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Post by antigua on May 17, 2020 18:32:02 GMT -5
I've done a test like this in the past, but it was a couple years ago, so I wanted to do a fresh redo, to compare with recent modelling experiments and other discussions. I put together this rig which makes the test as easy as it could be without some sophisticated automation. This test is to put the pickup in a parallel orientation with a "D string" tuned up to D3, excite the string with a Dayton brand exciter (a speaker without a cone), and measure the output of the pickup at different distances away from the string, 5mm and 10mm vertically, and 2mm increments horizontally. The Dayton exciter is rated at 20 watts, is being powered by a little 15 watt amp, and the Velleman PCSU200 is both serving the sine wave for the exciter, and reporting the Vrms of the pickup.  The pickups test are: Seymour Duncan SSL-1 Fender Japan Ceramic Strat Pickup Seymour Duncan Tele Quarter Pound Duncan Designed Jazzmaster neck Epiphone P-90 Epiphone P-90 with one of the AlNiCo bars removed Overall it appears that when the pole piece is closer, the sensing window is more narrow, or at least has a steeper voltage drop off with added horizontal distance. The AlNiCo and steel/ceramic Strat pickups appear to have the same overall sensing pattern. The "Quarter Pound" with the 1/4" pole pieces was also similar, except that the output was flat across with width of the wider 1/4" pole face. The broader coils of the P-90 and Jazzmaster pickups seemed to give them a slightly wider sensing width, but not by much. I think the P-90 would have been flatter like the Jazzmaster if the filister screws were as short as the Jazzmaster's AlNiCo pole pieces. On another forum there was talk about the effect of the sensing width of a P-90 if one of the AlNiCo bars was removed. I thought the sensing width would not change at all, someone else thought it would favor the side of the remaining magnet. I tested this several times over again, and it appears that there's virtually no difference in sensing area, whether the one AlNiCo bar is missing or not. I also noticed a unique issue with AlNiCo 5 pole pieces at close distance tot he string. The string movement towards and away from the pickup produce more voltage than string movement that is side to side, and the strong magnetic pull of the strong pull of the AlNiCo 5 can cause the guitar string to change from a circular movement to an up-and-down elliptical movement, and that increased up-and-down traversal results in a greater voltage output, possibly much greater, depending on the degree of pull. If I damped the string with my finger to present it from going elliptical, it read about 60mV output, but when I let go of the string, and the pull is strong enough to make the string vibrate in an elliptical pattern, the output jumped up to 90mV. I posted two pictures of this at the bottom of the post. This is a "best case scenario" for this string pull effect since the pickup is stationed at the middle of the guitar string, in a real guitar the difference would be a lot smaller. As a rough summary, I think it could be said that they sense above the noise floor about 10mm horizontally for 5mm string to pole piece distance, or 20mm horizontally at 10mm string to pole piece distance, which averages out to 15mm distance, or 30mm from one end of the pickup to the other. But then again, the centers are anywhere from +20dB to +40dB, while the edges are 0dB, so as a rouch approximation, suppose the mid-point is chosen as the cut off, that makes a 7.5mm aperture to either side, or 15mm across the