Increasing magnetic strength of a pole piece

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Increasing magnetic strength of a pole piece

Postby Mark Wybierala » Fri Nov 01, 2019 7:42 pm

I'm having a little adventure with designing pickups for my solid body mandolins and I've made an previous post about this some time ago. Over on another forum, it seems that having weak output of the high E-strings is a common complaint for many electric mandolins. I'm not surprised as the physical movement of a vibrating high E-string on a mandolin is quite small.

I'm wondering if I can improve the output by simply increasing the magnet field strength under just the weak string. The pickup that I made for my prototype is a P90 style but I could try a Fender style with individual magnetized pole pieces instead easily enough. I could use alnico2 for strings 2,3, and 4, and use alnico5 for only the first string(s)
How about simply attaching a small button neodymium to the bottom of the 1st string pole piece??? Anyone have any idea if attaching a button magnet to the bottom of a polepiece will actually work? I'm probably going to try this unless someone has some knowledge toward why this wouldn't work. Its not an expensive experiment. I think my local hardware store sells 4mm button neodymium magnets for just a few $s.

A complication to all of this is the available space for pickup thickness. There isn't a lot of room because the neck joint could be compromised if the pickup route is too deep so I need to keep the height of my coil as flat as I can.

I've seen firsthand the downside of having over-strength magnets. Typically I've encountered problems only with the low E and A-strings on a guitar resulting in intonation and a warbly tone when playing in the upper fretted positions.

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Re: Increasing magnetic strength of a pole piece

Postby Barry Daniels » Sat Nov 02, 2019 12:22 pm

You can increase the magnetic strength of an individual pole piece by passing it between a couple of large neodymium magnets.

I recently built some tele pickups from Stew-Mac parts kits and the alnico pole pieces came uncharged. So I bought two neo magnets about 4" long and set up a jig to hold them the right distance apart to let the pole piece just clear the gap. Pass it back and forth a few times and it comes out like a new magnet.
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Re: Increasing magnetic strength of a pole piece

Postby David King » Tue Nov 05, 2019 12:36 am

I mix and match AlNiCo formulations to balance out strengths but I'll avoid big jumps since those are rarely needed and the sonic differences can be jarring. I'll mix A3 and A2 or A2 and A4 or A4 and A5 or A5 and A8. I wouldn't try jumping from A3 to A5 for instance.
Another trick is to mix 3/16" and 5mm (aka .195") or simply stagger the magnet lengths as Fender did in so many of his designs.
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Re: Increasing magnetic strength of a pole piece

Postby Mark Wybierala » Tue Nov 05, 2019 12:56 pm

David, do you observe a noticeable difference in individual string output with your mixed slugs? From your reply I'll assume you do. I recently discovered a local source for pickup parts just down the road from me. They sell a wide range of bobbins and magnetic slugs at what I consider very reasonable cost. I need to check them out. I might even totally abandon crafting my own bobbins.

The e-string problem is subtle but once you recognize it it's bothersome. Another approach might be to work toward decreasing the magnetic strength of the other three magnets. I found a 3 band eq preamp on ebay and I ordered two of them. I'll be able to boost the output but the downside will be more singlecoil noise so shielding will become a consideration. I've never built a hum cancelling pickup but I know how. I'd rather avoid the extra effort but the prototype instrument I've got deserves a good pickup.

So many options. Its easy to get too many ideas going at once and then the conclusions get muddy.
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Re: Increasing magnetic strength of a pole piece

Postby Brian Evans » Sat Nov 09, 2019 10:41 am

traditionally didn't they adjust for variable output by changing the distance from the magnet (or steel pole piece) to the string? My 1946 Epiphone Zephyr has 6 magnets on threaded posts and you adjust by screwing them in and out. Humbuckers usually have screws to adjust string balance. Or is that sort of thing for show and I've been fooled for 50 years? :)
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Re: Increasing magnetic strength of a pole piece

Postby Mark Wybierala » Sat Nov 09, 2019 2:21 pm

Most certainly there is relationship for distance to the string. But I don't observe adjustable polepiece screws as having as much of an impact as moving both the coil and the magnet closer. I believe that the high tension of the mandolin E-strings is moving outside of the effectiveness window of the average magnetic pickup requiring additional compensation actions. I fully admit that I might be under false assumptions. Honestly, I'm looking for discussion.

I used to assume that the pickup operated by the string moving within and disturbing the magnetic field rather than the string becoming magnetized itself and its own magnetic field influencing the coil. I argued this point but have now changed my view.

I don't observe a lot of change when adjusting pole pieces that are not actually permanent magnets while pickups with individual permanent magnetic polepieces are typically not adjustable. There are some exceptions like Gretsch pickups that have a comparatively complicated magnet adjustment configuration.

I have a source for the parts and in a week or two, I'll have a 4-pole pickup with an AL5 for the E-strings and AL2s for the other three. I'm going to put all of the slugs at the same height and let you all know what happens.
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Re: Increasing magnetic strength of a pole piece

Postby Beate Ritzert » Sat Nov 09, 2019 5:29 pm

A P90 like pickup usuallyhas adjustable pole screws or at least easil may have. These can be used quite effectively to compensate for differences in the string response, even relatively large ones. I had to make use of this a lot especially in my problem child archtop.
Of course it should also be possible to increase the magnetic strength locall by using some additional magnets.

A third possibility is to use a dedicated coil for each pole piece and compensate for different output levels electrically. (for-fold weighted addition with a single op-amp).
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