Haven't you heard, John? According to Banquer and Khomenko, neither Ott nor Morrison know what they're talking about.
se
If Otto and Morrison say you can shield a magnetic field with copper or aluminum......they're right.
I had a Jacco moment Friday when I found several hundred Teledyne relays new in boxes rejected for lead free compliance. You're always showing off your stash of FET's.
Soundminded,
Reading your various posts it appears to me you are correct WRT
electrostatic and magnetic shielding properties of materials but there is
some misunderstanding WRT how these mechanisms play out *inside* an
enclosure.
Using a very thick aluminium enclosure constitutes an extremely low
impedance short circuit to any currents induced by external magnetic fields.
So the enclosure itself will act, for example like a) the transformer shorting
strap or b) the speaker driver pole piece shorting sleeve which oppose
magnetic field modulation inside the 'shorted turn' so to speak.
It's a totally different scenario to say a large open ended shielding plate
with on one side, a source of magnetic interference (transformer) and
on the other some sensitive electronics.
cheers
Terry
The eddy current losses in non ferrous materials are almost always negligable. It is difficult to contrive a situation where it isn't. The current travels a circular path transverse to the length of the sheet. This is why tranformer lamina are made thin and are insulated from each other, to minimize eddy current losses. The efficiency of conversion of magnetic flux into eddy currents is negligable in most cases. Even in the worst case of a standard efficiency transformer, a standard 75 KVA 480 volt delta primary/120-208 volt wye secondary for example, when all losses are included, eddy current, hysterisis, I squared R heating, and imperfect mutual transconductance coupling between the primary and secondary windings, the total losses are typically 8% or less. Only a small fraction of that is due to eddy current losses. One difficulty in generating eddy current losses in copper or aluminum is that their superior electrical conductivity may make it difficult to generate a sufficient voltage potential between two internal points to drive much current. Steel is not as good a conductor but it is satisfactory for an electrical ground and will produce some eddy currents. Hysteresis losses in copper and aluminum don't exist. You cannot induce a magnetic field in them no matter how strong the coercive force is.
I'm puzzled as to why this is even a topic of discussion, it seems pointless.
The good news is that there are very few areas in audio equipment that require magnetic shielding. About the only ones I can think of are magnetic phono cartridge bodies (near motors), tape heads, dynamic microphones, power transformers, and woofer magnets near CRT picture tubes. Invariably, other than these, induced hum and noise is the result of electrical fields including RF fields, not magnetic fields.
When the hot air dissipates, we can move on to other methods of shielding that are both cheap and relatively effective. How about a plastic case with a conductive coating applied? It works, mostly. Knowing the tradeoffs, and why some people don't find heavy shielding important can be discussed.
Eddy currents.
Whenever a time varying magnetic field impinges on a conductive material such as aluminum or copper, it induces eddy currents in the material. Because of the resistance of the material, the energy that went into producing the eddy currents is dissipated as heat. Which means there's less energy that gets through the material.
The shielding becomes more effective the thicker it is and/or the higher the frequency.
It's basically the same phenomenon that's responsible for skin effect in signal cables.
se
The attenuation in dB is approximately the thickness times the square root of omega (2 pi f) times the material's conductivity times the material's permeability.
se
To go on, there are two major kinds of airborne interference. One is called ELECTROSTATIC. The other is called ELECTROMAGNETIC.
What we have been debating is how much low frequency ELECTROMAGNETIC attenuation with a RELATIVELY THICK sheet of aluminum. For the Blowtorch, the original topic of this thread, the calculated attenuation is about 9dB, and the Blowtorch weighs a lot, and the cases ALONE, cost us $3,000 per set of 2 to have fabricated. The new Constellation Orion weighs about 3 times more, so it should have even better protection. Why do we go to such lengths for so little return?
Because higher frequency airborne interference is much more greatly attenuated than 60Hz, and we don't want ANY of the external interference in our preamps. The case is better damped too, and provides a wonderful heatsink that we can mount power devices directly onto, because it is so thick, we can drill and thread holes directly to the case.
