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PostPosted: Sun Mar 11, 2007 3:17 am 
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So guys, what's your take on this one?
Is it worth the trouble to build a box around a power transformer(toroid, R-core etc.)?

What would be the best material to use for such job? Copper?

I saw Audio Note uses entire copper chassis(see here for instance) for it's good screening properties. Again, is it worth the trouble? I didn't see any other company have their product's chassis built this way, hence the question.


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PostPosted: Sun Mar 11, 2007 8:30 am 
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Iron. Nickel (mu-metal, permalloy, etc). Something high-permeability, and make it thick.

Otherwise, there's little you can do to shield against low frequency magnetic fields short of building some kind of servo-controlled Helmholtz coil system (let's not go there) or using superconductors. Regular conductive materials like copper and aluminum do almost nothing at low frequencies - these are however useful for RF shielding (note that at RF, material thickness is not important due to skin effect).

Your best strategies are:

1) Minimize loop areas - applies to any circuit which may pick up or radiate magnetic fields. Basically this means tight, small circuit geometries, balancing send and return currents (controlling return current paths), physically symmetrical structures etc. Interference receivers tend to be sensitive circuits and ground loops. Emitters tend to be power wiring (including the path through rectifiers and filter caps) and power transformers. Choose trafos with low radiation characteristics such as toroidal or R-core. Some "shielded" toroids come with extra laminations wrapped around the outside of the windings to soak up some of the leakage flux.

2) Employ physical separation between sources and victims. The strength of the fields falls off by the square (or is it the cube? - forget which, off the top of my head) of the distance from the source. Parking your phono preamp chassis directly on top of it's power supply box may actually be worse than if you put the transformer in the same box. Neither sounds like a great idea to me. For this reason, wall-wart power supplies sometimes work better than expected, despite the bad rep they get (OT: I think it mostly has to do with snobbery, and a little to do with low quality or noisy switch-mode ones giving the rest a bad name).

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PostPosted: Sun Mar 11, 2007 3:48 pm 
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Actually, aluminum can be quite effective at 50-60 Hz. 0.25 inch material can give a factor of 5 to 10 shielding. The shielding will be even better as the frequency of the radiation goes up. At the same time, permalloy may not work as well due to saturation, all depends how strong the leakage field is. A leakage field of 5 Oersteds can saturate good permalloy. Thick aluminium (over 0.25 inch) would be a good start and is not a as complicated as shielding with high permeability (nickel-iron alloys) materials.


Bill


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PostPosted: Sun Mar 11, 2007 6:42 pm 
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Hi,
I had been led to believe that any conductive material could be used for electric field screening, but that magnetic field screening could only be achieved with magnetic materials. Copper and aluminium tend to work very well due to low resistivity.

The transformer field that causes most bother is the magnetic field and I would expect a thick iron, or steel, shield to be effective.

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PostPosted: Sun Mar 11, 2007 6:45 pm 
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Wow, I didn't realize aluminum (permeability 1) would be effective at such low frequencies. I remember using copper against magnetic fields at 15kHz, but...

Anyway, the only thing I can add here is one more variable: orientation. Sometimes you can find a null in the external field in a particular direction. Rotate the tranny such that the null (or minima) faces the circuitry in question.

jh


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PostPosted: Sun Mar 11, 2007 10:09 pm 
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Hi Andrew / Jim,

Here's one -

http://www.lessemf.com/mag-shld.html

also the full bottle on magnetics is (member) Raven whose work takes him there.

- you canna change the laws of Physics, Jim!

(Scotty -Star Trekkin across the Universe).
:mrgreen:
Greg


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PostPosted: Mon Mar 12, 2007 12:44 am 
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Yes, it is surprising how effective thick aluminum can be as an eddy current shield at low frequencies and the lower the resistivity the better the shield. 0.25" of Al can give a factor of 5 to 10 shielding.

