Just thinking in public...

[Chris Turner]

So, I was thinking...

In a Jazz Bass single coil pickup, you have small magnetized slugs in the middle of a tall, skinny coil. Would there be any merit in using a tall, skinny coil, with magnetic slugs as poles outside of the coil?

Kinda like so...

First the Jazz SC PU, and then the idea I was throwing around in my noggin.

Given that poles would be spaced apart as they are, and with both having the same polarity, would any field lines actually pass through the coil itself?

[Peter Naglitsch]

I *guess* that the field would look a bit like a Fender Jaguar pickup:

FEMM is a free (I think) tool that can produce this type of pictures. Download it and play around with it. And make sure to publish the result, both the image and the sound of the pickup...

This particular picture above can be found on Steven Kertings site

[Joe Gwinn]

FEMM is a free (I think) tool that can produce this type of pictures. Download it and play around with it

FEMM is free. It runs only on Windows machines, and is 2D (versus 3D).

[jason lollar]

There are several old designs like that- usually they will be mounted on a steel plate so if the outside magnets are south up the steel slug in the coil will be north up.
The steel guitar- either the national new yorker or the dynamic has a similar pickup but instead of several rods on each side it has one small block of alnico on each side in the middle of the coils length and there is a steel plate attached to the top of each alnico magnet about 1/16" thick, 1/4" wide and however long the coil is- the steel plates spread the magnetic feild along the length of the coil on each side.

[Chris Turner]

There are several old designs like that- usually they will be mounted on a steel plate so if the outside magnets are south up the steel slug in the coil will be north up.
The steel guitar- either the national new yorker or the dynamic has a similar pickup but instead of several rods on each side it has one small block of alnico on each side in the middle of the coils length and there is a steel plate attached to the top of each alnico magnet about 1/16" thick, 1/4" wide and however long the coil is- the steel plates spread the magnetic feild along the length of the coil on each side.

Is it necessary for the center core to be polarized at all? I was just gonna use rods that are already magnetized, with all north or south poles facing up.

Hell, I'll probably just build it and see how it turns out... I've played around with FEMM a bit, but I'm not skilled enough to accurately model what I've drawn up.

[David King]

The original Lace sensor pickups are kind of inside-out like that. They have steel comb-like center core instead of a bobbin and flexible ferrite magnets around the outside of the coils with a thick, U-shaped steel plate that wraps around the whole armature and ties the fields together.

[Mike Sulzer]

The original Lace sensor pickups are kind of inside-out like that. They have steel comb-like center core instead of a bobbin and flexible ferrite magnets around the outside of the coils with a thick, U-shaped steel plate that wraps around the whole armature and ties the fields together.

The coil produces voltage when time varying flux from the vibrating string passes through it. This requires that the string be magnetized, and if the pickup has a core in the coil, then the magnetization is most important right over the center of the coil. In fact, one could argue that magnetization away from the center is worse than a waste since it contributes to string pull without contributing much to the pickup signal. I am guessing that the exterior magnets couple to the core at the back (or bottom) of the pickup, thus magnetizing the core steel which in turn magnetizes the string over the core.

A steel structure around the pickup can help the flux from the string passing through the coil return to the string, thus increasing the output.

(The variable reluctance viewpoint can be helpful here. By decreasing the reluctance around the path outside the core, the change in reluctance due to the vibrating string is increased.)

[David Schwab]

The original Lace sensor pickups are kind of inside-out like that. They have steel comb-like center core instead of a bobbin and flexible ferrite magnets around the outside of the coils with a thick, U-shaped steel plate that wraps around the whole armature and ties the fields together.

Interestingly, they have fairly weak magnets. I have a Fender Lace Sensor here and it reads 230 G (N). They are also very quiet for single coils.

I also have a Transsensor P bass pickup that reads about the same. The Transsensor is a low-Z copper loop current pickup like the Alumitone.

[jason lollar]

like Mike says- I believe if you dont use a steel bottom plate the pickup will have less output- normally that sort of design has a steel plate that transfers magnetism to the steel blade in the coil.
There is one other design the fender trapazoid and box car pickups where the strings pass through the center of the coil- the coil is horisontal parallel to the strings and the magnets are on each end of the coil and there is a steel housing surrounding the assembly. If the hole in the coil is too big where the strings run through youll get alot less output- the coil has to be very close to the strings to work right.
http://img.villagephotos.com/p/2002-...ual8Pro_sm.JPG

[Chris Turner]

Thanks all...

Looks like I probably won't build it, then. Seems like too much trouble to get a usable signal out of it.

[jason lollar]

my experience is sometimes things work that you didnt think would or shouldnt work because of what youve read or been told- you never know for sure until you try it-

[Chris Turner]

my experience is sometimes things work that you didnt think would or shouldnt work because of what youve read or been told- you never know for sure until you try it-

You're right, of course...

Won't hurt to try it out. I'll report back with pics and clips when I get the chance.

[Chris Turner]

So...


I played with FEMM a bit more, and came up with this:


The outer blocks are Neo magnets (40 grade), and the center block is 1010 Steel (both materials from the default FEMM library). I didn't bother modeling the coil itself, as I just wanted to see what the field lines were doing.

The picture on the left shows the model with similar poles facing upward. The pic on the right shows the model with opposite poles facing upward.


Not being entirely sure what to make of both pics (lol), it sure seems there's a lot more going on in the core with similar poles facing upward.

[peskywinnets]

The coil produces voltage when time varying flux from the vibrating string passes through it. This requires that the string be magnetized

... does the string need to be magnetized?

I'd always been led to believe that all the vibrating ferrous string is doing, is disrupting the magnetic flux lines surrounding the copper pickup coil (thereby inducing an electric signal in sympathy).

whilst, the string obviously does need to have ferrous content (ie be magnetic) ...does it really need to be magnetized?

[David Schwab]

... does the string need to be magnetized?

The point is moot, since when the string is in the presence of a magnetic field, it is magnetized.

But the other stuff is true as well (variable reluctance).