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Post by JohnH on Apr 1, 2017 18:53:22 GMT -5
Here's a series of thoughts about what may be happening when a magnet is close to a string:
1. The magnet attracts the string, slightly deflecting it when at rest. But this in itself may not make much difference to its harmonic frequencies.
2. The key direction of vibration that is picked up by the pickups are the vertical modes, towards and away from the pickup, rather than across the pickup.
3. In a perfectly uniform field, the pull on the string is constant, so there is no change in pull-force as it moves, so no affect on vibration modes.
4. But with a real magnet close to the string, it experiences greater flux the closer the string moves towards it. So if the string is vibrating with an antinode over a magnet, as it naturally vibrates towards it, the flux increases, pulling it even closer. As it moves away in the next 1/2 cycle, the flux reduces and releases it to move even further away. Hence this harmonic is enhanced by the magnet.
5. But nothing can be for free, and the total energy in the string can only be what is first inserted by the force and deflection of the 'pluck'. Hence if one harmonic is enhanced, conservation of energy implies that it must be at the expense of reducing other harmonics.
Does that make any sense and or hold water?
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Post by antigua on Apr 2, 2017 2:26:36 GMT -5
I tried the magnet-over-string test again using the head stock vibration to excite the string instead of the EBow, and with the head stock, the result was somewhat different, the string overall lost a lot of amplitude when it was pulled on by a neodymium magnet. Harmonics diminished completely, which is to say they went below the noise floor, so I can't see what they did if anything. If the magnetic pull was near the 1st fret or the bridge pickup, the fundamental was retained a bit more, but as the pull gets close to the middle of the string, even the fundamental is heavily damped. This can be seen with the naked eye, the way the string movement becomes less and less as the neo magnet is slid from the end of the string towards the center. I think what it comes down to is that the physical excitation by way of sympathetic resonance is not strong enough to counter the pull of the neodymium.
The exciter is like a speaker's driver coil that interfaces directly against the head stock, and is set to the frequency of the string being tested. The vibrating string shows a lot more fundamental movement with this method than with the EBow, which means the EBow probably was promoting harmonic movement in the string. If you look at that the string that's being excited by the EBow, it moves perfectly up and down, by a vibrated string moves in a more circular pattern if excited from the end of the end of the headstock. If the exciter is attached directly below the nut, then the string does tend to move up and down, similar to the EBow. I got the most vibration out of the strings by having the exciter at the end of the head stock, so that's where it was for the testing.
It's very difficult to conduct this test with the acoustic exciter. The sympathetic resonance is dependent on how the entire body of the guitar vibrates, so just touching the body of the guitar would cause some movement to be absorbed by my finger, changing the harmonic balance. The neck is even more sensitive than the body, even lightly touching the neck upsets the harmonic balance. Also how the guitar was resting on my table would change the harmonic balance, too, so I had to be careful not to bump the guitar. When I held the neo over the strings, I couldn't touch the guitar at all, but the neo was pulling at the strings, so it was not east to suspect the magnet over the string.
I should still be able to test pulling effects from the pickups though, since they pull with less force than the neodymium magnet, and won't totally damped out the sympathetic vibration. This makes me more interested in creating a direct exciter coil that can work like an EBow. So far all I've got from the few test I've done it very faint vibration from the string, but I'm hoping someone will have some pointers on how to make a good exciter coil.
All of this is also presuming that there is something to see in a static state of string movement, as opposed to the attack/decay cycle of a plucked string. We might need a 3D spectrogram to really see anything. Someone also needs to come up with a consistent "plucking" scheme for attack/decay tests. I've tried several things in the past, and none have been consistent enough.