I think we often look for the best material rather than an effective material. Aluminium can be quite effective, but a well designed shield using magnetic material would give better shielding. Both eddy current shields and feromagnetic shields benefit from thickness. Eddy current shields benfit due to the skin depth and feromagnetic shields benefit by resistance to saturation.

Steel would be a good choice as it is a moderate permeability with high saturation magnetization so it will not saturate easily. Steel is better than using aluminium, however it will become permanently magnetized. Be careful if you use permalloy as it has several problems. It's high permeability can result in it saturating easily and bending, drilling or punching holes in it will result in very low local permeability.

Aluminum is particularly attractive for torroids as many torroids radiate a lot of junk. Make a wire loop, or small coil, connect it to your scope and look at the stuff that comes from many torroids (not a problem with all toroids, but often is).

Bill


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PostPosted: Mon Mar 12, 2007 1:12 am 
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Raven
Stupid question;
Where does nickel come in here since nickel is used in some audio transformers.


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PostPosted: Mon Mar 12, 2007 2:19 am 
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Mikeg,

Nickel-Iron alloy makes a soft (not easy to leave magnetized) material that has a high permeability. The B vs. H curve can be highly linear (audio folk like linear, right). Magnetic properties are a strong function of the specific alloy with big changes for a few percentage points change in the Ni/Fe ratio. Permalloy is usually considered to be 80% nickel and 20% Iron.

Hi permeability and a linear B-H curve are desireable for a linear transformer (but not the only considerations). Hi permeability is also desireable for a good feromagnetic shield.

Generally, magnetic field work is a bit more complex then electric field work. If only we had magnetic mono-poles.

Hope this helps,

Bill


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PostPosted: Mon Mar 12, 2007 2:26 am 
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Bill, you didn't say anything about copper... Is it because it's not good for the job or because there are cheaper things that do the same or are even better?


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PostPosted: Mon Mar 12, 2007 3:17 am 
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Bill,

Very interesting information - glad to hear from someone in the field (ohh, bad pun... sorry, i couldn't resist ahh-hahaha... oh, man). :lol: :mrgreen:

OK, seriously though... it's true what you say about searching for the best solution vs. an effective one, and every situation will have different constraints.

Admittedly, I have not personally made quantitative measurements of LF magnetic shielding before, I just try to apply what theory I've learned as best I can and hope that by following best practice and with attention to detail, I can avoid problems with what I build at home. It may be difficult or impractical to perform experimentation, especially since my DIY projects are mostly 1-off, and so there really isn't a good opportunity to make major changes once the thing is built (only tweaking, unless the problem is bad enough to warrant starting from scratch!). Finding a balance between overkill vs. 'sufficient' / cost effective design measures is definitely difficult for the hobbyist. I tend to err on the side of overkill, so long as it doesn't cost a lot.

At work, I try to apply best practice design where possible, but there are other people who pay attention to EMI for me, and I generally only have time to focus my attention on the circuit functionality of interest anyway. We have the luxury of several prototype stages to work with, plus fully equipped facilities and budget for all the toys and tweaks you could ask for.

To anyone interested in really learning the nitty-gritty of circuit grounding and shielding, I highly recommend Henry Ott's "Noise Reduction Techniques in Electronic Systems". It's been over a year since I last looked at this book, and I'm once again impressed at the quantity of good information packed in here... I've got the 2nd Ed. open in front of me, looking at an interesting graph (page 185 for those who have the book). Since it's copyright, I won't post a scan, but in essence, the graph is a comparison of near-field (0.1inch separation) shielding effectiveness (magnetic attenuation only) of various materials at different frequencies. At 1kHz, copper and aluminum sheet 40mil thick show about 5 and 3dB attenuation, respectively, while 40mil steel shows 15 or 16dB, and 30mil mu-metal around 18dB. At 100kHz, steel is only slightly better than copper. This data doesn't show to the difference at 60Hz, but one can extrapolate... at 50/60Hz, the difference is bound to be larger than 10dB. But it's still good to know you can get 15 or 20dB out of 1/4" Al if that's what you happen to have on hand (I know a lot of hobbyists use Al plate stock for chassis construction). Ott goes on to say later in the chapter that "Actual shielding effectiveness obtained in practice is usually determined by the leakage at seams and joints, not by the shielding effectiveness of the material itself". If using conductive material, seams should have as much overlap as possible, and should support good conduction across the joint. Therefore, bare aluminum will do better than anodized.

