UltraPerm sounds like a pain to fabricate. Although there are some drawbacks with UV and temperature constraints, you might like to take a look at this:
Magnetic Field Shielding Materials
"GIRON Magnetic Shielding Film, which does not contain Nickel, is unlike any other magnetic shielding material available on the market today. Suitable for applications requiring high saturation and good permeability, it is both thin and strong, and unlike Mu-metal™ it is very tolerant to bending or shaping without losing is shielding properties. GIRON is a woven, laminated material. Cuts neatly with snips or sheet metal tools, and can be used either flat or molded into shapes for best performance.Works with AC or DC magnetic fields (0-1000 Hz), and will tolerate temperatures from 32° to 122°F.Coating is UV sensitive, cover if installation is exposed to direct sunlight.
* Initial permeability: 500
* Relative permeability: 7000
* Saturation induction: 2,0 T
* Curie Temp.: 740°C (1364°F)
25.5 inch wide, 1 mm thick. Weighs 3.5 Kg/m² (about 15 pounds per 10 foot length). Edges are sharp!
GIRON (Cat. #A273) ............... $37.95 per linear foot "
Magnetic Field Shielding Materials
"GIRON Magnetic Shielding Film, which does not contain Nickel, is unlike any other magnetic shielding material available on the market today. Suitable for applications requiring high saturation and good permeability, it is both thin and strong, and unlike Mu-metal™ it is very tolerant to bending or shaping without losing is shielding properties. GIRON is a woven, laminated material. Cuts neatly with snips or sheet metal tools, and can be used either flat or molded into shapes for best performance.Works with AC or DC magnetic fields (0-1000 Hz), and will tolerate temperatures from 32° to 122°F.Coating is UV sensitive, cover if installation is exposed to direct sunlight.
* Initial permeability: 500
* Relative permeability: 7000
* Saturation induction: 2,0 T
* Curie Temp.: 740°C (1364°F)
25.5 inch wide, 1 mm thick. Weighs 3.5 Kg/m² (about 15 pounds per 10 foot length). Edges are sharp!
GIRON (Cat. #A273) ............... $37.95 per linear foot "
G-iron
I understand this post is quite old, but since I stumbled on it today, I thought I might as well comment and provide information for other users in the future.
The product sold under the name Giron in the States is actually called G-iron Flex and produced by G-iron Srl, an Italian company. It is used to shield the magnetic fields and works best when used in conjunction with an aluminum alloy sheet. It was developed for industrial use (shielding transformers, switchgear, busbars, etc.) but has some other applications. Visit Magnetic Shields, goodiron, underground shielding - G-iron s.r.l. for more information.
I understand this post is quite old, but since I stumbled on it today, I thought I might as well comment and provide information for other users in the future.
The product sold under the name Giron in the States is actually called G-iron Flex and produced by G-iron Srl, an Italian company. It is used to shield the magnetic fields and works best when used in conjunction with an aluminum alloy sheet. It was developed for industrial use (shielding transformers, switchgear, busbars, etc.) but has some other applications. Visit Magnetic Shields, goodiron, underground shielding - G-iron s.r.l. for more information.
I use this material from time to time in projects and can report it works fairly well. I have not compared it directly with ultra-perm or anything else, but it reduced the measured ripple on the output of a power amplifier by more than 12dB just by shielding part of the power transformer near the output transformer. (About 33% coverage - two sides closest to OPT covered)
Just for interest,
How do non magnetic materials become magnetic..
Its standard conduction..ie
If you have a ring of aluminium its the same as a ring of copper..
Think transformer winding.
If you have a shorted turn of aluminium in an AC field it will conduct and with that conduction comes a magnetic field just like a wire.
Just for interest. (Standard Squirrel cage theory)
I guess its back to reading the Beano...
Regards
M. Gregg
How do non magnetic materials become magnetic..
Its standard conduction..ie
If you have a ring of aluminium its the same as a ring of copper..
Think transformer winding.
