Giant Magnetic Resistance (GMR) Sensors for Guitar Pickups?

[Joe Gwinn]

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

[David King]

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.

[Joe Gwinn]

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.

[J M Fahey]

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.

Wouldn´t call it "sterile" by itself, it´s 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.

[David King]

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.

[Lollar Jason]

Uh huh- Dont be getting grabby inside a whurly!

[bbsailor]

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

[Joe Gwinn]

Wouldn´t call it "sterile" by itself, it´s 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.

[Mike Sulzer]

Isn't the low sensitivity a show stopper? I read this very fine student report (but watch out fo the use of "m". "micro" or "milla"?):

https://web.wpi.edu/Pubs/E-project/A...tar_Pickup.pdf

and that appears to be what they discovered.

One of the advantages of the normal electric guitar pickup is the very high sensitivity. Even with the preamp/amp gain turned way up to get huge sustain/distortion, noise (hiss) is not really a problem. (Although of course hum can be.) Also, the limited frequency response (resonance in mids to low highs) followed by a rapid roll off towards higher frequencies is usually a feature rather than a bug. And if you do want very flat response, you just put on less wire and move the resonance out past the required bandpass. There is still plenty of sensitivity.

The GMR pickup seems to require a preamp with a gain of up to about 1000 and it has an output impedance of about 5K. The output of the pickup is micro rather than milla volts, and the students worked hard to limit noise. This is too low for significant sustain/distortion with low noise, and even in "high fidelity" uses with flat response and no distortion, I think it would not be practical.

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

[Helmholtz]

The GMR pickup seems to require a preamp with a gain of up to about 1000 and it has an output impedance of about 5K. The output of the pickup is micro rather than milla volts, and the students worked hard to limit noise. This is too low for significant sustain/distortion with low noise, and even in "high fidelity" uses with flat response and no distortion, I think it would not be practical.

Maybe I missed something, but they say they got 2.84Vpp with a gain of 1000. That means 2.84mV from the PU alone. And why "milla" not milli?

Otherwise I agree with your assessment. And I don't like the necessary (?) bandwidth limiting to around 1KHz.

Also I think that the observed low-pass behaviour of the GMR PU is not a flaw of the device but real. If - as described - the GMR output voltage is directly proportional to the magnetic field and not (as with inductive PUs) proportional to the rate of change of flux/field (and consequently proportional to frequency), the output signal must decrease with increasing frequency at least compared to inductive PUs.
This is assuming that there is a natural tendency of guitar string amplitudes to roughly decrease proportional to frequency, which compensates for the frequency-proportional sensitivity of inductive PUs and thus allows for their more or less horizontal frequency response (in the absence of resonance).

In other words, the frequency response of the GMR PU is probably independent of frequency but as the frequency spectrum of the strings drops with increasing frequency, the overall response looks low-pass filtered.

[Mike Sulzer]

This is what confused me (page 38):

"As shown in Figure 38, the amplitude of the output signal is around 230mV. When the sensor is connected to the guitar amplifier, no output is heard. This is because the signal amplitude is too small so a preamplifier is needed to increase the amplitude to a detectable level."

A 230 millivolt signal should be just fine for a guitar amp.

Yes, you certainly have identified the source of the roll off towards higher frequency that the report identifies. I think the most important aspect of this for the sound produced is that string harmonics are not properly emphasized. Too dead even for jazz electric guitar.

Thus, equalization would be required, and this would bring up the noise in the low KHz range where the human ear has highest sensitivity by more than another 20 db.

Maybe I missed something, but they say they got 2.84Vpp with a gain of 1000. That means 2.84mV from the PU alone. And why "milla" not milli?

Otherwise I agree with your assessment. And I don't like the necessary (?) bandwidth limiting to around 1KHz.

Also I think that the observed low-pass behaviour of the GMR PU is not a flaw of the device but real. If - as described - the GMR output voltage is directly proportional to the magnetic field and not (as with inductive PUs) proportional to the rate of change of flux/field (and consequently proportional to frequency), the output signal must decrease with increasing frequency at least compared to inductive PUs.
This is assuming that there is a natural tendency of guitar string amplitudes to roughly decrease proportional to frequency, which compensates for the frequency-proportional sensitivity of inductive PUs and thus allows for their more or less horizontal frequency response (in the absence of resonance).

In other words, the frequency response of the GMR PU is probably independent of frequency but as the frequency spectrum of the strings drops with increasing frequency, the overall response looks low-pass filtered.

[Helmholtz]

A 230 millivolt signal should be just fine for a guitar amp.

Yes, the according statement confused me too.

There are more flaws in the report.