Now away from what my associates and I provide to our customers, what about the rest of you? Who, like me, would have to be GIVEN one of these units, as it costs as much as an automobile. I have one, because I designed the electronics, and that is part of the original agreement.
Is it really necessary? Yes, and no, depending on the performance that you want.
For example, if you use a Shure cartridge, like Soundmind, you don't need this kind of shielding. However, if you use an early Ortofon cartridge, you need it. You see, it just depends on your other choices, and what you want to live with.
I used to use a moving magnet cartridge, including the Shure for some years, but I went to early Ortofons, 45 years ago, and never looked back. For several years, I lived with the Ortofon transformer, later the Levinson JC-1, then the JC-2, then the JC-80, and finally, the Vendetta Research. My MC cartridges changed too, and anything below $1000, to me, is a real compromise. Hey, I'm in the business, and I get to hear the good stuff, and then lust for it. Joachim is the only person here that I am sure, knows what I am talking about. The rest of you are out of luck, so to speak, and should stay with the 2 buck chuck equivalent in wine, for your hi fi needs. If I were a wine drinker, I certainly would.
Still, that does not mean that good engineering design must necessarily be expensive, just realize that ultimate performance is hard and expensive to obtain. It is the same with telescopes, cameras, autos, etc.
I hope to talk about a 2'nd generation of phono stage, next. A two stage design, rather than single stage as described previously, that will give us a little more freedom to use different phono cartridges, and perhaps (you can bet on it) sound somewhat better, overall, except in a double blind test, where it will sound the same, as always. We will not let the parameters deviate enough to be easily detectable in ABX testing. Let's try to keep on topic, if possible.
What we have been debating is how much low frequency ELECTROMAGNETIC attenuation with a RELATIVELY THICK sheet of aluminum. For the Blowtorch, the original topic of this thread, the calculated attenuation is about 9dB, and the Blowtorch weighs a lot, and the cases ALONE, cost us $3,000 per set of 2 to have fabricated. The new Constellation Orion weighs about 3 times more, so it should have even better protection. Why do we go to such lengths for so little return?
Because higher frequency airborne interference is much more greatly attenuated than 60Hz, and we don't want ANY of the external interference in our preamps. The case is better damped too, and provides a wonderful heatsink that we can mount power devices directly onto, because it is so thick, we can drill and thread holes directly to the case.
Now away from what my associates and I provide to our customers, what about the rest of you? Who, like me, would have to be GIVEN one of these units, as it costs as much as an automobile. I have one, because I designed the electronics, and that is part of the original agreement.
Is it really necessary? Yes, and no, depending on the performance that you want.
For example, if you use a Shure cartridge, like Soundmind, you don't need this kind of shielding. However, if you use an early Ortofon cartridge, you need it. You see, it just depends on your other choices, and what you want to live with.
I used to use a moving magnet cartridge, including the Shure for some years, but I went to early Ortofons, 45 years ago, and never looked back. For several years, I lived with the Ortofon transformer, later the Levinson JC-1, then the JC-2, then the JC-80, and finally, the Vendetta Research. My MC cartridges changed too, and anything below $1000, to me, is a real compromise. Hey, I'm in the business, and I get to hear the good stuff, and then lust for it. Joachim is the only person here that I am sure, knows what I am talking about. The rest of you are out of luck, so to speak, and should stay with the 2 buck chuck equivalent in wine, for your hi fi needs. If I were a wine drinker, I certainly would.
Still, that does not mean that good engineering design must necessarily be expensive, just realize that ultimate performance is hard and expensive to obtain. It is the same with telescopes, cameras, autos, etc.
I hope to talk about a 2'nd generation of phono stage, next. A two stage design, rather than single stage as described previously, that will give us a little more freedom to use different phono cartridges, and perhaps (you can bet on it) sound somewhat better, overall, except in a double blind test, where it will sound the same, as always. We will not let the parameters deviate enough to be easily detectable in ABX testing. Let's try to keep on topic, if possible.