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Post by ms on Apr 2, 2017 6:49:04 GMT -5
I tried the magnet-over-string test again using the head stock vibration to excite the string instead of the EBow, and with the head stock, the result was somewhat different, the string overall lost a lot of amplitude when it was pulled on by a neodymium magnet. Harmonics diminished completely, which is to say they went below the noise floor, so I can't see what they did if anything. If the magnetic pull was near the 1st fret or the bridge pickup, the fundamental was retained a bit more, but as the pull gets close to the middle of the string, even the fundamental is heavily damped. This can be seen with the naked eye, the way the string movement becomes less and less as the neo magnet is slid from the end of the string towards the center. I think what it comes down to is that the physical excitation by way of sympathetic resonance is not strong enough to counter the pull of the neodymium. The exciter is like a speaker's driver coil that interfaces directly against the head stock, and is set to the frequency of the string being tested. The vibrating string shows a lot more fundamental movement with this method than with the EBow, which means the EBow probably was promoting harmonic movement in the string. If you look at that the string that's being excited by the EBow, it moves perfectly up and down, by a vibrated string moves in a more circular pattern if excited from the end of the end of the headstock. If the exciter is attached directly below the nut, then the string does tend to move up and down, similar to the EBow. I got the most vibration out of the strings by having the exciter at the end of the head stock, so that's where it was for the testing. It's very difficult to conduct this test with the acoustic exciter. The sympathetic resonance is dependent on how the entire body of the guitar vibrates, so just touching the body of the guitar would cause some movement to be absorbed by my finger, changing the harmonic balance. The neck is even more sensitive than the body, even lightly touching the neck upsets the harmonic balance. Also how the guitar was resting on my table would change the harmonic balance, too, so I had to be careful not to bump the guitar. When I held the neo over the strings, I couldn't touch the guitar at all, but the neo was pulling at the strings, so it was not east to suspect the magnet over the string. I should still be able to test pulling effects from the pickups though, since they pull with less force than the neodymium magnet, and won't totally damped out the sympathetic vibration. This makes me more interested in creating a direct exciter coil that can work like an EBow. So far all I've got from the few test I've done it very faint vibration from the string, but I'm hoping someone will have some pointers on how to make a good exciter coil. All of this is also presuming that there is something to see in a static state of string movement, as opposed to the attack/decay cycle of a plucked string. We might need a 3D spectrogram to really see anything. Someone also needs to come up with a consistent "plucking" scheme for attack/decay tests. I've tried several things in the past, and none have been consistent enough. Has anyone considered building a magnetic string plucker? That is, excite the string with an amp and coil, but use short powerful pulse to move the string aside and then release it. You would probably need quite a few watts, but that should not be a problem. It would take some experimentation to learn what pulse length you should use to get closest to the action of a pick.
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Post by stratotarts on Apr 2, 2017 11:01:07 GMT -5
I have some smaller electromagnets similar to this one: link . The ones I have are about 8mm in diameter so would fit over a string quite well. One way to get a giant pulse out of it is to charge a big cap and discharge it through the coil.
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Post by antigua on Apr 2, 2017 12:47:30 GMT -5
I have some smaller electromagnets similar to this one: link . The ones I have are about 8mm in diameter so would fit over a string quite well. One way to get a giant pulse out of it is to charge a big cap and discharge it through the coil. Can that be used for an oscillating field, or is only suitable for hold and release?
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Post by antigua on Apr 2, 2017 13:16:16 GMT -5
Has anyone considered building a magnetic string plucker? That is, excite the string with an amp and coil, but use short powerful pulse to move the string aside and then release it. You would probably need quite a few watts, but that should not be a problem. It would take some experimentation to learn what pulse length you should use to get closest to the action of a pick. I can see two issues; the harmonic content would be really low, as if you plucked the string with something even softer than a felt pick. Also, in order to get a good amplitude, you usually displace a string about 3mm and then the pick or finger lets the string break free. I just tried pulling the string with a neo, and the wound strings have enough permeability to allow for the string to be pulled a good distance before it breaks away, but the smaller plain strings can hardly be displaced at all. But this does lead to an idea, maybe the lower strings can show a consistent "pluck" by way of magnetic break away. I'll give it a try.
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Post by ms on Apr 2, 2017 14:19:36 GMT -5
Has anyone considered building a magnetic string plucker? That is, excite the string with an amp and coil, but use short powerful pulse to move the string aside and then release it. You would probably need quite a few watts, but that should not be a problem. It would take some experimentation to learn what pulse length you should use to get closest to the action of a pick. I can see two issues; the harmonic content would be really low, as if you plucked the string with something even softer than a felt pick. Also, in order to get a good amplitude, you usually displace a string about 3mm and then the pick or finger lets the string break free. I just tried pulling the string with a neo, and the wound strings have enough permeability to allow for the string to be pulled a good distance before it breaks away, but the smaller plain strings can hardly be displaced at all. But this does lead to an idea, maybe the lower strings can show a consistent "pluck" by way of magnetic break away. I'll give it a try. I do not see why the harmonic content would be low. The magnetic pulse could have sharp edges with maybe even more harmonies than a pick.
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Post by ms on Apr 2, 2017 14:41:52 GMT -5
I have some smaller electromagnets similar to this one: link . The ones I have are about 8mm in diameter so would fit over a string quite well. One way to get a giant pulse out of it is to charge a big cap and discharge it through the coil. They might be too slow even with a big pulse. Might destroy it trying to hit it hard enough. You might be able to do it with the low impedance side of a cheap audio transformer if you saw the core to make the best shaped field that you can. You would need to filter out the lows in order not to saturate it, but that might simulate the stretch and release of a pick.
