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Post by ms on Feb 21, 2022 18:10:19 GMT -5
The attached plot shows the results of measuring the impedances of a Fender-type strat pickup and the high-performance type (https://guitarnuts2.proboards.com/thread/9862/performance-strat-pickups-pickup-hobbyist). The upper plot is for a strat neck pickup I wound using 8000 turns of #42 wire and AlNiCo 5 magnets. This pickup was made to serve as a reference. In this measurement a voltage is placed across the pickup with a 2K resistor in series. Two voltages are measured: first across this series combination, and second, across the 2K resistor. You can compute the voltage across the pickup and the current through it. A driving signal containing all audio frequencies is used, and computation uses an FFT cross-spectral technique. The lower plot shows the results of the same type of measurement for a prototype of the high-performance pickups with 4500 turns per coil.  The inductances are close, but the capacitances are not. This is expected: fewer turns on a coil means lower capacitance; two coils in series lowers the capacitance still further. We expect the capacitance of the guitar cable to be larger than the coil capacitance, but with these new pickups the capacitance of the pickup itself can be almost ignored. The blue line shows the magnitude of the impedance. The resonant peaks are at different frequencies because the capacitances differ, but also the peak is higher with the new pickup. This indicates that the losses are lower. The Qs at 3 KHz Q are printed on the plots; the new pickup is significantly higher. The purple line is the imaginary part of impedance, modified by removing the effect of the capacitance. As long as this increases linearly with frequency, we have a very simple case: we are just seeing the inductance. But if it curves below the extended straight line, we usually say that the inductance is decreasing with frequency, and this mostly caused by eddy currents in the pickup cores. So in the case of the standard strat pickup, we see this start to happen at 5 or six KHz, significantly higher than we would see with higher conductivity steel cores. But the new pickup has a ferrite core, and we see the linear increase out to 50 KHz, and so we have very low conductivity. I doubt that this difference has much, if any, effect on the sound of the pickup since the frequency is so high, but we can say for sure that the new pickups are no worse in this way. When the new pickup is loaded by the several elements in the guitar circuit, we would expect the sound due to the circuit to be very much like that of a standard strat pickup. The new pickup would require the tone control to be turned down a bit to get exactly the same frequency response. None of this is of any use if the pickup is not sensitive to tie varying magnetic fields. In the next post we compare the sensitivities of the two pickups. |
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bajaman
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Post by bajaman on Feb 21, 2022 18:39:59 GMT -5
excellent work - I too have been experimenting with ferrites and neodynium magnets using a sidewinder design and .04mm copper wire - the limiting factor is the curvature of the strat pickup cover - i had to use 12 2mm diameter neo rods set up like a fender jazz pickup separated by 1mm - initial results are very promising - i used 6000 turns on each coil and stuffed 11 cut off 2.8mm diameter steel nails in each coil to bring the inductance up to 2.6H - the neo rods are 10mm long and so the nails contact the mid point which is neutral magnetically. cheers bajaman
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Post by ms on Feb 22, 2022 7:04:11 GMT -5
excellent work - I too have been experimenting with ferrites and neodynium magnets using a sidewinder design and .04mm copper wire - the limiting factor is the curvature of the strat pickup cover - i had to use 12 2mm diameter neo rods set up like a fender jazz pickup separated by 1mm - initial results are very promising - i used 6000 turns on each coil and stuffed 11 cut off 2.8mm diameter steel nails in each coil to bring the inductance up to 2.6H - the neo rods are 10mm long and so the nails contact the mid point which is neutral magnetically. cheers bajaman
Nice! "Sidewinder" is my favorite pickup geometry; I like the symmetry. But as you say, the curved end of the strat pickup severely limits the space for a "side" oriented coil. So I can see the reason for raising the inductance with the nails. But they are steel, and might lower the Q of the resonance. Have you tried using ferrite instead of nails? |
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Post by antigua on Feb 22, 2022 11:35:51 GMT -5
I'd be interested in seeing the output difference, that's what I think of when I think high performance, or the noise rejection ratio if it's humbucking, because for this pickup the high Q factor is impressive but in most cases the Q factor is brought down deliberately. Apparently for guitarists it's not enough to have tone knob, but there must be 125k to 250k ohm permanent load, so that our precious ears are never accidently burdened with the sound of a high Q factor.
A few years ago I was trying to create a passive "wah" effect on the guitar with momentary SPST button, but the effect wasn't very pronounced with a low Q factor. The higher the Q factor of the pickup, the better it would have worked.
