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Post by antigua on Feb 3, 2019 12:39:47 GMT -5
In another thread I can't seem to find at the moment, it was observed that the large steel bridge around the bridge pickup of a Telecaster caused noticeable attenuation due to eddy currents. I have a similar situation on hand, but instead of a thick steel surface, I have a Stratocaster pick guard with a thin, conductive adhesive layer that covers most of the pick guard, including the pickup holes. I measured a Strat pickup of 2.3 henries with and without the pick guard around the pickup, and found that there is no noticeable difference. I also connected the shielding to the ground size of the circuit just to see if there was a noticeable capacitance effect, and there was not. Unfortunately I don't know much about the coductive adhesive, such as the resistivity, or the actual thickness of the conductive layer, versus the backing and adhesive layers.
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Post by ms on Feb 3, 2019 14:26:45 GMT -5
The less metal you put in that stuff, the cheaper it is to make. Do not expect much.
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Post by JohnH on Feb 3, 2019 15:35:15 GMT -5
That's good info. We like shielding here on GN2 and its nice to see it making no change to measured pickup properties.
Given that no change was found in this case, I'd speculatively expect that to be a fairly general result with different Aluminium foils and shapes, since half or twice nothing is still nothing.
Maybe a tad of capacitance might be added to a fully wired guitar though.
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Post by antigua on Feb 3, 2019 17:12:28 GMT -5
The less metal you put in that stuff, the cheaper it is to make. Do not expect much. But does it shield effectively? Im guessing that even a thin metal with relatively high resistivity, supposing it is, gets the job done. Maybe a guitar pickup that isn't usually shielded could forego a bulky rigid cover for some sort of film like shielding around the bobbins.
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Post by antigua on Feb 3, 2019 17:16:53 GMT -5
That's good info. We like shielding here on GN2 and its nice to see it making no change to measured pickup properties. Given that no change was found in this case, I'd speculatively expect that to be a fairly general result with different Aluminium foils and shapes, since half or twice nothing is still nothing. Maybe a tad of capacitance might be added to a fully wired guitar though. The funny thing is that the Tele steel bridge assembly did show some real eddy current effects. I was expecting to see more of an effect here on principle, a large percentage of the lines of flux pass right though the shielding, being that its face is perpendicular to the magnetic axis. I've been meaning to test Tele base plates some more, of which I have some sitting around, and when I do I'll contrast it with thin foil cut to the same overall shape.
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Post by ms on Feb 3, 2019 17:21:42 GMT -5
The less metal you put in that stuff, the cheaper it is to make. Do not expect much. But does it shield effectively? Im guessing that even a thin metal with relatively high resistivity, supposing it is, gets the job done. Maybe a guitar pickup that isn't usually shielded could forego a bulky rigid cover for some sort of film like shielding around the bobbins. It's a function of frequency. To shield ac, charge must flow to follow the changing field, and there is capacitance. If it works at 60 Hz, that does not mean it works at 6 KHz. But as a practical matter, I do not think it takes much metal to shield in the whole audio range.
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Post by JohnH on Feb 3, 2019 20:18:25 GMT -5
We find that typical shielding doesn't do much to suppress the fundamental 50/60 hz hum, but it does help reduce the spiky high-frequency mains-related buzz due to switching transients from lights and other items.
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pj
Apprentice Shielder
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Post by pj on Feb 4, 2019 4:16:43 GMT -5
I think, as is rightly pointed out earlier, the mass of material, and it's permeability is the defining element here.
Experiential evidence has proved the tele bridge point to me. Hopefully now my healths back on track I'll get an integrator sorted and be able to add some objective data.