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Post by Charlie Honkmeister on Apr 2, 2017 14:42:21 GMT -5
Exciter coils shouldn't be a big issue. Just use the same one as you use for pickup testing. If you had a problem with 10 volts barely moving the string, the problem was current, not voltage. You have to get good current drive for the thang to work. The one I use is a plastic Strat bobbin without magnets/poles, and I had about 200 turns of #32 AWG on it. If you know the inductance and resistance of the exciter coil we could figure out the impedance. Parts Express and SparkFun have small speaker level power amp modules. Realistically, any small amp PCB capable of over about 100 milliwatts into 4-8 ohms should be fine. I have a coil I'd like to use with a DC resistance of 52.8 ohms, and the inductance is 0.67 mH, it must be about 150 turns of 41 AWG. That one is ideal because it's small and would be easy to position, but I also have a plastic Strat bobbin coil with 3.2 ohms resistance and 0.49mH inductance, which is much larger, about 70 turns of 26 AWG. I'm slightly worried about burning out the 41 AWG coil by exposing it to too much current. What do you think about a little amplifier like this www.amazon.com/Pyle-PCA1-30-Watt-Stereo-Amplifier/dp/B0012KZNP4/ref=sr_1_4?s=electronics&dd=lydIXCa_3W4e95ktHPGr5g%2C%2C&ie=UTF8&qid=1491065510&sr=1-4&keywords=small+audio+amplifier&refinements=p_90%3A8308920011 ? What I was thinking about is something like: LM386 Audio Amp BoardOn boards with an LM386, the restriction is that you have to have between 4 ohms and 32 ohms impedance load. It wouldn't be too much trouble to get that but it doesn't really match the coils you have already. The really low inductance of your two coils suggests you need a lot more turns for good magnetic field strength to couple to the string, which is a small target to intercept and react to flux. Wind a Strat coil with #30 to #32 AWG and end up with about 4 ohms DCR and you should end up with 250-450 turns and a lot more inductance. This would make a more suitable load either for the Pyle amp you are thinking about, or something smaller like a LM386. We could do this a bit more analytically with Salvarsan's coil calculator and some formulas, but you get the idea. The other idea is to put slugs, like humbucker slugs, into the plastic Strat bobbin that you wound the coil with, and both increase the inductance and focus the field. You could probably do that with the Strat bobbin exciter coil you have already, and it might work OK that way without having to wind anything new.
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Post by antigua on Apr 2, 2017 15:00:56 GMT -5
LM386 Audio Amp BoardOn boards with an LM386, the restriction is that you have to have between 4 ohms and 32 ohms impedance load. It wouldn't be too much trouble to get that but it doesn't really match the coils you have already. The really low inductance of your two coils suggests you need a lot more turns for good magnetic field strength to couple to the string, which is a small target to intercept and react to flux. Wind a Strat coil with #30 to #32 AWG and end up with about 4 ohms DCR and you should end up with 250-450 turns and a lot more inductance. This would make a more suitable load either for the Pyle amp you are thinking about, or something smaller like a LM386. We could do this a bit more analytically with Salvarsan's coil calculator and some formulas, but you get the idea. The other idea is to put slugs, like humbucker slugs, into the plastic Strat bobbin that you wound the coil with, and both increase the inductance and focus the field. You could probably do that with the Strat bobbin exciter coil you have already, and it might work OK that way without having to wind anything new. I can probably make a little bobbin with a steel slug, and wind it up pretty good. I have an Extech 380193 LCR meter, could the resistance mode be used to determine if I've achieved 4 ohms impedance with a combination of winds and steel core? 120Hz or 1kHz test frequency? My understanding is these impedance ratings tend to be imprecise anyway.
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Post by antigua on Apr 2, 2017 15:13:26 GMT -5
I can see two issues; the harmonic content would be really low, as if you plucked the string with something even softer than a felt pick. Also, in order to get a good amplitude, you usually displace a string about 3mm and then the pick or finger lets the string break free. I just tried pulling the string with a neo, and the wound strings have enough permeability to allow for the string to be pulled a good distance before it breaks away, but the smaller plain strings can hardly be displaced at all. But this does lead to an idea, maybe the lower strings can show a consistent "pluck" by way of magnetic break away. I'll give it a try. I do not see why the harmonic content would be low. The magnetic pulse could have sharp edges with maybe even more harmonies than a pick. That might be the case, but if you have a neo and an electric guitar on hand, and you see how strongly they react by holding them close together, I get the feeling you could saturate the string and still not get enough attraction to come anywhere close a forceful string pluck. I think that's why you get a slow buildup of amplitude with an EBow, it's not just that it's limited in output voltage, but limited by the string itself. Another curious thing that probably warrants research unto itself, that Chinese Tele still has the stock strings, and they appear to have much lower permeability than Elixers I usually install. With the Elxirs, I get a good break away effect from all but the B and high E string, so there's probably a string quality/permeability component to consider when setting up magnetic excitation. The neo actually sticks to the low E so well that pull away caused the string to impact against the frets, spoiling the harmonics yet again, so I might have to pull it sideways. A string is usually plucked sideways, and a consequence of that is a muted attack, since sideways movement is less inductive than up/down movement. If you plug a string top down, it has a different transient than when plucked from the side, even if you're careful to avoid an impact against the frets.
