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Thread: Giant Magnetic Resistance (GMR) Sensors for Guitar Pickups?

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    Giant Magnetic Resistance (GMR) Sensors for Guitar Pickups?

    Well, I had a brainstorm while reading IEEE Transactions on Instrumentation and Measurement - why not use GMR sensors to implement guitar pickups.

    Googled on "GMR Sensor guitar pickup" (without the quotes) and got many hits, so I'm late to that party. However, I don't recall it having been discussed here in the Pickup Makers Forum, and a forum search yielded nothing.

    It seems to me that we ought to investigate GMR sensors, and determine their advantages and disadvantages. If we have not already done so.


    The sensors are available from Digikey for $13 or so. One also needs an opamp.

    https://www.nve.com/Downloads/analog_catalog.pdf

    https://www.digikey.com/products/en?...=391-1046-5-ND

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    Here's a little historical background and primer on GMR. https://nationalmaglab.org/education...netoresistance

    This reminds me of a friend who wanted to build a pickup that tracked capacitance between the string and a fixed plate. The result was very sterile sounding and it was determined to be a dud. One always wonders if the magic of our primitive transducers is because we heard them first or if they really captured something we couldn't get to any other way.

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    Quote Originally Posted by David King View Post
    This reminds me of a friend who wanted to build a pickup that tracked capacitance between the string and a fixed plate. The result was very sterile sounding and it was determined to be a dud. One always wonders if the magic of our primitive transducers is because we heard them first or if they really captured something we couldn't get to any other way.
    Yeah, that's a fear. But if we use the standard magnetic structure, and sense flux variations as the strings move, we will get the same magnetic-circuit effects as always. We can probably use far lower magnetic filed strengths, reducing wolf tones and the like.

    What's different is the filtering due to the coil. There are two main effects to consider. First is that coils respond to the rate of change of flux, versus directly to the flux level. This biases coil response to higher frequencies, at the expense of low frequencies. The second is the effect of coil self-resonance due to cable and self capacitance. These effects can both be replicated electronically, but would now be easily controlled electronically, allowing us to access tradeoff spaces otherwise inaccessible.

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    Quote Originally Posted by David King View Post
    This reminds me of a friend who wanted to build a pickup that tracked capacitance between the string and a fixed plate. The result was very sterile sounding and it was determined to be a dud.
    Oh, I made one

    In fact, Wurlitzer pianos use a similar system, it applies high voltage and needs a very high impedance preamp near the strings (in a way similar to what happens with Capacitor microphones such as Neumann) because capacitance is very low.

    Wouldnīt call it "sterile" by itself, itīs simply very flat, all harmonics present, very similar to what a bridge Piezo picks up , just absolutely different from what an conventional magnetic pickup hears.

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    Juan Manuel Fahey

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    I think a possible benefit would be the ability to locate many sensors along the string length and having the ability to control which sensor is providing the signal depending on what note is fretted so that there would be harmonic consistency from note to note.

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    Uh huh- Dont be getting grabby inside a whurly!

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    Quote Originally Posted by Joe Gwinn View Post
    Well, I had a brainstorm while reading IEEE Transactions on Instrumentation and Measurement - why not use GMR sensors to implement guitar pickups.

    Googled on "GMR Sensor guitar pickup" (without the quotes) and got many hits, so I'm late to that party. However, I don't recall it having been discussed here in the Pickup Makers Forum, and a forum search yielded nothing.

    It seems to me that we ought to investigate GMR sensors, and determine their advantages and disadvantages. If we have not already done so.


    The sensors are available from Digikey for $13 or so. One also needs an opamp.

    https://www.nve.com/Downloads/analog_catalog.pdf

    https://www.digikey.com/products/en?...=391-1046-5-ND
    Using the GMR sensor instead of a transformer, you could use the technique that I described in the thread "Moving coil pickup for the technically curious" https://music-electronics-forum.com/...ically+curious to sense each string individually. Use an acoustic guitar tailpiece with a GMR sensor over the mechanical point where each string is attached to the tailpiece to sense the current in each string. Make a return loop with wire 1/10 lower in resistance than the string resistance to minimize losses. Bring this wire back to the common ground at the nut end of the string to make each string an independent string loop, moving coil pickup. There are many creative techniques that could achieve this such as using the neck truss rod conductivity for the path back into the guitar body. Lay weak plastic magnets from the heel of the neck to the bridge tapered closer to the string as it nears the bridge. With this type of pickup the longer the magnetic field the more the output on each string and the less dependent the sound would be on a narrower magnetic field location, like a traditional pickup . My raw string induced voltage measurements are between 1 to 3 mV. I was able to get near 100 mV out of an 8 ohm to 15K audio transformer (1 to 43 turns ratio) with the 8 ohm side bridging each string and the 15K side input to a 6 channel mixer with individual volume and tone adjustments for each string. With the GMR sensor replacing the transformer (in my above example), we could have individual outputs for each string and possibly move sensing guitar strings into a new direction.

    Thanks for the GMR information to get my mind looking forward.

    Joseph J. Rogowski

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    Last edited by bbsailor; 09-12-2019 at 03:28 AM.

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    Quote Originally Posted by J M Fahey View Post
    Wouldnīt call it "sterile" by itself, itīs simply very flat, all harmonics present, very similar to what a bridge Piezo picks up , just absolutely different from what an conventional magnetic pickup hears.
    I'd expect a GMR pickup to be like that as well, except that there will be some eddy-current effects to round the edges down.

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