If you have a shorted turn of aluminium in an AC field it will conduct and with that conduction comes a magnetic field just like a wire.
Just for interest. (Standard Squirrel cage theory)
I guess its back to reading the Beano...
Regards
M. Gregg
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G-iron use their product in conjunction with an aluminum alloy. It is like a hybrid system, made of G-iron Flex and an Al sheet. They get better results that way. you should contact them to know more about the alloy they use.
This product was developed more for industrial shielding (transformers, switchgear, busbars, power lines/cables, etc.) but it has few other applications as well. The bottom line is, it is an excellent product to shield fields ranging from 0 Hz to 10 kHz.
Interesting and still timely thread. Thanks giron, for resurrected it. I've got some 60 cycle bleeding between my PT and OPT right now. Double shielding with both high-mu and high-conductance metal (copper, aluminum) makes sense to me. I'm no expert, but my intuition tells me that the Cu and/or Al, in addition to the modest lo-freq (<1000Hz) attenuation contribution of the eddy-current induced counter-field, will attenuate the higher harmonics (all trannies distort). Giron and the like are good at the lo stuff. Cu/Al handle the hi end.
Using just the Giron is like turning down the bass. The hum is damped, but the harmonic buzz is still there. (Easy to simulate: Bass down, treble up. You'll probably still hear a 60Hz bzzzz in the tweeters). A layer of Cu/Al should help that.
Me, I'm going to try steel first. Soup can or the like. Add another layer and a beer can if that's not enough. Glue cardboard between layers. Rounded is good, I gather. Corners and edges focus and re-emanate magnetic fields. Keep bends and edges well away from what you're trying to shield and close to the source, but not touching. Small holes (>=1/4") don't hurt much, but best to point them away from sensitive things. Gotta let the heat out. And the wires.
Which brings up a question. Since the captured mag field is most intense at the edges of the shield, is poking the wires through a hole a bad idea? Better to run them out the center of the shield tube, away from any edge?
Using just the Giron is like turning down the bass. The hum is damped, but the harmonic buzz is still there. (Easy to simulate: Bass down, treble up. You'll probably still hear a 60Hz bzzzz in the tweeters). A layer of Cu/Al should help that.
Me, I'm going to try steel first. Soup can or the like. Add another layer and a beer can if that's not enough. Glue cardboard between layers. Rounded is good, I gather. Corners and edges focus and re-emanate magnetic fields. Keep bends and edges well away from what you're trying to shield and close to the source, but not touching. Small holes (>=1/4") don't hurt much, but best to point them away from sensitive things. Gotta let the heat out. And the wires.
Which brings up a question. Since the captured mag field is most intense at the edges of the shield, is poking the wires through a hole a bad idea? Better to run them out the center of the shield tube, away from any edge?
Not sure how you arrived at that. One useful place for a conductive shield is directly around the core.. a 'flux band'.
EMI screening and shielding
The thing that most DIY amp / preamp builders are trying to "screen out" of their audio circuit is some component of 60 Hz EMI. This either comes from the ubiquitous 60 Hz EM field all around modern dwelling areas, or more directly from a power transformer in or near the audio circuits.
If you plug an RCA cable into a amplifier's input and touch the center pin of the unconnected end with your finger, you get a strong 60 Hz hum in the output of the device. Here, your body is picking up mostly the electrical field component of the EMI and feeding it to the amplifier. This will have a lot of "buzz" to it , as higher harmonics of the 60 Hz field will be present along with fundamental and lower overtones.
So, signal wires like phono cables, single-ended line-level interconnects and the like have an ELECTROSTATIC shield - copper braid or a plastic foil with a thin aluminum coating- around the signal-carrying center conductor. This shield is connected to either ground or the "low" side of the input circuit. Voltage induced by the electric field in the area- mostly 60 Hz but some other frequencies too, especially if you are near a radio station- is carried to ground or "low" rather than getting into the signal line and getting amplified.
