How to get rid of transformer hum?

I have a pair of Pass X600s and an ac line with about 5% clipping of the sine wave. This gives me a large amount of transformer rattle (mechanical hum). I have built a line filter with two diodes and a couple of caps that has gotten rid of 80% of the noise but I'm looking for a inexpensive fix. My power company will not fix the power coming in unless it is clipping by more than 7%. Any suggestions?

Hi Tommy; I'm sorry I can't help you, but you've maybe helped me. I have a pair of McCormack DNA-2DX amps that hum at various levels. I've talked to Steve McCormack about this at length, and he says that most likely there is DC leakage into my AC lines, and that there is nothing they can do about it (is this the sine wave clipping that you mention?).

I have not yet contacted the power Company, but have a hard time believing they would have much interest in it? BTW, my amps are at SMc being upgraded, and I specifically asked them to check out their tendency to hum. Steve has told me that they do not hum in S. Calif.-- probably not enough power to muster up a hum ;>). We'll see. Craig.
The mechanical hum you are talking about could be from a few causes. Magnetostriction causes the transformer laminations to actually change size as the voltage changes. This effect is greatest at high voltages and is independent of the power the amp is delivering. At high currents transformers can have loose laminations that buzz, a loose winding that moves or the transformers stray magnetic fields can cause a steel chassis to buzz.

Do both amps behave identically?
Both amps hums just alike. Before building a filter made up of two caps and a diode I could hear the hum when I walked into the room. Now I hear it when I walk over to the amps. I'm just looking for a fix that doesn't cost 5 grand. Any thoughts?
Well, I think the transformers are either mismanufactured or poorly designed. When you design a transformer you make a deal with the devil. If you use lots of turns the transformer will be quiet, but the large number of turns means smaller longer wire which causes higher resistance so the transformer becomes less powerful. Or, you can make the transformer bigger without increasing the power, that costs more money.

If in fact the transformer is humming because it doesn't have enough turns on the primary, then the only thing one can do that I know of is to lower the voltage to the amp. This could be verifided by using a variac to lower the voltage and see if the hum goes away. More diodes will lower the voltage some. Or a stepdown transformer that dropped the voltage from say 120 to 110 votlts. Of course the step down transformer might hum. I presume you have two diodes in parallel connected anode to cathode. Each time you add one of these it will drop the line voltage about .6 volts. It also increases the lines harmonic distortion but that shouldn't be a problem.

If the hum is being transmitted into the chassis you might reduce hum by placing the amps on something soft. If there is a cover that is vibrating you could try removing it.
I realize that you have measured a 5% sine dip. But are you certian that this is the cause of the hum? I have tracked down a lot of hum problems and most are generated from within the home. I will relate my own personal experience. I always had a slight hum in my tv and Tice isolation transformer for my gear. I lived with it until I bought a Classe CA400. The amp hummed very loud! I finally isolated the source to my heat pumps outdoor unit. Something in the prehaet circuit of the compressor. Right now I just turn it off when I listen. The background is dead silent! I'm going to look into it deeper when I get the unit pm done. I would try turning things off in the house first to see if the hum is caused internally. (off means off at the breaker) Good luck!
A while back some funny Agon poster suggested that the way to stop an amplifier from humming was to teach it the words.
Usually physical damping of the transformer(s) helps a lot. Make certain the they are tightened down securely to the chassis. You might consider using some type of spacer if they will not tighten down properly. I have seen people place (wedge) hard rubber doorstops between power transfomers (not power supplies) that were spaced close together with good results. Just be carefull as these things can hold a lot of current even after they have been powered down for long periods of time. Anyway, I would try this out first before going the technical route.
PS: As far as using soft footers directly under the units, I disagree with Steve's comment. I have an external power supply on a Musical Fidelity amp (that sometimes hums). Using a soft footer in this manner just isolates the vibration to the unit which deteriorates the sound in this case. I have found it much more effective to draw some of the vibration from the unit to a Maple platform with the use of cones. However the platform is isolated with soft footers (Vibrapods) so that other vibration is not introduced to the platform. I will be simplifying this setup with the use Neuance shelving, but still recommed the use of cones/platform/soft footers as an inexpensive fix on amps and power supplies.
If it is a DC bias then a isolation transformer may help. If the isolation transformer wants to hum because of to high voltage then it may have a higher voltage tap you can try. You could also move the isolation transformer to the breaker panel. I have a 5kVA unit that I intend on wiring tothe breaker panel and then put my dedicated outlet on it.
Guys thanks for the feedback, I do have two diodes invertered, and prior to these amps I had Cary SLAM 100s which didn't rattle at all. I did speak with Nelson Pass and he is the one that gave me the filter idea. I can't tighten the transformers anymore so I'm looking for a way to clean up the sine wave. I may have to resort to a conditioner by PS Audio or Exact Power. Any other ideas.
Cleaning up the sine wave certainly won't hurt anything, but oddly enough it may not help. Magnetostriction is related to volt/seconds. The transformer stores up energy during each half cycle. If you begin to push the storage capacity to its limit it begins to saturate and hum. Since you are stuck with 60 hz only the volts are available to change. It is the area under the curve and not the shape of the curve that counts for volt/seconds.

It would be interesting to know what your line voltage is. Use an RMS reading voltmeter if you can, rat shack sells them along with some nice speaker cable (opps getting off the subject :-) if you don't have one of those a regular volt meter will work OK. If the line voltage is high then the following option might help.

There are ferroresonant transformers you can buy for probably 3 to 5 hundred bucks that clean up the waveform and stabilize the voltage. They also provide tremendous surge protection. The problems are that they are (at least the ones I have seen) very noisy. This is because they deliberately run them in saturation. So, you would have to locate the transformer somewhere away from your listening area. They also waste alot of power. Something like a third of their rating. So it is best to disconnect them when not using your system.

