Rare Earth P-90? Experienced Winders Please Have A Look?

[nerosrevenge]

Hi there. I have decided that I want to wind my own pick ups for my current build. I had a set of Seymour Duncan P-90's and somehow I have misplaced the box that they were in.

So here's is my dilemma, I can't seem to find a supplier up here in Canada for the Alnico 5's, so I did some digging around and found a few posts regarding neodymium (rare earth) magnets. Considering the strength of these little suckers, I felt that using something rather small might work. So...I have prepared this drawing in Sketchup to see how I might assemble one:

The magnets are indicated on the bottom in blue, they are 1/8" x 1/8", I am planning to wind them at about 8000 turns.

Can anyone offer any experience or advice on this idea?

Thanks so much,

STV.

[erikbojerik]

I don't know the P90 magnet dimensions or the field on the surface of A5 when fully charged, but it seems to me you want to find this out before you go ahead with the neos. The neos have a higher field for their size compared with A5, and in air the magnetic field drops with 1/D^2 where D is the distance away from the magnet.

A lot of the P90 sound has to do with the width and height of the coil, so you want to try & match the field strength above the pickup as closely as possible if you're using a regular P90 bobbin. So if a regular P90 gives you (let's say) 20 gauss @ 1cm above the pickup, you want a certain size and distance below the pickup surface so that your neos will match that. Otherwise you may wind up with (no pun) a pickup that is much stronger or weaker than intended.

Basically, you need a meter with a hall probe to measure the field.

[nerosrevenge]

Forgive me if this is a newbie type of question but I am...well...new to this is there a way to accurately calculate this in advance of building?

[Mike Sulzer]

Forgive me if this is a newbie type of question but I am...well...new to this is there a way to accurately calculate this in advance of building?

This is not that easy to calculate; if you are an expert with FEMM, you could model it. Those magnets are about the right size. I have made plenty of pickups with neos, and one nice thiing is that you can get several different sizes of magnets and try them all. No need to glue them on for testing.

[nerosrevenge]

Do you know if I can I get FEMM for Mac? I gave done a quick search just now but I'm not having any luck.

[Mike Sulzer]

Do you know if I can I get FEMM for Mac? I gave done a quick search just now but I'm not having any luck.

No, I have the same problem. But I have a new enough mac (intel chip) so that I can use a beta version of Apple's "boot camp" on it, whihc allows booting in either 10 or windows. But you have to get windows. I am working on this now.

[Mike Sulzer]

I don't know the P90 magnet dimensions or the field on the surface of A5 when fully charged, but it seems to me you want to find this out before you go ahead with the neos. The neos have a higher field for their size compared with A5, and in air the magnetic field drops with 1/D^2 where D is the distance away from the magnet.

A lot of the P90 sound has to do with the width and height of the coil, so you want to try & match the field strength above the pickup as closely as possible if you're using a regular P90 bobbin. So if a regular P90 gives you (let's say) 20 gauss @ 1cm above the pickup, you want a certain size and distance below the pickup surface so that your neos will match that. Otherwise you may wind up with (no pun) a pickup that is much stronger or weaker than intended.

Basically, you need a meter with a hall probe to measure the field.

The magnetic field falls off with 1/D^3 not (^2) except near the magnet. That is, the field is a very good approximation of a dipole (having + and - nearby) once you are "far" from it in terms of its size. Close up is complicated. (This is from, for example, page 186 of the latest version of Jackson.) Are you sure you want to stating that a lot of the sound has to do with the dimensions of the coil when the FEMM plots show that the string is affected by the permanent magnet only near the pole piece?

[SwedishLuthier]

Please Mike, don’t make this thread go down the same path as the recent P90 design-thread.

Do not underestimate the experience of a world of pickup winders. The common understanding among experienced winders is that the shape of the coil and the shape of the resulting magnetic field are among the most important factors that affect the tone in a pickup.

Are you sure you want to stating that a lot of the sound has to do with the dimensions of the coil when the FEMM plots show that the string is affected by the permanent magnet only near the pole piece?

According to you, in the earlier P90 thread, the bottom of the coil and the magnetic field in that part of the coil (please correct me if I have misunderstood you) had little to do with the output and tone of the pickup. Now you imply that the shape (the short, wide coil in this case) doesn’t really affect the tone (once again feel free to correct me if I have misunderstood you). As you have very specific opinions about what the factors are, it would be *very* interesting to hear your thoughts on what *does* affect the tone and what not.

