EMI/RFI Absorbers


#1

I noticed that 3M and others sell EMI/RFI absorbing material that you can buy in small sheets and relatively cheap from places like Mouser and even ebay. They come with adhesive backing so you can easily apply these sheets yourself over anything you want.

http://solutions.3m.com.hk/wps/portal/3M/zh_HK/electronics/home/productsandservices/products/TapesAdhesives/EMI_EMC/EMI_Absorbers/

Do products like this work in any practical sense and would applying one of these to the FPGA (or other places) in our DAC or other source components make much of a difference in sound quality?

I see they come in different thicknesses and are rated for specific frequency ranges. Which one would be ideal to put on the Spartan 6 FPGA in our Directstream? Are there other places inside the Directstream that would also make a difference? Or I suppose the better question is will it make much of a difference?


#2

The difference using some of the shielding material should be noticeable - people have used “audiophile” RFI absorbers and noticed the results. In general the biggest digital offenders are the biggest chips :slight_smile:

I don’t want to take the time to wade thru all of the options and make specific recommendations, but here are the best places to try and the frequency ranges of interest:

In Sr: The PIC/control processor/display processor on the back of the display board is the worst offender in the box. (80MHz plus all sort of impulses reading flash and ram, etc.) In the Jr the control processor is the one that says “PIC 32”.

Probably 2nd most offender is the Bridge (high clock rate - 100s of MHz and lots of current.) It’s the chip on the little daughter card on the Bridge II or on the little daughter card in the Jr next to the Ethernet connector.)

I don’t know which of the following causes the least or most RFI/EMI:

The USB processor is next to the USB connector, a little smaller and says XMOS. (About 400MHz + harmonics)

In the Jr the Ethernet PHY is near the network connector and says “ASIX” About 125MHz and probably not the worst offender.

The FPGA says “Spartan 6”. (some 22.6MHz, a little less 56MHz and a lot of 170MH - the number vary a little from release to release.)

The two small inductors near the USB chip might be a place to try if you can make sure that nothing can get shorted - they are part of the XMOS chip’s power supply so they have a lot of changing current.

In the DS Sr if your digital board still has rectangular epoxy PCB material just over where the audio transformers are, adding some magnetic shielding to the BACK of the digital board might be useful. Make sure to not short anything. There are no traces in those rectangle on any lower of the board.

There’s probably no reason to shield any of the other bigger chips (mostly regulators and higher current diodes.) But it shouldn’t hurt.

On the analog board there are only two places I’d try anything, both just under the ribbon cable: the oscillator module (22.6MHz) and the big 28 pin digital switch (which says “MC100E…FNG” - 11.3MHz.) Some people have reported good things when shielding them and others the opposite.

You might also find that wrapping some of the absorbing material around your power cord and/or you interconnects (especially digital interconnects) may make a noticeable difference, perhaps good, perhaps bad depending on the rest of your system.


Bit Perfect.... Or Not?
#3

Awesome info Ted. Much appreciated! It seems like getting material formulated for below 1Ghz is preferential then based on these numbers. Luckily enough, from 3M at least, it seems to be the cheapest.

I wonder if it might be more practical to build some kind of simple shroud around the display board if it indeed is the worst offender? Would a shield/shroud for this work better or maybe both absorption and shielding? The same could be asked about the toroidal transformer too. I don’t use a Bridge II so that’s one less issue to worry about. My UPnP/source component is in it’s own box luckily, though I may end up using some absorbing material in that as well, though Sonore claims the I2S output board and output I2S signal is very well isolated from noise/EMI, but I suppose it couldn’t hurt adding it.


#4

I didn’t know you were considering shrouds:

Yes one around the display board should make noticeable difference. I don’t know how much absorption you might need if you have a good shield.

I’ve got no idea how much of a problem the AC transformer causes - it’s specially built and specifically oriented to have minimal effect on the output transformers. I’m not sure I ever heard a problem but you can see some low level 60Hz with a good audio analyzer. Shielding the power supply transformer magnetically would be some work (the cage has to be fairly complete to work well.) You’d think that electrical radiation from the transformer would be small compared to the conducted output so I wouldn’t bother with electrical shielding.

FWIW Doing too much shielding around the audio transformers will muffle the sound noticeably. I haven’t personally tried mu metal for the output transformers but it has a chance of helping without messing things up.


#5

Ted, I still have some mu metal sheets at home…I could try, but I remember that such heavy shielding might cause magnetic fields, through their nearly complete blocking, to be then forced in directions that might cause other problems, heat etc…do you see a risk? Or just sound effects?


#6

I’m speaking from memory from about 10 years ago, so this could be wrong for many reasons :slight_smile:

mu metal is hard to work with since any bending of it causes it to need retreating. I think the biggest risk is making sure nothing ever shorts if the shield moves, etc. If I remember correctly, yes, it’s important to not have “holes” for the field to “leak” out. The easy 5 sided box is probably not worth while. Doing a complete box with the smallest possible holes for mounting, wires, etc. would be work… If money isn’t too much of a problem I’d rather just pay for output transformers that are already mu metal shielded. The manufacturer probably knows more about it than I do :slight_smile:


#7

This isn’t really a topic for me, since I’m very unlikely to open the case of my DSJr, but…

If you go nuts trying to shield different components inside the case, are you not at risk of either a) redirecting RF from one place to another, and/or b) messing up airflow for cooling?

