markm
Rookie Solder Flinger
Posts: 21
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Post by markm on Sept 10, 2019 0:59:37 GMT -5
I was thinking it would be great to be able to see the difference between beveled and non-beveled pole piece magnetic fields on a pickup. There's nothing like visual data to help you realize subtle effects. Then I saw a couple videos using "ferrofluid" (special dark liquid with suspended magnetic particles) between a couple pieces of glass and backlit for viewing. I couldn't find any videos of this being done with guitar pickups. A small bottle of the ferrofluid is ~$12, messy as anything, and doesn't last long, but it sounds like a cool experiment. Any thoughts?
I love a wider variety of pickups to take pictures of, but I just have standard AlNiCo single coils and humbuckers. I could swap A2,A5, and ceramic bar magnets out of a humbucker, which would be cool comparisons.
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Post by antigua on Sept 10, 2019 19:31:29 GMT -5
Some people have used the FEM software to model pickup magnets in a 2D representation. Another physical way to see it is with magnetic film, which you can buy on Amazon: As far as beveled versus non beveled pole pieces go, there's no cause to believe that it would make an audible difference to the sound. If the pole piece was sharpened to a point, maybe, but shaving a little off the edged leaves the pole piece unchanged for all intents and purposes. Another difference you can predict would be seen if you had a really good magnetic imaging tool, is you'd see steel pole pieces produce a slightly fatter and wider magnetic field, than AlNiCo 2, and you'd see AlNiCo 2 produce a field that is a little fatter and wider than AlNiCo 5, and all create a magnetic field that is fatter and wider than ceramic, as the permeability of the magnetic material drops, the magnetic geometry around the pole piece becomes less interactive within its own permeability, and instead conforms to simple superposition (it's for this reason that pole pieces need to be tall, in order to ensure that they extend lines of flux upwards towards the guitar strings). The difference would be very small though, and it was too small to be observed with a handheld magnetometer. You'd need really fancy imaging equipment to measure it. So there again, there's no cause to believe the permeability or coercivity of the pole piece effects the aperture of the pickup, such that it would make an audible difference.
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markm
Rookie Solder Flinger
Posts: 21
Likes: 2
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Post by markm on Sept 11, 2019 0:25:15 GMT -5
My current level of understanding is that the pickup magnetic field is a small part of the picture. The real game is in the way that the permiability of the pole allows the lines of flux from the sympathetic magnetic fields of the guitar string to be conducted, thus concentrating the flux in the inductor and increasing the signal output. So the permanent magnetic field essentially provides a way of inducing a magnetic field from the guitar string, but is otherwise just kind of in the way for visualization (like DC offset with an AC signal, but magnetic), fortunately flux in a coil doesn't care about that.
Maybe this could be better simulated without a full pickup by placing a normal rod magnet flat on the ferrocell glass and then placing a magnetized wire (on end or sideways) nearby, or just using some very small magnet to observe the comparative interaction with different magnets (or even steel slugs).
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Post by antigua on Sept 11, 2019 3:05:53 GMT -5
My current level of understanding is that the pickup magnetic field is a small part of the picture. The real game is in the way that the permiability of the pole allows the lines of flux from the sympathetic magnetic fields of the guitar string to be conducted, thus concentrating the flux in the inductor and increasing the signal output. So the permanent magnetic field essentially provides a way of inducing a magnetic field from the guitar string, but is otherwise just kind of in the way for visualization (like DC offset with an AC signal, but magnetic), fortunately flux in a coil doesn't care about that. Maybe this could be better simulated without a full pickup by placing a normal rod magnet flat on the ferrocell glass and then placing a magnetized wire (on end or sideways) nearby, or just using some very small magnet to observe the comparative interaction with different magnets (or even steel slugs). That's sounds about right, put more technically, the lower the reluctance path, the less magnetic "resistance" you get, the higher output will be. A pickup with steel pole pieces is generally louder than a pickup with AlNiCo pole pieces, even if the permanent magnetic flux is lower, because the reluctance of the steel pole pieces is so much lower than that of AlNiCo. The AlNiCo provides a stronger magnetic field in which the guitar string is immersed, but the AlNiCo has a much smaller reciprocal magnetic interaction with the guitar string, than does the steel pole piece. In addition to coil proximity to the guitar strings, this is another reason why inductance correlates with output, but doesn't cause it. The same low reluctance path that increases the inductance also increases the voltage output. It's one cause resulting in two effects. Here's a test that demonstrated how the steel pole pieces produced a higher output than AlNiCo pole piece pickups, all in a given test coil. This was done with a driver coil, so it didnt take into consideration the impact of the permanent magnetic field, but you can see that the steel pole piece is 4 to 5dB above the AlNiCo 2/3/4, and almost 7dB higher than the AlNiCo 5. Even if you accounted for the difference in magnetic strength (400G for steel over ceramic, 1050G for AlNiCo 5), it still wouldn't come close to closing that 7dB difference. Reducing the reluctance path has a far more significant effect than increasing the permanent flux density. It would seem that making pickups out of steel pole pieces would be a winning recipe, if not for the fact that a weaker permanent magnetic field has other tonal consequences in terms of how it pulls on the guitar strings, and the eddy currents in steel pole pieces diminishes the Q factor, which is a part of what gives vintage Fender style pickups their signature sound. On top of that, pickup makers tend to wind those steel ceramic pickups up to 8k turns, a DCR reading out 6k ohms, like an AlNiCo counterpart, which when paired with steel pole pieces, drives the inductance up past 3 henries, which produces a lower resonant peak, and a darker, thicker sounding pickup. Those steel poled pickups could be engineered to sound like an AlNiCo pickup (under-wind the coil to get the inductance correct, use laminated steel and/or higher resistance control pots, use neodymium instead of ceramic) but to date, no major pickup maker has shown an interest in trying. The payoff would be an potentially increased output for a pickup that still sounds "vintage", and doesn't require AlNiCo to be sourced.
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