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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
 
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
bryoncunningham OP
890 posts
 
Now that I think about it, if the steel I purchased is indeed non-magnetic, will it work for shielding EMI at all?!

bc
kijanki's avatar
kijanki
4,397 posts
 
T-304 won't shield against low frequency magnetic field. It might protect against it if you build Faraday Cage but even then it is very limited. I'm no expert on Faraday Cage but as far as I remember it is used mostly as electrostatic shield. It works against magnetic field but poorly at lower frequencies.

We don't even know what is the nature of the noise pickup. If switching power supply has fast transients (high frequency) even non-magnetic shield would help cutting on capacitive coupling and creating losses (eddy currents) with magnetic field. I would stay on the course, since you already ordered shields. It will give you shielding at high frequencies allowing to isolate the problem.

I absolutely agree with Al's assessment on the Cat6 improvement. Jitter is not only function of the signal noise (not likely since it is buffered) but also system noise (receiver threshold noise) that is influenced by noise injected by current induced in the cable that finds return to ground thru the system. Ethernet uses differential signaling that cuts on common mode noise but it still couples high frequencies thru input capacitance to signal ground. Currents traveling on the signal ground are system noise. Judging by improvements you experienced your noise pickup might be of high frequency and your non-magnetic shields will work.
rodman99999
6,018 posts
 
The TI Shield, that I mentioned earlier, will block all RFI/EMI and is VERY easy to work with(thin enough to cut with scissors, and bendable).
bryoncunningham OP
890 posts
 
Thanks, Kijanki, for that detailed response. I will follow your advice and add the regular steel shields to the copper shields already in place. I will report back with the results.

I think you and Al must be right that the Cat6 cable has reduced system noise, and thereby reduced jitter. It's the only plausible theory that also explains the dramatic change in sound quality.

It's worth noting that whatever the jitter-inducing system noise was, it was not audible as a NOISE FLOOR, i.e. hiss, buzz, and other grunge audible when playing music. The system has been very "quiet." Because of that, I (falsely) believed that the system didn't suffer from any significant noise problem. But now I see that my reasoning was flawed. Apparently, not all detrimental noise is audible as a noise floor. I wish I'd known that!

Bryon
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