Pickup theory question

[Steve Conner]

I think if there is room for reductionism in pickup design, it would go like this: The pickup can be thought of as two "filters", with the output of one fed into the input of the second, and the output of the second driving your amp.

A spatial filter made out of the geometry of the strings, magnets, and other ferromagnetic parts of the pickup. (This is Rick's "3-D density", a highly complex non-linear filter that can select some overtones, cancel others, and even create new ones that weren't in the string vibration.)

An electrical filter made of coil inductances, resistances, eddy current losses, and self-capacitances.

Now, something like using different kinds of wire is easy. Assuming that the geometry of the coil stays the same, then the spatial filter should be unchanged and only the electrical one should be affected, in a way that you can easily predict from the physics of resonant circuits.

But the problem is that many of the physical changes you can make in pickup design affect both filters. For instance, changing the magnet in a humbucker. Unlike a single-coil, the magnet is down at the bottom, far from the strings, so it shouldn't affect the shape of the field at the pole pieces.

But does it? It's not really that far. What if the magnet's own field gets up there directly, as well as travelling through the pole pieces? What if a stronger magnet saturates the pole pieces a little more and changes their permeability, changing the shape of the fringing fields up at the string ends? So maybe messing with the magnet changes the spatial filter, even if the new magnet is the same shape and size as the old one.

It also changes the electrical filter in the naive way that the magnet strength multiplies the gain. But the magnet will have complex permeability, leading to both a change in inductance and frequency-dependent eddy current losses. The pole pieces will also have complex permeability. I discussed changes in the real part above, but maybe the imaginary part changes with field strength too.

Finally, with our talk of resonant peak and Q, maybe we have the assumption that the electrical filter is a simple second-order lowpass. It could be more than that. There is no single capacitance, it's distributed amongst the coil. And humbuckers have two coils each with its own self-capacitance, not to mention both electric and magnetic coupling between them, the latter depending on the permeability of the magnet. They could have a fourth-order electrical filter, several resonances, ripples, notches, you name it.

I've heard it said that any "science" with more than seven variables is actually an art. I think I hinted at about 25 there.

Also, ears are great, but they're easily fooled. The research done into audio compression algorithms spells out exactly how to fool them. My own form of reductionism is that I refuse to spend time thinking about any mechanism in music electronics, unless I can prove to myself that it would make 3dB or more of a difference to the audio. I think any of the mechanisms I mentioned above would be good for a couple of dB at some frequency or other: I wouldn't have mentioned them otherwise. (I allow for the possibility that some of you might have "Golden ears" calibrated to 1.5dB. )

Oh, and Rick, you need to stick a dodecahedron on top.

[CRU JONES]

Steve, good post man. How come you aren't yelling? Don't you see that everyone here is yelling?

Do some tests bitches! Atleast mike does tests!

Cru

[Mike Sulzer]

............

An electrical filter made of coil inductances, resistances, eddy current losses, and self-capacitances.

.................

Finally, with our talk of resonant peak and Q, maybe we have the assumption that the electrical filter is a simple second-order lowpass. It could be more than that. There is no single capacitance, it's distributed amongst the coil. And humbuckers have two coils each with its own self-capacitance, not to mention both electric and magnetic coupling between them, the latter depending on the permeability of the magnet. They could have a fourth-order electrical filter, several resonances, ripples, notches, you name it.

For steel cored pickups, the filter function is indeed much more complicated than 2nd order low pass. (http://music-electronics-forum.com/t12276, and Helmuth Lemme's web pages about pickup measurements) Although the most obvious effect of eddy currents is at the resonant peak since the impedance is highest there, there are effects at lower frequencies, too; they are inductive as well as resistive, and involve the square root of frequency as well as the first power.

The coils do couple; this can be measured.

Effects involving the separate resonances of the two coils and their coupling are generally at pretty high frequencies. Remember, it is the cable capacitance that dominates. The resonance of an 8.5K humbucker (#42) can be as high as 16 KHz if it is run into a buffer rather than a cable.

