mu metal is over-hyped
hobbyists have little need for mu metal shielding - usually don't understand the real problem(s) and most won't know how to apply mu metal if needed in the correct manner
I have designed scientific and industrial instrumentation professionally - gains up to 4000 in strain gage amplifiers, 50+ ft cable runs to sensors - and I have never used mu metal in any instrument design
even in phono pickups twisted pair in the arm to RCA phono coax is normally all that is used - hum/buzz is more likely to enter as ground loop noise than to be introduced from magnetic field coupling to the signal wire's internal loop area in home environments
and “ground loop noise” is often chassis leakage currents flowing in common “ground reference” impedances – not so much power frequency magnetic coupling
twisting, star quad, coax all reduce magnetic field coupling - both radiation and pickup - so applying these to both power/motor wiring "threats" and to sensitive signals works fine in most cases - in the extreme wiring can be ran in steel conduit - not too handy for phonograph wiring
I have used welded corner steel boxes with EMI gasketing to house the amplifier circuitry, tinned steel internal shielding boxes around uC, DSP within inches of those Av 4000 strain gage amp circuits
but I never needed mu metal - never seen it used except in extreme low level signal transformers or RF tunning inductor housings
hobbyists have little need for mu metal shielding - usually don't understand the real problem(s) and most won't know how to apply mu metal if needed in the correct manner
I have designed scientific and industrial instrumentation professionally - gains up to 4000 in strain gage amplifiers, 50+ ft cable runs to sensors - and I have never used mu metal in any instrument design
even in phono pickups twisted pair in the arm to RCA phono coax is normally all that is used - hum/buzz is more likely to enter as ground loop noise than to be introduced from magnetic field coupling to the signal wire's internal loop area in home environments
and “ground loop noise” is often chassis leakage currents flowing in common “ground reference” impedances – not so much power frequency magnetic coupling
twisting, star quad, coax all reduce magnetic field coupling - both radiation and pickup - so applying these to both power/motor wiring "threats" and to sensitive signals works fine in most cases - in the extreme wiring can be ran in steel conduit - not too handy for phonograph wiring
I have used welded corner steel boxes with EMI gasketing to house the amplifier circuitry, tinned steel internal shielding boxes around uC, DSP within inches of those Av 4000 strain gage amp circuits
but I never needed mu metal - never seen it used except in extreme low level signal transformers or RF tunning inductor housings
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Seconded, mu metal is a pain to deal with, and must be annealed after any bending, you will not find it as a tape!
At audio frequencies, tight twisting is actually far more important then screening as far as magnetic coupling is concerned, and I would put the gram amp inside the plinth of the record deck and export low Z line level rather then worrying about exotic pain in the **** materials.
Regards, Dan.
At audio frequencies, tight twisting is actually far more important then screening as far as magnetic coupling is concerned, and I would put the gram amp inside the plinth of the record deck and export low Z line level rather then worrying about exotic pain in the **** materials.
Regards, Dan.
Percy sells something called TI shield which is supposed to have magnetic and EMI shielding
http://www.percyaudio.com/Catalog.pdf
BTW, an easy way to perform a relative magnetic field measurement is to use a compass, and see how much deflection of the compass there is. The stronger the magnetic fields, the more the compass will deflect from north.
http://www.percyaudio.com/Catalog.pdf
BTW, an easy way to perform a relative magnetic field measurement is to use a compass, and see how much deflection of the compass there is. The stronger the magnetic fields, the more the compass will deflect from north.
I have used mu metal to wrap toroid transformers, not sure if it helped but I seem to recall I slept better that night. I noticed on some plastic cases with digital equipment inside they spray a copper paint on the inside of the case and tie it to ground, I wonder if that paint can be purchased? Also is it worth the trouble for a Dac?
Bill
Bill
The conductive paint is actually mostly an RFI measure usually aimed more at radiated emissions control then anything and is a cheap fix late in the design process when you realise you will not meet the emissions limits for FCC/CE approval in a plastic box.
Nickel is somewhat more common then copper as a metal to load a conductive paint, but really, neither is particularly relevant to audio outside of very specific circumstances (Like say guitar control cavities).
Note that at low frequency (and all audio is nearly DC for this purpose), a thin copper screen is only effective against electric fields, it does little or nothing for magnetic induction, for that only reducing the loop areas, or going for a rather large and heavy hi mu material, helps.
The loop areas are easy to reduce, so that is always the first thing to try, and it almost always solves the problem.
Regards, Dan.
Nickel is somewhat more common then copper as a metal to load a conductive paint, but really, neither is particularly relevant to audio outside of very specific circumstances (Like say guitar control cavities).
Note that at low frequency (and all audio is nearly DC for this purpose), a thin copper screen is only effective against electric fields, it does little or nothing for magnetic induction, for that only reducing the loop areas, or going for a rather large and heavy hi mu material, helps.
The loop areas are easy to reduce, so that is always the first thing to try, and it almost always solves the problem.