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Post by antigua on Apr 2, 2017 16:36:43 GMT -5
Another thing I've noticed with string pull, that may not be obvious, is that introducing a magnetic field to an already vibrating string causes induces harmonic activity and intermodulations that then subside after some period of time, so when I bring the neo close to the strings, or raise the pickups, I have to wait ten to twenty seconds for the string movement to return to equilibrium.
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Post by JohnH on Apr 2, 2017 17:18:42 GMT -5
Another thing I've noticed with string pull, that may not be obvious, is that introducing a magnetic field to an already vibrating string causes induces harmonic activity and intermodulations that then subside after some period of time, so when I bring the neo close to the strings, or raise the pickups, I have to wait ten to twenty seconds for the string movement to return to equilibrium. That observation could be consistent with the effect I was trying to describe above, to do with varying flux density above the magnet. As amplitude reduces, the differences that the string experiences through its cycles of movement diminish too. I think it is not the flux itself but the variations in the flux through tbe range of string movement that may be causing these harmonic effects.
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Post by antigua on Apr 2, 2017 17:49:32 GMT -5
Another thing I've noticed with string pull, that may not be obvious, is that introducing a magnetic field to an already vibrating string causes induces harmonic activity and intermodulations that then subside after some period of time, so when I bring the neo close to the strings, or raise the pickups, I have to wait ten to twenty seconds for the string movement to return to equilibrium. That observation could be consistent with the effect I was trying to describe above, to do with varying flux density above the magnet. As amplitude reduces, the differences that the string experiences through its cycles of movement diminish too. I think it is not the flux itself but the variations in the flux through tbe range of string movement that may be causing these harmonic effects. For sure, the magnet will have continuous effects upon the string as it decays. The effect will be stronger when the magnet is closer to the string, and will be stronger initially, since the string displacement has an exponential decay.
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Post by stratotarts on Apr 2, 2017 17:51:51 GMT -5
I have some smaller electromagnets similar to this one: link . The ones I have are about 8mm in diameter so would fit over a string quite well. One way to get a giant pulse out of it is to charge a big cap and discharge it through the coil. They might be too slow even with a big pulse. Might destroy it trying to hit it hard enough. You might be able to do it with the low impedance side of a cheap audio transformer if you saw the core to make the best shaped field that you can. You would need to filter out the lows in order not to saturate it, but that might simulate the stretch and release of a pick. Another thing you could do, is energize the electromagnet with the rated 12V, manually push the string so it sticks to it, wait for the vibrations to settle down and then release it by turning off the 12V.
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Post by antigua on Apr 2, 2017 19:59:24 GMT -5
For this test I used an Epiphone Les Paul with Tonerider AlNiCo 4 humbuckers, so the magnetic pull on these pickups is weaker than what would be seen with AlNiCo 5, but about the same as AlNiCo 2. I attached an acoustic exciter to the headstock and varied the heights of the pickups, while analyzing the output from the pickups. The advantage of this test over the EBow is that there is no feedback or square wave to introduce unnatural effects, but the drawback is that having energy continuously input to the strings isn't natural either, but it is a lot closer. The effects of string pull probably compound as the string decays, but with energy inputted into the system, that's not allowed to happen as much. Neck pickup selected, neck pickup adjustedThese graphs show the neck pickup selected, and raised from the plastic mount to right up at the string. It shows that the second harmonic increases steadily as the pickup is raised. The third harmonic and higher only emerge once the neck pickup is set very close to the string. Overall, the effect is to increase harmonic response. The fourth harmonic is suppressed due to the neck pickup's position. The fundamental increased by about 5dB from lowest to highest, and interestingly seemed to be stronger when the pickup was raised only 4mm versus 6mm, suggesting that there is a sweet spot, where the string is close enough to induce a strong voltage, but far enough away to not present an impedance upon the string's vibration. Neck pickup selected, neck flush with mount Neck pickup selected, neck raised 2mm
Neck pickup selected, neck raised 4mm
Neck pickup selected, neck raised 6mm (1mm away from string)Neck pickup selected, bridge pickup adjustedWith the neck pickup selected, raising the bridge caused the overall amplitude to decrease, impacting the harmonics even more than the fundamental. The second harmonic was suppressed more aggressively than the third or fifth. The fourth harmonic can hardly be seen by the neck. The fundamental dropped by about 5dBV. Neck pickup selected (fixed at 2mm above mount), bridge flush with mount
