R.G.

Here's a thought - if what the lower coil does is mostly to get rid of hum, and it's not as effective at highs anyway, can it be designed so its natural rolloff is way low, just above power line. Kill the hum only, not the signal, and let the signal pass through. Maybe dinking with added capacitance can do something like that. Hmmmm... Then a bit later it strikes me that this is getting very, very abstract to avoid putting a battery inside a guitar. One possibility is to buffer the pickup right on the pickup, make the buffer be low current, and make it work from one or two hearing aid batteries out of a magazine of about six. Batteries don't have to be as big as PP3s. The voltage can be up-verted to as much voltage as you like to run the buffer from.

To measure the inductance, Measure the resistance so you can later subtract it out. Set up a test jig, with a 1M pot in series with a 10K resistor to ground, and the pickup in series with a 10K resistor to ground. Drive the top of the 1M and the top of the pickup with a 1kHz sine wave. Measure the voltage across the 10K under the pickup, then adjust the 1M resistor to give the same voltage across its 10K resistor. Measure the setting on the 1M pot. This value is the magnitude of the AC impedance of the pickup. The AC impedance of the pickup is Zp = Rp +jwL. The magnitude of Zp = square root of the sum of the squares of Rp and jwL. Solve Zp = SQRT(Rp^2 +(jwL)^2) for L. Note that I've ignored C. Generally you should be able to do this at 1kHz. If L is bigger than 3-4H, you may not be able to do this and you may have to look at phase angle on the scope. I think that will work. For 2 you're out of luck. You can't change only the inductance of a coil. You must change at least two things about it to get the same inductance. More/fewer turns needs less/more core area or less/more gap to compensate, and you probably have to change the wire diameter to keep the same resistance. All you can do is design a new coil with a different inductance keeping one thing constant that you care about - constant turns, constant area, constant gap, constant resistance, constant wire size, etc. But you can only have one.

As long as I'm going to get talked about here too, I guess I'm entitled to post a few comments. Pete - you needed a wake up call. I gave it to you. I did it in a way that I thought was far out enough that it could not be taken for anything except exaggeration, while at the same time getting your attention. Perhaps I should have been less subtle. But here are some facts from the real world that you need to face. If I was going to replicate your sustainer design, I would already have them available on the market. It is entirely possible to have a similar device produced and ready for sale in 1000's in a couple of weeks. That's not true about the people you have not heard from. You need to take Truth_David's warning as literal. Someone who's going to sell your idea is simply going to do it, not tell you they're going to do it. The very fact that I said anything at all is evidence of my good will. Unfortunately you can't "copyright it as soon as possible". Copyright does not protect anything except the expression of an idea, like a painting or page of artwork. Patent is the only protection for your "thin coil" driver, and it's probably not patentable now after public disclosure if it ever was. Even if you held a patent, you would have to enforce your own patent, and almost certainly in the USA and other countries, not Australia. You would also have to file the patent in those other countries. The expense and difficulty would mean that a well-financed infringer could simply spend you to death. Patent protection only works between financial peers. There are many ways to invalidate a less well-heeled competitor's patent. The big guys know them all. It is possible, and about 50-50 likely that when you sell your first stuff, one or more of the holders of the sustainer patents will sue YOU for infringement of their prior patents, which do exist. It's an unpleasant thought, but similar things have happened. Even if it's a blatent lie on the face of it, you could find yourself forced to defend yourself in court in the USA or Europe or Japan, or have summary judgement entered. You owe it to yourself to think about what you'll do if that happens. It may not. But it is one of the possibilities. I urge you, for your own sake, to listen: If you're serious about doing any of this commercially stop talking about it in public immediately and just quietly go do it. Every moment you spend posting about it gets you closer to someone else doing it.

Not reading the preceeding 72 pages, the question comes to mind - did anybody ever do my suggestion from Ampage, and rewind a neck pickup with about #24 wire to be a driver/pickup switchable? For sustain you drive it, and for pickup, you use an onboard transformer or high gain LOW input impedance preamp like a hifi moving coil preamp. Just curious. That seems easier than some of the permutations I've seen.

I used to think that as well. It turns out that a coil driver can push AND pull, but it needs a magnet to do it. The coil by itself does pull only, and at double frequequency on the string. However, if there is a permanent magnet already pulling on the string, the coil can reinforce and cancel the magnet pull, so the coil "pushes" to the extent that it cancels the permanent magnet's field that's already pulling. And that's why drivers need permanent magnets.

