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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
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almarg
9,670 posts
 
Hi Bryon,

Presumably the shielded cable is reducing the amount of rfi/emi that escapes from the cable and couples onto circuit points elsewhere in the system, most likely points that are "after" the reclocker circuitry. There are probably a number of ways in which that could occur, despite the shielding you have incorporated into the G68. One possibility would be that it couples onto the power wiring, and makes its way from there onto the grounds within the G68. Noisy grounds in the digital section could (and probably would) result in increased jitter. Noisy grounds in the analog circuitry would have unpredictable but conceivably significant consequences at audible frequencies, even if the noise itself is at rf frequencies (as a result of intermodulation, AM detection, or other effects that can cause rfi to affect audible frequencies).

That's my speculation, anyway.

Best,

-- Al
bryoncunningham OP
890 posts
 
Al, I think you may be right in your speculation that the reduction of EMI/RFI provided by the shielded cable is resulting in less jitter. That is consistent with the change in sound quality. If that theory is correct, the system now has a LOT less jitter.

I spent a couple hours listening last night and I continued to be stunned by the difference. I have NEVER experienced anything like this with any other cable. In the past, I've had two different $2k analog interconnects in the system, and they didn't have anywhere near the effect I'm describing. I currently have about $4k in power cords in the system, and they didn't result in a fraction of the improvement of this $7 cable. It's mind blowing.

FWIW, I very seriously doubt that other systems would be similarly effected by a shielded ethernet cable. My setup is so idiosyncratic that my results are most certainly not generalizable.

I'm inclined to conclude that there was a significant PROBLEM in the system that the shielded Cat6 cable has rectified, a problem that has gone undetected by me for years. Things have sounded pretty good, so when I listened to the system, it didn't sound like there was a problem. But this is such a dramatic effect, I must have been listening to/through this problem all along.

As a footnote, it's interesting that you mention intermodulation, because when I added shielding around the analog output stage, walling it off from the digital circuitry, I said to a friend: "It sounds as if there is less intermodulation." I'm not even sure I know what I meant by that, but that was the concept that immediately came to mind.

Bryon
kijanki's avatar
kijanki
4,428 posts
 
"I've been reading that copper is effective at shielding high frequency RFI, but not particularly effective at shielding low frequency EMI"

It is because copper, being non-magnetic, does not shield against electromagnetic wave. In cable this electromagnetic wave induces current but because of skin effect it travels on the outside of the cable - shield . Same happens with copper plates around components. Skin effect won't work at lower frequencies where copper becomes useless against magnetic field of 60Hz transformer or even 50kHz from switching power supply.

In case of low frequency radio interference you need at least 1/10 of the wavelength antenna to "receive" interference - not likely. You are more susceptible to direct magnetic fields like leakage field from power transformers.

MuMetal is good suggestion - it works much better than plain steel (permeability of steel is few thousand while MuMetal reaches 100,000). It comes in different permeability because higher permeability Mu Metal is easier saturated by magnetic field. In such cases sandwich of lower and higher permeability MuMetals gives best results. Your magnetic fields are most likely very weak but just in case don't place MuMetal close to the source (transformer etc.). Excessive bending of MuMetal is not advised (has to be re-annealed).
jea48
4,459 posts
 
Bryon,

You might want to do some research on the archives of Tweakers' Asylum regarding the use of ferrous, magnetic, materials. There may be better ways to shield for EMI.
Search shielding for EMI

http://www.hottconsultants.com/pdf_files/ground.pdf

http://www.q-audio.com/grounding.pdf
.
bryoncunningham OP
890 posts
 
Jea - Thanks for the advice. I will poke around on Tweakers and see what I discover.

Kijanki - Thanks for the help. You are always a good source of information. Much appreciated.

MuMetal is good suggestion - it works much better than plain steel.

I already ordered custom cut regular steel from an online retailer. :-( You think using regular steel is a waste of time?

Your magnetic fields are most likely very weak but just in case don't place MuMetal close to the source (transformer etc.).

Again, what if I use the regular steel I already purchased? It's T-304, which I believe is non-magnetic. If that's true, do I still have to be careful how close I get it to the power supply? How close is too close?

Bryon
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