These questions are boring. In case you do not know, why do not you google? It was explained in this thread probably hundred times, why do you ask again?? You may check the following links:
http://biblio.ugent.be/input/download?func=downloadFile&fileOId=578266
http://147.162.218.5/files/Lecture_36.pdf
Skin effect - Wikipedia, the free encyclopedia
Ever onward, for most practical situations, where impedances are not very low, electrostatic shielding is all that is really necessary, and it can be just a piece of aluminum foil that is connected to the ground of the preamp. This is OK with high level sources and impedances over 377 ohms (more or less) and is the usual sort of shielding that is used. Even coating the inside of a plastic box with special conductive paint can give this sort of protection.
sawreyrw try here Magnetic Shield Designs
Maybe pertinent EMI/RFI Shield Enclosure for EEG Laboratory at a University
On 9 February 2002, VitaTech Engineering was contracted by a university to design, install, test and certify a AC ELF EMI/RFI magnetic shield. Between May 10th and May 13th 2002, VitaTech installed a 9' x 12' x 8'(height) shield composed of conductive seam welded aluminum plates and extruded aluminum I-Beams. Additionally, an RF sealed door and filtered penetrations for HVAC and signal cables were installed to complete the design. Low (0.1-9.99 mG) AC ELF magnetic emissions emanated into room #240 from 60 Hz sources inside the room and from the electrical closet located across the hallway. In addition, the high frequency noise from multiple sources needed to be attenuated for the newly proposed EEG laboratory.
BEFORE DATA:
Inside Shielded Area 60Hz Magnetic Field Br Range @ 1-m
Min: 0.1 mG
Max: 2.64 mG
Mean: 0.46 mG
AFTER DATA
Inside Shielded Area 60Hz Magnetic Field Br Range @ 1-m
Min: 0.0 mG
Max: 0.0 mG
Mean: 0.0 mG
Must be using magic?
rgds
jms
Maybe pertinent EMI/RFI Shield Enclosure for EEG Laboratory at a University
On 9 February 2002, VitaTech Engineering was contracted by a university to design, install, test and certify a AC ELF EMI/RFI magnetic shield. Between May 10th and May 13th 2002, VitaTech installed a 9' x 12' x 8'(height) shield composed of conductive seam welded aluminum plates and extruded aluminum I-Beams. Additionally, an RF sealed door and filtered penetrations for HVAC and signal cables were installed to complete the design. Low (0.1-9.99 mG) AC ELF magnetic emissions emanated into room #240 from 60 Hz sources inside the room and from the electrical closet located across the hallway. In addition, the high frequency noise from multiple sources needed to be attenuated for the newly proposed EEG laboratory.
BEFORE DATA:
Inside Shielded Area 60Hz Magnetic Field Br Range @ 1-m
Min: 0.1 mG
Max: 2.64 mG
Mean: 0.46 mG
AFTER DATA
Inside Shielded Area 60Hz Magnetic Field Br Range @ 1-m
Min: 0.0 mG
Max: 0.0 mG
Mean: 0.0 mG
Must be using magic?
rgds
jms
And what do you do about those hand made $15,000 Japanese MC cartridges with bodies made out of wood and no two of the same model sound exactly alike? Paint them with aluminum paint? I'll be sticking with Shure and Empire, and you will be sticking with the MCs. The differences? My cartridges have ten times or more output and are therefore relatively immune to hum and preamp noise by comparison, track better at 3/4 gram than yours track at 2 grams which results in far less record wear. With Equalization, I can make the FR anything I want it to be or need it to be in the audible passband. To those who think that adding a single equalizer in a playback system will ruin their sound after there are at least 6 equalization stages in a non dolby vinyl record and at least 14 in a dolby A vinyl between the microphone and their preamp outputs, they are stuck with whatever happnens to be the the net result of multiple variables they have no control over and are rarely the same from record to record. Small wonder they spend their entire lives looking for a holy grail that doesn't exist. The magnetic shielding problem like all of the others is just a canard to obfuscate the real problem, the inherent inadequacy of the basic technology.
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