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Shielding components from EMI/RFI... Help please

A recent experiment with a product designed to reduce EMI/RFI left me curious about other ways to reduce EMI/RFI in my system. In the past ten days, I've stepped onto a slippery slope, at the bottom of which is surely some kind of insanity...

I've been experimenting with copper plates in an effort to absorb, deflect, diffract, and block EMI/RFI. I've tried copper plates under components, on top of components, and inside components.

This is the point where you tell me I don't know what I'm doing and I'm likely to short circuit something and/or electrocute myself. Consider me duly warned. This is also the point where you tell me to get some balanced interconnects, or at least to get some shielded interconnects for Chrissake. Consider me duly informed. Moving on...

I'm hoping you can help me make the most of this experiment, and help me avoid killing a component or myself. My strategy so far has been to:

1. Place copper plates at locations that generate a lot of EMI/RFI, e.g., components with switching mode power supplies or high frequency clocks. The system has a total of 3 SMPS and 3 clocks.

2. Place copper plates at locations that are vulnerable to EMI/RFI, e.g., under the amp, near the transformer.

3. Place copper plates inside noisy components -- in particular, my Meridian G68 preamp/processor. I've begun to build 2 partial Faraday cages, one for the SMPS, and one for the analog output stage.

4. Ground the copper plates either to the component chassis (when plates are used inside a component) or to an independent ground point (when plates are used above/below a component).

Has anyone tried this sort of thing?

Bryon
bryoncunningham
bryoncunningham OP
890 posts
 
The Meridian G68 has a ground wire running from the IEC input to the chassis. As discussed in earlier posts, the G68's PSU has a SECOND ground wire that is also connected to the chassis, but at a different point. You can see a diagram here. You can see a picture of the PSU's ground wire here.

Yesterday, while upgrading the AC wires running from the IEC input to the PSU, I removed the ground wire connecting the PSU to the chassis. The ground wire connecting the IEC input to the chassis is still in place. Works normally, but...

Have I created a safety hazard?

Thanks,
Bryon
almarg's avatar
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almarg
9,670 posts
 
Hi Bryon,

I think we had concluded earlier that the ground wire connecting the PSU to the chassis was for the purpose of connecting filter capacitors between the chassis and each of the two AC input wires (hot and neutral). Therefore removing that ground wire will not create a safety hazard, but it may reduce to some degree the effectiveness with which noise entering the unit from the AC line is filtered.

With the ground wire disconnected, what you have is the two capacitors in series (the series combination having a total capacitance equal to half of the capacitance of each of the capacitors) connected between the AC hot and AC neutral wires. That will provide some degree of noise filtering, but presumably less than if the ground wire were connected.

Whether or not the difference in noise filtering might have any audible significance is not predictable with any certainty, as I'm sure you realize.

Best,
-- Al
mapman's avatar
mapman
21,921 posts
 
I've applied mu metal foil that I was able to purchase on teh internet to shield my phono section from external EMI/RFI with good results.

My understanding is mu metal is designed specifically for this kind of application.

Check my "OHM Sweet OHM" system listing for pics of teh rather crude but effective mu metal enclosure I made around the Electrocompaniet step up transformer that I use with my low out mc cart and some discussion around same a couple years back. This effectively eliminated some external RMI/EMI issues from external devices that I was having with that particular device when I acquired it.
bryoncunningham OP
890 posts
 
Hi Al -- I didn't realize from our earlier discussion that removing the PSU ground wire would reduce the level of noise filtering. Back it goes!

Hi Mapman -- I've looked at Mu metal a few times. It seems like an interesting product. I elected to use TI Shield instead, along with regular old copper and steel. But I'll keep the Mu metal in mind for future diy projects. Thanks.

Bryon
bryoncunningham OP
890 posts
 
03-04-12: Almarg
...for shielding to be effective at high frequencies, as Dr. Johnson indicated the shield should be grounded at both ends. But I believe that the key element of what he is referring to by "grounding" is a connection at each end between the shield and the metallic structure of the component, rather than a connection to some external ground point. I believe that circulation of noise currents from the cable shield into the metallic structure of the components will dissipate their energy significantly, although perhaps less so in the case of the network switch due to its small size.

I've been giving more thought to this, and I decided to experiment further with an alternative grounding scheme. I disconnected the ground wire between the ethernet switch and the power conditioner (thereby removing the switch's connection to earth ground), and I added a braided grounded strap between the ethernet switch and a 6" x 12" aluminum plate. My hope was that, by "enlarging" the surface area of the ethernet switch, it would provide more adequate dissipation of RFI or other noise picked up by the long ethernet cable between the computer and the switch.

The results are at least as good, and possibly better, than the result of grounding the ethernet switch to the power conditioner (which, in turn, was grounded to the preamp, the amp, and earth).

Bryon
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