I've had a 1987 vintage NAD 7600 Monitor Series receiver that I've loved for 30+ years. It finally began to show various signs of age and, at just that moment, another that had been stored for most of its life, in the original packaging no less, came available on eBay. It wasn't cheap, but it seemed to be in "like new" condition, so I bought it.
Well, it's pretty all right, "like new" in appearance, all the circuits work as they should, even the pots are quiet--but the transformer makes a 60 Hz mechanical hum. As I understand it, transformers contain plates that are stabilized with glue which becomes brittle over time and can crack or loosen. In any case, my old, semi-functional but otherwise identical unit, didn't hum. So, after consulting various knowledgable friends, I contacted an authorized NAD service dealer about 200 miles away, took both units to him, and he swapped out the "old" transformer for the "new" one.
Maybe it was the 400 mile round trip in the car that caused some kind of break in the old, brittle glue bonds of the previously silent transformer, but installed in the "new" 7600, the unit still hums; not as loud as before, but we audiophiles are perfectionists, and it bugs me. It is, fortunately, a mechanical hum, as I mentioned, so I don't hear it in the speakers--although it does get picked up by the tuner.
Is there anything I can do to solve this problem? The transformer is securely mounted to the chassis, and I've tried putting additional damping materials under the feet of the unit. No improvement. I've tried different wall plugs, both with and without a power conditioner. No improvement.
Of course, it seems likely that the only solution is to replace the transformer with one that doesn't hum. But where would I be able to find that? And is it even worth doing, given what I was told about the time-degraded glue that keeps new transformers quiet?
The repair guy told me (and showed me) higher-cost transformers that are entirely encased in metal; those will never develop a hum, he says. Is it possible to encase one of these original transformers in some way in order to silence it? If so, who can do that?
Sorry for this probably futile plea. Obviously, one solution is to eat the $1,000 I've already spent on the "new" unit and just buy something else that's really new. But I don't want more than two-channel stereo, I don't stream, there are various smart features of the NAD that I would not want to sacrifice--and, best of all, it sounds great in every way except for this damn hum! I've had the option to replace it with a couple of different, and presumably better, devices, most recently a Primare A30.1 integrated amp, which a friend owns and is willing to sell. I had the Primare in my listening room for a month, and liked the sound of the NAD better. Plus, of course, it has a great phono circuit, a tuner, very useful semi-parametric tone controls, a powered balance control, and several other wonderful features.
The sort of mechanical vibration hum you have is caused by DC offset. Our homes are run off 240V but instead of running everything at 240V like they do in a lot of countries someone here decided that was way too dangerous for us fraidy cat Americans and so all our electrical panels are split right down the middle with only 120V going to each circuit. Except for water heaters, ovens, stuff like that, they still use 240V which explains why those breakers are twice as wide, they get the full 240V by connecting across both bus bars.
All the other stuff however - like your stereo - comes off just one of these legs. Now if everything is nice and even then this 120V goes perfectly evenly to both legs and there is no offset. But it is real easy to have something plugged into one leg that causes an offset and that is your DC offset.
Probably made all sorts of technical faux pas there but so what, this is a perfectly clear understanding more than adequate to our needs. You just need to know what it is, in order to figure out what to do about it.
First you can try going around flipping various breakers until you hopefully get lucky and find the offending circuit. Then if you do, try and figure out what is plugged in that is causing the problem. Unfortunately this is like tracking down other noise sources, you just have to try one after another until you get lucky or run out of things to try, or run out of patience. I usually run out of patience. A little noise doesn't bother me as long as the sound quality is high. Mine is sky high. (So is my noise level, I am sure someone will say you can barely hear the music over it.) YMMV.
Sometimes this works and there are stories of guys who find the culprit and live happily ever after. Lots more never do. Then you go to Plan B, which if your patience is like mine may in fact be Plan A. Either way: Hum Buster! This is an inexpensive power strip designed to eliminate DC offset hum.
It is a shame you’re only finding this out now. Because if I had seen it earlier I would have told you to not waste your time and money shipping. The DC offset was there the whole time, you were just lucky in having a tight mechanically solid component. Which after being bounced around on a truck a few days straight is one of the first things to go. Oh well. Hum Buster!