Neck pickup selected, bridge raised 2mm
Neck pickup selected, bridge raised 4mm
Neck pickup selected, bridge raised 6mm (1mm away from string) Bridge pickup selected, bridge pickup adjustedAs was the case with the neck pickup, when the bridge is raised, it sees more harmonic amplitude, though in this case it appears to favor odd harmonics, with the fifth harmonic becoming especially strong. The fundamental increased by about 10dBV from mount level to string. Bridge pickup selected, bridge flush with mount
Bridge pickup selected, bridge raised 2mm
Bridge pickup selected, bridge raised 4mmBridge pickup selected, bridge raised 6mm (1mm away from string)Bridge pickup selected, neck pickup adjustedKeeping the bridge at a fixed 4mm above the mount and raising the neck pickup caused an increase in harmonics also, most favoring the odd harmonics, except when the neck pickup was at the 4mm mark, the second harmonic was stronger, but then overtaken by the third once the neck pickup was set close to the string. The fundamental dropped about 13dBV with the neck pickup set close to the string. Interestingly, with the neck pickup set to 2mm and 4mm, it actually seemed to make the fundamental a little stronger. Bridge pickup selected (fixed at 4mm above mount), neck flush with mount
Bridge pickup selected, neck raised 2mm
Bridge pickup selected, neck raised 4mm
Bridge pickup selected, neck raised 5mm (2mm away from string)
Bridge pickup selected, neck raised 6mm (1mm away from string)There seems to be a trend of pickups pulling in more harmonic movement / voltage when they're closer to the string. When the neck is selected, and raised, it sees more harmonics. When the neck is selected, and the bridge is raised, the neck sees less harmonics. When the bridge is selected, and the bridge is raised, it seems more harmonics, especially odd ones. When the bridge is selected and the neck raised, the bridge again sees more harmonics, but looses a lot of fundamental power, too. There also appears to also be a sweet spot for a pickup, where it's close enough to see a good displacement and produce a higher voltage, but not close enough to cause significant damping via string pull. I suspect that guitarists tend not to raise a pickup so close that it dampens the string, because at that near of a distance, the string would hit the pickup when playing higher frets, but with a Fender featuring strong AlNiCo 5 pole pieces, it might happen with "standard" height settings.
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Post by Charlie Honkmeister on Apr 2, 2017 21:15:31 GMT -5
LM386 Audio Amp BoardOn boards with an LM386, the restriction is that you have to have between 4 ohms and 32 ohms impedance load. It wouldn't be too much trouble to get that but it doesn't really match the coils you have already. The really low inductance of your two coils suggests you need a lot more turns for good magnetic field strength to couple to the string, which is a small target to intercept and react to flux. Wind a Strat coil with #30 to #32 AWG and end up with about 4 ohms DCR and you should end up with 250-450 turns and a lot more inductance. This would make a more suitable load either for the Pyle amp you are thinking about, or something smaller like a LM386. We could do this a bit more analytically with Salvarsan's coil calculator and some formulas, but you get the idea. The other idea is to put slugs, like humbucker slugs, into the plastic Strat bobbin that you wound the coil with, and both increase the inductance and focus the field. You could probably do that with the Strat bobbin exciter coil you have already, and it might work OK that way without having to wind anything new. I can probably make a little bobbin with a steel slug, and wind it up pretty good. I have an Extech 380193 LCR meter, could the resistance mode be used to determine if I've achieved 4 ohms impedance with a combination of winds and steel core? 120Hz or 1kHz test frequency? My understanding is these impedance ratings tend to be imprecise anyway. Since you have an Extech, finding the impedance of an inductor at a certain frequency is pretty easy. Here's a reference Web page. www.arca53.dsl.pipex.com/index_files/elecprop.htm1. Measure the DC resistance. 2. Measure the inductance at 100 Hz or 120 Hz test frequency. Find the inductive reactance with the formula X(l) = 2 * pi * F * L , where F is the frequency you wish to find the impedance at, and L is the measured value of the inductor. You can pick 100 Hz as a reasonable starting value for F. since the reactance will rise as the frequency is increased (inductive load.) 3. Use the formula Z= sqrt ((DC resistance) ^2 + (inductive reactance from step 2) ^2). That's the impedance at the frequency you picked. 4. Plug numbers in again, for a frequency of 1 Khz and see if the impedance at that frequency is "reasonable" for the power amp you're using. (usually 4 to 16 ohms.) Probably could be above that range even, but power transfer to the coil would be limited in theory if there's a big mismatch between amp output impedance and coil impedance. Impedance measurement can be complicated with an actual electromechanical transducer like a speaker, because there's complex impedance changes and back-EMF caused by energy storage and dissipation in the moving cone, surround, etc. But for a simple coil with no moving parts, things can be pretty precise.