Regards, Dan.
mu metal saturates at very low H - up close to a power transformer it is useless
if you really must shield against magnetic field you should 1st use cheap Iron - low carbon steel - lower initial perm but is so cheap you can use mm thickness to handle much more flux
wrapping with tape would be wrong too - spacing mag shielding from the source is good - air is poor magnetic field conductor and even a little spacing add lots of reluctance to the magnetic circuit == reduced flux in the shield compared to little or no air gap with conforming tape
old school OEM power supplies used mm thick perforated steel boxes to allow air flow for transformer cooling and contain the EI transformer's leakage, rectifier/C charging current loop flux
any conductive material, surface coating << 1 Ohm/sq will be an effective electrostatic shield and reflect RF, EMI wavelengths shorter than the skin depth
if you really must shield against magnetic field you should 1st use cheap Iron - low carbon steel - lower initial perm but is so cheap you can use mm thickness to handle much more flux
wrapping with tape would be wrong too - spacing mag shielding from the source is good - air is poor magnetic field conductor and even a little spacing add lots of reluctance to the magnetic circuit == reduced flux in the shield compared to little or no air gap with conforming tape
old school OEM power supplies used mm thick perforated steel boxes to allow air flow for transformer cooling and contain the EI transformer's leakage, rectifier/C charging current loop flux
any conductive material, surface coating << 1 Ohm/sq will be an effective electrostatic shield and reflect RF, EMI wavelengths shorter than the skin depth
Lead is fairly horrible for EM screening, being a poor conductor compared to copper.
It is obviously useful against charged particle and EM once the photon energy gets up where you can reasonably talk of photons as opposed to EM waves.
Its one desirable feature is that it is dense making it good at stopping alpha and beta particles and reasonably effective against X ray and gamma, it does little against fast neutrons.
However for a gram amp we are probably not too concerned with stopping the gamma photons, as if there is much of that floating aroud the correct answer is to put down the records and leave the area rapidly.
Copper is what you want for electrostatic screening, with steel (Ideally a high carbon type) being useful for screening stray magnetic fields (Which are almost always the bigger problem to deal with).
73 Dan.
It is obviously useful against charged particle and EM once the photon energy gets up where you can reasonably talk of photons as opposed to EM waves.
Its one desirable feature is that it is dense making it good at stopping alpha and beta particles and reasonably effective against X ray and gamma, it does little against fast neutrons.
However for a gram amp we are probably not too concerned with stopping the gamma photons, as if there is much of that floating aroud the correct answer is to put down the records and leave the area rapidly.
Copper is what you want for electrostatic screening, with steel (Ideally a high carbon type) being useful for screening stray magnetic fields (Which are almost always the bigger problem to deal with).
73 Dan.
HI alls,
and thanks for precious suggestiuons from you , very expert technician.
But I remember to have seen a phono MC by KRELL, wich had the internal of all walls with a thin lajer of mu-metal.
And Mr. D'Agostino have said that the effect was little but audible .
So I have decided to try for myself in my DIY phono MC Hiraga preamplifer.....
Hope the cost will not be phroibitive for me.
So, skepticallysm apart, that i can understand, I search one kind person who show to me a good place where I can buy some thin lajers and long band ( as like scotch, I intend) to make my personal tests.
thanks for the helps, very appreciated,
Paolo,
Venezia - italy
and thanks for precious suggestiuons from you , very expert technician.
But I remember to have seen a phono MC by KRELL, wich had the internal of all walls with a thin lajer of mu-metal.
And Mr. D'Agostino have said that the effect was little but audible .
So I have decided to try for myself in my DIY phono MC Hiraga preamplifer.....
Hope the cost will not be phroibitive for me.
So, skepticallysm apart, that i can understand, I search one kind person who show to me a good place where I can buy some thin lajers and long band ( as like scotch, I intend) to make my personal tests.
thanks for the helps, very appreciated,
Paolo,
Venezia - italy
I realise that this thread is a little old now but it is very relevant to a problem I am currently trying to solve. I am working on a valve power amplifier design using toroidal mains and output transformers. Because of space limitations in the chassis, the two transformers are positioned very close together. Although flux leakage from the transformers is very low, it is enough for a hum current to be induced in the output transformer. Adjusting the relative rotational position of the two transformers enables me to reduce the audible hum considerably, but not to eliminate it entirely.
I started by wrapping a band of silicon steel core material around the outside of the mains transformer to see if this would have any effect, but the reduction in induced hum current was minimal. I am now wondering about winding a magnetic shield over the top of the audio transformer windings using some form of insulated mu-metal tape, in the same way that insulated copper tape is used for inter-winding electrostatic shields. Contrary to the comments above, there are plently of suppliers of mu-metal tape (LessEMF in the US and Magnetic Shields in the UK, for example), but I can't find anyone who does an insulated version.
Has anybody tried anything similar before or come across an insulated mu-metal tape?
I started by wrapping a band of silicon steel core material around the outside of the mains transformer to see if this would have any effect, but the reduction in induced hum current was minimal. I am now wondering about winding a magnetic shield over the top of the audio transformer windings using some form of insulated mu-metal tape, in the same way that insulated copper tape is used for inter-winding electrostatic shields. Contrary to the comments above, there are plently of suppliers of mu-metal tape (LessEMF in the US and Magnetic Shields in the UK, for example), but I can't find anyone who does an insulated version.
Has anybody tried anything similar before or come across an insulated mu-metal tape?
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