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Horizontal Pickup Magnets?

Discussion in 'Pickups & Electronics [BG]' started by onesandzeroes, Dec 23, 2016.


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  1. onesandzeroes

    onesandzeroes

    Dec 23, 2016
    I have some neodymium magnets that are too long to fit under the string in the usual vertical orientation.

    I was thinking of placing one magnet horizontally under each string with wire wrapped as pictured.

    Does anyone have any experience with this?
     

    Attached Files:

  2. Will_White

    Will_White

    Jul 1, 2011
    Salem, OR
    That's a lot of magnetic pull, a 3/16" N52 Neodymium magnet is roughly equivalent to a strong ceramic magnet and you're looking at 1" + Neos
     
    Last edited: Dec 24, 2016
  3. onesandzeroes

    onesandzeroes

    Dec 23, 2016
    Unfortunately, I'm still a beginner at this stuff. Would a lot of magnetic pull be a good thing?

    I did try building the pickup. About 6000 turns of 40 AWG using four 1/4" x 1" N48 Neodynium magnets laid out horizontally (as shown in the sketch).

    I found the pickup to be quite weak on the lower end (when compared to using a piezo...which is my only reference as I have no other pickups) and using the Ruby amp (Ruby). Higher end was still weak...but almost acceptable.

    I think I need more turns. I'll try with four 1/4" x 1/2" Neodynium magnets in the traditional vertical orientation and use 10k turns of 40AWG. Hopefully it's loud enough.
     
  4. MPU

    MPU

    Sep 21, 2004
    Valkeala Finland
    Your current magnet layout does not work well. If you put steel blades on end of your magnets it will work better.
     
    Will_White likes this.
  5. callofcthulhu

    callofcthulhu

    Oct 16, 2012
    I'd love to hear how it sounds.
     
  6. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    There's a basic problem with your concept. I'm not saying that you can't get it to work, but you are fighting against an inefficient design. Remember the basics of how a magnetic pickup works. The steel string wiggles within the magnetic field surrounding the magnet. The coil (the turns of wire) also have to be within that magnetic field. The wiggling string disrupts the magnetic field, which then causes a small current in the wire.

    The farther you go away from the magnet, the weaker the field is. To get the strongest signal current in the wire, both the string and the coil need to be in real close to the magnet. That's where the strongest field is. Also, on rod magnets, the field is even more concentrated right around the end of the rod. That's why most pickups have the rod magnet standing up vertically, with the coil wrapped tightly around it. The string wiggles right in the concentrated field at the end of the magnet, and the coil is tight in the field around the length of the magnet. High efficiency creating that signal.

    By laying the magnets down, you've put the coil between the magnet and the string. The wiggling string is in the less efficient area of the field, and it's too far away. So the signal created in the coil is weak.

    The best way to correct this is like MPU is suggesting: add steel plates at the ends of the magnets, which come up close to the strings. That will shape the magnetic field into a "U", wrapping the upper ends around the strings. Still not optimal, but much better.

    Adding more turns to your coil will increase the output signal, but not in an efficient way. It increases the multiplication of a weak signal, but it also puts additional load on the whole amplifier circuit. And, if adding more turns makes the coil physically bigger, then you have move the magnets farther down away from the strings, cancelling out the gains.

    I hope this helps. There are many, many ways to build pickups, and still plenty of unexplored territory.
     
    RSBBass and onesandzeroes like this.
  7. onesandzeroes

    onesandzeroes

    Dec 23, 2016
    Wow...thank you Bruce for that super detailed response! I can see now that I was confused about how magnetic pickups work (I thought that the string needed to vibrate near the coil).

    Out of curiosity (and before I read your response) I went ahead and built a prototype. I wound up going the humbucker route and wound up with two 10k ohm 42 AWG reversed pickups in series.

    For the reasons you outlined, I'm still not seeing a lot of signal. At least I believe I'm not. I don't have a real pickup to compare against.

    The only thing I can compare against is a piezo mic, which when hooked up to the Ruby amp generates way more volume (albeit with distortion at high gain).

    I guess my question is...do piezos generate more signal than magnetic pickups usually? Or are my pickups weak due to the reasons outlined above?

    I did try experimenting with orienting the pickups with the magnets vertical and near the strings...and the sound was louder, but still not nearly as loud as what the piezo produces.