[Joe Gwinn]

Project Pythagoras in the Age of Machines

I've got to build that test rig I designed. The thing was called "Project Pythagoras", and the monochord is to be fully adjustable for string length and tension, string after length and break angle at each end of the string, with a movable platform to carry and test magnetic pickups at various places along the string, pickup height adjustment, piezo pickups at each end of the string, various string pluck or drive devices also adjustable along the string length, etc. It is designed to stand vertically so gravity does not affect string motion. It will be built out of sand filled aluminum tubing (3 pieces) with movable platforms with a hole running down the center of each platform through which the string will stretch. The idea is to make it as non-resonant as is reasonable, and as free from flexure as possible.

With such a device, it should be possible to standardize many now-ethereal factors re. pickups including output from a standard string pluck, and hopefully allow quantifying some of these effects that seem to be present, yet have not been adequately described. We don't even have a proper spec for absolute phase of pickups yet. We're in the dark ages.

I have some construction ideas to offer.

First, aluminum is very elastic, and will prove hard to damp sufficiently unless you fill it with polymer concrete, and/or viscous dampers. The master of such construction is MIT Professor Alexander Slocum, author of "Precision Machine Design".

For instance http://www.me.ntou.edu.tw/coursemat/...al_design1.pdf. This is an excerpt from a course based the book "Precision Machine Design" by Alexander Slocum, published by the Society of Manufacturing Engineers (SME) in 1992. The book used to be available online in chapters somewhere in the MIT website. Chapter 7, especially section 7.4, is relevant.

Precision machine design - Google Books

Cast iron is very stiff, easily cast and machined, and has high damping, which is why most machine tool frames are made of cast iron. It strikes me that you could use the bed from an old worn out lathe as your instrument body. The bed would be mounted vertically and the string stretched between carriage and tailstock, the headstock (chuck end) having been removed. Such lathes go for a few hundred dollars or even free, and most of the fancy machining is already done.

[Rick Turner]

Joe, fabulous idea as long as the iron of the machine doesn't "tilt" results with magnetic circuits and all that. There's a used machinery place nearby here; I'll check them out next week.

The other possibility would be carbon fiber. I'd considered the polymer concrete idea with the aluminum. By having three columns with the string running down the center, bending issues should be fairly under control. There are also polyurethane foam compounds used for viscous damping.

One disadvantage to reuse of a lathe is that this test bed would be hard to reproduce. I'd like to come up with something that could be published and then reproduced at reasonable cost.

[Joe Gwinn]

Joe, fabulous idea as long as the iron of the machine doesn't "tilt" results with magnetic circuits and all that. There's a used machinery place nearby here; I'll check them out next week.

If the pickup is kept reasonably far away from the iron, the tilt should be reasonable. Cast iron isn't all that permeable, as ferrous alloys go. The easy test is to measure the inductance and AC resistance of a singlecoil as a function of distance from the iron bed of one of your machines. A 9" lathe is designed for a 4.5" height of rotation axis (and thus convenient string location) above the bed.

Pianos have cast iron frames. Has anyone ever made an electromagnetic pickup for a piano?

The other possibility would be carbon fiber. I'd considered the polymer concrete idea with the aluminum. By having three columns with the string running down the center, bending issues should be fairly under control. There are also polyurethane foam compounds used for viscous damping.

Carbon fiber isn't well damped. Polyurethane foam isn't stiff enough to provide much damping for the three-column structure. Slocum invented a damper that consists of a polished steel rod smeared with silicon grease and then epoxy potted in a aluminum or steel tube. The grease acts as mold release and as the viscous layer. The details may be found in his patents.

One disadvantage to reuse of a lathe is that this test bed would be hard to reproduce. I'd like to come up with something that could be published and then reproduced at reasonable cost.

Use a South Bend lathe bed of a common size. South Bend lathes are common as dirt, and worn out ones are easy to come by, at least in the US.


If you really want to fabricate a frame, I would guess that square 6061 aluminum tube filled with granite chips in polyester would work. But, I would go over Slocum's book and patents first, with particular attention to the prediction of dynamic damping coefficient.

Also important is the resonant frequency, which should be a factor higher than the string fundamental.

And then there is acoustic impedance: The string must be terminated in something far stiffer than the string, so string motion causes little motion of the mounts. It's hard to beat a big piece of metal here.


