Magnetic shields for transformers

[dsavitsk]

After a long and painful search for hum, I finally found that I am getting magnetic coupling between a power transformer and some output transformers. It is very low, so low that you can't really hear it during the day, but a night it is there. I found that moving the power transformer back about 3 inches is enough to eliminate the hum -- problem is that 3 inches is off the chassis. Everything is already at right angles, etc.

So, either I need to find some clever method of shielding, or I need to build a new chassis which I definitely do not want to do. So, is there some method to add a magnetic shield to the power and/or output transformers? Maybe some mu metal bell ends, or a way to pot them? Thoughts?

[Sherman]

You might try picking up a metal basket at The Container Store. They have various sizes. Choose one made of steel of course. The ones I've seen there are painted, usually silver or black.

You can mount them to the chassis using plastic wire clips, the kind that form a loop and have a hole for a screw to go through. I used them in one project. I decided to use them for cosmetic reasons, open frame OPTs look ugly to me and these would mostly hide them while also preventing fingers from touching the B+ connection on the trafos.

I initially mounted the OPTs much closer to the PTX and had hum in the left channel. Dropping the basket over both OPTs "cured" the hum. I was surprised since they are baskets with 1/8" openings between the steel weave. I eventually moved the OPTs about 1.5 inch farther from the PTX and the hum is gone even with the basket off. The basket is still used for aesthetics though.

[BudP]

dsavitsk,

With a picture of your amp and a mention of the chassis material I can begin to help.

Just for starters, did you orient the OPT 90 degrees from the power, in three axis? If so, then airborne EMF coupling can almost be ruled out. If your chassis is non ferrous and the transformers are decoupled in three axis, then it is very likely due to antenna effects from the wiring. Perhaps the power rectification circuit is coupling with either the OPT leads or coil orientation.

Bud

[dsavitsk]

The amp can be seen at http://www.ecp.cc/semha.html

As you can see, transformers are 90 degrees, but slightly off axis -- to be fair, I was warned that this could be problematic, but there really isn't anywhere else to put them. After redoing nearly every part of the amp, I removed the power transformer, placed it far away and ran some long wires to it. This solved the problem completely. I also oriented it as if flush mounted and put it right behind the chassis and again silent. However, maintaining the flush mounting orientation but setting it right where it is, there is still some hum -- maybe a little less, but it is still there. Maybe this transformer radiates more than others (I have a Hammond with the same specs to try to see if this is correct), but I think I either need a shield, or a bigger chassis.

[tvi]

I think what Bud means by 90° in 3 axis would put you power transformer like the one in the attached pic.

Regards
James

[dsavitsk]

That is what I mean by flush mount orientation, which didn't help. The measured hum, if you can trust my DMM, is ~300uV. Low, but not quite low enough here.

[BudP]

dsavitsk,

What you have is only a single axis of rotation. If you laid the transformer on it's side and pointed the short side at the OPT's that would be very close, with two axis of rotation. Then rotation of the OPT's from vertical to horizontal mounting, still with mounting end bells, would be the last test for compatibility.

You can shield the core of the power transformer. You need at least grain oriented M6 applied over a layer of thin, two sticky side tape, with the overlap on top, if the chassis plate is steel and down if it is aluminum. Ideally the material width you use should extend beyond the core stack edges by about .050" on either side.

You can also wrap a copper shield around the coil and up over the laminations and core field suppression band. This is a shunted shorted turn and likely should be grounded at the power ground point.

If you look for the hum with a scope attached, look at power lead outs and surface of endbells and core with a sniffer coil. Look for spikes in the sine wave. A leading edge spike comes from the coil and a trailing edge spike from the core. You may also be able to find these at the OPT.

I do have some M6 shield stock already cut to size for specific transformers. If we have similar sizes you may be able to use it. If not, Lamination Specialties in Chicago and Yates Electro-Magnetics on the East coast, will usually cut to order. Also investigate the conetic alloys of Mu Metal that are available. You can purchase a "samples" set of sheet stock, but I no longer remember from whom. The cut to size strips of M6 will likely be cheaper.

Another avenue is a 0.060 thick CRS box, that has been heat treated to marstenite/osteonite temperature, to render it dead soft after forming the box. This will provide 100% EMF containment, even with constant voltage transformers.

Bud

[dsavitsk]

Originally posted by BudP
What you have is only a single axis of rotation. If you laid the transformer on it's side and pointed the short side at the OPT's that would be very close, with two axis of rotation. Then rotation of the OPT's from vertical to horizontal mounting, still with mounting end bells, would be the last test for compatibility.

Okay, I see. I'll give this a try and see if it helps.

Originally posted by BudP
You can shield the core of the power transformer. You need at least grain oriented M6 applied over a layer of thin, two sticky side tape, with the overlap on top, if the chassis plate is steel and down if it is aluminum. Ideally the material width you use should extend beyond the core stack edges by about .050" on either side.

You can also wrap a copper shield around the coil and up over the laminations and core field suppression band. This is a shunted shorted turn and likely should be grounded at the power ground point.

The transformer is custom made, so I will call the maker on Monday and see if he can do some, or all of these things to this transformer. It is certainly the easiest option from my perspective, as realignment will require a new top plate, and otherwise a new chassis.

Originally posted by BudP
Another avenue is a 0.060 thick CRS box, that has been heat treated to marstenite/osteonite temperature, to render it dead soft after forming the box. This will provide 100% EMF containment, even with constant voltage transformers.

Any idea where to get such a box?

[dsavitsk]

Originally posted by dsavitsk
Okay, I see. I'll give this a try and see if it helps.

Nope. The hum might be slightly lower, but not appreciably.

[tvi]

Less EMF have a range of "Magnetic Field Shielding" materials often sold by the foot, might be of some help?

Regards
James