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Thread: low-impedance pickups thin

  1. #36
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    Could be Monel. Guess that wouldn't be very magnetic. The electric version is called 'American Steel'. That's why I assumed Stainless Steel. Maybe just Nickel plated Steel. I don't know.

    Better yet, get this and use whichever strings you want: http://www.ikmultimedia.com/products/irigacousticstage/
    The sound from a MEMS mic is virtually transparent. Specs for the new MEMS mic is impressive at 101dBA, and the multifunction preamp is 32bit A/D-D/A conversion! Can't imagine anything better for the money.
    Last edited by Fluoroscope 5000; 04-05-2017 at 11:37 AM.

  2. #37
    Pickup Maker David Schwab's Avatar
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    Gibson and Rotosound made Monel strings too.
    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein

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  3. #38
    rjb
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    Quote Originally Posted by David Schwab View Post
    Gibson and Rotosound made Monel strings too.
    Google tells me:

    Gibson started making Monel strings in the 1930's. They had been discontinued, then started again with the Sam Bush mando strings.
    I "misspoke" before- should have made the connection: Bill Monroe -> Gibson F-5 mandolin -> Gibson strings.
    (Also, Lester Flatt -> D-28 Martin guitar -> Martin strings; Earl Scruggs -> Gibson Mastertone banjo -> Gibson strings)

    Rototsound has made flat wound Monel bass strings since the early 1960's.

    What this has to do with thin low impedance pickups, I have no idea.

  4. #39
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    Background
    In this (kinda old) thread Antonio Surdo has made a low impedance pickup by sandwiching neo magnets between steel plates, making a slim and tidy pickup for acoustic guitar. The output of the pickup is intended to be input to a microphone preamp, via XLR cable.

    I found this design very interesting and want to ask some semi-newbie-ish questions about it.

    -- Is there something about this design, like maybe the very strong magnets used, that keep the metal plates from being an issue? Seems like eddy currents would have been mentioned with the loss of highs, etc.

    -- If I make a similar style of pickup with non-conductive bobbin material--forbon or whatever--is it reasonable to expect it to sound noticeably brighter (closer to flat)? I understand that the metal is a significant part of the design--it's the outside cover of Antonio's pickup, so it makes a visual statement. But in my case I'd be using this in a bass, maybe under a wood cover, and neither the metal nor the extreme thinness would be a feature I need.

    -- I want to come up with an on-board application for this kind of low or low-ish impedance pickup, and I'm thinking about using a differential op amp, in particular the Burr Brown INA126 in place of the mic preamp. The output of this op amp would go to my preamp, either an MM 2-band or one of the Sallen-Key circuits that are being discussed here and elsewhere. Any issues with this?

    Thanks in advance for helping me develop this concept for my own builds.

    John

  5. #40
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    Quote Originally Posted by JayGunn View Post
    Background
    In this (kinda old) thread Antonio Surdo has made a low impedance pickup by sandwiching neo magnets between steel plates, making a slim and tidy pickup for acoustic guitar. The output of the pickup is intended to be input to a microphone preamp, via XLR cable.

    I found this design very interesting and want to ask some semi-newbie-ish questions about it.

    -- Is there something about this design, like maybe the very strong magnets used, that keep the metal plates from being an issue? Seems like eddy currents would have been mentioned with the loss of highs, etc.

    -- If I make a similar style of pickup with non-conductive bobbin material--forbon or whatever--is it reasonable to expect it to sound noticeably brighter (closer to flat)? I understand that the metal is a significant part of the design--it's the outside cover of Antonio's pickup, so it makes a visual statement. But in my case I'd be using this in a bass, maybe under a wood cover, and neither the metal nor the extreme thinness would be a feature I need.

    -- I want to come up with an on-board application for this kind of low or low-ish impedance pickup, and I'm thinking about using a differential op amp, in particular the Burr Brown INA126 in place of the mic preamp. The output of this op amp would go to my preamp, either an MM 2-band or one of the Sallen-Key circuits that are being discussed here and elsewhere. Any issues with this?