pickup. ********************************************** The 5mm and 10mm vertical offset is from the top of the bobbin, rather than the tops of the pole pieces, so in cases where pole pieces stick out, there is a close distance between the string and pole piece. The screws of the P-90 and the middle pole pieces of the Strat pickups are about 2mm high, the rest are flush or about 1mm high. mV = Vrms "PE" = pole piece edge "CE" = coil edge Some of the mV values differ slightly from the dBm, because the value would bounce around by about 5% at close distance, and up to 50% at the noise floor. Seymour Duncan SSL-1 Strat Pickup
Pole distance 5mm
Dist. mV dBm dBm+55
0mm 85 -19.3 30.7 *******************************
2mm-PE- 58 -22.4 27.6 ****************************
4mm 36.5 -26.5 23.5 ************************
6mm=CE= 22 -30.8 19.2 ********************
8mm 14.5 -34.7 15.3 ****************
10mm 8.5 -39 11 ***********
12mm 5 -43.5 6.5 *******
14mm 3 -48.2 1.8 **
16mm 2 -51.8 -1.8
18mm 2 -51.8 -1.8
20mm 2 -51.8 -1.8
22mm 2 -51.8 -1.8
24mm 2 -51.8 -1.8
26mm 2 -51.8 -1.8
28mm 2 -51.8 -1.8
30mm 2 -51.8 -1.8
Pole distance 10mm
Dist. mV dBm dBm+55
0mm 25 -29.82 20.18 *********************
2mm-PE- 23 -30.5 19.5 ********************
4mm 20 -31.8 18.2 *******************
6mm=CE 16 -33.7 16.3 *****************
8mm 12 -36.2 13.8 **************
10mm 9 -38.7 11.3 ************
12mm 7 -40.1 9.9 **********
14mm 5 -43.8 6.2 *******
16mm 4 -45.7 4.3 *****
18mm 3 -48.2 1.8 **
20mm 3 -48.2 1.8 **
22mm 2 -51.8 -1.8
24mm 2 -51.8 -1.8
26mm 2 -51.8 -1.8
28mm 2 -51.8 -1.8
30mm 2 -51.8 -1.8
Fender Japan Ceramic Strat Pickup
Pole Distance 5mm
Dist. mV dBm dBm+50
0mm 153 -14.0 36 ************************************
2mm-PE- 119 -16.2 33.8 **********************************
4mm 70 -20.7 29.3 ******************************
6mm=CE= 47 -24.2 25.8 **************************
8mm 30 -28.1 21.9 **********************
10mm 19 -32.3 17.7 ******************
12mm 10 -37.3 12.7 *************
14mm 5 -42.7 7.3 ********
16mm 2 -51.1 -1.1
18mm 2 -51.1 -1.1
20mm 2 -51.1 -1.1
22mm 2 -51.1 -1.1
24mm 2 -51.1 -1.1
26mm 2 -51.1 -1.1
28mm 2 -51.1 -1.1
30mm 2 -51.1 -1.1
Pole distance 10mm
Dist. mV dBm dBm+50
0mm 54 -23.0 27 ***************************
2mm-PE- 47 -24.2 25.8 **************************
4mm 40 -25.8 24.2 *************************
6mm=CE= 35 -27.0 23 ***********************
8mm 27 -29.4 20.6 *********************
10mm 20 -31.5 18.5 *******************
12mm 13 -35.1 14.9 ***************
14mm 9.5 -38.1 11.9 ************
16mm 7 -40.4 9.6 **********
18mm 5 -44 6 ******
20mm 3 -48.3 1.7 **
22mm 2 -51 -1
24mm 2 -51 -1
26mm 2 -51 -1
28mm 2 -51 -1
30mm 2 -51 -1
Seymour Duncan Quarter Pound Tele Bridge
Pole Distance 5mm
Dist. mV dBm dBm+50
0mm 195 -12.0 38 **************************************
2mm 184 -12.4 37.6 **************************************
4mm-PE- 149 -14.5 35.5 ************************************
6mm=CE= 106 -17.3 32.7 *********************************
8mm 73 -20.6 29.4 ******************************
10mm 38 -26.0 24 ************************
12mm 20 -31 19 *******************
14mm 11 -36.9 13.1 **************
16mm 5 -42.3 7.7 ********
18mm 4 -45.8 4.2 *****
20mm 2 -52.1 -2.1
22mm 2 -52.1 -2.1
24mm 2 -52.1 -2.1
26mm 2 -52.1 -2.1
28mm 2 -52.1 -2.1
30mm 2 -52.1 -2.1
Pole distance 10mm
Dist. mV dBm dBm+50
0mm 60 -22.2 27.8 ****************************
2mm 58 -22.3 27.7 ****************************
4mm-PE- 52 -23.3 26.7 ***************************
6mm=CE= 46 -24.6 25.4 **************************
8mm 36 -26.7 23.3 ************************
10mm 26 -29.2 20.8 *********************
12mm 19 -32.0 18 ******************
14mm 12 -35.8 14.2 ***************
