I have built a 12AX7/EL84 SET, but need to eliminate some noise coming from the Power Tranny being too close to the output trannys. I had quite a bit of hum which was only noticable through headphones, but I moved the Output Trannys away which reduced the hum to a tollerable level though Headphones. I would like to eliminate it completely and I'm sure a proper shield would help even more. Is there a ready made, easilly available shield? What do you guys use? I could fabricate one, but what should I use?
Thanks for the reply. I have already found the positions with the least amount of hum which is very low. I'm sure if I had more room on my chassis, Id be able to get rid of it all, but I don't. So I'm thinking I need to make a shield. I've heard MU metal mentioned, but haven't found it yet. What exactly is it? Remember, I'm using this for headphones as well as Fostex/MLTL. The headphones are really sensitive and I want I silent noise floor.
Several years ago I was working on a problem where the noise from the display (400Hz) was corrupting the VCO in a two way radio. Our mechanical engineering team had made some shields from several materials and I tested them all. The Mu-Metal shield worked the best, followed by cold rolled steel. I don't remember the details (20 years ago), but these two were the only materials that worked good enough. There were about 10 different ones including copper and aluminum.
Mu_Metal is a material designed for magnetic shielding. The metal case that a phono cartridge is made from is likely Mu-Metal.
Mu_Metal is a material designed for magnetic shielding. The metal case that a phono cartridge is made from is likely Mu-Metal.
really?
so that means that i DO have to buy that nuclear fusion lifetime power supply on e-Bay!! Neat!!
So it works for subatomic particles....does that mean it is useless for magnetic shielding?
my experiments only covered earthed sheets of pertinax unetched pcb's
they did not make an audible difference. I gave up on magnetic shielding because someone said it was useless, but now i think of it, they shield speakers with steel too!
I WAS goint to add a steel shield between my trannies and the electronics, now my hopes are up for effectiveness!
Bas
so that means that i DO have to buy that nuclear fusion lifetime power supply on e-Bay!! Neat!!
So it works for subatomic particles....does that mean it is useless for magnetic shielding?
my experiments only covered earthed sheets of pertinax unetched pcb's
they did not make an audible difference. I gave up on magnetic shielding because someone said it was useless, but now i think of it, they shield speakers with steel too!
I WAS goint to add a steel shield between my trannies and the electronics, now my hopes are up for effectiveness!
Bas
Hello,
it is also important to have an enclousure made out of non magnetic material like aluminium, copper or stainless steel. If you have a steel chassis it will conduct the magnetic radiation of the power transformer. Apart from that I'm afraid there is not much you can do to shield against magentic interference of the strenght a tube amp mains transformer emits apart from more space.
Best regards
Michael
it is also important to have an enclousure made out of non magnetic material like aluminium, copper or stainless steel. If you have a steel chassis it will conduct the magnetic radiation of the power transformer. Apart from that I'm afraid there is not much you can do to shield against magentic interference of the strenght a tube amp mains transformer emits apart from more space.
Best regards
Michael
Only ferrous metals can screen a DC magnetic field (such as the leakage from a loudspeaker's permanent magnet). It's sometimes possible to buck an AC magnetic field with a really low resistance shorted turn - that's why you sometimes see a copper foil shorted turn around the outside of a power transformer. Save your lead for damping vibration...
Neg feedback would solve alot of hum problems by reducing the effective output stage impedances. However I get alot of queries about SE orientation of transformers and the old adage of carefully putting transformers a long distance from each other in opposite corners usually works..
Mumetal is a luxury and not viable against cost.
I don't agree about steel chassis not viable. Every amp in the early days was put on a steel chassis and even drop through transformer types which are the worst offenders for circulating fields. If one can just pick out hum with headphones then the s/n is around 60dB down and is hard to improve on that figure unless an output attenuator is used.
Are you using an output attenuator for headphones or is directly connected to o/p tranny sec ?
richj
Mumetal is a luxury and not viable against cost.
I don't agree about steel chassis not viable. Every amp in the early days was put on a steel chassis and even drop through transformer types which are the worst offenders for circulating fields. If one can just pick out hum with headphones then the s/n is around 60dB down and is hard to improve on that figure unless an output attenuator is used.
Are you using an output attenuator for headphones or is directly connected to o/p tranny sec ?
richj
c-core transformers put out less magnetic fields
could it be true that they dont PICK UP on magnetic fields as much?
Mij c-core opt's are at 4 cm from the input channel. I tested it for hum by connecting the secondaries to the input of my sand amp, cranked up the volume to max and i got ollmoat NO hum (apart from the sand amp hum)
Bas
could it be true that they dont PICK UP on magnetic fields as much?
Mij c-core opt's are at 4 cm from the input channel. I tested it for hum by connecting the secondaries to the input of my sand amp, cranked up the volume to max and i got ollmoat NO hum (apart from the sand amp hum)
Bas
The problem with shielding an AC magnetic field is at you will get current in the shield and at that current will in its turn induce a new magnetic field outside the shield. In some cases, succesive shields will just transport the magnetic field.
It is only 2 solutions: use mu-metal: its expensive, but it work and you must take in account this shield when calculating the transformer; or use the distance as allready suggested.
You can even put the redressors and the main caps of the alimentation with the transformer.
It is only 2 solutions: use mu-metal: its expensive, but it work and you must take in account this shield when calculating the transformer; or use the distance as allready suggested.
You can even put the redressors and the main caps of the alimentation with the transformer.
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