Electronics > Projects, Designs, and Technical Stuff
Electric guitar pickup - without permanent magnets: Core material, etc
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TinkeringSteve:
Howdy,

so I saw this YT video (can't watch & link proper time index right now - will search this later), called something like "Uli Jon Roth rig rundown" (or gear rundown or such).


Somewhere in the video, UJR speaks about his "Sky Guitar" pickups, which very much intrigued me:

Instead of permanent magnets like regular electric guitar's pickups have, his uses an extra coil*, to provide the magnetic field to magnetize the steel strings whose movements then induce the current in the regular pickup coil. (he did  not say it quite like that explicitly, but that's what I imagine - making a coil optimized for pickup tonal properties vs. one to magnetize a core with DC would be different requirements I guess, so I'd assume 2 stacked coils or such).

The advantage of this is that he can switch more fully between single-coil pickup vs. humbucking pickup modes, in that the single coil mode will have only the magnet for one coil active, while the other one is off, and thus not additionally (mechanically) dampening the strings, like you would have with the regular "split coil switch" on some guitars with humbucking pickups, where, for single coil mode, one pickup coil is "hanging in the air", while still 2 rows of magnets are impacting the strings. And that times 2 or 3, depending on how many pickups a guitar is fitted with.

I wonder now, what kind of material would be needed for the core of such a doubly tasked coil - producing a steady field by DC, and picking up audio range (60 to 8k Hz maybe) changes?
And other considerations to make?

Wouldn't it be nice to 3D-print the plastic parts of such a thing and make one's own custom pickup... especially with this principle - I won't quite dish out the money for the Sky Guitar just for that nice little trick ;)

* and external power supply via 1 wire of a stereo TRS cable, as a battery won't last long, he said.
Kleinstein:
To pick up a magnetic field nicely the core would need to be magnetically soft. As there is a air gap there is no need for super high ยต and it is very unlikely to get a high DC field. So a soft ferrite is probably the right choice for the core material.

I somewhat doubt it would make a big different in damping from the open coil and permanent magnet. This usually should not give much damping as there is not much current flow in an open coil, and the frequencies should be low enough that the winding capacitance should not yet be an issue to make it electrical resonant.

At small scale an electromagnet is poor efficiency. So it would mainly produce heat. So I don't think this is a good idea. Modern permanent magnets are quite powerful in comparison.
bob91343:
You need a magnetic field for coil motion to generate voltage.  A second coil can provide that.  The only caveat is that the second coil's field be strong enough to accommodate the signal amplitude to avoid distortion.

Another approach could be a nonmagnetic one.  Piezoelectric, for instance, or a resistance that varies with motion or pressure.  A crystal microphone element for instance.
TinkeringSteve:

--- Quote from: Kleinstein on June 05, 2019, 03:29:13 pm ---I somewhat doubt it would make a big different in damping from the open coil and permanent magnet. This usually should not give much damping as there is not much current flow in an open coil, and the frequencies should be low enough that the winding capacitance should not yet be an issue to make it electrical resonant.

--- End quote ---

This is about mechanical dampening of the string vibrations by the magnetic fields, reducing the duration which the strings continue to vibrate (sustain). Since even adjusting pickup height (distance to strings) makes a difference, this should  definitely make a difference. (I haven't measured it myself, it's common wisdom. Maybe to be taken with a grain of salt)
If you have a pickup which can switch off half of its magnets when only one half of it is used for picking up the signal, it's mechanical dampening of the strings is 1/2. A guitar with 3 single coil pickups - assuming roughly the same magnetic strength and string gauge & material & distance of magnets to strings, might have 1/6 the dampening of a guitar with 3 humbucking pickups?
TinkeringSteve:

--- Quote from: bob91343 on June 05, 2019, 04:25:58 pm ---Another approach could be a nonmagnetic one.  Piezoelectric, for instance, or a resistance that varies with motion or pressure.  A crystal microphone element for instance.

--- End quote ---

Electric guitarists (heh, that sounds funny) don't want their guitars to be microphonic, I would say. This invites feedback on stage. Also, you won't have the typical sounds and sound options from switching between differently positioned pickups.

This really is about emulating more closely having both, a Stratocaster type guitar, and a Gibson SG or Les Paul one, in one instrument, at least sound wise. So every other aspect of this would have to be "traditional", I guess.

The hint about distortion is duly noted.
I guess measuring the magnets' strengths of some typical pickups would be a good idea as a start.

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