16mm 9 -39.2 10.8 ***********
18mm 6 -41.4 8.6 *********
20mm 4 -43.9 6.1 *******
22mm 2.5 -49.9 0.1 *
24mm 2 -51.0 -1
26mm 2 -51.0 -1
28mm 2 -51.0 -1
30mm 2 -51.0 -1
Duncan Designed Jazzmaster Neck Pickup
Pole Distance 5mm
Dist. mV dBm dBm+45
0mm 113 -16.9 28.1 *****************************
2mm-PE- 90 -18.9 26.1 ***************************
4mm 60 -22.1 22.9 ***********************
6mm 42 -25.4 19.6 ********************
8mm 29 -28.6 16.4 *****************
10mm 20 -32 13 *************
12mm=CE 11 -37 8 ********
14mm 7 -40 5 *****
16mm 5 -43.8 1.2 **
18mm 5 -43.8 1.2 **
20mm 5 -43.8 1.2 **
22mm 4.5 -44.7 0.3 *
24mm 4.5 -44.7 0.3 *
26mm 4.5 -44.7 0.3 *
28mm 4.5 -44.7 0.3 *
30mm 4.5 -44.7 0.3 *
Pole distance 10mm
Dist. mV dBm dBm+45
0mm 44 -25.1 19.9 ********************
2mm-PE- 40 -25.7 19.3 ********************
4mm 31 -27.7 17.3 ******************
6mm 24 -29.7 15.3 ****************
8mm 18 -32.3 12.7 *************
10mm 13 -35.8 9.2 **********
12mm=CE 10 -37.7 7.3 ********
14mm 7.5 -40.1 4.9 *****
16mm 6 -42 3 ***
18mm 5.5 -43.1 1.9 **
20mm 5 -43.5 1.5 **
22mm 5 -43.5 1.5 **
24mm 4.5 -44.6 0.4 *
26mm 4.5 -44.6 0.4 *
28mm 4.5 -44.6 0.4 *
30mm 4.5 -44.6 0.4 *
Epiphone P-90 Rhythm Pickup
Pole distance 5mm
Dist. mV dBm dBm+50
0mm 220 -10.9 39.1 ****************************************
2mm-PE- 202 -11.7 38.3 ***************************************
4mm 114 -16.6 33.4 **********************************
6mm 80 -19.7 30.3 *******************************
8mm 47 -24.2 25.8 **************************
10mm 30 -28.3 21.7 **********************
12mm=CE 18 -32.7 17.3 ******************
14mm 10 -37.6 12.4 *************
16mm 5 -43.2 6.8 *******
18mm 2 -49.5 0.5 *
20mm 2 -51 -1
22mm 2 -51.8 -1.8
24mm 2 -51.8 -1.8
26mm 2 -51.8 -1.8
28mm 2 -51.8 -1.8
30mm 2 -51.8 -1.8
Pole distance 10mm
Dist. mV dBm dBm+50
0mm 35 -27.1 22.9 ***********************
2mm-PE- 33 -27.3 22.7 ***********************
4mm 28 -28.7 21.3 **********************
6mm 24 -30.2 19.8 ********************
8mm 19 -32.0 18 ******************
10mm 14 -34.6 15.4 ****************
12mm=CE 10 -38.0 12 ************
14mm 7 -40.8 9.2 **********
16mm 4 -45 5 *****
18mm 3 -48 2 **
20mm 2 -51.8 -1.8
22mm 2 -51.8 -1.8
24mm 2 -51.8 -1.8
26mm 2 -51.8 -1.8
28mm 2 -51.8 -1.8
30mm 2 -51.8 -1.8
P-90 single magnet test (with string excitation cranked up for improved resolution)
Pole distance 10mm, 1 magnet removed; side w/magnet:
Dist. mV dBm dBm+50
0mm 64 -21.7 28.3 *****************************
2mm-PE- 63 -21.9 28.1 *****************************
4mm 52 -23.5 26.5 ***************************
6mm 39 -25.9 24.1 *************************
8mm 27 -29.4 20.6 *********************
10mm 18 -32.6 17.4 ******************
12mm=CE 11 -36.7 13.3 **************
14mm 7 -40.9 9.1 **********
16mm 4.5 -44.6 5.4 ******
18mm 3 -48.2 1.8 **
20mm 2 -50 0
22mm 2 -50 0
24mm 2 -50 0
26mm 2 -50 0
28mm 2 -50 0
30mm 2 -50 0
Pole distance 10mm 1 magnet removed; side w/o magnet
Dist. mV dBm dBm+50
0mm 64 -21.7 28.3 *****************************
2mm-PE- 57 -22.6 27.4 ****************************
4mm 40 -25.5 24.5 *************************
6mm 28 -28.7 21.3 **********************
8mm 19 -32.3 17.7 ******************
10mm 12 -36.3 13.7 **************
12mm=CE 9 -38.9 11.1 ************
14mm 6.5 -41.4 8.6 *********
16mm 4.5 -44.2 5.8 ******
18mm 3 -48.2 1.8 **
20mm 2 -50 0
22mm 2 -50 0
24mm 2 -50 0
26mm 2 -50 0
28mm 2 -50 0
30mm 2 -50 0
Normal string movement:  Elliptical string movement when the string is above the AlNiCo 5 pole pieces:  Jazzmaster pickupL  P-90 with magnet removed:  |
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Post by aquin43 on May 18, 2020 3:54:10 GMT -5
I am not sure what you are measuring here.
