Pickup Placement

[Bakeacake08]

I'm currently building my 1-year-old son his own little guitar he can drag around and beat up since he shows much interest in my Taylor acoustic (which he cannot drag around and beat up). After I finish that project, I want to make one for myself. I have a humbucker pickup from an old Epiphone SG style body I acquired awhile back. These guitars only use three strings, so I was wondering of there would be anything beneficial to arranging the pickup so that more of the magnets are under the strings. That is to say, instead of setting it up in the normal fashion and only having half of it covered, I could angle it--or even turn it a full 90 degrees--so that the strings rest over all the magnets. What would this do for the signal? I'm inclined to think it would make it stronger. But I'm also inclined to think there could be some kind of phasing issue as well. But really, I have absolutely no clue, so I'm asking the experts.    Your thoughts?

[Roly]

This diagramme shows some of the oscillatory modes that may be present on a struck string, fundamental (black), 2nd harmonic (red), 4th (yellow), and 6th (violet).

Notice that a pickup placed at "6" will sense all these harmonics and so they will be well represented in the output. But as we move to the left first the violet, and then the yellow, pass through null or nodal points where these is no string movement for that harmonic, and therefore the pickup at these positions won't sense these particular harmonics.

This is a very simplified view of a much more complicated reality, but it should give you an idea of why a pickup may sound quite different in a position that is only slightly different.

I'd be inclined to stick with perpendicular.  It's only a guess but I would expect that placed parallel to the strings (some of the) higher order harmonics might get cancelled out.

When in doubt, try some experiments.   :dbtu:

[Bakeacake08]

Oh, so if I'm understanding that right, the part of the pickup "outside" of the strings would actually be picking up more of the harmonics. Was I right, thought, in thinking that the signal would be stronger (higher voltage) with more of the pickup closer to the strings? If that's true, I'd think I'd be trading off the richness (fullness? quality?) of sound for a stronger signal. Which would maybe work better for a bass-type instrument? I will definitely try to rig something up to play around with it.

[J M Fahey]

As Roly said, perpendicular (or maybe a slight angle, as on a Strat) is best; it will have the richest sound; a pickup along the string will have a lot of self cancellation, specially at higher harmonics, and sound dull and undefined.

We already know that a humbucker with its 2 coils has a wider capture field and in no way can have the brightness and "quack" of a single coil ... unless you short a coil or wire them out of phase, which kills *other*  harmonics and most fundamental.

[Roly]

...um...  the part of the pickup that is not more or less directly under the strings doesn't really play much of a part. 


Perhaps it would help if we revisit how a guitar pickup works.

A simple guitar pickup consists of a bar magnet with a thousand or more turns of wire around it;



The first detail missing above is that unlike the common kids bar magnet that is magnetised end-to-end, this bar is magnetised top-to-bottom.

The string being made of a magnetic alloy provides a path of much less resistance than the air to the magnetic lines of flux, so it "captures" them, somewhat like this;



It is a common misconception that the strings generate the voltage in the coil by moving through the lines of flux.  This would generate a voltage in the string, but the field around and through the coil wouldn't move, and therefore no voltage would be generated.

What actually happens is that the flux captured by the string moves to follow it when it vibrates, causing the entire field pattern to also move, and the moving part that cuts the turns of the pickup coil induces a voltage.


Your next thought is "true, but...".  The closer the string to the pickup the greater the flux capture and movement of the overall field with respect to the coil, so you do get more output.  The "but" is that the string also suffers more electro-dynamic damping so it rings for a shorter time (which may be heard as a loss of upper harmonics).

So we have two effects in opposition and we have to compromise; far away the harmonics are weak in the pickup, while close up the magnetics damp them, so there is an optimum spacing for each situation of pickup magnet strength, string alloy, and personal taste. 


With a bass guitar the strings are pretty massive and the upper harmonics lower in frequency and also stronger, so spacing normally isn't as much of an issue, but again there are so many variables in a given situation that pickup height is set by the "suck it and see" method.  It may be theoretically predictable but it would be so much trouble the SIAS method is a lot more straight forward.


Another issue which is important but often overlooked is the electrical load the pickup coil sees.  I've got an article discussing the issues here;

http://www.ozvalveamps.org/pickups.htm

...but in short the tenor guitar pickup is resonant in the upper range and the resistance and capacitance across the pickup coil has a strong influence on the upper harmonics, bright or dull.

HTH