My caution would be that the DACs are voiced as manufactured, and any internal mods are just as likely to make things worse as better.

Now, feel free to educate me. nerd_gif

Chris


#8

interesting topic. These 3M products remind me of, or are very similar, if not the same as the stealth cloth offered by Stillpoints, and probably marked up considerably too. Regardless one has to wonder though if purchasing a Shakti Stone and placing it on top of the DS or DS jr might not be just as effective and for sure far easier to use. Its my understanding these devices passively perform the same thing on a broader scale, although I have no direct experience with them.

Hope this helps.


#9

Ted, yeah a 5 sided box around the display board using MuMetal seems like a cheap and easy way to shield the rest of the DS from all the nasties coming from it. I don’t know if I have the skill or patience to try anything fancier. There are some 3M AB5050 2 inch x 2 inch absorbers I see on eBay for cheap as well. I think I’ll play around and see what kind of results I can get based on your recommendations.


#10

You might consider adding resonance-damping to the caps. I’ve seen this done as part of many aftermarket electronics upgrades. Theory is that the caps have some mechanical resonance, and that this can quiet the unit noticeably. The usual technique seems to be putting a circular piece of self-stick damping material on the flat end of the caps.


#11

Does having the display set to turn off (which is how I usually keep it set) negate the need to treat the board for rf?


#12
christophervalle said

If you go nuts trying to shield different components inside the case, are you not at risk of either a) redirecting RF from one place to another, and/or b) messing up airflow for cooling?

Putting boxes around components or sets of components in the DS Jr could indeed mess with needed air flow and might risk a future short circuit in either the Jr or Sr. Little square tabs of EMI dampening (not shielding) on top of the aforementioned bigger chips (NOT the power supply regulators in the DS Jr.) is worth trying for the adventurous (they don't run hot), make sure you can remove it if it doesn't do what you want.
EldRick said

You might consider adding resonance-damping to the caps. I’ve seen this done as part of many aftermarket electronics upgrades. Theory is that the caps have some mechanical resonance, and that this can quiet the unit noticeably. The usual technique seems to be putting a circular piece of self-stick damping material on the flat end of the caps.

Only the power supply caps on the power supply card. The DS's analog card doesn't have any electrolytic caps near the analog section - it uses film for the audio proper and high quality low impedance 8 legged caps for power supply bypassing, local power storage. In the DS Sr it probably wouldn't hurt things to try to dampen the electrolytics that are there, but in the Jr they need all the heat radiation they can get.
emailists said

Does having the display set to turn off (which is how I usually keep it set) negate the need to treat the board for rf?

Turning the display off will quiet some audio noise down about 120dBFS - it's audible for some. Even so the display processor/control processor is still a big RF radiator which might have more of an effect in some systems than others.

Damping inside your Directstream?
#13
Ted Smith said

Putting boxes around components or sets of components in the DS Jr could indeed mess with needed air flow and might risk a future short circuit in either the Jr or Sr. Little square tabs of EMI dampening (not shielding) on top of the aforementioned bigger chips (NOT the power supply regulators in the DS Jr.) is worth trying for the adventurous (they don’t run hot), make sure you can remove it if it doesn’t do what you want.

Is anybody here eager to show us how to accomplish it? It doesn't seem to be rocket science, but I don't have a furthest idea on how to start it. :)

#14

If it were easy (and cheap enough) you bet it would already be done :slight_smile:

I don’t encourage anyone to try it that isn’t in the mood for “audiophile hacking”: This is the type of thing that some audiophiles love to experiment with: they don’t know if or where an idea might work but they are compelled to experiment and find out.

I tried to answer the original question in the spirit I believed it was asked in: I’m not saying that this is (or is not) the best way to spend time and money to augment a device, but for those interested in experimenting with things like this I tried to give a good start. It’s certainly cheaper than some of the aftermarket “3D” EMI absorbing devices that people have had success with in the past.

The original poster seemed to have some idea on which kinds of sheets to try - I’m sure there are some that will make a difference, but I have no idea which might do the best job or be the most cost effective.

The explicit shielding (using conductive or mu metal boxes is dangerous: you have to make sure the boxes will NEVER slip, slide, vibrate too much, etc. or they very likely will blow chips, caps, etc. up.

I figure putting scraps (small squares) of the adhesive backed material is reasonably safe since there’s nothing that’s likely to cause them to move and 3M (if anybody) knows how to make adhesives that do what they want. I’d just make sure that I could take things back off if something doesn’t work as well as expected. You can almost certainly use an X-ACTO knife to shave it off if the adhesive is too good but that might be a problem with some locations you might want to experiment on.


#15

Has anyone tried absorption and/or shielding? What are the results?

MuMetal can be covered with tape or lacquer to prevent electrical shorts no?