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Help! Cure for transformer hum?

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. 

Thanks for any advice you can give me.
snilf

Showing 2 responses by jea48

jea48
4,788 posts
 
@ snilf

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.

.
jea48
4,788 posts
 
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.

Here is a quote from Nelson Pass:

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 +.
https://www.diyaudio.com/forums/solid-state/2080-dc-filter.html#post15973



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