Thanks for this tip; I was hoping for something like this. Of course, I'm puzzled that there was no hum before, when the "old" transformer was installed in the original unit. Nor is there any hum in any other transformer-driven device in my home. But hey--I'll give it a try.
"It" being the "hum buster." What is that? I just searched Amazon and eBay, and I only found a few ground loop hum eliminators. This is NOT a ground hum, of course.
Can you name a brand for this device? A source for it? Cost is almost irrelevant; I'm willing to try anything within reason.
You had no hum before simply for the same reason most don't notice, a combination of low level and solid construction. Lots of times there's background noise that can be there sometimes for a very long time, until something happens you notice and then OMG that noise is killing me! One person watching a movie is so engrossed they don't even notice the popcorn muncher beside them, while another can think of nothing else. Same noise. Completely different response.
The amp is the same way, you should know better than anyone, you had one that made no noise, then another same one that did, then the first one comes back makes noise but in between the other two. Perfect example.
Hum Buster is the name PSAudio sold theirs under. I forget the exact circuit, it ain't much. If they don't make it any more someone does. Just one of those names like Thermos people use even though a lot of insulated containers aren't actually Thermos name brand.
I doubt that what you are referring to is related to a ground or AC line problem.
When transformers get old the shellack or similar material they use to "pot" the coil windings get brittle and breakdown. This could also be from hot/cold thermal cycling and or moving the equipment around. I have found toroid transformers are much less prone to this problem. I have swapped out a couple old style "standard" transformers with toroids, but that's not always an option due to rail voltage differences. I have used "white glue" or epoxy to repot the transformers that I could not replace with toroids. It's a pain, but can be done if you're patient and have time. My favorite amps have become MUCH quieter after potting. If the transformers are encased in metal, you will need to drill one or two holes (VERY CARFULLY) to fill with glue completely. After filling with a suitable potting material, I have always let the transformers "cure" at room temperature for weeks to months. I would not try this unless you have some electronics and mechanical experience. Good luck!
The other thing I should have mentioned first, is to check the power supply caps assuming they haven't been replaced. If they are dry or leaking, that could put additional load on the transformer.
I’ve had a hum in my subwoofer amp location for a while. It would come intermittently and for no obvious reason. I finally concluded that I was a good candidate for the DC offset issue that was mentioned above. I purchased the Emotiva cmx-2 and the hum is gone. I would rather not use it like a bandaid because I’m using a premium chain and this unit may not be as premium as the rest of it, but the hum seems to be gone.
I came here to update my thread regarding subwoofer hum and that it was fixed. So this fits here too. It retails for 140 US. Good luck.
Are you sure you didn't inadvertently move them to another circuit, or start using another bit of electronics?
Certain digital power supplies, especially in lights, can introduce DC. Before judging it to be the transformer's fault, try turning off everything else in the house. Also, move it to another circuit even after that.
If the hum goes away, it was never the transformer. :) You need a DC blocker, or transformer.
Big difference between solving mechanical hum in a transformer and solving 60/120Hz hum coming from your speakers.
The cable ground loop isolator mentioned above can break a ground loop that is causing hum in your speakers but will do nothing to solve mechanical hum due to DC in your AC. Using a DC blocker will not solve hum coming from your speakers but may reduce or eliminate mechanical hum coming from a power transformer.
Often, various legitimate solutions are ineffective because of user error.
So many helpful suggestions here! Thank you all. This is Audiogon at its very best.
A few comments:
millercarbon: thanks yet again for the links to specific products. Your audio knowledge is vast, and I'm grateful for it. (BTW, although I do not share your enthusiasm for Schumann resonance generators, I nevertheless have one hooked up to my system.)
fiesta75: Sadly (for me), I suspect your advice is likely the most accurate regarding this problem. About the power supply caps, however: when the NAD service tech swapped out the transformers, he checked the caps, and they were OK (not leaking, at any rate; I'm not sure how thorough a test he made of them). In any case, no, they have not been replaced. I know they probably should be.
