Magnetic fields

Pickups, magnets, microphones, amps, speakers, cabs, whatever...
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Mark Wybierala
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Joined: Fri Feb 03, 2012 10:14 am
Location: Central New Jersey

Magnetic fields

Post by Mark Wybierala »

Does a non-magnetic metal such as brass have an effect on the magnetic field of a magnet? For the sake of discussion, assume that the alloy in use is completely nonmagnetic.

The intent is to understand the implications of using brass for a pickup bobbin. I made a few of these some years ago but never actually tested them. The idea here is that the bobbin itself if grounded would provide some shielding and brass polishes up to a mirror finish -- years later it will still always polish up to a mirror finish. The downside is that the coil can short to the bobbin but it wouldn't be difficult to insulate the interior of the bobbing with paper and polyurethane.

I need to build some pickups for a cigar box guitar. A P-Bass pickup is the easiest. I'm also thinking maybe plastic sewing machine bobbins -- too easy, but something unique would be cool too.
Brian Evans
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Re: Magnetic fields

Post by Brian Evans »

It's a good question, and the answer seems to be "maybe". Brass is, in it's simplest form, alloyed of two non-magnetic metals, but in it's common form might have a number of semi-magnetic components. So it depends what brass you get. But - while brass isn't ferromagnetic it is paramagnetic (or diamagnetic, couldn't find a perfect answer) which means that while it can't be magnetized it does react to external magnetic fields while they exist. So it's a suck it and see situation if your brass bobbins will work, will work exactly the way a plastic or non-metallic bobbin would work, and if a difference might be bettter or worse for your application. My gut is that they will work just fine, there are and have been a lot of pickups with a lot of metal in them, and I suspect a lot of plated brass pickup covers or shrouds. The music gets out somehow...

Edit - if you think about it, brass is mostly copper and copper is what the coil is wound with, so the magnetic field interacts with the copper just fine. The more copper, the more the interaction. It's not like the copper winding shields the flux from getting to the middle of the coil.
David King
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Re: Magnetic fields

Post by David King »

Brass being an alloy of copper and zinc and often with 0.5% lead is always nonmagnetic. However brass is conductive and it has a quite audible effect in a pickup by nature of eddy currents, i.e. current flows that are induced by electrons moving back and forth in the adjacent coil. Brass will potentially dull the sound, attenuating the higher frequencies which may be a good thing or not in your intended instrument. Insulating a brass bobbin against coil shorts for the long term may be a lot harder than it first appears. I'd go with a high quality masking tape and overlap all the joints slightly.

If you want a good demonstration of eddy currents in action then drop a small neodymium magnet down a section of copper pipe and see what happens. Youtube has hundreds of demonstrations but seeing it with your own eyes in way more compelling.
Halgeir Wold
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Re: Magnetic fields

Post by Halgeir Wold »

In RF circuits, ferrous coil cores will increase inductance, while aluminium or brass will lower the unductance somewhat... I've used this effect sometimes in radio circuits.. No real idea what it will do in a PUP......
John Eisenhower
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Location: UK

Re: Magnetic fields

Post by John Eisenhower »

It’s is important to understand that all matter responds in some way or another to magnetic fields ... even seemingly ‘inert’ stuff like wood, humans, ceramics. But metals are especially prone to magnetic influences - all metals for the purpose of this discussion. Ceramic materials, too. That is Ceramic materials that are formed by the oxides of metals. They are mixed and sintered, then used as cores in many electronic circuits.

If you place metal in the presence of a **changing** magnetic field, a voltage in induced in the metal. From a guitar strings point of view, the vibrating string sort-of stirs up the static field of the pickup magnets. This then induces a voltage in the pickup coil. All good so far. If you put your coil around a brass former that, too, will have a voltage induced in it. But ... it works to **oppose** the voltage induced in the coil. (Yes, I know this is a contraction of what really happens, but for this purpose it is sufficient).

Because the vibrating string produces harmonics as a normal process, the induced voltage in both coil and brass are frequency dependant over a fairly wide range. The brass contributes little opposition to the coil voltage, but it is there! The end result tends to roll off the higher harmonics. Brass shielding over pickups also causes this problem and is one of the reasons covered humbuckers, for example, tend to sound ‘darker’ than single coil pickups.

Putting copper foil inside pickup cavities contributes, too. There is a mathematical formula that relates frequency (musical pitch) to the metal in question and other parameters to give a thickness of metal that will stop a signal going through the metal thickness: for power line frequencies - 50Hz or 60Hz - it requires about a 10mm thickness of copper plate to achieve!!!

Returning to the original post, metal is never used as a pickup former for the above reasons.

John
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