I guess the point I was trying to make earlier was that when it comes to dealing with low-frequency magnetic fields in an audio system, I would first turn to some basic design principles before I would consider shielding materials. Shielding would be icing on the cake, but maybe not so necessary if the other pieces are done right. Also consider if you need to shield the entire power trafo, or just install a small shield around a particularly sensitive circuit.

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PostPosted: Mon Mar 12, 2007 12:03 pm 
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hifizen wrote:
Parking your phono preamp chassis directly on top of it's power supply box may actually be worse than if you put the transformer in the same box. Neither sounds like a great idea to me. For this reason, wall-wart power supplies sometimes work better than expected...



Indeed.
It is also good idea to leave the rectifier diodes out of the Phono pre. So, the external PSU must be AC/DC.
A practical example of a bad design is the Rega Fono phono preamp.
The external PSU is AC, the rectifier diodes are inside the phono pre. The enclosure is small, and there's a full copper shield PCB on the bottom.
It happens that if you remove this full copper shield PCB you end up massively reducing the (audible) noise. Because it spreads the diode noise to all over the circuit.
A better option then, is removing the diodes from inside the preamp and using a DC PSU.
All in all, a stupid design from a company that has tradition in turntables and arms.

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PostPosted: Mon Mar 12, 2007 12:43 pm 
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roiibm,

Copper is better than aluminum due to it's lower resistivity.

As always, abstinence is more effective than shielding.

If you choose to use eddy current shielding, remember that it shields against fields perpendicular to the surface, and shields only behind it. If the field is at 45 degrees, the shielding drops off by root 2.

Using steel for shielding is far more complex. Feromagnetic shields only works against fields that are parallel to it, fields normal to a feromagnetic shield will only get eddy current shielding (not the greatest in iron) When using feromagnetic shielding remember that flux will concentrate at all discontinuities, like corners and edges.

I usually shield my transformers, particularly torroids, with at least 0.25 inch aluminium. The torroids radiate much higher frequencies than the AC line frequency ..... all that diode hash also turns to magnetic hash in the transformer core. Now we are looking at much higher frequencies than 50-60 Hertz and aluminium (or copper) plate can be quite effective.

Bill


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PostPosted: Mon Mar 12, 2007 7:22 pm 
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Hi Bill,

Quote:
I usually shield my transformers, particularly torroids, with at least 0.25 inch aluminium. The torroids radiate much higher frequencies than the AC line frequency ..... all that diode hash also turns to magnetic hash in the transformer core. Now we are looking at much higher frequencies than 50-60 Hertz and aluminium (or copper) plate can be quite effective.


For sure, the familiar diode spike 'buzz', is a FS of harmonics starting at maybe 1KHz (~1/10 th of 1/2 cycle charging) right into peak hearing acuity at 3Khz. By comparison 50-60 Hz is innocuous due to Fletcher-Munson effect, unless it's bloody loud!

Greg


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PostPosted: Tue Mar 13, 2007 7:38 am 
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I finally had to dig up a 20 year old text. This 5 to 10 reduction in 60Hz magnetic field by 1/4" aluminum just seemed too good to be true. Well, shux, I guess it is (if you meant dB). Shielding consists mainly of reflection and absorption. In this case, almost none of the field is reflected. It all gets burned off as heat inside the metal.

The empirical formula (as given by Vasaka, 1954) is:

A = 3.34 * t * sqrt(f * u * G); // dB

For aluminum, permeability (u) is 1.0, conductivity (G) is 0.61, both relative to copper. Thickness (t) is in inches. Frequency (f) in Hz. So for a 1/4" aluminum plate, we get 5dB absorption at 60Hz.

jh


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