Depends on where you place the power supply. It's common with commercial equipment that the separate power supply is in a chassis similar to the preamp, which invites people to stack the two. That's often worse than putting the supply in the same chassis as the preamp.
Tom
As the hum was intermittent and the mains connection of CageyH's house is apparently a bit feable, it might just be voltage regulators going into drop-out when the mains voltage is too low. That's assuming there are any voltage regulators, of course.
I would have thought this to be 'common sense.' To get away from the temptation, both chassis will now be made in a pyramid shape.
I would do, but the trees are no more, so skyhooks now needed to support the µ-metal.
'Hum', not 'him'. It's an easy experiment to do with that preamp. Install a shield of aluminum and then later one of iron and see if either make a difference in the hum level. If the hum is magnetically induced, then the iron will likely work better. If the hum is electrostatically induced, then either material will work similarly well.
You need to put a shield barrier in there even if if just for looks, I mean, even if its wood...or plastic...or acrylic... I mean come on man!
My experience is that aluminium shield caps on tubes have zero effect on hum. What is strange, steel shield caps had the same zero effect, too.
What helped in my case is 2 layers of mumetal sheets between the transformer and the input stage, and about 25 cm distance. The transformer is a toroid.
What helped in my case is 2 layers of mumetal sheets between the transformer and the input stage, and about 25 cm distance. The transformer is a toroid.
What thickness of mumetal is required for an effective shield?
I still don’t think I need one, as I have no hum.
I still don’t think I need one, as I have no hum.
I'm not surprised. Those cans, if grounded, would shield against capacitive coupling. What you're dealing with at mains frequency is inductive coupling. Then all you get is a bit of attenuation of the field from the metal of the can and that's about it. This does assume that the hum couples in via the tubes in the first place.
Not much. Most µ-metal has a relative permittivity that's 10-100x that of mild steel, so even a thin sheet would provide a lot of attenuation.
That said, note that field lines can go around the shield, so for the best shielding you really want the offending part enclosed in a µ-metal box.
Tom
Hi @CageyH,
Yes, aluminium shielding can bring an improvement. Here's what I did on my SRPP Anzaï custom-modded tube preamp - discard the shielding cylinders around the preamp tubes at left : I removed them since they proved unnecessary here.
The steel chassis is GND'ed of course, plus covered with a an aluminium adhesive foil (available at Casto or Leroy 😉). The transformer is wrapped in aluminium GND'ed foil screen too (not passing through the center of the tore !), plus shielded with screens (2mm and 0.5mm).
Toroïdal transformers proves to be "generous omnidirectional speaders" of hum, buzz, spikes disturbances - more than the classic EI transformers - by my personal excperience...
T
Yes, aluminium shielding can bring an improvement. Here's what I did on my SRPP Anzaï custom-modded tube preamp - discard the shielding cylinders around the preamp tubes at left : I removed them since they proved unnecessary here.
The steel chassis is GND'ed of course, plus covered with a an aluminium adhesive foil (available at Casto or Leroy 😉). The transformer is wrapped in aluminium GND'ed foil screen too (not passing through the center of the tore !), plus shielded with screens (2mm and 0.5mm).
Toroïdal transformers proves to be "generous omnidirectional speaders" of hum, buzz, spikes disturbances - more than the classic EI transformers - by my personal excperience...
T
If you do add a shield to try to deal with radiated mag fields, it’s important to make sure it forms a loop around the offending source. A straight sheet interposed between the source and the receiver and earthed will only provide protection against capacitive (electric field) coupling and little or nothing against mag field coupling. The radiated field from a toroid is out from the circumference at perpendicular to the vertical axis and normally substantially less than that from EI devices. However, with toroids, the field is not uniform but quite ‘lumpy’, so should be rotated back and forth through about60 degrees to find the null point.
As already mentioned, a thin piece of aluminium as used in a tube shield is all but useless against mag fields.
As already mentioned, a thin piece of aluminium as used in a tube shield is all but useless against mag fields.