Shielding

WerTicus · « **on:** 4 Dec 2003, 07:49 am »

[Moved, by request, from the Starting Block to The Lab -- JohnR (SB fac)]

would aluminium foil work for sheilding?

i was thinking of cutting a4 sheets and putting them into plastic sleeves for insulation and then wrapping them around my amp's boards and maybe another sheet around the torriodals....

or wouldnt aluminium work?

IanATC · « **Reply #1 on:** 4 Dec 2003, 10:47 am »

Quote from: WerTicus

would aluminium foil work for sheilding?

i was thinking of cutting a4 sheets and putting them into plastic sleeves for insulation and then wrapping them around my amp's boards and maybe another sheet around the torriodals....

or wouldnt aluminium work?

I don't think so.

AL is no-ferrous, so I believe that is will not TRAP RFI/EMI but: it is a light metal, and the molecular density is too low to sheild. At least that's what I have observed.

WerTicus · « **Reply #2 on:** 4 Dec 2003, 01:55 pm »

okay then i wont waste my time... ill just get some copper foil instead from a craft shoppe i suppose?

IanATC · « **Reply #3 on:** 4 Dec 2003, 06:06 pm »

That is where I got my foil. It was from a UK craft supplier called Fred Aldous.

In metric: 30cm x 30cm x .156

English: at least 7" x 9" x .005 or .006 inch thick

it's cheap! And it works!

WerTicus · « **Reply #4 on:** 5 Dec 2003, 06:12 am »

you know the english use metric too now

its only those crazy americans who have stuck with the imperial system.

lonewolfny42 · « **Reply #5 on:** 5 Dec 2003, 07:53 am »

Quote from: WerTicus

you know the english use metric too now

its only those crazy americans who have stuck with the imperial system.

Careful

or I'll get my ruler out ! !

IanATC · « **Reply #6 on:** 5 Dec 2003, 10:12 am »

According to Frank Van Alstien, the best material to use for shielding is "MU METAL." He said it has a looser molecular structure that is the best at sheilding magnetic/rfi/emi.

BTW: Here in the UK:
English for road distance.
Weights: Pounds and stones. Not to be confused with Pounds currency.
"That cabbage is 1 pound per pound??"
Metric: MM and CM for materials, components drivers etc.

WerTicus · « **Reply #7 on:** 5 Dec 2003, 01:41 pm »

MU metal.. what the hell is that... MU... MU.... sorry that dosnt ring a bell ... i dont believe its an element!

IanATC · « **Reply #8 on:** 5 Dec 2003, 03:17 pm »

Quote from: WerTicus

MU metal.. what the hell is that... MU... MU.... sorry that dosnt ring a bell ... i dont believe its an element!

Mu-metal

Mu-metal is a nickel-iron alloy (77% nickel, 15% iron, plus copper and molybdenum) that is very efficient for screening magnetic fields. It is used to shield vacuum chambers for experiments with low-energy electrons.

The name of the material refers to the Greek letter μ, which is the symbol for magnetic permeability. Mu-metal has a high value of μ.

You are right, it's not an element.

WerTicus · « **Reply #9 on:** 6 Dec 2003, 04:30 am »

okay sweet.. so where might one obtain MU metal sheets then?

_scotty_ · « **Reply #10 on:** 10 Dec 2003, 04:20 am »

Mu-Metal can be ordered from McMaster-Carr at http://www.mcmaster.com/
sorbothane sheets are also available in various thicknesses and durometers

randytsuch · « **Reply #11 on:** 10 Dec 2003, 04:39 am »

Quote from: IanATC

According to Frank Van Alstien, the best material to use for shielding is "MU METAL." He said it has a looser molecular structure that is the best at sheilding magnetic/rfi/emi.

I thought mu metal was supposed to be for magnetic shielding. I had not heard it makes a good EMI/RFI shield material. TI shield (from Michael Percy) claims to do both, but I never tried the stuff myself.

I also would not discount aluminum foil so quickly. I have seen it used at an EMI lab, and it made a huge difference when it was wrapped around a cable. Of course, it needs a low impedence path to ground to work. I have also seen aluminum foil used in shielded cables.

That said, I also think copper would be better, but it's more expensive too.

Does this belong in the beginners circle?

Randy

IanATC · « **Reply #12 on:** 10 Dec 2003, 06:12 am »

Quote from: randytsuch

I thought mu metal was supposed to be for magnetic shielding. I had not heard it makes a good EMI/RFI shield material. TI shield (from Michael Percy) claims to do both, but I never tried the stuff myself....

Could be, I believe it would work because of it's density and copper content.

Quote from: randytsuch

I also would not discount aluminum foil so quickly. I have seen it used at an EMI lab, and it made a huge difference when it was wrapped around a cable. Of course, it needs a low impedence path to ground to work. I have also seen aluminum foil used in shielded cables.
....

Foil does seem to work wrapped around power cables. But I am thinking at this point that it is not the best thing to have as a case lid material. Cu might work better.

And this probably does belong in the lab forum....

WerTicus · « **Reply #13 on:** 10 Dec 2003, 02:22 pm »

I bought a metre square of copper shimmy... its thicker than copper foil...so i checked it out and its still quiet bendable so i should be able to wrap it around stuff....

but this raises the question... WHAT?