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Post by ms on Feb 22, 2022 13:03:39 GMT -5
I'd be interested in seeing the output difference, that's what I think of when I think high performance, or the noise rejection ratio if it's humbucking, because for this pickup the high Q factor is impressive but in most cases the Q factor is brought down deliberately. Apparently for guitarists it's not enough to have tone knob, but there must be 125k to 250k ohm permanent load, so that our precious ears are never accidently burdened with the sound of a high Q factor. A few years ago I was trying to create a passive "wah" effect on the guitar with momentary SPST button, but the effect wasn't very pronounced with a low Q factor. The higher the Q factor of the pickup, the better it would have worked. For fun I have tried passive "wah" with pickups with Q like these, and it works well. But what I want from a tone control is Q control, not shift in resonance, and so I do not use a capacitor, but rather a resistor which gives the minimum Q I want to be able to adjust for. So no "wah". The output level is very good; I will try to post the info tonight. |
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Post by ms on Feb 22, 2022 18:06:56 GMT -5
In this post we compare the response to time varying magnetic fields of the Fender-type strat pickup described earlier and the new high-performance pickup also described earlier. The measurements use a driver coil that is small enough so that it covers part of a pole piece of the first pickup. In fact, it sits on one of the pole pieces, neither the tallest nor the shortest. It sits on the plastic face of the other pickup over the ferrite blade, and so the measurements differ a bit in the coil location. The coil is driven by a broad band signal and the response is measured by a cross spectral technique. A very weak drive is used to make the hum, harmonics of 60 Hz, visible. The same level signal is applied to both pickups. The top plot shows the response of the Fender-type pickup. Notice that many hum harmonics are visible. Also notice the response at 800 Hz, at a level indicated by the green line. The lower plot shows the response of the new high-performance pickup. There is no visible hum. The response at 800 HZ is a bit less than 10 db higher.  |
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Post by antigua on Feb 22, 2022 21:14:54 GMT -5
10dB is pretty significant, do you think it owes to a high permeability ferrite core? Do you have pictures of the new pickup?
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Post by ms on Feb 23, 2022 5:39:32 GMT -5
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Post by aquin43 on Feb 23, 2022 5:47:18 GMT -5
If the extra 10db also appears in the sensitivity to string movement then it would be possible to greatly reduce the coil windings and lower the impedance of the pickup so that it could be used with a lower value volume pot such as 22k or 47k and be almost unaffected by the cable capacitance while still fully driving a normal guitar amp. |
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Post by ms on Feb 23, 2022 6:10:30 GMT -5
If the extra 10db also appears in the sensitivity to string movement then it would be possible to greatly reduce the coil windings and lower the impedance of the pickup so that it could be used with a lower value volume pot such as 22k or 47k and be almost unaffected by the cable capacitance while still fully driving a normal guitar amp. Yes, but to keep the standard strat sound, you would have to: 1. Use larger wire (to occupy approximately the same space) to keep the Q up. 2. Add parallel C (before the volume control) so that the cable C and additional C together would give the correct resonant frequency. My gut feeling is that keeping the inductance at the traditional level and getting more output is what most guitarists want. |
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Post by ms on Feb 25, 2022 10:08:12 GMT -5
In the discussion in the pickup forum, I mentioned that the field resulting from the thin neos on top of the ferrite blade falls off more quickly than that of an AlNiCo pole piece. Here are the results of FEMM models. The model for the blade is actually that for an infinitely long blade because of the 2D solution, but it should be good except near the ends of the real blade. The AlNiCo pole piece is modeled with the cylindrical mode. The 1D plots are the result of selecting a "contour" above the pole. The two vertical scales were different, and so I squashed one and stuck it on top of the other. Thus the two can be seen together. As expected, they are nearly the same at large distances, but the blade with neo is stronger for short distances. The neo case is a bit stronger than I measure; maybe the magnets are not N42 grade as they should be (and used in the model), or maybe my simple minded linear model of the ferrite permeability is not accurate enough.
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veliko
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Post by veliko on Jun 22, 2022 17:39:58 GMT -5
The top plot shows the response of the Fender-type pickup. Notice that many hum harmonics are visible. Also notice the response at 800 Hz, at a level indicated by the green line. The lower plot shows the response of the new high-performance pickup. There is no visible hum. The response at 800 HZ is a bit less than 10 db higher. Is the lack of hum not expected then you have a 2 coils with reversely wound? Also I did some digging and and some companies provide ferrites that have a permeability of 15 000 - 20 000 as opposed to the 640 stated by magnetic permeability table the 1500 - 3000 stated by google. Wouldn't a solid ferrite core, instead of a perforated one, improve even more the permeability of the core? That should be possible with a rotary switch with resistors right? Generally speaking, a pickup with a a high resonant peak freq. and a high Q factor (such as ms's pickup) along with 2 rotary switches one with capacitors and 1 with resistors should be a able to provide any possible pickup tone, right? We have the high resonant freq. and and high Q from the pickup that we wouldn't be otherwise able to increase after the pickup is made and have the capacitor rotary switch shift the peak right-left on the frequency spectrum while the resistor rotary switch would shift the Q up-down. That should provide "all possible combinations" of course within a given increment of rotary switches. I know it's quite probably more complicated than that, but please feel free to enlighten me. |
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Post by ms on Jun 23, 2022 14:36:05 GMT -5
Soft ferrite (that is, with little if any permanent magnetism) is availalbe with many different permeabilities, including, as you said, some quite high. Very high permeability is only fully effective in a closed core, such as a toroid or other such shape as used in transformers. For the short open cores used in pickups, you can only get a few times improvement in performance no matter how high the permeability. Higher does not hurt, but it does not help much either. (Also yes, I would prefer solid rather than with holes.) So you might choose the type of ferrite based on something like "OK, this material has a permeability of at a least a few hundred, and, hey, it comes in the right size, and I can buy it now!"
I agree that you can get a very wide range of sounds with resistors and capacitors. This is one good reason for using a basic pickup design capable of high frequency and Q: it is not that hard to reduce either or both. Whether you want to have all that flexibility on one guitar, or use different guitars for different types of sound, is a matter of personal preference.
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