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Post by ms on Dec 5, 2016 14:24:34 GMT -5
I'm not sure what you mean by the further strings requiring a separate path, because for example, this version of the pickup has all six strings utilizing both routes that the current can take, by having reverse magnetic polarities, like a typical humbucker: The difference with the split-magnets version on the Alumitone that I have on hand is just that they arbitrarily decided to magnetically polarize the guitar string in one location instead of two, so it so happens that three string use one route, and the other three use the other. On the example I have on hand, the polarities of the magnets are reversed, but since the strings are only being charged by one magnet or the other, the two magnets could be of the same polarity, and it would still work fine. I suppose that making the two split magnets opposite polarities helps prevent a dead spot if you are bending the string in the middle of the pickup, between the two magnets. You said some details that you're unsure of are not made clear by the pictures of the disassembled Alumitone, do the patent drawings illustrate the unseen feature you're referring to? Just so it is known, the patent does say explicitly that the inventor believes the "8" figure induces hum cancelling: "It should be appreciated that the shape of the primary winding 320 produces a hum canceling effect due to the current flow therethrough." Normal hum bucker sized version: There are two loops that share a common leg. A hum field causes a changing flux through both loops, and thus induces voltages around both loops. The idea is indeed just like a standard humbucker: the voltages are added out of phase by using the appropriate electrical phasing. Magnets are placed so that each string induces voltage around each loop, and the magnets have opposite polarity so that the signals add rather than cancel. The loops complete by passing around the transformer core. This is the primary winding. The core must have a high enough permeability so that the primary has enough inductance to allow the voltage to develop at all guitar frequencies. The very high turns ratio means that the required much higher voltage can develop at the secondary. (I think that there should be a piece of the transformer core that connects the two core ends together, put on after the core ends are placed through the loop. This would complete the flux path around the transformer and greatly increase the inductance. Otherwise there is a huge gap. Is such score piece part of the construction? If so, then one one can be sure there is one transformer. Otherwise, you could look at it as two transformers with some coupling between them.) "Since the phases of EMI cancels out at any point along the center leg..." I do not understand this. I do not see how this relates to voltages induced around a path by changing flux through the path. The single coil wide version: It appears to be the analog of two coil humbuckers where one coil is used for three strings, the other for the other three strings. Or two and two for the more familiar bass pickup. This interpretation requires that there be two loops, one for the three strings further from the transformer, and the other for the strings further away. I have identified the loop path for the strings further away. I do not see why you say there are not two loops, if that is what you mean, I might be misunderstanding.
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Post by ms on Dec 5, 2016 7:18:33 GMT -5
"Shockingly, the stagger is "modern", as in, a "G" pole that is a lot lower than the "D" pole. See the pic below. Of all the sets for Fender to apply a modern twist to, this should be the last on their list, I'd think. Who makes these decisions? Why do they make these decisions?"
Almost nobody is going to go back to using a wound G string, and so they must have decided to get the string volume balance right for modern strings. This is all about marketing. It is more important for a guitarist to believe that he is vintage correct than to actually take the trouble to do it.
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Post by ms on Dec 5, 2016 7:05:35 GMT -5
I'm not sure where you see a cut, it looks to me like a fully continuous "8" shape. This is my understanding: when the magnetic flux increases due to the moving string closer to the pickup, the current would travel one direction or another, depending on the polarity of the of the magnet, but since the magnet is to one side of the 8 or the other, current would flow down one direction on the magnet's side of the 8, and flow up the other direction though the center and the farther side of the 8. So in that respect, it would be a loop around the whole perimeter of the pickup. But the transformer core only wraps around the center of the "8", so it will only (mostly) see current travelling through the middle of the 8, and not that which travels around the outside. Magnetic noise would hit the "8" at some angle, and it would induce currents around the entire perimeter of the 8, as well as around the two side loops of the 8, and in the middle of the 8, they would be meeting each other in opposite directions and cancel out, and it's at that point where the current is tapped into by the transformer. If the transformer were around either of the two outer loops alone, it would not humbuck because it would never see that cancelling current from the other side of the 8, coming in the other direction. The fact that the ceramic magnets are to one side of the 8, or the other, is what allows the current from the strings to not cancel out in the middle. Therefore, I don't think you have to have two secondary coils to get EMI cancellation from the primary, the secondary only has to be a humbucker so that the secondary doesn't itself become a source of noise. This makes sense in my mind, so let me know if I'm getting my facts wrong. If I visualize the Alumitone as being bent into a tall, straight strip of metal, sticking straight out of the guitar, we then have what looks like a traditional transformer, with the primary and secondary side by side, with a shared core. The 8 shape just means that some of the induced current in the single turn is 180 degree out of phase with itself, and cancelling, while some of the current remains intact and get passed along to the secondary. I was misunderstanding what periphery you meant, but that does not change what I am saying. There must be a continuous very low impedance path through which changing flux from the vibrating string passes, and which results in changing flux through the transformer core. Thus this path must go around the transformer core. The only way I can see this happens is if the end of the core sticks through the path. This appears to be the case at the end of the pickup where the protruding part of the core appears to stick through the path where it bends over the end of the pickup. I do not see how this happens near the middle of the eight. This path describes how the strings further from the end where the transformer is located are sensed, as seen in the picture of the disassembled pickup. Once this path is identified, it is apparent that there must be another path for the strings closer to the pickup. I cannot see all of it, but it appears that it is a similar path.