Erik: I am sure I have it plugged into the same circuit (via a power conditioner, btw). In any case, I've tried it in several different plugs, and I've removed it entirely from the system, plugged it in to a remote outlet via a good extension cord, opened it up, and stuck my ear up close; this is how I know the sound is coming from the transformer. In any case, before I made that long drive with both units, one hummed and the other didn't when plugged into the very same outlet.
tvad: thanks for the PSAudio troubleshooting guide link. I'll read it this afternoon (I've got a busy day until then).
Finally, vinylvalet: Yes, this is a mechanical hum, NOT a ground loop hum. There is no sound coming out of the speakers, and, as I just explained, my ear has located the source of the mechanical hum with good confidence as coming from the transformer. Your further comments tend to confirm millercarbon's advice, which is very encouraging.
Just an FYI - If you service an amp with any issue, of that age, it is ALWAYS good to replace the electrolytic capacitors. This is a normal procedure for any good tech. I would take it back and have them do it correctly. Maybe you should get your "old" transformer back and re-pot it. After all it's yours, you bought a new one? Please also confirm the hum/buzz is not coming from your speakers. Good luck!
There is an IFI power conditioner that plugs in the wall (about 100 dollars) that is designed to eliminate your very condition. Check it on Amazon.com....if it doesn’t work, ship it back free.
I just looked at the inside of the NAD 7600 in US Audio Mart. It looks like the power transformer has strange metal strap/bracket of the top holding the transformer. Maybe when the tech replaced it, he didn't fasten it back down correctly. Still I would take it back because the problem you brought it in for was not corrected. But it could be a relatively easy fix if you want to pull the cover off and take a look.
the electrical panel might require some tightening of all the connections, including the ground, and then some snapping of the breakers and resetting of said breakers. Just some additional aspects that should be considered.
I don’t think your problem is DC offset on the mains. The power transformer in the NAD 7600 looks like an EI, not a Toroidal. As a rule an EI is not affected by DC offset on the ac Mains.
I would carry the receiver to the kitchen. Place it on the counter and plug it *directly* into an outlet above the countertop and check for mechanical hum/buzzing of the transformer. Kitchen outlets (above countertops) are on 20 amp circuits, #12awg copper wire minimum. You don’t need the speakers hooked up to the receiver for the test. If you are worried just turn the volume control all the way down.
You never said if the receiver’s power transformer made the humming/buzzing sound at the repair shop. Did it? How about after the tech put the other transformer in, was it quiet?
All EI transformers hum/buzz somewhat. Some just more than others.
FWIW, I have read a few posts in recent years where a high amount of harmonic distortion on the AC mains caused an EI transformer to hum/buzz louder than normal.
I bought the iFi DC Blocker right away and it works for me, different problem.
jea48 made the big point. Was it quiet in the shop after the surgery? If so power it up at another location before rooting around your house or buying unecessary stuff. One thing for sure, your car IS NOT a truck.
I had dedicated 20 amp lines installed 4 wire dual ground , one ground common ,the other ground seperate isolated,insulated ground awg 10 Copper wire even lower resistance ,and use quality Copper goldOutlets .My audio lines are now dead quiet.
Another big thanks to everyone. You guys are great.
So, as to particular suggestions and questions:
fiesta75: Yes, I know the opinion among many is that electrolytic capacitors should be replaced after even 20 years. But then, others have told me (and they're usually the ones who might actually get paid to do the job, so are motivated to recommend it) that if they're not leaking, and if they measure within spec, there's little point in "re-capping." I've been repeatedly assured that SQ will not change, and that, especially if they have not been warmed and cooled much from lots of use (and this unit was stored for most of its life, as far as one can tell and as the seller claimed), electrolytic caps can last 50 years or more. Of course, they're now 34 years old, so it makes sense to follow your advice and have them replaced. I'll do that if I have to take it back to the tech, but I'd rather solve the problem myself without making another 400 mile drive.
As for the "strange metal strap/bracket on the top" of the transformer: wow, thank you for going to so much trouble as to search out photos of the guts of this thing. I have my own photos, as you can imagine, and would be happy to send them to you if you think a virtual visual inspection might result in more advice. Let me say, though, that I have indeed checked to be sure the transformer is securely fixed to the chassis. I was tempted to swap out the two of them myself, but didn't want to screw things up; as I've mentioned, the unit does work in every other respect, and superlatively well.