    Thanks in advance for helping me develop this concept for my own builds.

    John
    In general, low impedance pickups will be "flat" response because the self-resonance between the pickup inductance and wiring capacitance is outside the audio range, over 20 KHz. That's one of the biggest attractions of a low-Z design.

    The steel plates will direct the magnetic field of the neos to magnetize the strings. Eddy currents aren't likely to be a significant issue if you build one like the OP, but if you wish to experiment, it's possible to guide the magnetic field up to the strings without forming a continuous loop around the pickup.

    Preamps or mic preamp IC's can be very good quality but unless you get power from an external power supply to the instrument, you can run into battery life issues with these if you use an onboard battery. If you are using a typical 9V battery, you need to keep current drain low as possible and in most cases under 2 mA, ideally under 1.5 mA, to avoid the issue of having to change batteries too often. Look at the current consumption specs on the preamp solution you use.

    Noise specifications are very important with low-Z pickups because if you go with direct onboard amplification instead of a step-up transformer or an external mic-level preamp, you need anywhere from 25 to 30+ dB gain. Many mic preamps (discrete or IC) trade off noise for current consumption, as you will see when you start going through the spec sheets of high quality opamps and mic preamps.

    I have done a successful preamp with a low-noise JFET as the gain element, and bootstrapping the gain with a X1 (unity gain) op amp buffer. That allows a nice low-noise solution since you are not getting gain with the more noisy op-amp. If you are interested I can provide the schematic for the 5V version. I think it draws about 1.5 mA total and is easily settable for more or less gain with 1 resistor.

    -Charlei
    Last edited by charrich56; 06-22-2017 at 02:58 AM.

  6. #41
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    Quote Originally Posted by charrich56 View Post
    The steel plates will direct the magnetic field of the neos to magnetize the strings. Eddy currents aren't likely to be a significant issue if you build one like the OP, but if you wish to experiment, it's possible to guide the magnetic field up to the strings without forming a continuous loop around the pickup.
    -Charlei
    For the experiment I'd build one pickup with solid magnetic stainless plates and another with each plate having a slot cut out from one edge to the middle. Does this make sense? Then observe whether the slotted pickup has an extended high end. (And also observe whether I like the extended high end; sometimes warmer is better). I might do it, though I'd have to buy more 10 x 20 x 2 neos, actually a little under 2mm thick. I have 12 pcs here, coincidentally purchased a year before this thread was started. Antonio is stacking 3 to get to 5mm thickness, so I could actually use all 12 in a single pickup for 4 strings.

    One thing I am concerned about is string pull, since these rare earth magnets are quite strong, naturally, and 12 magnets in a single pickup seems really intense. Antonio uses 9 of them in his pickup, but acoustic guitar strings must be harder to magnetize than the strings we use on a bass.

    I bought the parts recently for making the simple gauss meter based on the Allegro A1302 hall effect transistor (9v battery, 7805 regulator, 10uF cap and the A1302). What I am thinking about doing is to measure a pole of the 2 basses I have on hand here, a MIM Fender Jazz and an entry-level Yamaha with a clone P-bass pickup. Then see how many and what size neos produce a reading in the same ballpark. I'd measure them all at the same distance, like maybe 3/16". Besides the qty12 10x20x2 neos I also have qty8 5x10x1 and also at least a dozen 5x2 discs, which I use for control covers. If I do this testing I'll definitely report the results here, because I have not been able to find on the forum any rules of thumb for how strong neos should be to achieve a decent balance between output and string pull.

  7. #42
    rjb
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    Quote Originally Posted by charrich56 View Post
    The steel plates will direct the magnetic field of the neos to magnetize the strings.
    I question that statement.
    K&J's field calculator indicates that the steel plates will reduce the magnetic field at the strings, and confine it to an area spread around the magnets' perimeters. Which is probably a good thing.