For a semi-good explanation of ground loops, read the "Spyder Power Supply" article at GEO - http://www.geofex.com. Ground loops happen because: (a) ground is different voltages to different circuits ( AC circuits leak © ground conductors have some resistance, like all conductors (d) loops of any kind pick up EM interference from the ether.

Fray a wire and glue it to the underside of the steel with conductive epoxy...

Hahahahahahahahahahahahahahahahahaha..hee..ho..ha... Oh, my sides ache. What a patent. That makes a number of things clear. - there are a few guys who are fairly nuts over magnetic sustainers working in the market - they hate one another - they think they're going to get rich by excluding others from building the Final Sustainer and having the world beat a path to their door. - they'll say or do anything to get there. Let's look at some things. - Prior art list. Ohmigod - SIXTY US PATENTS cited as references! - And the same names come up over and over and over. If you spend the money and time to buy a patent, you do NOT appreciate someone else horning in and trying to exclude you from some aspect you didn't think of yet. So these guys love each other. - Forty-seven claims with subparts, and I suspect when we see the drawings, circles and arrows and a paragraph on each one describing how they're to be used in evidence against ... someone... (sorry Arlo. ) Let's look at the more humorous claims: They claim that they have at least one pole pair to drive each string (b ) and they let the pole extensions get closer to some strings (narrowing the gap ). Well, duh. And we take another full paragraph to point out that not only does the magnetic pole cup extend beyond the coil, it's separated into prongs. This means "We claim it works on ordinary style electric guitars with neck and bridge pickups". Ohmigod!! (slaps forehead) we can use a JACK and PLUG to get the sound out to the amplifier!!! EGAD!!! We'll use an AC power supply with its own plug-and-jack to get powre in! Nobody's ever powered anything like that before!! Near as I can tell, they just claimed to have invented a fuzz box with tone controls by using the switching mode of their driver amplifier. Cool. If we can't keep the driver output out of the signal to the amp LETS FEATURE IT!!! um... they've patented the pickguard holding pickups... Translation: the sustainer goes between the neck and bridge pickups. Claims 27, 28, and 29 reinvent the pulse width modulator. Translation: "If you use a bipolar or FET to amplify your driver signal, you'll hear from our lawyers! You're violating our patent claim". Pity they didn't claim vacuum tubes, Gallium Arsenide FETs and Josephson junction devices too. Then there's some fun in the text description: Translation: "We're patenting the idea of stuffing the battery and driver inside the output jack cavity!! Look Ma! No routing!!" There's a whole mess on switched-mode drives. I'm not sure that the guys who invented PWM drive for servomotors would think this was too novel. I'm getting tired of typing. This style of patenting is one where you claim ownership of the moon, the stars, air, water, heaven and earth, then send nastygrams to everyone who walks, breathes, drinks or looks at the sky. You include enough claims that if a court invalidates some of them, you have a zillion others to fall back on. There are a few good points in there; not sure how novel they really are, but then a court is the only place where that can be determined. The novel bits are: - moveable magnetic shunts (I think that this qualifies as "obvious to one skilled in the art" as I came up with them on my own in about fifteen minutes, but only courts get to call this). You know, now that I think about it, I was a few centuries late. Back when sailing ships were made out of wood and the compas was up on deck, they used big chunks of iron to cancel out the effect of the cannons on the earth's magnetid field so the compass would read true. - using a layer of the coil as a shield; good idea, but it's been used in transformers before. I remember some transformer teaching stuff from the 50's using that idea. Reading patents as humor is usually rewarding; it can also cure insomnia.

There are two ideas there. One - use a center blade/side wrapper to focus the field on the strings. This works best if the magnet itself is inside the coil, not powering the iron from the bottom of the stack. Otherwise, you do get high leakage from the middle of the iron stack out to the exterior wrapper before it gets to the area of space that the strings occupy. This is all going to work better with inherently long magnets like Alnico rather that short magnets like ceramic and near as I can tell, neodymium. You want the magnet to separate flux that comes out one end (north, for example) from that which comes out the other end. Alnico has best field strength if it's longer in the magnetic direction than it is wide. Ceramic works best if they're short and fat. The point of the close-to-the-coil wrapper is that it moves the magnetic gap from along the length of the magnet up to across the face of the magnet. It actually probably ought to have an opening like a blunderbus, so it makes it progressively harder for flux to jump from center to wrapper as it gets nearer the business end. This format forces more flux out to the space above the center lams than a tight wrapper does. Second, the focusing cup is not equal to the magnetic shunt. The magnetic shunt is some distance away from the whole magnet/coil affair, not connected to it at all. It's a garbage collector - it sucks up the stray field before it makes its way to another pickup. Half the distance to the next pickup is a good starting point.