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Post by ms on Apr 3, 2017 7:41:01 GMT -5
I do not see why the harmonic content would be low. The magnetic pulse could have sharp edges with maybe even more harmonies than a pick. That might be the case, but if you have a neo and an electric guitar on hand, and you see how strongly they react by holding them close together, I get the feeling you could saturate the string and still not get enough attraction to come anywhere close a forceful string pluck. I think that's why you get a slow buildup of amplitude with an EBow, it's not just that it's limited in output voltage, but limited by the string itself. Another curious thing that probably warrants research unto itself, that Chinese Tele still has the stock strings, and they appear to have much lower permeability than Elixers I usually install. With the Elxirs, I get a good break away effect from all but the B and high E string, so there's probably a string quality/permeability component to consider when setting up magnetic excitation. The neo actually sticks to the low E so well that pull away caused the string to impact against the frets, spoiling the harmonics yet again, so I might have to pull it sideways. A string is usually plucked sideways, and a consequence of that is a muted attack, since sideways movement is less inductive than up/down movement. If you plug a string top down, it has a different transient than when plucked from the side, even if you're careful to avoid an impact against the frets. It is very hard to saturate a small piece of magnetic material. Saturation occurs easily when you cause a field around a closed loop, such as an audio transformer. The magnetic field induced in a small piece of magnetic material is almost independent of permeability as long as the permeability is fairly large. For example, this is shown in the MacDonald (Princeton) derivation of pickup response. I think the fact that a neo pulls the string very hard means as that you could that with an electrically driven coil. Maybe not as strong as a strong pick, I do not know. The eBow is a very underpowered device since it runs off a battery and drives the coil with a standard op amp. It cannot saturate the string. The fact a neo can pull so hard shows that.
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Post by antigua on Apr 3, 2017 11:54:26 GMT -5
I can probably make a little bobbin with a steel slug, and wind it up pretty good. I have an Extech 380193 LCR meter, could the resistance mode be used to determine if I've achieved 4 ohms impedance with a combination of winds and steel core? 120Hz or 1kHz test frequency? My understanding is these impedance ratings tend to be imprecise anyway. Since you have an Extech, finding the impedance of an inductor at a certain frequency is pretty easy. Here's a reference Web page. www.arca53.dsl.pipex.com/index_files/elecprop.htm1. Measure the DC resistance. 2. Measure the inductance at 100 Hz or 120 Hz test frequency. Find the inductive reactance with the formula X(l) = 2 * pi * F * L , where F is the frequency you wish to find the impedance at, and L is the measured value of the inductor. You can pick 100 Hz as a reasonable starting value for F. since the reactance will rise as the frequency is increased (inductive load.) 3. Use the formula Z= sqrt ((DC resistance) ^2 + (inductive reactance from step 2) ^2). That's the impedance at the frequency you picked. 4. Plug numbers in again, for a frequency of 1 Khz and see if the impedance at that frequency is "reasonable" for the power amp you're using. (usually 4 to 16 ohms.) Probably could be above that range even, but power transfer to the coil would be limited in theory if there's a big mismatch between amp output impedance and coil impedance. Impedance measurement can be complicated with an actual electromechanical transducer like a speaker, because there's complex impedance changes and back-EMF caused by energy storage and dissipation in the moving cone, surround, etc. But for a simple coil with no moving parts, things can be pretty precise. That makes sense, but won't the "R" function of the LCR meter do something similar? I'm not aware that there's a distinction between AC resistance and impedance. That might be the case, but if you have a neo and an electric guitar on hand, and you see how strongly they react by holding them close together, I get the feeling you could saturate the string and still not get enough attraction to come anywhere close a forceful string pluck. I think that's why you get a slow buildup of amplitude with an EBow, it's not just that it's limited in output voltage, but limited by the string itself. Another curious thing that probably warrants research unto itself, that Chinese Tele still has the stock strings, and they appear to have much lower permeability than Elixers I usually install. With the Elxirs, I get a good break away effect from all but the B and high E string, so there's probably a string quality/permeability component to consider when setting up magnetic excitation. The neo actually sticks to the low E so well that pull away caused the string to impact against the frets, spoiling the harmonics yet again, so I might have to pull it sideways. A string is usually plucked sideways, and a consequence of that is a muted attack, since sideways movement is less inductive than up/down movement. If you plug a string top down, it has a different transient than when plucked from the side, even if you're careful to avoid an impact against the frets. It is very hard to saturate a small piece of magnetic material. Saturation occurs easily when you cause a field around a closed loop, such as an audio transformer. The magnetic field induced in a small piece of magnetic material is almost independent of permeability as long as the permeability is fairly large. For example, this is shown in the MacDonald (Princeton) derivation of pickup response. I think the fact that a neo pulls the string very hard means as that you could that with an electrically driven coil. Maybe not as strong as a strong pick, I do not know. The eBow is a very underpowered device since it runs off a battery and drives the coil with a standard op amp. It cannot saturate the string. The fact a neo can pull so hard shows that. The neo pulls on the higher quality wound string really well, but not the plain ones, so I think it could work, but would work best with lower strings. I'll try making a small coil with a steel core and find out. Do you think 41AWG can handle the current that would be required, or should I get ahold of something thicker?