A step down transformer would be cheaper. It can be an autoformer, since you are not looking for isolation. An autoformer has no secondary. This makes them smaller and therefore cheaper.

Try four or five more of those diode pairs. If they don't help you are not out much. Try the cheapest stuff first.

I have decaded 20A lines to each amp and voltage reads a solid 119 plus change on each one. Do you think any of the power conditioners will fix this problem? Do I put the second set of diodes across the first set? Thanks Tommy
Geeez, Tommy, it just doesn't make sense that they are humming at only 120 volts. If it weren't for the fact that both are doing the same thing I would think it was something mechanical.

I don't know much about the audiophile line conditioners. I did look up a couple of brands on the internect in response to a post someone had made. It said something like "No capacitors, no inductors, no transformers to interfer with the purity of the sound!". It cost 6 or 7 hundred dollars and they were advertising that there was nothing useful in the box?! The ferroresonant transformer I mentioned won't help because the line voltage is not too high.

The diodes are like little valves. One is pointed in each direction because AC needs to go both ways. It takes .6 volts to open the valve. ten pairs would give you a 6 volt drop. So, you just make some more pairs and put them in series with the one you have. They can be soldered to the one you have. They can go anywhere in either leg and will work the same, just not in parallel with each other. You could just temporalily cobble it all together with alligator leads to see if it helps before taking the time to do it right.

I'm not familiar with the amps. Is the manufacturer available for advice?
I have in the past used a vpi brick (currently on top of my Ls7 pre) and it brought the hum down a lot. It may not do what you want on a electical basis but it is the cheap fix you may be looking for.
A Shakti Stone electromagnetic stabilizer may be a useful fix for this application (at least one for each amp)

The mfg said that is the power that is causing the hum and sure enough when I made the filter it got rid of about 80% of the hum. I posted this thinking that someone else might have a easy cure. I'll clip some more diodes across to see what happens since they cost about 60 cents a piece. Thanks again for your input.
Tommy, Sqjudge has the right idea. I had a Levinson 335 with the same problem, Madrigal technical guys here in London UK suggested the transformer (in my case 4KW) which works perfectly
Lets see what we got so far. The mechanical hum couldn't been a design problem, for that would have surfaced and licked at the maker's QC dept.
I agree with Stevemj 5-14-01 post part about loose laminations or windings acting up. Varnish which is used to fill gaps in trans. or motor windings do deteriorate with time. Tommy, it might be worth your while to call a motor/transformer coilwinder workshop to check if all you need is a bake and re-varnish to seal off your misery! This problem is very common thing to electrical maintenance people. Get opinions of a couple of coil winders & their price to the job.
I've not had this same problem with my audio so far, but I have solved similar transfr hums (in my work) by tightening mounting screws or a re-bake/varnish at the winder's. I cannot guaranty this will work for you but a coat of insulating varnish does no harm, and I dont think is expensive. But try not start your conversation with the winder saying the usual hi-end audio stuff, you know what i mean :)
Tommy, have you tried to power these amps at another local? Away from your home? I saw that the mfg stated that its a power problem. I may not be an EE, but I have over 20 years in the field and 9 times out of 10 its not the equipment or the power but another source of noise on the line saturating the transformer causing it to hum. I know we all hate the power company and sometimes they are at fault. I'm just curious as to whether the problem follows the amps when they are powered somewhere else.
Here is the diode thing with a filter.

The amps were at another home with the same power company. They didn't hum there. I listened to the my parasound amps in my surround system and they hum too, they are behind a wall and the transformers are about 10% of the size of the Pass. I've had the power company out and I saw the sine wave on there scope it was chopped off at the top. Just not enough for them to work on fixing it.
While it may not be a feasible solution for some, try moving. The transformer in my DAC hummed audibly (hearable from ~2 feet away) and it was almost on its way to the factory for repair. There was also a seemingly unrelated hum through the speakers that floating the preamp's ground removed, though the overall sound was better with the ground enabled.

Well, I moved last week. Bigger and much more modern place. Especially the wiring. Previous place was a unit in a '50's era fourplex; new one is a "condo" (though NO shared walls) that's about ten years old.

When initially set up in the condo the DAC still hummed and the pre-problem was actually more pronounced. A couple of days later the DAC hum is completely gone and the speaker hum caused by the pre- is only audible from a scant few inches away.

Can't explain it, but what a relief!
Fpeel, this is a different problem than yours. This does not sound like a ground problem but it sounds like the culprit is power line harmonics. The chopped off top of the sine wave suggests that something looking similar to a squarewave is feeding the device. A squarewave's spectral makeup is the sum of all of the odd harmonics. If you looked at the power line on a spectrum analyzer you would probably see lots of odd harmonics. I believe it is these harmonics that are making the transformer laminations loosen and vibrate.
Tommy, I apoligize for my over simplified explaination but have you thought of something like the PS audio box that recreates or regenerates the sine wave? Another possibility is a used ac power source. I have seen some manufactured by Elgar at some of the used test equipment houses for about $1.5K to $2K. Good luck.
I'm not sure this was a ground issue, Liguy. Not sure it wasn't, either. The grounding at the old place was originally suspect, but am pretty sure it is more than adequate now (long story). Given the neighborhood it may well have been similar , but different to your situation. This area is a lot more residential; the last one had a lot of commercial property nearby. Lord knows what kind of shape the power was in.

Yes, I'm rambling and don't have an answer. Just thought the ruminations might be of some help. Good luck getting it resolved!