Pickup height on a new SG

[Liam]

The farther the pickup is from the strings, the weaker the output. It actually drops off at the square of the distance.

Not quite, the field close to a magnet doesn't obey anything as simple as inverse square law. The output most likely drops off at the same rate as the flux density at right angles to the string movement. The "B" part of the equation is generally referred to as "how much flux is being cut by the strings". (You need to picture magnetic field lines eminating from the top of a polepiece here.)

This means that when the string is close to the pickup, it stays within an area of magnetic flux that is fairly uniform in density and direction. So string movements are fairly linearly translated into current flow.

However when a string is further away the flux density is higher towards the centre of the magnet, and of course the direction of the flux lines off-center are no longer at right angles to the string's movement. So you're right, we lose efficiency, but not linearly. Large amplitude string vibrations are reproduced less effectively than small amplitude. So pickups maybe sound a little thinner, and obviously have less output. But the thing I really like is that if you hit the strings quite hard, the early part of the note has a little less attack, and as the note dies there is a compressor like effect that enhances the sustain as the string is vibrating close to the magnet centre line. I appreciate the signal will potentially be noisier, but it's not a problem with humbuckers so far.

FWIW I remember telling someone what I liked about the sound of slightly bigger string to pickup clearances before I figured out what it is likely to be caused by (he thought I was mad). Years later as I learnt more about e.m. theory I decided this was a good explanation for what I was hearing.

I tend to run single coil pickups at slightly closer spacing, but then they have such different tonal and note shaping characteristics...

I know none of this is new - but I still think it's valid to do the experiment for yourself - it does more than just weaken the output. And whether the theory works or not, I bought a Les Paul a couple of months back, tried playing it, thought "euch!", and had to back the pickups off to where they sounded good to me. When I think about it I'll measure where the "sweet spot" was for me.

Had enough yet, David? I could type all night (forgot how much I enjoy heated debate on ampage - thank you!)

[David Schwab]

at right angles to the string movement.

And which way does the string move? It's moving in a circular motion. Part of the time it's moving parallel to the surface of the pickup, and the other half of the time it's perpendicular.

This means that when the string is close to the pickup, it stays within an area of magnetic flux that is fairly uniform in density and direction. So string movements are fairly linearly translated into current flow.

Depends on how the magnetic poles are set up. Look over the two Bartolini patents for an example of shaping the flux field to pickup certain motions of the strings.

However when a string is further away the flux density is higher towards the centre of the magnet, and of course the direction of the flux lines off-center are no longer at right angles to the string's movement.

Once again, this is making assumptions on the magnetic poles of the pickup. What if it has blade poles? Then of course a humbucker is quite different from a single coil with rod magnets.

So you're right, we lose efficiency, but not linearly. Large amplitude string vibrations are reproduced less effectively than small amplitude. So pickups maybe sound a little thinner, and obviously have less output. But the thing I really like is that if you hit the strings quite hard, the early part of the note has a little less attack, and as the note dies there is a compressor like effect that enhances the sustain as the string is vibrating close to the magnet centre line. I appreciate the signal will potentially be noisier, but it's not a problem with humbuckers so far.

As I pointed out previously... check out the two Bartolini patents, especially the first one. He used a wide flat topped magnetic pole to distribute the field close to the top of the pickup, so the lines of flux are more parallel to the top. This picks up more up and down motion than side to side.

As far as the distance from the strings... check out the chart by Mike Sulzer as discussed over at Project Guitar:

http://projectguitar.ibforums.com/in...ic=32514&st=90

This figure (http://www.naic.edu/~sulzer/nlmPickupResp.png) shows the result discussed this post (and part of the next) . First consider the set of curves near the top of the plot. The five peaked plots show the magnetic field with the string located at five different heights above the pole piece. (We are looking at the field along a vertical contour extending up from the pole piece .05 inches from its center. This contour passes through the "string".) Also, one line (dark blue) shows the field with no "string". It is the baseline that the other fields approach as the distance from the string is increased.

The lowest set of curves is like the upper set, but with the baseline subtracted. The green line running along the peaks of the curves is the the field with no string, scaled by 40% in order to show that the variation with distance of the intensity of the magnetization of the string is very nearly the same as the field that causes it. We would expect this to be true because the string has very little material compared to the pole piece. But we would not necessarily expect such a simple result if we put a much larger piece of steel over the pole piece. Magnetic problems can be quite complicated, and it is good to find a simple result!