If you hear more of a "zzzzzz" than a "hummmm" this is likely the result of electrical field "contamination" of your signal line, many higher harmonics are present because this unwanted signal is coupled into your audio by the equivalent of a capacitor, acting as a high-pass filter. (This sort of noise contamination can also "ride the ground" if you have some kind of capacitance path to the power line in one of your connected audio components- this is sometimes called a GROUND LOOP.)
If, however, you hear mostly lower frequencies - a HUMMMMMM - then you have an inductively coupled 60 Hz contaminant, meaning the magnetic field of some EMI source is inducing a noise current someplace in your signal chain. This type of noise ingress is essentially a low pass filter.
If you have BOTH hummmm and buzzzzz then your signal is getting fundamental and harmonics, you could have TWO or more sources of noise, one capacitively one inductively coupled; or you may have a resistively coupled problem someplace....
The strength of the general 60 HZ EM field is pretty weak, you do not hear 60 Hz hum induced into speakers, for example. But at the other end of the audio chain, a high-impedance input followed by gain is pretty susceptible to picking up noise. ANY place there is gain can be a source of noise.
I am ignoring any ripple on the DC supplying the audio circuits here.... but if you have noise issues, don't forget to check that too!
Look at an audio output transformer in a tube amp, if it's next to the power transformer 60 Hz and lower harmonics can get coupled in to the output transformer- it would not result (usually) in enough voltage and current to drive the speaker with a 60 Hz hum, but the output stage likely uses feedback - and this noise will get coupled into the amp's feedback loop, where there is gain, and now your have amplified noise being output to the speaker.
Capacitively coupled 60 Hz EMI - the electrical field component of the 60 Hz EM field- is pretty easy to get rid of - any good electrical conductor can shield your input stages against this. (This gets harder to do at RF frequencies, where the path to ground can appear as a fairly high impedance to the RF, and so the RFI won't 'flow' to ground....)
Inductively coupled 60 Hz hum is harder to fight. This is the magnetic field component of the EM field. It falls off as the cube of distance, and so it is strongest around the power transformer. Different kinds of transformers have different 60 Hz magnetic leakage field characteristics - I think traditional H- and L-core transformers have the most, followed by toroids, and then R-core, if I remember correctly. So choice of transformer can make a difference here.
Physical placement also helps, which is one of the reasons to put your power supply and audio circuits in separate chassis. (Aside from magnetic field EMI, the other reason is so that you can safely separate your audio signal ground from the safety earth ground of the AC power plug, which often has lots of noise on it, especially the higher harmonics.)
Some circuits are more susceptible to "contamination" by the 60 Hz magnetic field around a power transformer than others. An example is the Beta 22 headphone amplifier by AMB. An excellent design, but nonetheless it seems much more sensitive to the field around a power transformer than other circuits. I have not figured out why - but generally you see these headphone amplifiers as a two-chassis build.
Placing good electrical conductors completely around the amplifier's circuits or around a source of 60 Hz electrical-field EMI, is the strategy to use to avoid 60 Hz capacitively coupled noise. This is fairly easy to do.
Providing shielding against the AC magnetic fields that can inductively couple to your audio circuit is trickier. Distance is often the easiest - separate the input circuit from the power transformer. Next comes various things like low-carbon steel, etc. You need some experimentation to find ways to reduce this interference, but sometimes no practical amount and placement of sheet iron can be found, either to shield the circuit picking up the field, or the transformer, source of the field.
Here's what Piltron Transformer Company says about lower noise from toroid transformers => Toroidal Advantages | Plitron Manufacturing
Traftor has some nice graphics of magnetic leakage => Toroidal inductors and transformers advantage: leakage flux - Traftor
You can use an old cassette machine tape head or even a simple hand wound coil, connected to the input of a small amplifier or to your oscilloscope as a "probe" to help you locate magnetic fields that may be causing trouble for you.