    (as you may notice from the photo...it's not a traditional electric bass I'm building, but something bowed more akin to a cello. Hope it's okay I posted the question here).
     

    Attached Files:

  8. Toptube

    Toptube Supporting Member

    Feb 9, 2009
    generally speaking, you built a pretty typical pole piece pickup-----and turned it on its side. Which puts the pole pieces (the main strings sensing areas) far away from the strings. Even saying that you tried turning them the other way....string distance is key. In a properly setup bass with pickups which suit it----2 turns on the height screws can make a noticeable difference. We are talking a couple of millimeters difference in height.

    If you have a regular electric bass, try making huge changes in the pickup heights. You will gain an immediate understanding of how important pickup height/string distance, is.

    With your idea of turning the coils, you should have a look at sidewinders (often two coils, but one blade or one set of pole pieces). A very cool way to get a humbucker, which can have a more focused tone, kind of like a single coil.
     
    onesandzeroes likes this.
  9. Use one or two of the magnets on their sides, perpendicular rather than parallel to the strings. Connect four iron/steel screws to them, one under each string, and then wrap the wire around these. Congratulations, you now have basically a Rickenbacker high-gain pickup.
     
    onesandzeroes likes this.
  10. MPU

    MPU

    Sep 21, 2004
    Valkeala Finland
    Take four pieces of steel strip , mount them on the end of the magnets so that they face the strings and you'll have a lot more signal.
     
    onesandzeroes and Will_White like this.
  11. Will_White

    Will_White

    Jul 1, 2011
    Salem, OR
    Something to note from my own experiments with NEO mag pickups is that they seem to have a very concentrated magnetic field and need to be fairly close to the strings to get a good signal. I would put the two (I'm assuming) single coils together North to North with a steel plate between them and viola you have the makings of a nice hot sidewinder.
     
    onesandzeroes and Sartori like this.
  12. micguy

    micguy

    May 17, 2011
    Neodymium magnets are very good at keeping a constant field strenth. Magnetic pickups on basses and guitars rely on being able to change the flux from the magnet to get a good amount of signal, so a magent that doesn't want to change is usually counterproductive. Yes, some pole pieces on the ends of your magent will help, but if you don't know what you're doing, you'll likely get more output with weaker magnets (alnico, ferite), as they change field strength more easily. I built a neodymium pickup many years ago, and it had almost no output - I've been exactly where you are now, and learned a couple lessons about high energy magnets at that time.

    But don't stop experimenting - you will find something that works well eventually. I don't think any of the guys that designed guitar pickups in the early days had PHD's in magnetics, and they did fine with a few magnets, some wire, and a lot of persistence.
     
    onesandzeroes and Will_White like this.
  13. onesandzeroes

    onesandzeroes

    Dec 23, 2016
    I've borrowed an oscilloscope and voltmeter and have being testing the output of my pickup to see if it would be reasonable to use it (I guess I kind of got attached to it).

    I picked up a cheap pickup for comparison (Amazon.com: 1set 4 String Noiseless Pickup Black for Precision P Bass Replacement: Musical Instruments) and also tested a piezo and my iPhone headphone jack to see what kind of output they get.

    I positioned the purchased pickup as close as I could to the strings without it touching them.

    Also, I've tried placing metal plates between the pickups (they actually stuck together even though they would normally repel), and adding plates in all combinations to magnetic ends, but could not find any noticeable difference in output.

    Here's the AC RMS voltage I found for each device:

    iPhone headphone jack: ~ 250mV
    Piezo mic (plucking single string): ~ 60mV
    Pickup I bought on Amazon: ~ 30mV
    My pickup: ~ 10mV

    So it looks like my pickup is about 3x weaker than the one from Amazon (and I can see now why the piezo seemed so much louder than my pickup in the previous tests). This make me feel slightly hopeful I may still be able to use the pickup I made.

    Maybe I just need an extra amplification stage? Maybe something like this? Discrete FET Guitar Preamp

    Or something else?
     
  14. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    The purpose of the plates is to shape the magnetic field up around the strings. What you need are eight small pieces of steel which are stuck to the ends of the magnets, and extend straight up to within about 1/8" of the strings. One for each end of each magnet. So you end up with four U-shaped magnets, facing up, almost touching the strings. That puts the string within the concentrated field at the tips of the U. As the string wiggles, it disrupts the field, which is also passing down around the U, through the turns of the coil. You've got to get that field wiggling to create that signal.
     

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