My amp does emmit a low level buzzy type hum sound and this is idealy what i want this stuff to eliminate... the sound always remains the same volume level regardless of how loud the stereo is and can 'just' be heard from the listening position (so its pissing me off!)

I think it might be due to the close proximity of the twin 300va torroidal's inbetween the left and right channel circuitry... they come within a couple of mm of the power filter caps so im guessing this is a good place to wedge some shimmy!

i suppose i should wrap it with electrical tape first though? But that wont use much shimmy really.. what would you do with the rest of it?

also do i just need to block line of sight or should i 'wrap' the shimmy around each separate piece?

IanATC · « **Reply #14 on:** 10 Dec 2003, 06:20 pm »

I think the best idea here is to make a sheild around the toroid, between it an any smaller components.

The *BEST* manner is to unscrew the toroid, make sure the copper foil sits under it [yes poke a hole for the mounting screw] and make a sheild around it like a case.

Essentially, it's like a Faraday cage, trapping/blocking the emi that comes off the toroid. The copper below the toroid will help stop the case from bouncing emi across the bottom which is equally bad. The ultimate thing would be to line the bottom , sides and top of the case with copper foil. Couterpoint used to do this.

Just leave a bit of gap between the copper and the toroid itself just so as not to close it it for heat purposes, and ensure whatever shape you bend is rigid enough not to move into something and short it. Also, roll the edges to reduce possibility of vibration.

Good luck...

BTW: just finished a Rotel cd player power transformer sheild. Worked like a champ. This $400 Rotel now sounds like a Naim cd-5 at least.

boxhead · « **Reply #15 on:** 10 Dec 2003, 06:57 pm »

After working in a calibration lab for the last 20 some years the copper shielding is called a faraday shield. It is used with HF and VHF transmitters to block unwarranted RF energy bleeding into the transmitters or recievers. It is very useful for all types of electronics, even blenders that cause the static on video displays.
As for cables the testing is done using inductors to measure the difference of current to frequency response. When we test cables it is usually a good or bad scenario. Of course there are parameters for frequency response but these cables are for use in anything from HF to Microwave applications. I measured basic speaker cable all the way up to the high end cables and the only difference was as more current was applied (2 amps) the frequency response rolled off at around 100KHZ. The best way for a cable to remain coherant is to have proper termination. Soldering the cables with a high quality solder(Nickel-silver) to the binding posts solves a lot of mirophonic problems with vibration.
As far as isolation of electrical circuits we use variacs for a proper voltage or current source to achieve tolerances. They vary in price but you can get them from military auctions for cheap. I would recommend them if you have a lot of voltage variations in your area.
Isolation from vibrations is not what we measure but it would make sense as this would also cause microphonics through circuit boards.
I would open up any consumer electronics and inspect the soldering inside. I had an old Sony surround RCVR and the soldering was abysmal! After I retouched the entire board it actually sounded better. Cold solder joints can be quite annoying.
Also on power transistors we use a material that isolates it from the case to provide a floating of the case used with a plastic insulator to assure no chance of grounding to the case. This material resembles tothpaste and is usually found in electronic supply stores.
All of this is to help any one who would like to improve the sound of their systems.

Now as for tubes there is a whole lot of discussion on the biasing of tube. If you are not afaid of putting it on a bench with the cases opened up then I would suggest get a good grounding mat,wrist strap attached to the mat and a grounding mat for the floor. This will also help with conventional electronics as well. The chips have a very low tolerance to static and even the slightest amount will cause degredation over the life of the chips. Especially those DSP chips. With chip technology going into the 2V range you can imagine rubbing your foot across the carpet and touching a circuit board. I would highly recommend anti-static mats under each piece of equipment grounded to the mats to a known ground.

Hope this helps with the audio group and enjoy. Cheers.

_scotty_ · « **Reply #16 on:** 11 Dec 2003, 03:54 am »

IanATC advice is okay but he neglected to mention that you should under no circumstances have a complete ring of electrically conducting material that passes through the center of the toroid. This constitutes a dead short of the toroidal transformers magnetic field. The transformer will heat up and be permanently damaged. Air gaps or insulation should be left between the mounting bolt and any shielding and insulation should be used between the upper dished plate and any shielding. All you need for a dead short is to electrically connect the upper mounting plate to the chassis.

WerTicus · « **Reply #17 on:** 11 Dec 2003, 06:40 am »

well currently the mounting bolt is earthed to the chassis...

if i were to make a box of copper around the torroidal it would not touch the top of the bolt.

and i would have to cut a hole in the bottom of the box to put the bolt through... so i can make it so it dosnt touch this also....

i could easy make it so the copper box dosnt earth at all? by putting insulation under the terroid area.

from what you said i understand that i just need to avoid having the bolt conact the copper box?

Rob Babcock · « **Reply #18 on:** 11 Dec 2003, 07:16 am »

If I was a newbie reading this thread, I think I'd chuck my stereo and take up model trains or something.

WerTicus · « **Reply #19 on:** 11 Dec 2003, 09:50 am »

yeah a mod should move this to the lab.