BTW, I did keep the "old" transformer--which is really the newer transformer, since it was originally mounted in the unit that saw much less use. If it can be "re-potted," I would definitely want to do that. In fact, I hinted at that as a possible solution in my original post, except that I described it ineptly. Do you know anyone who does that kind of thing?
jea48: You're probably right that it's not a toroidal transformer. It's disappointing to learn that millercarbon's original suggestion--a DC offset filter--wouldn't solve the problem if that's the case.
I will try the test you suggest: plugging it in over the range in the kitchen. But I don't understand quite how this would be diagnostically meaningful. Is it that a 20 amp circuit wired with #12awg copper won't have the DC issue? If it doesn't hum in the kitchen, does that mean that a DC blocker would, or wouldn't, help? If it doesn't hum in the kitchen, does that indicate whether or not it's a toroidal transformer?
As for whether or not it hummed in the repair shop (which fuzztone also wonders about): well, I can't say! You should have seen that shop! A beautiful mess in an industrial park, old equipment everywhere (for parts). The repair guy was an old Japanese man on the verge of retirement who has been fixing audio equipment since the seventies, and has been an authorized NAD tech for most of that time. But the place was noisy; traffic, lots of other distractions. And we're talking about a fairly subtle sound here. I'm lucky to live in the country, where I rarely hear anything other than breezes in the trees and birds. In my listening room, I have complete silence, and no nearby neighbors. The noise would be obvious to almost anyone here at home, but in the tech's shop, I wouldn't have been able to hear it if I'd tried.
So, to sum up: 1) do you guys think a DC blocker is a likely fix, or not? 2) How might I get the newer transformer "re-potted"? 3) If the unit still hums when plugged into the kitchen outlet, what do I conclude from that?
Thanks in advance for your sleuthing and speculating.
millercarbon9,401 posts06-01-2021 12:18pmThe sort of mechanical vibration hum you have is caused by DC offset. Our homes are run off 240V but instead of running everything at 240V like they do in a lot of countries someone here decided that was way too dangerous for us fraidy cat Americans and so all our electrical panels are split right down the middle with only 120V going to each circuit.
Thank you Sir.
Here I was thinking having 240v was an impediment limiting purchasing choices.
I used to work for a company that repaired and rewound electric motors and generators. See if you can find a company that does this. They can do what they do with the motor stators after rewinding. They put them in an oven and heat them, then dip them in 555 varnish until they cool, lift them out and leave them to drip then put them back in the oven to slowly bake overnight. We did this in batches every night, so it should be quite inexpensive for them to just pop your transformer in with all the other stuff. If it is a shrouded transformer you will need to remove that first. https://nsshah.in/product/jyoti-555-fd/
As an aside, if the transformer is vibrating, this can also be the windings vibrating as well as the iron laminations. Over time this can cause the windings to short. A dip and bake will stop that.
If you don’t mind upsetting your good lady (assumption mad here) you can buy the 555 varnish and do it yourself. The oven only needs to be 80 to 90 degrees to thin the varnish when dipped to allow it to penetrate. Then leave to drip and bake at 80 to 90 degrees for an hour or so. Do it on a fine day as you will need the windows and doors open 555 is smelly stuff :^)
I had a similar problem with my transformers humming. I replaced the capacitors in the power supply and went to home depot, hot some appropriately sized rubber washers and mounted them between the transformer and chassis. Hum gone.
Curious to see if it goes away when plugged into the stove outlet. I use that one for testing stuff myself.
Items that can cause DC on the mains are, Dimmers, Hair Dryers, portable electric heaters, LED lights, CFL lights, electronic ballasts, and cheaply made switch mode power supplies.
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 +.
Don't know if anyone is still looking at this thread, but just in case:
I tried the kitchen wall outlets; no change in the hum. What does this mean? Does it mean that the problem is not "DC offset"? That I would not therefore benefit from a DC Blocker? (I notice that the "Humdinger" site says that device will NOT cure a hum caused by a faulty transformer, although it might reduce it.) Remember that the "old" unit never hummed in 30+ years of operation, and none of my other audio devices hum even now. So the problem does seem to be in the transformer, and not in the wall power. Still, I'd love to believe a DC blocker would help; that would be the easiest, and cheapest, solution.
chris_w_uk: thanks for your suggestion! I think, finally, it's my best bet here: a DIY "re-potting" with 555 varnish and an oven. But the link you provide is not, as far as I can tell, to a business that could do this for me. For one thing, it's in Mumbai! No way I'm sending this very heavy, and apparently vulnerable transformer to India.