The thing that most DIY amp / preamp builders are trying to "screen out" of their audio circuit is some component of 60 Hz EMI. This either comes from the ubiquitous 60 Hz EM field all around modern dwelling areas, or more directly from a power transformer in or near the audio circuits.
If you plug an RCA cable into a amplifier's input and touch the center pin of the unconnected end with your finger, you get a strong 60 Hz hum in the output of the device. Here, your body is picking up mostly the electrical field component of the EMI and feeding it to the amplifier. This will have a lot of "buzz" to it , as higher harmonics of the 60 Hz field will be present along with fundamental and lower overtones.
So, signal wires like phono cables, single-ended line-level interconnects and the like have an ELECTROSTATIC shield - copper braid or a plastic foil with a thin aluminum coating- around the signal-carrying center conductor. This shield is connected to either ground or the "low" side of the input circuit. Voltage induced by the electric field in the area- mostly 60 Hz but some other frequencies too, especially if you are near a radio station- is carried to ground or "low" rather than getting into the signal line and getting amplified.
If you hear more of a "zzzzzz" than a "hummmm" this is likely the result of electrical field "contamination" of your signal line, many higher harmonics are present because this unwanted signal is coupled into your audio by the equivalent of a capacitor, acting as a high-pass filter. (This sort of noise contamination can also "ride the ground" if you have some kind of capacitance path to the power line in one of your connected audio components- this is sometimes called a GROUND LOOP.)
If, however, you hear mostly lower frequencies - a HUMMMMMM - then you have an inductively coupled 60 Hz contaminant, meaning the magnetic field of some EMI source is inducing a noise current someplace in your signal chain. This type of noise ingress is essentially a low pass filter.
If you have BOTH hummmm and buzzzzz then your signal is getting fundamental and harmonics, you could have TWO or more sources of noise, one capacitively one inductively coupled; or you may have a resistively coupled problem someplace....
The strength of the general 60 HZ EM field is pretty weak, you do not hear 60 Hz hum induced into speakers, for example. But at the other end of the audio chain, a high-impedance input followed by gain is pretty susceptible to picking up noise. ANY place there is gain can be a source of noise.
I am ignoring any ripple on the DC supplying the audio circuits here.... but if you have noise issues, don't forget to check that too!
Look at an audio output transformer in a tube amp, if it's next to the power transformer 60 Hz and lower harmonics can get coupled in to the output transformer- it would not result (usually) in enough voltage and current to drive the speaker with a 60 Hz hum, but the output stage likely uses feedback - and this noise will get coupled into the amp's feedback loop, where there is gain, and now your have amplified noise being output to the speaker.
Capacitively coupled 60 Hz EMI - the electrical field component of the 60 Hz EM field- is pretty easy to get rid of - any good electrical conductor can shield your input stages against this. (This gets harder to do at RF frequencies, where the path to ground can appear as a fairly high impedance to the RF, and so the RFI won't 'flow' to ground....)
Inductively coupled 60 Hz hum is harder to fight. This is the magnetic field component of the EM field. It falls off as the cube of distance, and so it is strongest around the power transformer. Different kinds of transformers have different 60 Hz magnetic leakage field characteristics - I think traditional H- and L-core transformers have the most, followed by toroids, and then R-core, if I remember correctly. So choice of transformer can make a difference here.
Physical placement also helps, which is one of the reasons to put your power supply and audio circuits in separate chassis. (Aside from magnetic field EMI, the other reason is so that you can safely separate your audio signal ground from the safety earth ground of the AC power plug, which often has lots of noise on it, especially the higher harmonics.)
Some circuits are more susceptible to "contamination" by the 60 Hz magnetic field around a power transformer than others. An example is the Beta 22 headphone amplifier by AMB. An excellent design, but nonetheless it seems much more sensitive to the field around a power transformer than other circuits. I have not figured out why - but generally you see these headphone amplifiers as a two-chassis build.
Placing good electrical conductors completely around the amplifier's circuits or around a source of 60 Hz electrical-field EMI, is the strategy to use to avoid 60 Hz capacitively coupled noise. This is fairly easy to do.
