Pure 50/60Hz is almost(?) always magnetic induction from the transformers.
For this distance is the best cure. 25mm extra is MUCH better than any amount of mu-metal. If you have to use mu-metal, use it around the sensitive parts. Mu-metal has low saturation and putting it around the transformer will have little effect. Steel chassis complicate matters as the evil flux is directed & concentrated in complex ways.
But cos our hearing is worse at LF, 50/60Hz is not usually a cause of complaint.
Far more audible is BUZZ, 100Hz and other 50Hz harmonics. This is caused by the very sharp charging impulses of the rectifiers. If the earthing system is poor, these pulses get into clean earths and cause most problems. The VERY high currents give large IR voltages in your earth tracks.
Good earthing arrangements and PCB layouts deal with this. They ensure the high currents are confined ONLY to dirty earth tracks and keep sewage from clean earths. But the charging currents don't only cause IR voltage noise.
High currents flowing in a loop also induce a magnetic field. The larger the loop, the larger the area and volume of the dirty field. Any other loop in this dirty field will pick up this induced field and generate a voltage. This is how transformers work.
The Lundahl is probably better shielded as well as presenting a smaller pickup loop to external fields.
Good PCB PSU layout will not only be very careful about the earthing but also make the charging current loops as small as possible.
For layout, this means twisting all transformer wiring together and carefully laying out any wires/tracks that carry charging current so the area between to & fro current paths is as small as possible.
If you do this properly, and it is not easy, you can have big nasty power supplies in the same box as very sensitive circuits. You want to do as much of the layout on a PCB so when you get it right, it will stay right. Then you will only have a small amount of info on EXACTLY where you want the transformer & mains wiring to go, to teach the person building the box.
You can tell if a manufacturer understands Low Noise just by looking at how he lays out input circuits and PSU wiring.
For this distance is the best cure. 25mm extra is MUCH better than any amount of mu-metal. If you have to use mu-metal, use it around the sensitive parts. Mu-metal has low saturation and putting it around the transformer will have little effect. Steel chassis complicate matters as the evil flux is directed & concentrated in complex ways.
But cos our hearing is worse at LF, 50/60Hz is not usually a cause of complaint.
Far more audible is BUZZ, 100Hz and other 50Hz harmonics. This is caused by the very sharp charging impulses of the rectifiers. If the earthing system is poor, these pulses get into clean earths and cause most problems. The VERY high currents give large IR voltages in your earth tracks.
Good earthing arrangements and PCB layouts deal with this. They ensure the high currents are confined ONLY to dirty earth tracks and keep sewage from clean earths. But the charging currents don't only cause IR voltage noise.
High currents flowing in a loop also induce a magnetic field. The larger the loop, the larger the area and volume of the dirty field. Any other loop in this dirty field will pick up this induced field and generate a voltage. This is how transformers work.
The Lundahl is probably better shielded as well as presenting a smaller pickup loop to external fields.
Good PCB PSU layout will not only be very careful about the earthing but also make the charging current loops as small as possible.
For layout, this means twisting all transformer wiring together and carefully laying out any wires/tracks that carry charging current so the area between to & fro current paths is as small as possible.
If you do this properly, and it is not easy, you can have big nasty power supplies in the same box as very sensitive circuits. You want to do as much of the layout on a PCB so when you get it right, it will stay right. Then you will only have a small amount of info on EXACTLY where you want the transformer & mains wiring to go, to teach the person building the box.
You can tell if a manufacturer understands Low Noise just by looking at how he lays out input circuits and PSU wiring.