OP, when troubleshooting my subwoofer amp, I connected it to a dedicated microwave receptacle in the kitchen and got the same hum. I still purchased the Emotiva cmx-2 like I mentioned above and the hum went away. I’m not sure if components in the house have to be on and running fir the interference because it would happen to me during the quietest time of the mornings, when no lights were on, certainly no blow driers etc... Anyway, I couldn’t pinpoint the cause but the cmx-2 worked for me. Good luck.
555 varnish equivalent: http://www.glyptal.com/products/1202tech.pdf 1202: Clear General Purpose Insulating and Finishing Alkyd Varnish. Air dry. Excellent heat resistance. Up to 145C on non flexing equipment. Although primarily an air-drying varnish, the oil, moisture, acid, and salt water resistance are greatly improved by baking. http://www.glyptal.com/glyptal_products.html
I had power conditioner and tube amp transformer vibration or hum years ago, did the back to back caps on AC line, helped. Ultimately, I discovered it was over voltage from power company causing hum, 124-126V was typical. Ever since then I monitor voltage 24/7, has stayed 119-121v for well over three years, no transformer hum since, no longer have the caps on AC line.
Some updates on this issue, for those who may be interested.
First, I tried a DC blocker (the iFi unit). No effect. Frankly, I had thought this was a long shot; after all, the unit didn't formerly make any noise--it had been dead quiet for 30+ years--and no other transformers on my home's power, audio or otherwise, make any noise.
The "dip and bake" suggestion seems to me to be the best proposal, short of just living with the noise or replacing my beloved old NAD unit with something newer. However, I'm not sure I want to mess with heating varnish in my kitchen's oven, for the obvious reasons. Chris_w_uk is the Audiogoner who described this process, identified the varnish I would need to use, and then even provided me with a link to a company that may be able to do this for me (at his suggestion, I tried to locate such a company in my area, but none of the likely candidates really understood what I needed). So I've emailed the company Chris sent me the link to, and I'm still waiting on their reply. But I want to thank Chris for his well-informed help.
Re-capping has also been suggested, and I know this is something I probably ought to do for lots of other reasons as well. But my intuition tells me it is not likely to resolve this particular problem. I won't go into my reasoning here, but if any of you think I'm wrong about this, let me know.
Finally...the "noise" is, after all, very subtle. I can't hear it at all while I'm listening to music (except between tracks), it does not seem to affect the SQ in any other way, and even with the room completely silent, the transformer hum is faint. My home's deluxe HVAC system is louder, and so is the refrigerator two rooms away in the kitchen. I have an audiophile friend with a designated listening room full of far more expensive equipment than mine, and he doesn't seem to be bothered by an ambient hum from some household device in the vicinity that is much louder than my transformer hum. So, really, this problem is more psychological than musical. And yet...we audiophiles are perfectionists. I can tolerate a noise in a fridge (although I chose one based on how quiet it was); I can tolerate wind noise in the trees outside, or birds, or the occasional plane overhead. These are not flaws in my audio system, unless the term "system" is construed broadly to include everything in the audio environment, including one's own ears (and who among us has perfect hearing, or doesn't suffer from minor tinnitus?). Be all that as it may, I am insanely intolerant of any perceived flaw in my audio equipment itself. So perhaps this "problem" is really mental. Maybe I should learn a lesson from it.
Thanks for everyone's contributions to this thread.
I have posted this before... Hum is very rarely caused by DC on the line. I have a PS Audio Humbuster, still NIB which did nothing in my case.I actually had 2 causes: In my tube preamp one of the toroidal transformers needed to be rotated about its axis to minimize hum, my repair tech did this while I was on the phone with him. My other cause was 20 year old filter capacitors in my SS amp. I replaced those myself, and all hum was gone. Please don't burn your house down by putting varnish in your oven.
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