I had a reasonably hot tele sized p90 made for a semi hollow tele alike that lacked mids and lows. It helped solved the guitars lack, but in the process of changing I noticed how permeable the bridge seemed. I wondered how this might affect the field line return path to the magnets beneath the plate. Assuming it was mild steel, my interest was piqued. So, i swapped to a half cut Wilkinson type that does not surround the pickup. There was a real audible difference. How much of this was change to the magnetic circuit, or eddy currents, or the conductive loop around the coil (or all three) I dont know but the pickup sounded clearer and more 'dynamic'. Now i subsequently changed this pickup to a mpre traditional al5 tele construction with more coil inductance and the difference in the lows is less apparent but the percieved increase in Q is more obvious (assuming the presence of the steel core of the previous pickup diluted the audible results)
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Post by antigua on Feb 4, 2019 13:28:35 GMT -5
I think, as is rightly pointed out earlier, the mass of material, and it's permeability is the defining element here. I'm slightly confused about the contribution of permeability when it comes to eddy currents, because while it's true that is amplifies the magnetic flux of the eddy current, I believe it should also amplify the flux of the invoking magnetic field by exactly the same amount, and so it should be a wash.
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Post by ms on Feb 4, 2019 14:24:26 GMT -5
I think, as is rightly pointed out earlier, the mass of material, and it's permeability is the defining element here. I'm slightly confused about the contribution of permeability when it comes to eddy currents, because while it's true that is amplifies the magnetic flux of the eddy current, I believe it should also amplify the flux of the invoking magnetic field by exactly the same amount, and so it should be a wash. Permeability affects the skin depth, forcing the eddy currents to flow in a thinner region. en.wikipedia.org/wiki/Skin_effect
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pj
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Post by pj on Feb 4, 2019 14:47:28 GMT -5
I can see what your issue is with the concept and it does technically seem to be a grey area, as is magnetism generally.
The fact that it (the core) is an open loop 'magnetic circuit' greatly lessens the effect of the higher permeability, thats fairly well documented in theory, but still as you say the density is far greater in the steel pole example. If thats what you mean by a wash then you could well be right and my ears were being bent by psychological priming. I suppose the only way to really test that is to get the string output on a spectrum analyser and some consistent excitation.
The reason for my logic is that I'm making the supposition that the lines of flux returning from the string furthest from the core are likely to be the least effected by the core material. Hence placing the mild steel bridge plate in the way would diffract the field lines further, kicking the pickup in its one good leg so to speak. For the record this bobbin was packed full with 42 gauge and by my reckoning the outer part of the coil was running shy of flux in the first place.
With the more recent alnico 5 pickup I would expect the field to balloon out more parallel to the surface of the guitar, once again having that permeable material in proximity and alignment with this (even more so at the outer poles next to the ashtray lip) I would expect to have some measurable effect in the same way having a ground plane beneath a cb antenna polarises the field, only in reverse. How much of the audible effect i experienced was self kidology Id be glad to know.
My curiosity about eddy currents and the effect of permeable material in the vicinity ties into thoughts I had around the Tillman pickup window simulation. Does narrowing the magnetic window reduce phase cancellation at some audible frequencies? Could the effect of substantial ferrous material below be seen with magnetic film? I know your excellent tests on strat base plates showed little variation. Some years ago I had a Jerry Donahue strat with a steel plate recessed behind the scratch plate to ape a tele bridge. Id love to have the opportunity to interrogate that one again.
Im ruining your thread with pseudo science and train of consciousness waffle, apologies. Hopefully I can prove which end of the reality wedge my thoughts and experiences sit when i get the tools to do so. Thanks for your times as always
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Post by antigua on Feb 4, 2019 16:38:45 GMT -5
My curiosity about eddy currents and the effect of permeable material in the vicinity ties into thoughts I had around the Tillman pickup window simulation. Does narrowing the magnetic window reduce phase cancellation at some audible frequencies? Could the effect of substantial ferrous material below be seen with magnetic film? I know your excellent tests on strat base plates showed little variation. Some years ago I had a Jerry Donahue strat with a steel plate recessed behind the scratch plate to ape a tele bridge. Id love to have the opportunity to interrogate that one again. The metal parts are generally too far away from the strings to widen the magnetic window enough to cause phase cancellations. The strings harmonics have physical widths, and if you look at the harmonic divisions that are similar in size to the pole pieces, they are high frequency, above 5kHz, effectively beyond the frequency range in which things matter. The window would have to be a lot larger in order to effect the audible range, like half an inch in diameter or larger. You can test this in the Tillman demo by setting the frequency, pickup position and the aperture width, and seeing what happens up until the 5kHz mark as you tweak the parameters. In order for extraneous steel parts to effectively widen the window, the steel would have to be placed on top of the pickup, in between the strings and the coil, because the strength of the magnetic field drops off too rapidly with distance in order for the steel parts to interact significantly with the guitar strings if it's anywhere else (not counting the core of the coil). The idea that the base plate on a Telecaster bridge pickup widens the magnetic field in any way that impacts the magnetic window, or ultimately the tone, is not plausible. The base plate is about 3/4 of an inch away from the strings, imagine lowering the guitar pickup itself 3/4" away from the strings, and you can intuit an idea of how important the pickup parts are at that distance. If you actually were to put a steel plate on top of the pickup, it would get darker, but not primarily due to any widening of the magnetic window, the steel would cause an increase in both inductance and eddy current losses, which would attenuate the treble much more substantially.
