inverse square eraser

[Stadler Guitars]

I've built an electric banjo for a guy who complained that the pickups were, "too high." I was going to move them closer to the string once the final setup was dialed in, but years of playing acoustic banjos is conflicting with this players desire for a great electric tone. Running into the covers just messes with him. So...

I'm replacing the hot single coils with something else and I'm hoping to come up with a design that mitigates the minimum of 3/8" separation between the pickups and strings. I know the signal to noise ratio is going downhill, so it will have to be a humbucker.

Does anyone have some insight into projecting a flux that far while keeping the sound clean, fat, and dynamic?

[CRU JONES]

Pickups for Banjo! Cool! Can you move the position? ... Probably already tried that.

A dual coil, humbucker design may be a good option. If you look in books on magnetic circuits, there are examples of circuits that are designed with a certain "reach" intended. For instance, a refrigerator magnet only sticks notes to the fridge up to a certain thickness. The fridge magnet's "pole spacing" is very small. It is constructed of very small regions of alternating North/South megnetic regions... A fridge magnets "reach is low"... It's "magnetic gradient" is high. You want the opposite of that!

I'm looking at chapters 8 and 9 of "Permanent Magnet Design and Application Handbook".

So, I am thinking a humbucker design may work for you. The wider pole spacing will give you a lower magnetic gradient, and higher reaching power.

One of the things that is quite interesting, that Mike Sulzer does, is place a Neodymium magnet on top of the pole piece. This placement, he points out, is to increase the field strength where you want it; at the strings!

I think what you are doing is really interesting! And who knows how it will effect the tone... Especially for a banjo!

So, good luck! Check out the book I mentioned above and you will find chapters 8 and 9 very interesting... Maybe drop your design into FEMM as well (that is cheating though;-))

Cheers,
Ethan

[Stadler Guitars]

I ended up making a humbucker with 5 alnico poles laid parallel to the strings and wound like a stacked single coil. I figured the pole spacing couldn't get much more separated than that, and it worked. It was way louder and much quieter that a P-90 at a given distance, but it was still too quiet and had poor string to string balance, although when placed at a proper distance it had good balance and was phenomenal. I'm going to start stuffing them in ebony cases and put them on my archtop pickguards. You learn something new every day!.

I ended up making a pickup with adjustable poles and tucked it between the bridge and the bridge pickup. It was slim and out of the way enough to please the player. Thanks CRU, your post confirmed my thinking and led me down a rewarding path!

[Mike Sulzer]

I ended up making a humbucker with 5 alnico poles laid parallel to the strings and wound like a stacked single coil. I figured the pole spacing couldn't get much more separated than that, and it worked. It was way louder and much quieter that a P-90 at a given distance, but it was still too quiet and had poor string to string balance, although when placed at a proper distance it had good balance and was phenomenal. I'm going to start stuffing them in ebony cases and put them on my archtop pickguards. You learn something new every day!.

I ended up making a pickup with adjustable poles and tucked it between the bridge and the bridge pickup. It was slim and out of the way enough to please the player. Thanks CRU, your post confirmed my thinking and led me down a rewarding path!

Not sure if you have tried this: The magnetic field strength at the strings is limited; too much gives string pull. If the pickup is further from the strings than normal, you can partly compensate for loss in signal by using stronger magnets.

[CRU JONES]

I ended up making a humbucker with 5 alnico poles laid parallel to the strings and wound like a stacked single coil. I figured the pole spacing couldn't get much more separated than that, and it worked. It was way louder and much quieter that a P-90 at a given distance, but it was still too quiet and had poor string to string balance, although when placed at a proper distance it had good balance and was phenomenal. I'm going to start stuffing them in ebony cases and put them on my archtop pickguards. You learn something new every day!.

I ended up making a pickup with adjustable poles and tucked it between the bridge and the bridge pickup. It was slim and out of the way enough to please the player. Thanks CRU, your post confirmed my thinking and led me down a rewarding path!

I think this is a very interesting solution! I am confident that at a distance of 3/16" from the top of an alnico pole piece the field strength would be greater than the field strength at 3/16" from its side... However, I do think it is quite possible that at a distance of 3/8" the field strength is greater from the side. Plus the aperture is greater, also achieving more of your goals! Excellent use of your magnets, nice work my friend!
Ethan