I need some advice for killing hum that my MC transformers pick up from nearby mains transformers, or even mains cables.
The transformers are Lundahl LL1678, they are built in a plastic box that I had at hand. The constuction of a Lundahl transformer is the following: it is built in a tiny metal (permalloy?) case, and wrapped around with some (permalloy?) ribbon. It seems this is not enough. Or perhaps the unshielded wires from the input/output RCA connectors pick up stray magnetic field.
I don't have access to bigger permalloy sheets for building a magnetic shield for the whole assembly of two transformers and wiring. But I have some tinned steel plate that I could bend and solder to form a box. What is a minimum thickness that would keep the magnetic flux away? What other alternatives exist? (I will try shielded internal cables anyway, but not sure if they would prevent magnetic pick up)
The transformers are Lundahl LL1678, they are built in a plastic box that I had at hand. The constuction of a Lundahl transformer is the following: it is built in a tiny metal (permalloy?) case, and wrapped around with some (permalloy?) ribbon. It seems this is not enough. Or perhaps the unshielded wires from the input/output RCA connectors pick up stray magnetic field.
I don't have access to bigger permalloy sheets for building a magnetic shield for the whole assembly of two transformers and wiring. But I have some tinned steel plate that I could bend and solder to form a box. What is a minimum thickness that would keep the magnetic flux away? What other alternatives exist? (I will try shielded internal cables anyway, but not sure if they would prevent magnetic pick up)
Without spending any money, my best suggestion would be to play with the orientations of the transformers with respect to each other. Coupling is stronger when the axes are in alignment and weaker when one is transverse with respect to the other.
Any ferrous metal box will shield the transformers from hum many, many times better than a plastic enclosure. It does not need to be very thick.
Hi +1 on the ferrous metal enclosure will be the best. Plastic does nothing. I have used the 1678s in a small steel enclosure roughly 4X4x2 inches(I think it was made by Bud). I keep the signal grounds separate from the housing and case. I also run each signal ground through from input to output and not tie the channels together. Still is is best to keep it away from transformers with AC on them. Sometimes tilting the box on its side or twisting it 90 degrees can help in sever situations.
Do I need to create a box with all sides closed? Or can I omit the front and rear sides where the cables go from the connectors to the transformer pins (about 2")?I have a tape recorder head demagnetizer that I use for testing sensitivity to magnetic field. The strange thing is when I walk along the output shielded cable with this device, there are peaks and nulls that follow each other at regular intervals, like 3 or 4". I know a shielded cable is shielding against electric field, not against magnetic field. Strange anyway.