Nerosrevenge: If you whant a P90 sound get some ALNICO5 bar magnets. They are the same size as the once used in traditional Gibson HBs, exactly like these:

http://www.stewmac.com/shop/Electronics,_p...ckup_Parts.html

these:

http://www.guitarjonesusa.com/catalog/prod...products_id=474

these:

http://www.guitarpartsusa.com/item--Guitar...Magnet--PROD391

or these:

http://www.allparts.com/store/pickup-cover...000,Product.asp

At a few dollars I find it unwise to start with alternative designs for your first winds. Start with something close to the design icons, and when you get the feel of what factors to tweak, start doing it then. Take it from one of the experienced winders you asked in the thread title

[Mike Sulzer]

Please Mike, don’t make this thread go down the same path as the recent P90 design-thread.

As you have very specific opinions about what the factors are, it would be *very* interesting to hear your thoughts on what *does* affect the tone and what not.

Peter, I have attempted to answer your questions on that other P-90 thread as best as I can. Remember, you introduced Faraday's law of induction, and you asked me to explain why the component through the coil is what matters. (And you have not yet commented on that explanation.)

The question is why does a P-90 sound very different from, for example, a Fender type SC? Multiple reasons, but I think the most important difference is the frequency response. The P-90 has a lower resonant frequency than a strat type pickup. Different resonant frequencies mean that they sound different. The coil inductance is one very important factor in determining the resonant frequency, but it is not the only one. And the dimensions of the coil are a factor in determining the inductance, but not the only ones.

But what I was really referring to in my post that you quoted is the belief that a wide coil samples the string over a greater part of its length than a narrow coil. The FEMM plots do not support that claim. Is it still not clear why?

[nerosrevenge]

Thanks for the advice Peter. I am having difficulty sourcing Alnico magnets in my area (although I was searching for the rod type, not the bars) I will dig around again and see it I can find some. I think I am going to build one based on the drawing anyhow and see how it turns out, but your advice is well heeded. Thank you very much for contributing!

This is so much fun!

STV.

[erikbojerik]

IIRC P90s do not have magnetic slugs; the magnets are bar magnets underneath the pickup, which contact the pole pieces.

Mike you are correct, the field drops as 1/D^3 (I mistook it as the force applied to a charged particle, which does go as 1/D^2....).

Coil geometry and induced fields will indeed have an effect on the 3D distribution of field lines above a pickup (lines which a vibrating string interacts with), no doubt.

[SwedishLuthier]

Peter, I have attempted to answer your questions on that other P-90 thread as best as I can. Remember, you introduced Faraday's law of induction, and you asked me to explain why the component through the coil is what matters. (And you have not yet commented on that explanation.)

The question is why does a P-90 sound very different from, for example, a Fender type SC? Multiple reasons, but I think the most important difference is the frequency response. The P-90 has a lower resonant frequency than a strat type pickup. Different resonant frequencies mean that they sound different. The coil inductance is one very important factor in determining the resonant frequency, but it is not the only one. And the dimensions of the coil are a factor in determining the inductance, but not the only ones.

But what I was really referring to in my post that you quoted is the belief that a wide coil samples the string over a greater part of its length than a narrow coil. The FEMM plots do not support that claim. Is it still not clear why?

Mike, I left that discussion as I have a policy of not taking part of discussions were people (not you Mike, not you) start to call each other BS

My problem with the last P90 thread we had was your way of using science in a way that isn’t contributing to the discussion.

1 It actually scares a lot of guys away because they do not understand what you are talking about. I have 2 yeast of university in physics and math (god knows that is 20 years ago) and I have a hard time following you.

2 The world is seldom as simple as we try to describe it. Especially as the pickup is a complex system, involving a lot of different disciplines of science and we need to understand all of them in depth to be able to draw conclusions. Very few of us have that ability. I pulled out the law of induction to help people understand the basic functionality of a pickup. If I thought that it would scare people off I would have left it behind, and I might do that in the future.

3 The science gives us very few *practical* leads to how we can enhance the existing pickup designs. In real life there are only a few factors that we can change; the shape of the coil, the shape and strength of the magnetic field, the amount of turns and the resistance (via different wires).

You are saying that the main factor that make up the difference in sound between a P90 and a fender style SC is the frequency response. Well, yeah, the frequency response is what I in layman’s terms call “the sound”. That didn’t make me any smarter. The coil inductance? Of cause it will change the entire signal circuit and thus change the sound. But which of the factors that we actually can manipulate are the main contributing factors? That is in my opinion a much better question to ask. And I would very much like to hear your thoughts on that.