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Post by ms on Apr 3, 2017 15:58:32 GMT -5
Do you think 41AWG can handle the current that would be required, or should I get ahold of something thicker? I think it needs to be bigger if you are driving it with a power amp. The pulse that drives the string could be a short burst of sine wave at the string resonant frequency. Picking can excite any frequency, depending on the string resonant frequency. It can be thought of as a driving function that contains a broad spectrum of frequencies. We should not have to waste amplifier power doing that. If we think of filtering out the unneeded frequencies from the driving pulse and keeping only the ones near the resonant frequency, we are left with a pulse with some rise and fall time and which oscillates at the resonant frequency. If you used a short burst of a square wave, you would be exciting harmonics as well. A burst of triangle wave would give lower levels of harmonics. Or one could design a waveform containing the desired level of harmonics. I think that the main thing is that we want to be looking at the decaying oscillation, like what happens after picking, as opposed to a steady state driven string as with the eBow.
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Post by Charlie Honkmeister on Apr 3, 2017 20:44:21 GMT -5
That makes sense, but won't the "R" function of the LCR meter do something similar? I'm not aware that there's a distinction between AC resistance and impedance.
It's the same thing for our purposes, I think. Now that you pointed that out, you can read AC resistance directly with your Extech, but only at the test frequency you selected (100/120 Hz, 1 Khz, 10 Khz, etc.) The impedance/AC resistance at 1 Khz is probably good enough for a quick sanity check of the coil impedance, but the formulas I gave will work for any frequency.
I'm not familiar with the Extech since I have a DER EE DE-5000 LCR meter, but they basically would do the same thing.
I would agree with MIke that #41 AWG is probably too skinny to let you get good current through the coil. You would have to use a lot higher voltage drive signal to get that current, and would actually run into resistive heating effects you didn't want.
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Post by ms on Apr 5, 2017 19:36:33 GMT -5
There is another thing about the eBow that I think we have not discussed. This is the fact that it introduces its own string pull (but how strong?) because the driver coil uses a permanent magnet for a core.
Why? Because that way you can drive it with the same frequency that you generate, and thus it is possible to use feedback and a simple amplifier.
However, for the purposes of driving the string in order to test string pull from another magnet, we do not want the driving device to pull the string also. The problem is this: if you drive the coil at the frequency you want to excite using an ac waveform, the string is pulled twice as often as it should be, and goes almost no where. This is because the string is pulled in the direction in which the magnitude of the field is increasing, independent of whether the field is positive or negative, or, that is too say, whether it is acting as a north or south pole. That is why magnets always attract things in which they induce a field using either pole. Introducing a "dc" bias field means that the total field increases and decreases, but does not change sign, and then the eBow works as expected.
To drive the string with an amplifier and coil, perhaps using a steel core, but not a permanent magnet, you need to make sure that the coil is driven just once during each cycle. One way to achieve this would be to half wave rectify the driving sine wave. Another way would be to shift the sine wave by a dc level so that it is always either positive or negative. This could be achieved by capacitively coupling the amp out followed by a diode clamping circuit.
Edit: If you drive the string as suggested above, you are pulling on the string in one direction during the driving pulse. I think hat is OK because that is what a pick does.
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frankfalbo
Meter Reader 1st Class
Posts: 74
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Post by frankfalbo on Apr 5, 2017 20:09:13 GMT -5
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Post by antigua on Apr 5, 2017 23:07:09 GMT -5
There is another thing about the eBow that I think we have not discussed. This is the fact that it introduces its own string pull (but how strong?) because the driver coil uses a permanent magnet for a core. Why? Because that way you can drive it with the same frequency that you generate, and thus it is possible to use feedback and a simple amplifier. However, for the purposes of driving the string in order to test string pull from another magnet, we do not want the driving device to pull the string also. The problem is this: if you drive the coil at the frequency you want to excite using an ac waveform, the string is pulled twice as often as it should be, and goes almost no where. This is because the string is pulled in the direction in which the magnitude of the field is increasing, independent of whether the field is positive or negative, or, that is too say, whether it is acting as a north or south pole. That is why magnets always attract things in which they induce a field using either pole. Introducing a "dc" bias field means that the total field increases and decreases, but does not change sign, and then the eBow works as expected. To drive the string with an amplifier and coil, perhaps using a steel core, but not a permanent magnet, you need to make sure that the coil is driven just once during each cycle. One way to achieve this would be to half wave rectify the driving sine wave. Another way would be to shift the sine wave by a dc level so that it is always either positive or negative. This could be achieved by capacitively coupling the amp out followed by a diode clamping circuit. Edit: If you drive the string as suggested above, you are pulling on the string in one direction during the driving pulse. I think hat is OK because that is what a pick does. Good observations. I measured about 400 gauss at the two far ends of the EBow, along where the string would be positioned. I assume one is the pickup and the other the driver, which is interesting, because it means they must have found that a magnet aided with excitation, which means I might be in for a tough time trying to achieve similar excitation without one. Therefore, the string pull is probably stronger than a PAF, but not as strong as a Strat single coil, dependent upon distance of course. These fields are closer to the string than a pickup, with only 1mm to 2mm clearance. I'm not planning to use an EBow for further testing, unless there's a situation where all of the effects it imparts wouldn't matter. Regarding the two pulls per one cycle, I'll try half the frequency as well as a DC offset, and see how each works out. I'm making a coil right now. I had to wait for epoxy to set before I could wind it up. I'm not sure why the magnet helps increase the string oscillations, but I notices this when I tried to use a steel pole pickup as an exciter, it worked much better with a magnet was present. Is it similar to how a speaker works, perhaps? At the moment, I'm almost more concerned about a visualization approach that will make time dimensional differences visible to the naked eye. 3D spectrograms are the best, but I have to find a program that can take a wav file and give back a good graphic representation. That's almost more daunting than the consistent plucking problem.