    K&J Magnetics - Steel Plate Thickness Calculator
    Try entering
    Grade: N35
    Length: 20mm
    Width: 10mm
    Thickness: 2.1mm
    Steel Plate Thickness: .009 in (.23mm) - the thickness of a typical "tin" can in U.S.A.

    Then swap the Length and Width values to see a change in field "spread".
    Of course, this doesn't show the interaction between adjacent magnets.

    Enjoy.

    -rb

    EDIT: Of course, this also assumes that the K&J simulation is valid....
    Last edited by rjb; 06-22-2017 at 11:22 PM.

  8. #43
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    Quote Originally Posted by JayGunn View Post
    For the experiment I'd build one pickup with solid magnetic stainless plates and another with each plate having a slot cut out from one edge to the middle. Does this make sense? Then observe whether the slotted pickup has an extended high end. (And also observe whether I like the extended high end; sometimes warmer is better). I might do it, though I'd have to buy more 10 x 20 x 2 neos, actually a little under 2mm thick. I have 12 pcs here, coincidentally purchased a year before this thread was started. Antonio is stacking 3 to get to 5mm thickness, so I could actually use all 12 in a single pickup for 4 strings.

    One thing I am concerned about is string pull, since these rare earth magnets are quite strong, naturally, and 12 magnets in a single pickup seems really intense. Antonio uses 9 of them in his pickup, but acoustic guitar strings must be harder to magnetize than the strings we use on a bass.

    I bought the parts recently for making the simple gauss meter based on the Allegro A1302 hall effect transistor (9v battery, 7805 regulator, 10uF cap and the A1302). What I am thinking about doing is to measure a pole of the 2 basses I have on hand here, a MIM Fender Jazz and an entry-level Yamaha with a clone P-bass pickup. Then see how many and what size neos produce a reading in the same ballpark. I'd measure them all at the same distance, like maybe 3/16". Besides the qty12 10x20x2 neos I also have qty8 5x10x1 and also at least a dozen 5x2 discs, which I use for control covers. If I do this testing I'll definitely report the results here, because I have not been able to find on the forum any rules of thumb for how strong neos should be to achieve a decent balance between output and string pull.
    Here is an experiment to obtain some useful information.

    1. Cut a plastic credit card or thin plastic sheet .25" wide by 3" long to make a picking device.
    By striking the string with this device, you will be making a near consistent pluck of the string so that if you measured the total time of the amplified string sustain down to the noise level it would be very close to the same with multiple plucks.

    2. Do the same thing with magnets of various strengths and distances from the string and measure the total time of the string sound decay. Any decrease in the time from doing step 1 above, is due to the magnet damping the string vibrations.

    The mass of the string, strength of the magnet and magnet distance from the string are all variables that you can measure by doing this experiment. One additional thing you can do is to lay additional magnets under the same string to increase the length of the magnetic field.

    In my thread on this forum "moving coil pickups for the technically curious" describes how the string can be the coil in a pickup by just attaching the 8 ohm side of a miniature audio transformer (8 ohms to 20K or higher) across a string and listen to the output on the high side of the transformer. But in this case the longer the length of the magnetic field increases the output.

    With guitar pickups there is only one thing that matters: how it sounds to your ear but at low listening levels it is different from live playing at a higher level blending with other instruments.

    It is curious that most high impedance guitar pickups have a resonance frequency in the same range where the human ear is most sensitive at low listening levels. See the Fletcher Munson curve.

    I hope this helps,

    Joseph J. Rogowski

  9. #44
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    bbsailor/Joseph: thanks for giving me a manageable way to try out different magnet strengths. Fortunately neos are cheap and I can buy a good collection of sizes to work with. For an Antonio-style pickup (neos sandwiched between magnetic plates) I am pretty convinced, intuitively, that 12 magnets like Antonio's will be a lot more pull than I need and want. If I drop down to 4 stacks of 2 magnets each stack (instead of 3 magnets deep) then I only have about 3.7mm between the plates, under 3/16". So I am planning to order some neo rods, like 10mm diameter 10-12mm long and 6mm diameter 10-12mm long, which should reduce the field strength considerably. I'll use your method to check them out.