Nerosrevenge: Shoot me a PM and I will send you a pair of A5 bar magnets in the mail. I buy in bulk so it won’t cost me that much more than the stamps. I can also send you a CAD pdf with P90 bobbin drawings.

[Mike Sulzer]

Coil geometry and induced fields will indeed have an effect on the 3D distribution of field lines above a pickup (lines which a vibrating string interacts with), no doubt.

Thank you for responding. This is interesting. Current flowing in the coil could indeed cause a magnetic field which could influence the strings, but it would have to be large to do so. The effect would be to damp the string vibration because energy would be dissipated in the electric circuit. But this is not a significant effect. If it were, you would notice the sustain dropping as you raised the pickup. (But a strong permanent field can do this: string pull, or stratitis.)

[nerosrevenge]

With reference to the bar magnets, I realized that about half way into this. The problem is that there doesn't seem to be a whole lot of information out there on the exact construction of the original P-90's (by information, I mean photographs, dimensions, etc.) that is not to say that there is no information out there, just not an abundance. Without having one to look at (or two, like I originally had ) it made the whole experience a little frustrating.

Just for kicks, if anyone has an original P-90 and would like to share some photographs of the top, bottom and sides of the pickup with the cover off, along with dimensions (lol, not asking too much huh?!) I would be more than happy to do up another model based on the original design.

Just a thought.

Anyhow, I got my RE magnets today, as I said I am going to wind one up with those just for kicks, I am starting the bobbin tomorrow (I hope) now I just need to find somewhere nearby (Toronto perhaps) where I can get some wire!

Cheers!

STV.

NICE - Seems I spoke too soon, just after I posted this I found this image:

That should help!

Edited September 24, 2007 by nerosrevenge

[Mike Sulzer]

"Mike, I left that discussion as I have a policy of not taking part of discussions were people (not you Mike, not you) start to call each other BS"

OK, Peter, I understand. I am stubborn, and figured the best thing to do was to ignore it and keep going.

"My problem with the last P90 thread we had was your way of using science in a way that isn’t contributing to the discussion. "

I do not see any way to understand this stuff without science. I will try to keep it simple.

"...the pickup is a complex system, involving a lot of different disciplines of science and we need to understand all of them in depth to be able to draw conclusions."

Very true. That is what makes pickups interesting.

"3 The science gives us very few *practical* leads to how we can enhance the existing pickup designs. In real life there are only a few factors that we can change; the shape of the coil, the shape and strength of the magnetic field, the amount of turns and the resistance (via different wires). "

In addition to those things, you can use different types of magnetic materials to good advantage. But you really do need to understand the science.

"Frequency Response": I mean the effects of the things in the circuit. The law of induction says the sensitivity continues to increase with frequency. But that is not what gets to the amplfier. The circuit elements make a resonant low pass filter. One can get an idea of the importance of this by going to the Seymour Duncan web page and looking up the resonant frequencies of various pickups. Do these frequencies correspond with how the various types of pickups sound in general? I think so, at least approximately. So it is important to study the pickup circuit and understand how it works.

I will try to think of better ways to explain these things.

Mike

[Myka Guitars]

I don't have anything to add to this thread since I am not versed in pickup making but I do want to say that I appreciate both the layman's explanations as well as the hard core science behind it. I really do not think that Mike was out of line by getting deep into it. I actually thought it was a refreshing approach. Most people (it is true) just want to know what to do and not why. But there are a few of us that really enjoy the challenge of wrapping our minds around the science at work behind the scenes. Some of the more involved tangents may not yeild a simple 'you need a bigger pole piece' but it is valuable just the same.

~David

[Sock Puppet]

Maybe you've seen these p90 kits:

http://www.thomann.de/gb/goeldo_puk90_pick...artner_id=25293

Not particularly cheap though.

S.

[Mike Sulzer]

I don't have anything to add to this thread since I am not versed in pickup making but I do want to say that I appreciate both the layman's explanations as well as the hard core science behind it. I really do not think that Mike was out of line by getting deep into it. I actually thought it was a refreshing approach. Most people (it is true) just want to know what to do and not why. But there are a few of us that really enjoy the challenge of wrapping our minds around the science at work behind the scenes. Some of the more involved tangents may not yeild a simple 'you need a bigger pole piece' but it is valuable just the same.

~David

Thank you, David.