Providing shielding against the AC magnetic fields that can inductively couple to your audio circuit is trickier. Distance is often the easiest - separate the input circuit from the power transformer. Next comes various things like low-carbon steel, etc. You need some experimentation to find ways to reduce this interference, but sometimes no practical amount and placement of sheet iron can be found, either to shield the circuit picking up the field, or the transformer, source of the field.
Here's what Piltron Transformer Company says about lower noise from toroid transformers => Toroidal Advantages | Plitron Manufacturing
Traftor has some nice graphics of magnetic leakage => Toroidal inductors and transformers advantage: leakage flux - Traftor
You can use an old cassette machine tape head or even a simple hand wound coil, connected to the input of a small amplifier or to your oscilloscope as a "probe" to help you locate magnetic fields that may be causing trouble for you.
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Aluminum Tape
Hey check this out. Cheap and easy to use.
https://www.walmart.com/ip/Duck-Brand-HVAC-Aluminum-Foil-Repair-Tape-1-88-x-50-yds/17300797
Hey check this out. Cheap and easy to use.
https://www.walmart.com/ip/Duck-Brand-HVAC-Aluminum-Foil-Repair-Tape-1-88-x-50-yds/17300797
The higher the magnetic permeability of the material, the better job it will do to shield against penetration of magnetic fields. In a sense, permeability is a magnetic property of a material that is analogous to conductivity of materials in terms of electric current. The higher the permeability of a material, the more it will "contain" the magnetic penetration. This is true for any magnetic field, either DC or AC in origin.
Mild steel is very good, it has a permeability around 4000- while high-carbon Swedish steel has a permeability around 100- such high-carbon steel is good for tools and blades, but not good for magnetic shielding. Specialized materials have been developed with MUCH higher shielding abilities, such as Mu-Metal which has a permeability around 20,000 to 100,000 and amorphous metal glasses (Metglass) which has a permeability around 1,000,000. Metglass is the standard for magnetic shielding against which all other materials may be judged- but it is very expensive. See the Permeability of Materials table at this link => Permeability (electromagnetism - Wikipedia) From that table you can see that mild steel is 4,000 times better than aluminum in terms of shielding against magnetic fields. Aluminum is really not effective in terms of magnetic shielding at all.
That said, for audio frequencies, ordinary mild steel is probably the best material to use as it is cheap, widely available and easily worked.
Mu-Metal is also sometimes used in electronics - some lab-grade oscilloscopes employ it to shield their CRTs from stray fields, allowing for more accurate measurements. When using Mu-Metal, however, it is important to anneal the material at about 1150°c in pure dry hydrogen after it has been worked or bent, then allowing it to slowly cool at a rate of about 5°C per minute. This is not really practical for DIY audio builders unless you have access to elaborate facilities. Failing to re-anneal after working or bending Mu-Metal causes it to lose a fair amount of it's very high permeability.
Metglass is available in sheets, foils, tape and films. It is rather brittle and so cannot be bent beyond a certain minimum radius without breaking.
Mu-Metal and Metglass are often available on eBay if you wish to experiment with them. Flat sheets of Mu-Metal can easily be employed as long as they are not bent, hammered, etc. Sometimes you can find pre-formed cans and other shapes of Mu-Metal that might suit a project you are working on.
Various special considerations have to be taken when trying to shield against AC magnetic fields at microwave frequencies, but these factors need not trouble people working on audio equipment.
Mild steel is very good, it has a permeability around 4000- while high-carbon Swedish steel has a permeability around 100- such high-carbon steel is good for tools and blades, but not good for magnetic shielding. Specialized materials have been developed with MUCH higher shielding abilities, such as Mu-Metal which has a permeability around 20,000 to 100,000 and amorphous metal glasses (Metglass) which has a permeability around 1,000,000. Metglass is the standard for magnetic shielding against which all other materials may be judged- but it is very expensive. See the Permeability of Materials table at this link => Permeability (electromagnetism - Wikipedia) From that table you can see that mild steel is 4,000 times better than aluminum in terms of shielding against magnetic fields. Aluminum is really not effective in terms of magnetic shielding at all.