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Post by ms on Dec 4, 2016 18:49:18 GMT -5
There is hum cancellation in the secondary, but not the primary. That is a possibility, but in this way can you cancel both the hum from the loops and the hum pickup from the transformer coils? I think you need control over the phase at both the primaries and the secondaries to do both. (But I will think about this no more for a while!)
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Post by ms on Dec 4, 2016 18:40:10 GMT -5
"I understand they use aluminium to keep the resistance very low, is resistivity the sort of loss you're referring to?" Copper would be significantly lower, but yes it is restive loss that i refer to. "If a typical coil is a lot of turns in series, isn't this essentially a similar number of turns in parallel?" Yes, that is one way to think about a single low resistance turn. "Does this mean that a thicker aluminium structure would, or should, have a higher Q factor?" I think so, but there probably are other factors involved as well. "I'm not real clear on this part of it. This forum allows editing of posts indefinitely, so I'll remove information that is incorrect so that the first post is not misleading. I'm not entirely clear as to whether the low impedance of the primary coil is low-noise due to the fact that it is low impedance. As you said earlier, voltage is induced around a path, and here I count three paths, one around the entire perimeter, and two paths on either side. That being the case, the current running through the middle of the "8" should be cancelling, because they two outer paths are meeting there, going in opposite directions. Then they arranged it so that the string induces current on only one side of the "8" or the other, so that where it meets the transformer at the middle, it is not cancelling itself out. That looks to me like a humbucking arrangement right there, but no, I don't have a firm grasp of all that is going on there. " It makes sense to me that the transformer is a humbucker in its own right, to prevent noise from entering the circuit there, but I figured that if the aluminum "8" served not technical advantage, they would just do the single "0" loop, as was depicted in the patent www.google.com/patents/US5831196?dq=5831196 The patent says " It should be appreciated that the sensor assembly 10 may be configured to act as a humbucker or a noise compensating single coil." Schematic #10 is below:" Low impedance reduces electrical noise, as does shielding with a conductor. Magnetic noise must be canceled; if you shield with magnetic material, you will affect the signal as well. I do not see a path all the way around the perimeter; I see a cut on one end that interrupts it and dangerously assume that the other end is configured to make the following explanation work. I agree that the purpose of the "8" is to allow hum bucking performance. I think it effectively makes two loops, one for the wound strings and the other for the plain strings. I think it is just a matter of getting the signals to the transformer coils in such a way that the voltages from the changing flux of hum fields cancel. My assumption is that the protruding parts of the transformer cores visible in the disassembled photo stick through the loops on the end where they bend "vertical", enabling the primary excitation when assembled. Then you can see how the loop on the side away from the transformer excites flux through the core. The black stuff might be hiding something we need to see in order to understand how the close side loop is connected. This would imply that each loop has its own transformer, connected together in order to mutually cancel their hum pickup. As you noted earlier, the transformer cores are laminated, and I think this eliminates eddy currents in the transformer as an issue.
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Post by ms on Dec 4, 2016 8:23:55 GMT -5
In understanding a pickup such as this, I think it is important to emphasize how the law of magnetic induction applies equally to all pickups. The law says that a voltage is induced around a path. In a multi turn coil, the voltages of the individual turns naturally appear in series. This is a simple, effective, and elegant way to achieve a higher voltage, and it is hard to believe that a single turn pickup has any advantage other than novelty. And it has some significant practical disadvantages, mainly how to handle the very low impedance without extra loss.
Current flows when a load is attached, no matter how many turns you have, and how much current can flow is limited by the properties of the coil, both its resistance and inductance. You can work out the scaling. The same power is available with any number of turns if the permeability of the core is the same, and the same space is filled with the turns. (Fewer turns have less resistance linearly, but you can use larger wire and so you get a squared relationship, always obtaining the same effective resistance for copper with any number of turns in the same space. The inductance varies as the square of the number of turns for turns in the same space, and so both parameters lead to constant available power, which goes as the square of the voltage.)
I suspect that the low Q is at least partly a result of higher effective resistance. Not only does the "coil" occupy less space the a normal pickup, and so has a higher effective resistance, it is very difficult not to incur extra losses with such a low impedance.
Magnetic noise is not canceled as you say. The changing flux of the noise field through the coil induces voltage around the turns. Cancellation must be specific here as in a normal hum bucker. In this case you also have the potential to induce hum directly in the transformer. This is why transformers intended for low level audio are normally shielded. Here it appears that a hum bucking transformer has been created instead of using a shield. This appears to be part of a system in which the signals into the two coils add while the hum subtracts, but I have not worked out the details.
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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 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 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 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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