    Quote Originally Posted by rjb View Post
    K&J's field calculator indicates that the steel plates will reduce the magnetic field at the strings, and confine it to an area spread around the magnets' perimeters. Which is probably a good thing.
    -rb
    Is it a good thing because the magnets seems so strong that the strength of the magnetic field they create should be reduced/confined?

    Thanks to both of you!

  10. #45
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    I took a look at that calculator. Look at the effect on the magnetic field outside of the plate (in this case represented by the field lines below the plate) as affected by the plate thickness, and as reflected in the pull strength.

    The plate I have in stock is 0.040" about 1mm. With that thickness there's almost no field below the plate, having been reduced as predicted by @rjb
    k-jthicknesscalculator040.jpg

    Here is the effect of cutting the plate thickness in half. Looks like Antionio used something more like this, and I should too, at least on the top of the pickup.
    k-jthicknesscalculator020.jpg

  11. #46
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    Quote Originally Posted by rjb View Post
    I question that statement.
    K&J's field calculator indicates that the steel plates will reduce the magnetic field at the strings, and confine it to an area spread around the magnets' perimeters. Which is probably a good thing.

    K&J Magnetics - Steel Plate Thickness Calculator
    Try entering
    Grade: N35
    Length: 20mm
    Width: 10mm
    Thickness: 2.1mm
    Steel Plate Thickness: .009 in (.23mm) - the thickness of a typical "tin" can in U.S.A.

    Then swap the Length and Width values to see a change in field "spread".
    Of course, this doesn't show the interaction between adjacent magnets.

    Enjoy.

    -rb

    EDIT: Of course, this also assumes that the K&J simulation is valid....
    Thanks for going in-depth on this, my statement was intended to be very general and was not technically very accurate.

    -Charlie

  12. #47
    Senior Member salvarsan's Avatar
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    The K&J Magnetics sims are good as far as they go.

    We spent a lot of time on 2D magnetic simulations using FEMM back in the '00s. Steve Kersting put up the best of them on his site.

    explicit link:
    Magnetic fields in pickups

    They have both colored gradient images and graphs of magnetic field strength at string height. A baseplate can be a good thing to even out a magnetic field.

    For a taste, here is a 2D sim of an AlNiCo5 PAF.

    The field strength peaks ~0.0275 Tesla = 275 Gauss at 3/8" string height. FYI, Tele and P-90 pickups typically deliver 100 Gauss (or slightly less) at the string. You don't need much to get the sound you want, and probably should keep the field strength as low as is reasonable to minimize string damping.

    hbfield.pnghba5plot.png



    It also shows that the magnetic aperture (50-100%) is about 2" which is at the heart of the PAF "vowel" sound because of comb filtering effects as discussed in Don Tillman's guitar pickup analysis.

    explicit link:
    Response Effects of Guitar Pickup Position and Width

    Someone, if they were motivated, could relearn that clunky FEMM4.2 editor interface and put up a few sims of neodymium buttons with a skinny baseplate since I suspect that's all you need in a skinny pickup.

    explicit link:
    Finite Element Method Magnetics: HomePage
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  13. #48
    rjb
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    I Betcha There's Gotta be a Dot Product in There Somewhere....

    Personal Background:
    During a previous life, I took a class in electromagnetic fields, which had a prerequisite of vector calculus. However, I subsequently joined the circus (figuratively speaking), so never used that div, grad & curl stuff, and have long since forgotten it.

    Question:
    Is the basic argument/miscommunication/whatever between Mike & Salvarsan that "Sal" is looking at absolute values of field strength, while Mike is considering the y-components of field lines through the coils? Assuming that sentence made any sense?

    Getting back to Thin Low-Impedance Pickups:
    Is there a consensus that if you want to make a TLIP by sandwiching neo magnets between two sheets of steel, then the top sheet shouldn't be much thicker than "tin can" metal?

    -rb

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