That said, for audio frequencies, ordinary mild steel is probably the best material to use as it is cheap, widely available and easily worked.
Mu-Metal is also sometimes used in electronics - some lab-grade oscilloscopes employ it to shield their CRTs from stray fields, allowing for more accurate measurements. When using Mu-Metal, however, it is important to anneal the material at about 1150°c in pure dry hydrogen after it has been worked or bent, then allowing it to slowly cool at a rate of about 5°C per minute. This is not really practical for DIY audio builders unless you have access to elaborate facilities. Failing to re-anneal after working or bending Mu-Metal causes it to lose a fair amount of it's very high permeability.
Metglass is available in sheets, foils, tape and films. It is rather brittle and so cannot be bent beyond a certain minimum radius without breaking.
Mu-Metal and Metglass are often available on eBay if you wish to experiment with them. Flat sheets of Mu-Metal can easily be employed as long as they are not bent, hammered, etc. Sometimes you can find pre-formed cans and other shapes of Mu-Metal that might suit a project you are working on.
Various special considerations have to be taken when trying to shield against AC magnetic fields at microwave frequencies, but these factors need not trouble people working on audio equipment.
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That's incorrect. If that were true, than ANY conductor would shield against AC magnetic fields, and Techtronix and Hewlett-Packard would have used aluminum to shield their lab 'scopes instead of mu-metal which costs fifty times more.
If you don't believe me, try this test: make a coil of wire - any wire - 20 turns or so a couple inches in diameter, and connect the ends of the coil to an oscilloscope. Now, place the coil next to an operating 60 Hz power line transformer of any kind - a high-intensity desk lamp's base, or any wall-wart power adapter that outputs AC voltage will do - On the 'scope you'll see the sine wave indicating that the coil is picking up the AC magnetic field of the transformer. Now place an aluminum sheet between the coil and the transformer, you'll note on the 'scope there's less than 0.1 dB of attenuation of the signal magnetically induced in the coil. Now do the same experiment with a piece of mild steel and you'll see quite a bit more attenuation- I saw more than 3 dB. That's more than an order of magnitude more than the effect of the aluminum, which is barely noticeable.
Aluminum makes a good shield against electric fields / capacitively coupled noise, but is mostly useless in terms of shielding against magnetic fields / inductively coupled noise.
2 questions:
1. When a mild steel "flat" sheet between 2 coils attenuate 3bB, how much attenuation is expected to see when whole coils are wrapped with the same mild steel?
2. To achieve -3dB attenuation without any shield object, distance between coils should be doubled? How can we calculate it?
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It is true of ANY conductor. Obviously not as effective as mu-metal or something similar.
This is from a site that doesn't have the page anymore:
"Lossy magnetic shielding depends on the eddy-current losses that occur within highly conductive materials (i.e., copper, aluminum, iron, steel, silicon-iron, etc.). When a conductive material is subjected to a time-varying (60 hertz) magnetic field, currents are induced within the material that flow in closed circular paths - perpendicular to the inducing field. According to Lenz's Law, these eddy-currents oppose the changes in the inducing field, so the magnetic fields produced by the circulating eddy- currents attempt to cancel the larger external inducing magnetic fields near the conductive surface, thereby generating a shielding effect."
Here John Swenson talks about it as well: Hammond Chokes w/ Quickie - Cheap and Worthwhile!
"You can actually do magnetic shielding with aluminum! Thick aluminum (say 1/4" to 1/2" thick) has a high enough conductivity that external fields induce eddy currents which generate their own fields which counteract the original field. You can get 5-6db or so at 120Hz with 1/4" thick aluminum. It actually gets more effective as the frequency goes up."
So i'd be curious if you use an 1/4 inch thick aluminium plate in your experiments....
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