Or, the brutally simple approach: Get a big solid bar of 6061 aluminum and have a standard 3/8" or 1/2" T-slot milled into it and the top milled flat. (There may also be a extrusion available from catalogs.) Then, one can clamp various aluminum blocks to the beam. Damp the beam by gluing a thin piece of 6061 sheet to the beam using a viscous rubber of some kind, to implement a constrained-layer damper.

But I would start with the old lathe bed, and work the details out first, before undertaking the construction of a custom T-slot bar et al.

[Rick Turner]

Joe, re. mag pickups for pianos, yes, it's been done. That is the Helpinstill pickup: Helpinstill Piano Pickups

Duly noted and thanks re. the suggestions. I'll check Rowe Machinery for a lathe bed.

[Joe Gwinn]

Joe, re. mag pickups for pianos, yes, it's been done. That is the Helpinstill pickup: Helpinstill Piano Pickups

Cool. I'll have to look up Helpinstill's patents. It the video, it looks like he is using a very long but otherwise ordinary blade (or double blade) pickup.

Duly noted and thanks re. the suggestions. I'll check Rowe Machinery for a lathe bed.

Another thing that occurs to me is that it probably is not necessary to make the string vertical, as the string tension is far larger than the weight of the string.

[Joe Gwinn]

Piano pickup patent

I found the two relevant patents, one for piano pickups (4,212,220), the other for acoustic guitars with steel strings (4,145,944), both to Ezra Charles Helpinstill II. The design is dead simple, and he uses rubber magnets. All of Lace's patents cite 4,145,944.


There is a Ezra Charles Helpinstill III also inventing piano-related things; I assume that he is the son.

[CRU JONES]

Or, the brutally simple approach: Get a big solid bar of 6061 aluminum and have a standard 3/8" or 1/2" T-slot milled into it and the top milled flat. (There may also be a extrusion available from catalogs.) Then, one can clamp various aluminum blocks to the beam. Damp the beam by gluing a thin piece of 6061 sheet to the beam using a viscous rubber of some kind, to implement a constrained-layer damper.

Joe,
What do you think about the usefulness of this (aluminum extruded T-slot):
80/20 Inc. - The Industrial Erector Set®

I have used this stuff for the creation of many jigs and machines in the past and have found it to be very useful. There is also a CAD plug-in for their parts that would make drafting plans and sharing part numbers very easy.

Cru

[Joe Gwinn]

Joe,
What do you think about the usefulness of this (aluminum extruded T-slot):
80/20 Inc. - The Industrial Erector Set®

I have used this stuff for the creation of many jigs and machines in the past and have found it to be very useful. There is also a CAD plug-in for their parts that would make drafting plans and sharing part numbers very easy.

That stuff looks pretty useful. The 3030 cross-section could work, especially if the center cavity is filled with granite chips and polyester resin or some other damping fill.

My fear is that the resonant frequency could be too low, although I suppose we would not have an active length (string length) exceeding three feet. Anyway, this looks like a reasonable design approach.

[David King]

Why not string up a Chinese granite surface plate from Enco? You can't beat the price aside from delivery and the resonances should be minimal with the elastomer layer and thin aluminum plate damping method if necessary. It's still useful for sanding (or boat anchor) when you finally give up your Quixotic research.

[David Schwab]

When AlNiCo is the pole piece, the inductance of a pickup with AlNiCo2 is about 2x that of AlNiCo5, and the ratio of the resonant frequency Al2/Al5 ~ 0.7.

Also, as a reference, the inductance with AlNiCo5 is only about 1.5x the coil by itself.

I thought that was the case, but I wasn't sure. I had seemed to remember reading that someone once though.

[David Schwab]

The famous example is the Barden Blade which uses ceramic magnets (viz., non-conductive). Copper foil electrically connects the blades since the pickups are otherwise too bright sounding.

I'm sure that was actually for grounding purposes. Every blade pickup I made that way. Do you think it would alter the tone without the two blades connected together? My blades are also connected together like that.

Also, any ideas of how eddy currents act on a grounded surface? I know they don't sound any different if you lift the ground on the blades.

[Joe Gwinn]

Also, any ideas of how eddy currents act on a grounded surface? I know they don't sound any different if you lift the ground on the blades.

Eddy currents go in closed loops, like like ... eddies.

Grounding at exactly one place has no effect on eddy currents.