How do these measurements relate to the problem of mapping the pole's response to the string in the normal situation? You are moving the string relative to the pole and so altering its magnetisation. The change in magnetisation is unknown and probably depends on the remanent magnetisation. How much of the mapped response is attributable to the change in magnetisation? The measurement doesn't address the effect of the distribution of flux within the string, which is a crucial component of the pole response.
Would it be possible to use unmagnetised versions of the pickups and magnetise the string with a separate magnet fixed relative to it?
Would it be practical to replace the string by magnetised fragment of ferrite to represent a small dipole source? This might allow mapping of the pole response to a dipole which, combined with the reasonably well known flux distribution within the string would give the sensitivity map and hence the pole window.
Arthur
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Post by ms on May 18, 2020 6:54:24 GMT -5
Based on aquin43's comment, and your measurement technique, measuring the pole response (that is, the pickup window without the effect of the spatial distribution of string magnetization) would be useful. You just need to mount a magnet in the arm of one of those fancy holder things and use a p-90 with both magnets removed, or a humbucker coil under the string (or alternatively vibrate a very small neo or ferrite as a43 suggests). Once you have that measurement you can make a set of measurements with the pickup coil centered under the string and the magnet at different locations along the string. Then somebody could spend some time constructing a forward model and use an inverse technique to get the string magnetization and the pickup window. (I was thinking this should just be basically a convolution, which could be attacked with linear regularization. But I believe it is a non linear problem requiring an iterative technique)
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Post by aquin43 on May 18, 2020 8:07:43 GMT -5
Based on aquin43's comment, and your measurement technique, measuring the pole response (that is, the pickup window without the effect of the spatial distribution of string magnetization) would be useful. You just need to mount a magnet in the arm of one of those fancy holder things and use a p-90 with both magnets removed, or a humbucker coil under the string (or alternatively vibrate a very small neo or ferrite as a43 suggests). Once you have that measurement you can make a set of measurements with the pickup coil centered under the string and the magnet at different locations along the string. Then somebody could spend some time constructing a forward model and use an inverse technique to get the string magnetization and the pickup window. (I was thinking this should just be basically a convolution, which could be attacked with linear regularization. But I believe it is a non linear problem requiring an iterative technique) I wonder if, instead of vibrating a small magnet, a small single turn current loop could be used. This could be driven with the highest possible frequency within the pickup bandwidth, to maximise sensitivity. It would be oriented with its axis at right angles to the pole, i.e. as if it were wrapped round the string, and traversed across the pickup. This would, I think, provide a mapping of the contribution of a dipole on the string to the flux in the pickup that produces output.
Arthur
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Post by ms on May 18, 2020 9:04:32 GMT -5
Based on aquin43's comment, and your measurement technique, measuring the pole response (that is, the pickup window without the effect of the spatial distribution of string magnetization) would be useful. You just need to mount a magnet in the arm of one of those fancy holder things and use a p-90 with both magnets removed, or a humbucker coil under the string (or alternatively vibrate a very small neo or ferrite as a43 suggests). Once you have that measurement you can make a set of measurements with the pickup coil centered under the string and the magnet at different locations along the string. Then somebody could spend some time constructing a forward model and use an inverse technique to get the string magnetization and the pickup window. (I was thinking this should just be basically a convolution, which could be attacked with linear regularization. But I believe it is a non linear problem requiring an iterative technique) I wonder if, instead of vibrating a small magnet, a small single turn current loop could be used. This could be driven with the highest possible frequency within the pickup bandwidth, to maximise sensitivity. It would be oriented with its axis at right angles to the pole, i.e. as if it were wrapped round the string, and traversed across the pickup. This would, I think, provide a mapping of the contribution of a dipole on the string to the flux in the pickup that produces output.
Arthur
I have worked with tiny three turn coils before and the sensitivity was OK; keep the current drive high, as well as the frequency, and use a long fft in the analysis. Orient it as you say since that is the way the string magnetization points. Probably it would be a good idea to do this measurement more than one way and look at how well the results agree. We also should have a check on the string magnetization. Zollner's measurements should be fine, but I do not like his explanation of how all the flux lines escape from the string because it saturates. I think the saturation just limits how much flux density is present to magnetize the string away from the magnet where saturation becomes near saturation and so on down the hysteresis curve. |
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Post by aquin43 on May 18, 2020 10:49:14 GMT -5
I wonder if, instead of vibrating a small magnet, a small single turn current loop could be used. This could be driven with the highest possible frequency within the pickup bandwidth, to maximise sensitivity. It would be oriented with its axis at right angles to the pole, i.e. as if it were wrapped round the string, and traversed across the pickup. This would, I think, provide a mapping of the contribution of a dipole on the string to the flux in the pickup that produces output.