Here is a bit more on the subject that Eric brought up near the end of previous page. It is useful to consider what these things have in common: power or audio transformers, electric motors, electric generators, and speakers. First, they all use magnetic fields; but more important, they all use magnetic field to transfer energy as efficiently as practical. They are what I would call "tightly coupled" systems. When you put power on the primary of the transformer it is transferred to the circuit in the secondary and the circuit in the primary feels the effect of the "load". Motors and speakers turn electrical energy into mechanical; generators do the reverse. Generators are hard to turn because of the electrical energy produced and going into the circuit. Motors and speakers draw energy from the source so that it can be turned into mechanical energy,and some into sound in the case of the speaker. All these devices use strong magnetic fields and confine the fields to the device as much as is reasonable. They use ferromagnetic material to do this, in continuous enclosed structures as much as possible and small "air gaps" where necessary.

A pickup is very different. The permanent field is kept pretty weak so that it does not disturb the vibration of the string. The poles are short rods, or similar, that do not enclose the field lines. As a result, the pickup couples weakly to the string, and so the string does not transfer a lot of energy to the coil and attached circuit, and the tiny current flowing in the coil does not have a significant on the magnetic field or the string. You can think of the coil as just "along for the ride". You can analyze magnets, pole pieces and strings without considering what happens in the coil. Then you need to analyze what happens in the coil and circuit.

[nerosrevenge]

Peter - I have pm'd you.

Sock Puppet, thanks for the link. I have seen several kits around but I think I would rather make them from scratch if I can.

Thanks to everyone for their contributions, this is definitely a learning experience.

STV.

[nerosrevenge]

Update...I scored some Magnet wire on Ebay. 3 lbs for $30! I figured that was a pretty good deal! It is 44 AWG rather than 42, so I guess that means fewer winds to get the desired resistance, correct?

Also, I managed to get the bobbins made today. Here they are with the wire:

I have a line on a used sewing machine so I am hoping to use that as a winder.

I'll keep you all posted!

Steve.

[Mike Sulzer]

Update...I scored some Magnet wire on Ebay. 3 lbs for $30! I figured that was a pretty good deal! It is 44 AWG rather than 42, so I guess that means fewer winds to get the desired resistance, correct?

Also, I managed to get the bobbins made today. Here they are with the wire:

I have a line on a used sewing machine so I am hoping to use that as a winder.

I'll keep you all posted!

Steve.

No, you want to use the same number of turns, not wind for the same resistance. The output is not proportional to the resistance, it is proportional to the number of turns. If everyone uses #42, then you can use the resistance as an alias for the number of turns. You will need to wind for a higher resistance to get enough turns.

[nerosrevenge]

Thanks Mike. Seems as though the original P90's were wound in the 10k range. So my plan is to wind the neck pickup at that and the bridge pickup at around 8.5k.

I would like to reverse wind the bridge pickup for hum canceling. Can anyone comment on how effective that is? Also, do I need to change the orientation of the magnets to make it more effective?

I also have a question about the wire. Is there any benefit to using one wire (say a newer insulated/shielded wire) over something more vintage like the original Gibson type metal shielded wire?

Thanks!

Steve.

[nerosrevenge]

DONE!!

I have finished my first pickup!

Here are the specs:

P90 Style

Wooden bobbin

Brass baseplate

44 AWG wire

Alnico 5 bar magnets (special thanks to Swedish Luthier)

Zinc coated steel pole screws

7000 winds

10k ohm resistance

Pics:

Rear view...

Side view...

As always comments are welcome. This was my first go at it. I was an enjoyable learning experience, now, I have to finish the second one and make up a set of covers!

Cheers,

STV.

[SwedishLuthier]

Way to go Steve. That is looking much better than my first pickup. Did you use the 44AWG wire? The coil look a bit "skinny". And the coil isn't the most even I have seen (or is it a distortion in the picture?), but once again, much better then my first coils. Have you tested it yet? Have you "potted" the pickup? That is almost a must if you want to avoid microphonic feedback.

For the next try (I'm sure there will be a next one) pay extra focus on getting the coil as even as possible. Also maybe make it a bit hotter and go higer on the turn count. Original P90s had 10000 turns of wire.

[nerosrevenge]

Yeah the coil isn't as even as I would have liked. I had intended to wind the coil to 10k winds, but as a result of the coil being uneven I was concerned that it would run off the bobbin.

I will be potting both pickups but I will do that once the next one is finished.

I suspect that this one will be a bit thin sounding as a result of the coil so I will likely use it in the neck position to help balance that out a bit.

I tested it for resistance and continuity but I have not tried it in a guitar yet.

Thanks for the positive comments!

STV