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In Reply to: RE: Not my idea posted by unclestu on July 31, 2015 at 20:03:40
Thanks for you comments and suggestions. They are helpful to me.
I was aware that HK and Counterpoint had thought of using copper plated steel. I came across an article written by Michael Elliott in the Singapore based "High End" magazine, back in the Nineties. I do not know the thickness of the copper plating. I imagined it would be at least a couple of mm.
I think fitting a folded whole sheet of copper around input/output sockets, even with the PCB and magnetics displaced would be a difficult option, given the need to remove oxidants and other contaminants from the copper before applying the conductive adhesive.
Maybe layers of copper tape would be the easiest option. The chassis is big, so the aesthetics would suffer.
BTW I took a look at your Website. The article on recorded signal polarity is the most useful I have seen.
Thanks
Follow Ups:
NOt really. The counterpoint copper plating was standard and rather thin.
The idea is that the copper will absorb the E component of EMF. all you need is a continuous plane: ie. to be able to measure continuity from one point to another across the copper. Actually the steel itself is conductive, but the copper simply offers way less resistance and thus "speeds" up the electrical component being generated.
For my copper foil experiments I simply used 15 mil stuff. and that worked well ans I suspect I could have gone thinner but it gets harder to work with. I fabricated mu metal covers for my CD players and transformers ( tube type) and covered them with copper tape. Fields being what they are, and old fingers being what they are, I found that 100% coverage was not necessary.
I came to realize that in dealing with fields, you are not really shielding anything. Rather, you are channeling them away from the critical elements. May seem the same but there's a subtle difference. Shielding employs rather brute force: channeling employs imaging the field and then diverting it away. You can not really end the EMF field but you can move it away to a more benign area.
The metal chassis /copper foil needs the copper to be grounded to work properly, or else the charge on the surface ( rather small in reality) builds up. Without an outlet ( ground wire) a portion of its effectiveness is wasted. Want to know where the field is going to? Just imagine the magnetic lines of force emanating from your transformers and such and attach the ground wires away from the source if possible.
Remember those high school experiments with ferrite powder on a piece of paper over a magnet? You'll remember that the lines want to complete a loop. That's the key. You want to short the loop and thus shrink the field. Often times rather than trying to brute shield the entire chassis, the transformer's field can be shrunk by placing a smaller steel or iron copper lined plate under the transformer itself.
If you have adjacent transformers, for example, a simple L shaped piece is quite effective, and more so if grounded. The fields are still there but the interaction magnetically between the two transformers is significantly reduced.
The magnetic field entering a steel chassis tends to shoot along all lengths and actually extends the field instead of limiting it. Again refer to the ferrite on paper experiment. Attach a nail to the magnet and you will see the lines of force shoot along the nail, and for quite a long way before escaping and completing the loop.
The chassis shielding works great for RFI where the incoming vectors are everywhere. Shielding for transformers is a bit different, though.
Hope this helps
Thanks for your further thoughts.
Yes I largely follow what you are saying and I plan to treat the likely source of my hum problem - ie stray magnetic fields from the power transformer and choke, by "nested shielding" of them along the lines you have suggested. That may suffice but it seems to me that there could be EM fields from the circuit itself interacting with the local steel chassis. Probably not first order effects, but when has that stopped an audiophile tweaker?
A good quick-and-dirty method is to use cheap stick-on copper foil. Make a structure out of cardboard (aka cereal packet) or plastic sheet, then cover with copper foil. (This is made for things like shielding pickups, and is very thin, 50 microns or so).
Using this technique it's easy to make fairly complex shapes to shield various parts.
For extra performance you can also use a layer of ERF paper.
And as Stu says, don't forget to make sure it's grounded. You can solder to the foil OK.
Thanks. These are good ideas to try, particularly for RF shielding.
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