Arthur
I have worked with tiny three turn coils before and the sensitivity was OK; keep the current drive high, as well as the frequency, and use a long fft in the analysis. Orient it as you say since that is the way the string magnetization points. Probably it would be a good idea to do this measurement more than one way and look at how well the results agree. We also should have a check on the string magnetization. Zollner's measurements should be fine, but I do not like his explanation of how all the flux lines escape from the string because it saturates. I think the saturation just limits how much flux density is present to magnetize the string away from the magnet where saturation becomes near saturation and so on down the hysteresis curve. I have thought a bit further about this approach and realised that the measurement we actually want (for vertical string movement) is the gradient of this measurement with height above the pole, for it doesn't matter how much flux couples in if it doesn't vary as the string moves. This would indicate how the signal coupling flux varies as the source moves in the pole-axial direction.
The easiest way to do this would to make an arrangement of two identical coils, one above the other with their centres separated by 1mm, say, and alternate the exciting current between them. The measurement would be in the modulation of the pickup output at the alternation frequency.
Perhaps the coils could be vias on a PCB.
I agree that there is no need to invoke saturation to describe the external field of the string.
Arthur
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Post by antigua on May 18, 2020 11:00:07 GMT -5
I am not sure what you are measuring here.
How do these measurements relate to the problem of mapping the pole's response to the string in the normal situation? You are moving the string relative to the pole and so altering its magnetisation. The change in magnetisation is unknown and probably depends on the remanent magnetisation. How much of the mapped response is attributable to the change in magnetisation? The measurement doesn't address the effect of the distribution of flux within the string, which is a crucial component of the pole response.
Would it be possible to use unmagnetised versions of the pickups and magnetise the string with a separate magnet fixed relative to it?
Would it be practical to replace the string by magnetised fragment of ferrite to represent a small dipole source? This might allow mapping of the pole response to a dipole which, combined with the reasonably well known flux distribution within the string would give the sensitivity map and hence the pole window.
Arthur
I can possibly get a remnant-magnetization-only reading with an air coil pickup, and I even bought really tiny neodyium magnets that are about the circumference low E string which I can epoxy to a nylon string and test that way, but keep in mind that the voltage output of a pickup is already rather low under normal circumstances, and these isolations literally decimate the output to the point where it's difficult to get signal above the noise floor. I'd probably have to put a noise cancelling coil into the mix and hope that's enough. The string vibration can be made very strong, but if the string is displacing by 5mm or more, then it undermines the idea of accurate x,y,z spacial precision. From having worked with driver coils for normal pickup measurements, I also know that a coil that is small and fine like a guitar string would also be difficult to drive. I have a 15 watt amp but I'm afraid I'll fry something with a tiny coil. I'm willing to try other things though. What method did Manfred Zollner use? |
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Post by aquin43 on May 18, 2020 11:45:31 GMT -5
I am not sure what you are measuring here.
How do these measurements relate to the problem of mapping the pole's response to the string in the normal situation? You are moving the string relative to the pole and so altering its magnetisation. The change in magnetisation is unknown and probably depends on the remanent magnetisation. How much of the mapped response is attributable to the change in magnetisation? The measurement doesn't address the effect of the distribution of flux within the string, which is a crucial component of the pole response.
Would it be possible to use unmagnetised versions of the pickups and magnetise the string with a separate magnet fixed relative to it?
Would it be practical to replace the string by magnetised fragment of ferrite to represent a small dipole source? This might allow mapping of the pole response to a dipole which, combined with the reasonably well known flux distribution within the string would give the sensitivity map and hence the pole window.
Arthur
I can possibly get a remnant-magnetization-only reading with an air coil pickup, and I even bought really tiny neodyium magnets that are about the circumference low E string which I can epoxy to a nylon string and test that way, but keep in mind that the voltage output of a pickup is already rather low under normal circumstances, and these isolations literally decimate the output to the point where it's difficult to get signal above the noise floor. I'd probably have to put a noise cancelling coil into the mix and hope that's enough. The string vibration can be made very strong, but if the string is displacing by 5mm or more, then it undermines the idea of accurate x,y,z spacial precision. From having worked with driver coils for normal pickup measurements, I also know that a coil that is small and fine like a guitar string would also be difficult to drive. I have a 15 watt amp but I'm afraid I'll fry something with a tiny coil. I'm willing to try other things though. If you have noise problems you will have to raise the working frequency as high as is practicable and reduce the bandwidth of your measuring system.
