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Post by stratotarts on Nov 10, 2016 9:58:03 GMT -5
I've recently completed studies of eddy current geometry in pickups, and presented some novel prototype pickup covers that exhibit extremely low losses. It's too big to attach here, so here is a link: study (PDF format) |
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Post by antigua on Nov 10, 2016 12:59:05 GMT -5
This is highly valuable information about how eddy currents geometrically react with guitar pickups. It shows that the more a metal directly counteracts the coil, from a physical orientation standpoint, which is to say, create overlapping anti-coils, the greater the losses will be. This means a pickup cover can be designed to provide through EMI shielding, without also acting as an anti-coil. Based on this information, I can even conceive of ways in which steel slugs and screws could be modified, or produced, that would reduce the eddy losses they impart. For example, if a slug had is cut down the middle like this:  it would prevent current from circulating in the necessary circular manner, and reduce eddy current losses by a good amount. This could conceivable improve the tone of cheap ceramic single coil pickups, as well as overly muddy humbuckers. |
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Post by stratotarts on Nov 10, 2016 13:12:36 GMT -5
Absolutely! First I thought it would be too hard to slice a pole in half, but as you show, it's not necessary. That could be done in a vice with a diamond saw or hacksaw. Definitely worth trying (with eye protection, of course)!
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Post by JohnH on Nov 10, 2016 14:28:43 GMT -5
I've recently completed studies of eddy current geometry in pickups, and presented some novel prototype pickup covers that exhibit extremely low losses. It's too big to attach here, so here is a link: study (PDF format)That is a brilliant study, very interesting and easy to follow. Everybody should read it! (and I will read it again more slowly) A question: The original purpose of a cover is to reduce externally induced buzz. The concept of a Faraday cage, wrapping around an object, rather than a series of separate isolated panels so that the induced unwanted signals circulate and dissipate can apply here too. So, if you take one of the low-eddy slotted covers and measure the signal level of induced buzz in a controlled way, how does the shielding performance compare to a similar un-slotted cover? |
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Post by stratotarts on Nov 10, 2016 14:58:55 GMT -5
When it's working as an electrostatic shield, small slots don't have any effect at wavelengths much greater than the slot diameters. This is because the current travels across the surface of a cage at almost light speed. At GHz frequencies, the slots will produce delays that imbalance the fields on alternate sides of the shield, thus allowing some transmission of signal. In fact, some clever microwave circuits make use of slots to transform impedances and construct virtual components. But those frequencies are so heavily attenuated by the coil and cable reactance, that none of it will get out to the amplifier anyway (also the amp will have limited susceptibility to it). So in practice, the shielding with slots should be just as good as a solid box with none.
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Post by JohnH on Nov 10, 2016 15:55:02 GMT -5
When it's working as an electrostatic shield, small slots don't have any effect at wavelengths much greater than the slot diameters. This is because the current travels across the surface of a cage at almost light speed. At GHz frequencies, the slots will produce delays that imbalance the fields on alternate sides of the shield, thus allowing some transmission of signal. In fact, some clever microwave circuits make use of slots to transform impedances and construct virtual components. But those frequencies are so heavily attenuated by the coil and cable reactance, that none of it will get out to the amplifier anyway (also the amp will have limited susceptibility to it). So in practice, the shielding with slots should be just as good as a solid box with none. I expect so too. But i still think it deserves a test or two so that that aspect is a solidly proven as the eddy tests. It wouldnt need the integrator, just a screened cable from pickup to a buffer (a Boss pedal would do), an annoying but cobsistent source of buzz (not just 50/60 hz but more spiky, which is where shielding and covers help most) , then straight into some recording software where relative db can be estimated |
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Post by antigua on Nov 10, 2016 16:06:31 GMT -5
With a Faraday cage, even rather large holes in the mesh will block out frequencies in the gigahertz range, so I can't imagine the slots would be an issue there.
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Post by ms on Dec 1, 2016 13:13:21 GMT -5
I've recently completed studies of eddy current geometry in pickups, and presented some novel prototype pickup covers that exhibit extremely low losses. It's too big to attach here, so here is a link: study (PDF format)The question discussed here is this: How effective is the KW proposed slotting scheme, which prevents currents around the whole cover and also around the area around individual pole pieces? The answer to be explained here: it is better than his measurements show. The reason is that currents around the individual pole pieces are more important than his measurements indicate. This has been determined by using a more realistic magnetic source, that is, one that is much smaller, producing a field more like that produced by a string magnetized by a pole piece. The coil was made by fastening four pins into holes in a scrap of standard perf board forming a square, one empty hole between the pins on each side. Ten turns using a scrap of magnet wire were wound and the ends soldered into nearby holes. A twisted pair was used for connection. This is driven with a few hundred ma of random noise. I prefer to generate noise digitally, but as a result of some equipment failure, I used a 100K resistor operating into a FET preamp driving an SS power amp. Bass is boosted and mid and treble are cut in order to give roughly uniform SNR over the measurement bandwidth. The spectra of both the current through the exciter coil and the voltage from the pickup are measured and the former is divided into the latter in order eliminate variations in excitation with frequency. The response is reduced by 6db per octave, of course. Two loops were constructed with heavy bus wire, a small one just bigger than the exciter coil, and a large one that matches the boundary of the pickup coil. The pickup is an old telecaster bridge pickup I wound years ago to 6.9K. In the attachment the red is the response with no loop, the green with the large loop, and the blue with the small loop. KW's measurements show more attenuation with the large loop than with the multiple small loops. I believe that this is a result of the large source for the magnetic field. The results here show essentially the same attenuation over guitar frequencies, with a suggestion that the small loop falls off somewhat faster out side the guitar bandwidth. The results suggest that measurements should be made with a small coil. The small square coil is a step in the right direction, but I believe that a rectangular one would be better, preferably even smaller. Plenty of signal would be obtained by driving it with about an ampere.  |
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Post by antigua on Dec 1, 2016 18:48:01 GMT -5
Welcome ms, thanks for joining in and doing an experiment. I finally have some time to make a new driver coil. I'm thinking a popsicle stick with a coil wound around one end, using 42 AWG, would be pretty good. I did this experiment with magnetic film, and it looks like a very thin, long coil would make a field similar to that of the strings.  |
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Post by ms on Dec 1, 2016 20:02:45 GMT -5
"I did this experiment with magnetic film, and it looks like a very thin, long coil would make a field similar to that of the strings."
