Shielding and dummy coils

[peskywinnets]

My own (albeit brief) sortie into dabbling with dummy coils in a stacked arrangement weren't particularly promising either! I parked the project to take up 'bus tickets of the world' collecting instead.

Once I've managed to complete the ticket collection (only an 'Easter Island airport express/shuttle' to complete it now), I intend having another pop - but my approach will be a little different - rather than use 'ambient hum' to subjectively ponder whether situation x is better than situation y, I'll generate a chunky amount of 50Hz hum via a 'transmitter' coil in proximity to both coils under test ....then wap them into an active preamp to magnify the issue so to speak & have a dabble from there. Else it felt I was just grasping in the dark....I wanna see the differences in the coil hum magnitudes on a scope, whilst changing the variables.

Humtastic times beckon.

[Bruce Johnson]

David;

Yes, that makes sense. The four M-pickup coils are very independent. Back when I was first working with the idea, I wired up a set in my test mule so I could listen to each coil separately. There was no audible signal coming from the string next door. I had wondered what kind of mixing of the magnetic fields I would get because they're so close together. The answer seems to be: not much. The fields are apparently very narrow across the string and very long along the string. That's what I was trying to do.

Joe;

Yes, the dummy coils that I played with were concentric with the group of M-pickup coils. The group of pickup coils is 2" long x 2 3/4" wide. The dummy coil bobbin was 1 3/4" long x 2 5/8" wide x 1/2" thick, centered right under the group.

The M-pickup coils are 5000 turns of #41, measuring 1.90K DCR each, wired together in series for a total of 7.60K DCR

I wound up three different versions of the dummy coil, all on the same bobbin parts. They ranged from 1250 turns of #32 measuring 106 Ohms, up to 10,000 turns of #41, measuring 6.63K DCR.

I tried each both in series and in parallel with the M-pickups in the test mule. Lifting the mule up towards the florescent lamp over the bench creates a nice 60hz hum. Switching the dummy coils in and out, I couldn't hear any reduction or change in the hum, in any of the combinations. After some of the discussions above about plates and pole pieces, I tried some quick mockups with steel plates between the pickup coils and the dummy coil; no change.

As I said, I gave up because I wasn't seeing any results at all that I could hear. And putting the dummy coil in the circuit was messing with the tone, as expected. In comparison, rewiring the M-pickups into a true humbucking group had a dramatic effect: The hum on the bench or up near the lamp was basically gone.

So, my conclusion is, to get the hum canceling effect the counteracting coils need to be pretty close to mirror image in characteristics to get a significant effect. I would have thought that the 10,000 turn 6.63K DCR dummy coil would have been close enough to group of pickup coils to do something, but I guess not.

An interesting note: I did one 5 string bass using a cluster of five M-pickups in an unbalanced humbucker configuration: Three norths and two souths, alternating. Even though it wasn't truly symmetrical, it worked and was very quiet.

[Joe Gwinn]

Yes, the dummy coils that I played with were concentric with the group of M-pickup coils. The group of pickup coils is 2" long x 2 3/4" wide. The dummy coil bobbin was 1 3/4" long x 2 5/8" wide x 1/2" thick, centered right under the group.

The M-pickup coils are 5000 turns of #41, measuring 1.90K DCR each, wired together in series for a total of 7.60K DCR

I wound up three different versions of the dummy coil, all on the same bobbin parts. They ranged from 1250 turns of #32 measuring 106 Ohms, up to 10,000 turns of #41, measuring 6.63K DCR.

My suspicion is that the dummy coils were not large enough physically

I tried each both in series and in parallel with the M-pickups in the test mule. Lifting the mule up towards the florescent lamp over the bench creates a nice 60hz hum. Switching the dummy coils in and out, I couldn't hear any reduction or change in the hum, in any of the combinations. After some of the discussions above about plates and pole pieces, I tried some quick mockups with steel plates between the pickup coils and the dummy coil; no change.

As I said, I gave up because I wasn't seeing any results at all that I could hear. And putting the dummy coil in the circuit was messing with the tone, as expected. In comparison, rewiring the M-pickups into a true humbucking group had a dramatic effect: The hum on the bench or up near the lamp was basically gone.

So, my conclusion is, to get the hum canceling effect the counteracting coils need to be pretty close to mirror image in characteristics to get a significant effect. I would have thought that the 10,000 turn 6.63K DCR dummy coil would have been close enough to group of pickup coils to do something, but I guess not.

An interesting note: I did one 5 string bass using a cluster of five M-pickups in an unbalanced humbucker configuration: Three norths and two souths, alternating. Even though it wasn't truly symmetrical, it worked and was very quiet.

The dummy coil should have worked; I have no idea what went wrong.

[Bruce Johnson]

I should mention that I also tried reversing the leads on the dummy coil, putting it in both additive and subtractive modes. Hardly any difference that I could hear.

I also tried comparing it in the shielded cavity below the M-pickups versus holding it by hand over the strings, centered over the pickups. Still, almost nothing.

So, I assume that all means that it simply wasn't working as an antenna.

[Joe Gwinn]

I should mention that I also tried reversing the leads on the dummy coil, putting it in both additive and subtractive modes. Hardly any difference that I could hear.

I also tried comparing it in the shielded cavity below the M-pickups versus holding it by hand over the strings, centered over the pickups. Still, almost nothing.

So, I assume that all means that it simply wasn't working as an antenna.

As I said, it isn't obvious to me why it failed; it should have worked. But it would be good to know why it didn't.