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pj
Apprentice Shielder
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Post by pj on Feb 4, 2019 16:56:29 GMT -5
Thanks that puts the proportional effect of the concept much clearer in my mind. I'll have to chew it all over and feed it back into everything else Ive learned here.
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Post by blademaster2 on Feb 4, 2019 17:25:03 GMT -5
So now I am curious. The addition of metal covers onto pickups has a measurable (and definitely audible) impact, as I observed myself in 2018.
When shielding is added it is not significantly thicker than the covers - at least some of the shielding I have seen. True, it is thicker but I am guessing less than an order of magnitude.
So what governs the apparently negligible effect of shielding in such close proximity to the pickup, whereas the covers (and a Tele steel bridge as I understand it, but I do not have a Tele) create a non-negligible difference?
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Post by antigua on Feb 4, 2019 17:48:13 GMT -5
So what governs the apparently negligible effect of shielding in such close proximity to the pickup, whereas the covers (and a Tele steel bridge as I understand it, but I do not have a Tele) create a non-negligible difference? In the other Tele bridge plate testing thread, I just tested a piece of aluminium foil around the pickup, and the difference there was more substantial than with the adhesive shielding sheet on the Strat pickguard, although I was testing with different pickups, so it was perfectly apples to apples. The pick guard shielding might have a higher resistivity, like nickel silver, for example, and that could account for the diminished effect. The thick brass plate showed heavy attenuation, and AFAIK the brass bridge plate has the most in common with the foil and the shielding tape, since none of the above are magnetic (relative permeability ≈ 1), so I'm going to say it's governed by the thickness and resistivity, but that's an observation based guess. I'd be interested in know what sort of adhesive shielding film in on the pick guard, but google searches aren't yielding close visual matches. Most shielding tape is adhesive, foil and cellophane, but this shielding is a matte gray, powdery appearance, as seen in the picture. So if anyone happens to know what that conductive film layer is, tell me so I can research it.
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Post by antigua on Feb 4, 2019 21:13:37 GMT -5
I was able to pull away some of that adhesive shielding from the pickgaurd and carefully scrape away the stick paper backing from a small portion, and use a digital micrometer to measure the thickness, and it came out to 0.015mm, which probably still includes some amount of adhesive in the measurement, but it's thinner than the Reynolds Wrap, which is 0.022mm thick. The plot below shows that that <0.015mm adhesive shielding is almost on par with no shielding at all (black and blue), so it could be placed over, and probably around, a pickup and cause no adverse effects whatsoever. The Reynolds Wrap shows a slight drop (red) is also innocuous in a loaded context. I also stuck a sheet of brass in there, it was a lot thicker at 0.26mm, which is about 12 times as thick as the Reynolds Wrap, and as can be seen it drops the resonance by about half of what it was. All in all I'd say the thickness of the shielding is very much at play, and in practice, the thinner the shielding the better. As far as electrostatic shielding goes, my understanding is that it does not have to be thick, as opposed to electromagnetic shielding, which at audio frequencies would have to be several inches think in order to be effective, so that's not even under consideration, and instead humbucking coils gets the job done instead.
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