Good experiment; that looks about like the field I was expecting. I prefer to make a narrow rectangle on perf board. Or, for a hum bucker, a single piece of board would have two coils with opposite phase, one over the corresponding pole piece of each coil.
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Post by JohnH on Dec 1, 2016 20:45:06 GMT -5
I've recently completed studies of eddy current geometry in pickups, and presented some novel prototype pickup covers that exhibit extremely low losses. It's too big to attach here, so here is a link: study (PDF format)The question discussed here is this: How effective is the KW proposed slotting scheme, which prevents currents around the whole cover and also around the area around individual pole pieces? The answer to be explained here: it is better than his measurements show. The reason is that currents around the individual pole pieces are more important than his measurements indicate. This has been determined by using a more realistic magnetic source, that is, one that is much smaller, producing a field more like that produced by a string magnetized by a pole piece. The coil was made by fastening four pins into holes in a scrap of standard perf board forming a square, one empty hole between the pins on each side. Ten turns using a scrap of magnet wire were wound and the ends soldered into nearby holes. A twisted pair was used for connection. This is driven with a few hundred ma of random noise. I prefer to generate noise digitally, but as a result of some equipment failure, I used a 100K resistor operating into a FET preamp driving an SS power amp. Bass is boosted and mid and treble are cut in order to give roughly uniform SNR over the measurement bandwidth. The spectra of both the current through the exciter coil and the voltage from the pickup are measured and the former is divided into the latter in order eliminate variations in excitation with frequency. The response is reduced by 6db per octave, of course. Two loops were constructed with heavy bus wire, a small one just bigger than the exciter coil, and a large one that matches the boundary of the pickup coil. The pickup is an old telecaster bridge pickup I wound years ago to 6.9K. In the attachment the red is the response with no loop, the green with the large loop, and the blue with the small loop. KW's measurements show more attenuation with the large loop than with the multiple small loops. I believe that this is a result of the large source for the magnetic field. The results here show essentially the same attenuation over guitar frequencies, with a suggestion that the small loop falls off somewhat faster out side the guitar bandwidth. The results suggest that measurements should be made with a small coil. The small square coil is a step in the right direction, but I believe that a rectangular one would be better, preferably even smaller. Plenty of signal would be obtained by driving it with about an ampere. View AttachmentHi ms and welcome to GN2. I see you have a strong interest in the theory! We need mote members like yourself. So may I ask what are your particular areas of involvement with these issues? Cheers J |
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Post by antigua on Dec 2, 2016 0:37:30 GMT -5
Here is the new Popsicle stick driver coil. The first test plot came out great. I had to send a lot more juice to the coil, though, about 4Vpp where as 0.6Vpp was sufficient with the larger coil. There was also less noise in the plot line with this new coil for some reason, might be environmental. The resonance peaks and Q came out the same with both coils, for this particular pickup.  |
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Post by ms on Dec 4, 2016 7:37:36 GMT -5
Hi ms and welcome to GN2. I see you have a strong interest in the theory! We need mote members like yourself. So may I ask what are your particular areas of involvement with these issues? Cheers J Well, I have always like electric guitars, and I made some, as well as the pickups, when I was in high school. I became an engineer and a scientist, and then took up electric guitars as a hobby some years ago. |
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Post by ms on Dec 4, 2016 7:49:47 GMT -5
Here is the new Popsicle stick driver coil. The first test plot came out great. I had to send a lot more juice to the coil, though, about 4Vpp where as 0.6Vpp was sufficient with the larger coil. There was also less noise in the plot line with this new coil for some reason, might be environmental. The resonance peaks and Q came out the same with both coils, for this particular pickup. I like how the end of the stick allows you to set the height of the coil from the pickup. That is a nice gizmo for holding the stick and setting the position. Can you tell me what it is called an where it can be obtained? Your coil is quite high, which make the field more directional than that from a string. If you use only turns close to the string, you will lose some level, but perhaps not too much since the the turns further from the string contribute less than the closer ones. |
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Post by antigua on Dec 4, 2016 13:07:16 GMT -5
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