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Post by ms on Apr 6, 2017 7:09:07 GMT -5
I measured about 400 gauss at the two far ends of the EBow, along where the string would be positioned. I assume one is the pickup and the other the driver, which is interesting, because it means they must have found that a magnet aided with excitation, Without the permanent magnet in the eBow excitation coil, there would be very little excitation. Both halves of the driving sine wave cycle would pull the string, canceling out nearly all the drive at the driving frequency. Another way to drive the string without a permanent magnet would be to use two coils placed so that they would pull the string in opposite directions. You would use diodes to steer the positive half of the signal to one coil, and the negative half to the other. Both would pull, but in opposite directions on alternate halves of the cycle. This is mechanically much more difficult, but it would lower the dc current through the coil and allow more drive per watt of amplifier power and more drive without saturation than with the diode clamp circuit. Another way to drive using one coil would be to use a dc coupled amp, and drive it with a signal that only has one polarity, such as a repeating (at the driving frequency) positive only pulse, which could be square or some other waveform to cut down on the harmonics in the generating signal.
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Post by antigua on Apr 8, 2017 15:15:08 GMT -5
I made one coil, but it looks like trash, so I'm making a new one. The first one has a resistance of 5 ohms at 1kHz.
Does anyone know if I should expect the coil to get a little warm, or is that a sign that it's taking more power than it can handle?
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Post by ms on Apr 8, 2017 16:14:46 GMT -5
I made one coil, but it looks like trash, so I'm making a new one. The first one has a resistance of 5 ohms at 1kHz. Does anyone know if I should expect the coil to get a little warm, or is that a sign that it's taking more power than it can handle? It is OK to run a bit warm, or even a bit hot. What is its inductance?
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Post by antigua on Apr 8, 2017 18:42:06 GMT -5
I made one coil, but it looks like trash, so I'm making a new one. The first one has a resistance of 5 ohms at 1kHz. Does anyone know if I should expect the coil to get a little warm, or is that a sign that it's taking more power than it can handle? It is OK to run a bit warm, or even a bit hot. What is its inductance? The inductance measured 2.8mH
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Post by antigua on Apr 8, 2017 18:59:18 GMT -5
I've just made a new coil, probably 200ish turns of 26AWG around a steel screw with steel washers glued about 15mm apart. 13.5mH inductance @120hz, 6.8 ohms resistance at 120Hz, 26 ohms resistance at 1kHz. This works really good. I have it being powered by a little amp, and it looks like it gets a pretty good voltage, and the coil didn't get hot at all. It looked like a good vibration occurred with 80 Vpp, so I didn't push it any harder than that. You were right that 98Hz worked a lot better than 196Hz, for the G string, due the two "pulls" per cycle. 196Hz wanted to induce harmonics. The only issue now is to find a graphic program that give us good representation of the decay harmonics.
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Post by ms on Apr 9, 2017 8:07:15 GMT -5
I've just made a new coil, probably 200ish turns of 26AWG around a steel screw with steel washers glued about 15mm apart. 13.5mH inductance @120hz, 6.8 ohms resistance at 120Hz, 26 ohms resistance at 1kHz. This works really good. I have it being powered by a little amp, and it looks like it gets a pretty good voltage, and the coil didn't get hot at all. It looked like a good vibration occurred with 80 Vpp, so I didn't push it any harder than that. You were right that 98Hz worked a lot better than 196Hz, for the G string, due the two "pulls" per cycle. 196Hz wanted to induce harmonics. That looks great! Using half the frequency, you are exciting with a full wave rectified waveform (http://powercircuits.net/fourier-series-of-full-wave-rectifier-circuit/). There is a dc component; the first ac component is at twice the generator frequency as you say, and then there are harmonics that decrease with about the square of the harmonic number. Can you excite enough string motion with a short burst of 90 Hz, or do you need to leave it open cw? One good way to study the effect of the neo is to excite the string, then cut the drive, and then start the spectral analysis to study the effect of the neo on the spectrum, comparing to the spectrum without the neo.
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