only solution in that case.
Psag, I agree. But, in my system, DC offset was eliminated when I installed dedicated lines and RFI was eliminated when I installed Verizon FIOSS. Apparently, my outside lines must be fairly clean. Installing dedicated lines and FIOSS resulted in a significant improvement in my system performance.

Good Post...I would add that the LED light fixtures that are sold today are extremely noisy on line. I have to ask the wife to shut her office light down for interference on am stations, and sometimes on my amp when playing. I think finding amp hum/buzz can be very frustrating, but remember this, if you build an amp, those wire talk to each other, and it is something even experienced builders have to deal with it, so BE PATIENT!! 

Johnne in Seattle 

Loved your descriptive story. Gremlins...laf
One other tweak worked for me with little gremlins try iFi Audio
AC iPurifier.