Since you are working at a fixed frequency, it should be possible to filter out hum with a high pass filter and add a bandpass centred on the working frequency to help remove other noise. If your oscilloscope can do trace averaging, that is another excellent way to reduce noise.
The proposal I make in an earlier reply would make for much easier measurements by allowing the use of a much higher frequency
I have tried a single turn coil of ~1mm diameter driven by 35mA rms and can get a clean 15mV from a strat pickup. I am using a CCIR noise filter, part of an ancient analogue noise meter, which has a steep low frequency roll off and driving at a test frequency of 6kHz.
Arthur
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Post by antigua on May 18, 2020 14:24:32 GMT -5
I can possibly get a remnant-magnetization-only reading with an air coil pickup, and I even bought really tiny neodyium magnets that are about the circumference low E string which I can epoxy to a nylon string and test that way, but keep in mind that the voltage output of a pickup is already rather low under normal circumstances, and these isolations literally decimate the output to the point where it's difficult to get signal above the noise floor. I'd probably have to put a noise cancelling coil into the mix and hope that's enough. The string vibration can be made very strong, but if the string is displacing by 5mm or more, then it undermines the idea of accurate x,y,z spacial precision. From having worked with driver coils for normal pickup measurements, I also know that a coil that is small and fine like a guitar string would also be difficult to drive. I have a 15 watt amp but I'm afraid I'll fry something with a tiny coil. I'm willing to try other things though. If you have noise problems you will have to raise the working frequency as high as is practicable and reduce the bandwidth of your measuring system.
Since you are working at a fixed frequency, it should be possible to filter out hum with a high pass filter and add a bandpass centred on the working frequency to help remove other noise. If your oscilloscope can do trace averaging, that is another excellent way to reduce noise.
The proposal I make in an earlier reply would make for much easier measurements by allowing the use of a much higher frequency
I have tried a single turn coil of ~1mm diameter driven by 35mA rms and can get a clean 15mV from a strat pickup. I am using a CCIR noise filter, part of an ancient analogue noise meter, which has a steep low frequency roll off and driving at a test frequency of 6kHz.
Arthur
I have tiny neodymium that is barely thicker than a guitar string that I attach in parallel with the axis of the string. I can either use steel strings or nylon, which do you think would yield a more valid result? I suspect the test above would give similar results as if a nylon string were used, because both method replace the virtual "row of magnets" with air (or nylon as the case may be). The oscilloscope is a Velleman PCSU200, I'll have to see if it has any averaging or noise cancelling features. If has an FFT view that shows voltage by frequency, but the noise floor is made up of broadband noise:  The individual spikes are hum, but the ocean of blue is broadband noise that eats everything. Ill try messing with high frequency, low voltage into tiny coils around a steel string. If that works, and all you have to do is wrap a piece of small wire around the guitar string and measure the voltage at the output jack of the guitar, that should be a very easy test, requiring virtually no fabrication, just a function generator, and oscilloscope and some spare pickup gauge wire. Have you tried this testing already? In your LtSpice model you had used an guess approximation for what this testing could indicate empirically, isn't that true? |
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Post by aquin43 on May 18, 2020 16:57:23 GMT -5
Since you have the string system already made, I think that something worth trying would be to use a plain nylon string and wrap a single turn of fine wire round it. Twist the wire together as it exits the loop to make a fairly long twisted lead running straight away from the string horizontally. Drive this loop with as much current as you dare at 5kHz.
Vibrate the string as before and this time place the pickup in the normal orientation to the string as in a guitar. Move the pickup along the string. The output of the pickup should be a 5kHz sine wave modulated by the string frequency. If you consider that waveform as an amplitude modulated carrier, the modulation is the required result which should reflect the point by point sensitivity of the coil to a dipole in the moving string. That, multiplied by the string magnetisation pattern should give the pole window.
The raw 5kHz represents the amount of potentially signal generating flux coupled into the pickup from a dipole in the string. The modulation on the 5kHz represents how much this flux varies as the string is moved. It is this variation that produces the output.
The same result could be obtained in principle by moving the coil directly by the vibrator, but those vibrators produce a large radiated magnetic field.
The virtues of this approach are that the coil is much shorter than a magnet and so more dipole like and the signal is made larger and moved away from the major interfering frequencies.