[Liam]

And which way does the string move? It's moving in a circular motion. Part of the time it's moving parallel to the surface of the pickup, and the other half of the time it's perpendicular.

I've never bought that. Probably elliptical (I'm only any good at Fourier analysis in 2 dimesions), but I reckon it's got to have the major axis roughly parallel to the pickup. Unless you're big into finger or thumb picking, might just get circular in that case, but more likely still an offset ellipse.

Depends on how the magnetic poles are set up. Look over the two Bartolini patents for an example of shaping the flux field to pickup certain motions of the strings.

Let's just limit ourselves to the guitar in question for the moment - I'm not as well read as you on Bartolini patents - so have been trying to understand humbuckers.

Once again, this is making assumptions on the magnetic poles of the pickup. What if it has blade poles? Then of course a humbucker is quite different from a single coil with rod magnets.

That's why they use ferrous pole pieces, the rectangular magnets are purely a driver, and IMO it's the shape of the pole pieces that shape the field (I say IMO because I've not measured it.) Having said that, if the shape/magnetism of the bars determined the field, humbuckers would not have any output.

The graph is lovely, but only shows differing height, doesn't show distance off-center. You've got to admit, it's nowhere near inverse square law though, at very best it's just inverse. Need to read up on this and see what's measured, and what's from theory. Good threads, thanks for the links.

You've given me a lot of interesting stuff to read up on...so once again - thank you!

Liam

[Liam]

As promised, string to pickup heights on my favourite Les Paul. 57 Classics, I'm afraid, so YMMV.

Bridge pickup: Low E side - 0.100" (2.5 mm), High E side - 0.110" (2.8 mm)
Neck pickup: Low E side - 0.115" (2.9 mm), High E side - 0.105" (2.7 mm)

After all the above, disturbingly close to factory recommendations...and they're pretty well parallel to the strings!

Ah well.

Liam

[David Schwab]

I've never bought that. Probably elliptical (I'm only any good at Fourier analysis in 2 dimesions), but I reckon it's got to have the major axis roughly parallel to the pickup. Unless you're big into finger or thumb picking, might just get circular in that case, but more likely still an offset ellipse.

Yep, that's how you can get to many tones with your fingers! Bill Bartolini points out that on an acoustic guitar, the bridge is confined to mostly move up and down, so you mostly hear the string's motion perpendicular to the sound board. He figured that contributes to what makes an acoustic instrument sound the way it does. So he restricted the field on the pickups to mimic that.

Let's just limit ourselves to the guitar in question for the moment - I'm not as well read as you on Bartolini patents - so have been trying to understand humbuckers.

That's why they use ferrous pole pieces, the rectangular magnets are purely a driver, and IMO it's the shape of the pole pieces that shape the field (I say IMO because I've not measured it.) Having said that, if the shape/magnetism of the bars determined the field, humbuckers would not have any output.[/quote]

I was fascinated with Bartolini's designs because they rely on square coil cores and wide planar "shaping faces" on top of those. He also originally used a multitude of coils, all sharing a common magnet and "shaping faces". Some of the other designs have trapezoidal pole tip faces.

I haven't tried any of these ideas, but it's interesting to put that much attention into the shape of the magnetic field.

Similarly, Seth Lover didn't want adjustable pole pieces in the original humbucker. He also didn't like the screws extending out the back of the baseplate.

The graph is lovely, but only shows differing height, doesn't show distance off-center. You've got to admit, it's nowhere near inverse square law though, at very best it's just inverse. Need to read up on this and see what's measured, and what's from theory. Good threads, thanks for the links.

If you read that thread over at PG, you see where they did tests with a small coil that was near the top of the core, then they would flip the coil over so it was away from the strings.

I did some tests with a stacked tele pickup I made. Both coils were would the same, and the magnets were on the bottom, P-90 style. The bottom coil sounds drastically different from the top, just be virtue of it's distance from the strings, even though the pole hasn't moved. The top coil was twangy and bright, and the bottom coil sounded like a jazz guitar... mellow and warm.

You've given me a lot of interesting stuff to read up on...so once again - thank you!

Thank you too. Always nice to have a debate on such things. Sopmetimes we get stuck with certain ideas, and then you think about it and say to yourself "hey wait a minute... that does make sense" Then you figure out how to fit that into your model of the universe.