Man this place is all over... lemme see.
Copper and aluminum are useful because they are conductive. Cu because it's cheap and next to the most conductive element, just falling short of silver. Aluminum is used also because it's cheap, and more conductive than copper by weight. In any case, a conductor is used to act as a shorted turn to the EM fields in an area.
Iron and other ferromagnetic metals are useful because they form a magnetic short circuit, and also work for static fields as well. I hear most steel has a mu=1 or 2 around 3kHz, so it's useless for anything RF. Steel is not a good electrical conductor.
Lead is *not* a magnetic conductor, and not very good at electricity either. Its only shielding advantage comes from its density, where there are more protons and neutrons per volume than other materials, blocking more beta and gamma rays than other materials. Although x-rays are considered to be EM radiation...
Tim
Copper and aluminum are useful because they are conductive. Cu because it's cheap and next to the most conductive element, just falling short of silver. Aluminum is used also because it's cheap, and more conductive than copper by weight. In any case, a conductor is used to act as a shorted turn to the EM fields in an area.
Iron and other ferromagnetic metals are useful because they form a magnetic short circuit, and also work for static fields as well. I hear most steel has a mu=1 or 2 around 3kHz, so it's useless for anything RF. Steel is not a good electrical conductor.
Lead is *not* a magnetic conductor, and not very good at electricity either. Its only shielding advantage comes from its density, where there are more protons and neutrons per volume than other materials, blocking more beta and gamma rays than other materials. Although x-rays are considered to be EM radiation...
Tim
There are lot's of rumours out there. If you try yourself you may see that this isn't very true. 
If you want to bend or deflect a magnetic field (at low frequencies) the material in question must have magnetic permeablity greater than air. Aluminium has just about that. There are only a few magnetic materials and the most common one is soft iron.
If we talk higher frequencies (way over the audio band) the material must be a good conductor.
If you want to bend or deflect a magnetic field (at low frequencies) the material in question must have magnetic permeablity greater than air. Aluminium has just about that. There are only a few magnetic materials and the most common one is soft iron.
If we talk higher frequencies (way over the audio band) the material must be a good conductor.
Regardless of this shielding thing I think the wiring is the main problem if you are having problems with hum. I have _never_ succeeded to get hum only by the closeness of a toroid, even in a preamp. If you check my SMD headphone amp, transformers are close to the circuits and those transformers are regular ones!
Use sheiled cables for input and output signals and keep them away from the transformers.
Sheilding a toroid is very rare and needed only in rare occations. Focus on the real problem instead. Do you have problems with grounding? If it's a soft hum, you have grounding problems (probably) and if you have a harder 120 Hz hum with many overtones you have problems with you power supply in some way.
OK, the thread starter asked only in general and the short answer is no, no problems. Min distance has to do with how sensitive the circuit is and how mant amperes you take out from the transformer.
Safe distance is pretty close, 20 mm or so. If you are thinking of a MC preamp the distance will not be that cloce, 50-100 mm maybe.
Use sheiled cables for input and output signals and keep them away from the transformers.
Sheilding a toroid is very rare and needed only in rare occations. Focus on the real problem instead. Do you have problems with grounding? If it's a soft hum, you have grounding problems (probably) and if you have a harder 120 Hz hum with many overtones you have problems with you power supply in some way.
OK, the thread starter asked only in general and the short answer is no, no problems. Min distance has to do with how sensitive the circuit is and how mant amperes you take out from the transformer.
Safe distance is pretty close, 20 mm or so. If you are thinking of a MC preamp the distance will not be that cloce, 50-100 mm maybe.
Pass DIY Addict
Joined 2000
Paid Member
Yes, the copper sheet removed the hum caused by my toroid. I can't really comment if it was capacitive coupling or not that was blocked by the shield, but the hum in my speakers went away after the shield was added.
The loose cables are a function of time that I don't have all that often to go back and tidy up a bit. Things were very quiet before I put the components into the chassis, and then very noisy once inside... I moved various wires around, but it did not result in very much difference in hum. Adding the shield made the hum go away, so I stopped fiddling... I just haven't been motivated to go back and tie everything down.
The closeness of the secondary to the primary wiring is really a function of the angle of the photo, they are not as close as they look in the 2D image.
CU -- did I mention this before?
back when I was a Physics lab rat (several decades ago, a summer in SS Physics which encouraged me not to go into Physics as a profession) we used a brass enscreened room to do our very low noise measurements -- the power was all DC from storage batteries (which do have some noise as I have subsequently found out).
oh, the physics really helps when you do analysis of financial derivatives!
back when I was a Physics lab rat (several decades ago, a summer in SS Physics which encouraged me not to go into Physics as a profession) we used a brass enscreened room to do our very low noise measurements -- the power was all DC from storage batteries (which do have some noise as I have subsequently found out).
oh, the physics really helps when you do analysis of financial derivatives!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.