Arthur
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Post by aquin43 on May 19, 2020 4:45:46 GMT -5
; The oscilloscope is a Velleman PCSU200, I'll have to see if it has any averaging or noise cancelling features. If has an FFT view that shows voltage by frequency, but the noise floor is made up of broadband noise: The individual spikes are hum, but the ocean of blue is broadband noise that eats everything. Ill try messing with high frequency, low voltage into tiny coils around a steel string. If that works, and all you have to do is wrap a piece of small wire around the guitar string and measure the voltage at the output jack of the guitar, that should be a very easy test, requiring virtually no fabrication, just a function generator, and oscilloscope and some spare pickup gauge wire. Have you tried this testing already? In your LtSpice model you had used an guess approximation for what this testing could indicate empirically, isn't that true? Most of the scope noise will be at frequencies way beyond the those of interest but will still obscure the measurement. With a flat noise spectrum, the total noise is proportional to the square root of the bandwidth. You should construct a small preamp with a gain of 10X or so and a restricted bandwidth to go in front of the scope. Any low noise op amp should do and it could be battery powered and housed in a tin box. With a dual op amp you could add a high pass filter to remove 60Hz interference. I would guess that the mains interference you mostly experience is 60Hz and its 3rd harmonic at 180Hz. The preamp should bring the noise floor down considerably. If you are measuring with a moving magnet, i.e. your expected signal is at the string frequency, you should also put a capacitor across the pickup to limit its bandwidth and lower the impedance at high frequencies. This will help to reduce interference.
I have tried a static measurement with a single turn loop at 6kHz and found that the induced signal remains high even quite far (12mm or more) from the pole. This doesn't mean that the pole window is that wide, however, because it is the change of this signal with vertical movement of the coil that matters and I haven't the apparatus to make that measurement.
Arthur
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Post by ms on May 19, 2020 5:36:50 GMT -5
I have tried a static measurement with a single turn loop at 6kHz and found that the induced signal remains high even quite far (12mm or more) from the pole. This doesn't mean that the pole window is that wide, however, because it is the change of this signal with vertical movement of the coil that matters and I haven't the apparatus to make that measurement.
Arthur
That agrees with what I measure. I am using a 10 turn coil of #41 wound in a groove on a fancy tooth pick. Also the signal gets very weak nearly over the center of the pole. (Not quite over the center because I am using a humbucker; that upsets the symmetry some. I need to change to a single coil.) It would seem that measuring the gradient is very important. Edit: I did some early morning measurements with one coil from a humbucker and also an old underwound fake tele bridge. Very similar response vs. distance. I will construct a dual coil set up using two fancy toothpicks sanded flat on one side to get the coils close enough together. Since I have two independent output channels, I can run them on close but sufficiently different frequencies and so make measurements on both at the same time. At least that is a plan. |
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Post by antigua on May 19, 2020 9:00:14 GMT -5
Since you have the string system already made, I think that something worth trying would be to use a plain nylon string and wrap a single turn of fine wire round it. Twist the wire together as it exits the loop to make a fairly long twisted lead running straight away from the string horizontally. Drive this loop with as much current as you dare at 5kHz.
Vibrate the string as before and this time place the pickup in the normal orientation to the string as in a guitar. Move the pickup along the string. The output of the pickup should be a 5kHz sine wave modulated by the string frequency. If you consider that waveform as an amplitude modulated carrier, the modulation is the required result which should reflect the point by point sensitivity of the coil to a dipole in the moving string. That, multiplied by the string magnetisation pattern should give the pole window.
The raw 5kHz represents the amount of potentially signal generating flux coupled into the pickup from a dipole in the string. The modulation on the 5kHz represents how much this flux varies as the string is moved. It is this variation that produces the output.
The same result could be obtained in principle by moving the coil directly by the vibrator, but those vibrators produce a large radiated magnetic field.
The virtues of this approach are that the coil is much shorter than a magnet and so more dipole like and the signal is made larger and moved away from the major interfering frequencies.
