That oscilloscope noise pickup is quite typical -- your body is a decent capacitive coupler to the ambient 60Hz powerline fields.
As for the audio inductors, back in the 1950's and 60's there were abundant 88mH (or 77mH ?) toroid inductors, used in the old telephone networks, and these showed up a lot in audio filter designs. The advantage of a toroid is that it is fairly immune to extrnal magnetic fields. I've never used one of these telco inductors myself, and I have no idea if they can still be found surplus, but you can always wind a new one on the appropriate core material.
It's interesting that I had never really noticed that amount of 60 Hz noise on my scope when touching a probe, but I suspect that's because I've typically been looking for HF signals on it. This time I was specifically looking for mains noise.
I've definitely heard of the surplus telco 88 mH chokes but I don't recall ever seeing any at a hamfest or the like for sale. I imagine those worked well, since the toroid is self-shielding. I'm not familiar with the type of core material you would use for these types of high-value inductors. Any hints in pointing me in the right direction would be greatly appreciated.
I think modern audio inductors and transformers are wound on a laminated core made of steel ribbon, but they may be iron powder or ferrite blends that will work.
Rather than a toroid, the easier closed-magnetic-loop option is to use a pot-core. This lets you wind the wire on a bobbin, then surround it by the two-piece core sections. There are high-permeablility materials, often used in switching power supply applications, that might be appropriate for this. Or you can buy toroid inductors in the 10-100 mH range from Chinese sources (unknown characteristics) or domestic sources such as DigiKey (expensive). This is why we generally do this with op-amp filters now.
I know this isn't very helpful, but I've had essentially zero experience working with audio-range inductors.
That oscilloscope noise pickup is quite typical -- your body is a decent capacitive coupler to the ambient 60Hz powerline fields.
As for the audio inductors, back in the 1950's and 60's there were abundant 88mH (or 77mH ?) toroid inductors, used in the old telephone networks, and these showed up a lot in audio filter designs. The advantage of a toroid is that it is fairly immune to extrnal magnetic fields. I've never used one of these telco inductors myself, and I have no idea if they can still be found surplus, but you can always wind a new one on the appropriate core material.
-Paul / wb6cxc
It's interesting that I had never really noticed that amount of 60 Hz noise on my scope when touching a probe, but I suspect that's because I've typically been looking for HF signals on it. This time I was specifically looking for mains noise.
I've definitely heard of the surplus telco 88 mH chokes but I don't recall ever seeing any at a hamfest or the like for sale. I imagine those worked well, since the toroid is self-shielding. I'm not familiar with the type of core material you would use for these types of high-value inductors. Any hints in pointing me in the right direction would be greatly appreciated.
Thanks for the comment!
I think modern audio inductors and transformers are wound on a laminated core made of steel ribbon, but they may be iron powder or ferrite blends that will work.
Rather than a toroid, the easier closed-magnetic-loop option is to use a pot-core. This lets you wind the wire on a bobbin, then surround it by the two-piece core sections. There are high-permeablility materials, often used in switching power supply applications, that might be appropriate for this. Or you can buy toroid inductors in the 10-100 mH range from Chinese sources (unknown characteristics) or domestic sources such as DigiKey (expensive). This is why we generally do this with op-amp filters now.
I know this isn't very helpful, but I've had essentially zero experience working with audio-range inductors.