Arthur
Is there a reason why the loop around the string should be of a single turn, and not a five or ten? It should improve the S/N ratio without making the pseudo-dipole too much larger. A problem I'm perceiving with this 5kHz loop, or a test using a neodymium glued to a nylon string, is that the magnetism of the loop or the neodymium is a fixed intensity, but in a real guitar string, if modeled as a row of magnets, the magnets that are farther from the pole piece are weaker magnets. I could turn the amplitude of the loop voltage down at further distances, but of course I don't know what the correct voltage would be at any given distance. I think the only way to get overcome that is to attach a tiny segment of steel guitar string to a nylon guitar string, so that you would have isolated a narrow segment of steel string in the same vibrating system as a whole steel string, but in that case, a fraction of the string means a fraction of the output, and so the noise eliminating measures would have to be very good. I think the test above, although you don't think it's telling of a pickup's sensitivity, at the very least the string's flux at any given distance is a consequence of the flux it is receiving from the pole pieces, plus some amount of remnant magnetism. I don't have the same lack of confidence in my test, and i'm interested in seeing if another approach agrees or disagrees with what I've measured so far. Once I have a nylon string on the rig, any of these tests should be rather easy, but the noise issue seems to be the most daunting thing. Putting together band pass filters and op amps is doable but it would push my time table back, since that's a little project unto itself. I have stacked humuckers that can stand in for single coils too. |
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Post by aquin43 on May 19, 2020 10:20:43 GMT -5
Is there a reason why the loop around the string should be of a single turn, and not a five or ten? It should improve the S/N ratio without making the pseudo-dipole too much larger. A problem I'm perceiving with this 5kHz loop, or a test using a neodymium glued to a nylon string, is that the magnetism of the loop or the neodymium is a fixed intensity, but in a real guitar string, if modeled as a row of magnets, the magnets that are farther from the pole piece are weaker magnets. I could turn the amplitude of the loop voltage down at further distances, but of course I don't know what the correct voltage would be at any given distance. I think the only way to get overcome that is to attach a tiny segment of steel guitar string to a nylon guitar string, so that you would have isolated a narrow segment of steel string in the same vibrating system as a whole steel string, but in that case, a fraction of the string means a fraction of the output, and so the noise eliminating measures would have to be very good. I think the test above, although you don't think it's telling of a pickup's sensitivity, at the very least the string's flux at any given distance is a consequence of the flux it is receiving from the pole pieces, plus some amount of remnant magnetism. I don't have the same lack of confidence in my test, and i'm interested in seeing if another approach agrees or disagrees with what I've measured so far. Once I have a nylon string on the rig, any of these tests should be rather easy, but the noise issue seems to be the most daunting thing. Putting together band pass filters and op amps is doable but it would push my time table back, since that's a little project unto itself. I have stacked humuckers that can stand in for single coils too. I lean towards the single turn coil because it is the smallest and I can overcome the noise problem. You are doing audio measurements using a system with MHz bandwidth and a noise level to match.
The reason for using a constant magnet strength at all distances is so that the variation of the response of the pickup to a fixed dipole moment can be measured along the string. Once this is obtained it can be combined with the known variation of the magnetisation in a real string to calculate the window.
The flux in any part of the string depends on the presence of the rest of the string. It is impossible to isolate a piece and achieve the same result.
Arthur
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Post by antigua on May 19, 2020 11:47:55 GMT -5
The reason for using a constant magnet strength at all distances is so that the variation of the response of the pickup to a fixed dipole moment can be measured along the string. Once this is obtained it can be combined with the known variation of the magnetisation in a real string to calculate the window.
The flux in any part of the string depends on the presence of the rest of the string. It is impossible to isolate a piece and achieve the same result.
Arthur
This makes sense, but so much so that it leaves me pessimistic about the experiment. If I use a nylon string, there is no "rest of the string", or any string at all for that matter. Combining a fixed magnetization result with a "known variation" of magnetization at distance introduces both a degree of approximation, as well as an opportunity for calculation error. I'm not sure how "known variation" would be nailed down here. If the end result contains approximations, I feel like that's what I've already achieved, more or less, using a real vibrating guitar string, but with a spatial imperfection rather than a material imperfection. Theoretically speaking, how do you suppose the results of an ideal test measurement would differ from the result yielded by the method above? Do you think the sensing width would expand a little further out? Have a higher intensity at close range? |
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Post by aquin43 on May 19, 2020 12:48:10 GMT -5
...If the end result contains approximations, I feel like that's what I've already achieved, more or less, using a real vibrating guitar string, but with a spatial imperfection rather than a material imperfection. Theoretically speaking, how do you suppose the results of an ideal test measurement would differ from the result yielded by the method above? Do you think the sensing width would expand a little further out? Have a higher intensity at close range? The string magnetisation pattern has been measured by Zollner. What hasn't been measured is the spatial response of the pickup to a magnetic field. The two together would give the window shape. Your experiment varies too many things at once by taking the whole string away from the pole. Who knows how much of the drop in level is caused by the change in the string magnetisation and how much by moving which part of the string? I would guess that you will end up with a result that is slightly narrower than the true window but that result will be fortuitous and will bear little relation to the physics of the actual working string.
Arthur
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