See http://www.geofex.com/FX_images/Magdriver and shunt.pdf

I think a really useful thing to do is to try the magnetic shielding. M-field is a square law thing. Each doubling of distance weakens the field by a factor of four. If you put a magnetic shunt covering the driver, it shunts the strong(-ish) field right at the driver and the strings get little driving. If the strings are closer than the magnetic shunt by a factor of two, they see a driver field of 4X the field that the shunt does. I'd put a soft iron shunt around the driver, but spaced away from the driver at least three times the distance from the driver to the strings. That would mean that the strings are still getting 9/10 of the driver field, and the shunt is working on 1/10 the field. The shunt's job is to cut that fringing field down to unnoticeable over at the next pickup. Accordingly, the first shot at a shunt ought to be a ring of soft iron around the driver at 3X the driver-to-string distance from the outside of the driver. This should suck in the driver fringing field and shunt it away from the other pickup(s). You might get 40-60db of shunting. A shielding shunt around the other pickup as well should clean things up a lot. I think of magnetic fields in iron like water ditches in dirt. The ditch carries maybe 1000 to 10000 times the water that just level dirt does, but some water always leaks out no matter how you dig the ditch. Without iron, M-field spreads out like water on level dirt.

Sorry, I wasn't being clear. You're quite right. Any amplifier working from a limited voltage like a 9V battery is going to be power limited. A perfect amplifier can only get +/-4.5v out of a 9.0V battery, and into 8 ohms, you only get 4.5V/8 = 0.56A peak, 0.39A rms. That makes the max sine wave power 1.266W. But if you want a smallish coil of thicker wire, you can use a transformer to convert that to a lower voltage and a higher current. Something like a 5:1 stepdown gets you to 0.9V peak and 2.8A. While this is the same power into the driver coil, you can now use 1/5 the number of turns in the coil and get the same ampere-turns of magnetic drive. The inductance of a coil is proportional to the square of the number of turns for a constant form factor, so the inductance of the coil drops by a factor of 25. So now the coil is quite agile and can be driven to a given signal quite quickly. That moves the problem from designing a critical pickup to designing an amp/transformer to drive it, which moves the design problem a bit away from the mechanically limited driver coil. Actually, a strat pickup wound with #24 wire to a few ohms of resistance was my first suggestion for a sustainer driver.

Oops, forgot. One possible cure for your driver-to-pickup crossfeed problems is to put a big magnetic shunt between them. The bleedover of the driver M-field outside of the string is presumably what you don't want, as that will squeal at whatever frequency the whole mess is resonant at, not the string frequency. You *want* feedback through the string. The M-field feedback comes from the driver field getting over to the active pickup directly. While there are no magnetic insulators other than superconductors, you can make a magnetic shunt between the two pickups with some iron/steel. With a thickish piece of steel between the two, the steel will shunt excess drive field before it gets to the pickup. Ideally this would encircle the driver completely so that the fringing field was shunted on all sides and there was no air gap for it to overcome. That improves the efficiency of the shunt a lot. Iron is several thousand times better "conductor" of M-field than air, so if you put an iron shunt around the driver, it potentially cuts the fringe field that the pickup sees by a lot.

Yes, it will. And the resistance will be lower than the nominal speaker impedance of the speaker. However, it's close enough to let you decide what it is, because there are only a few values. Most 4 ohms speakers measure 3.2 to 3.6 ohms DC. Most 8 ohms speakers measure around 6 ohms DC. Most 16 ohms speakers measure around 11-14 ohms DC. The DC resistance will always be less than the speaker nominal impedance, because part of the speaker impedance goes to putting power out as sound. The DC resistance is the part that wastes electricity as heat inside the speaker. The two parts added together are the impedance that the speaker is rated at. The actual impedance, not the nominal, rated one, varies by at least 3:1 as frequency varies, sometimes more than that.

Where I left off my thinking about a sustainer driver a year or two ago, I had proposed the idea of taking a single coil pickup, cutting off the windings and rewinding with thicker wire, the consequence of needing to have lower voltages drive the necessary ampere-turns to move the string. I think it's in the archives over at ampage or DIY. The ampere-turns of magnetic field is what determines the "grip" the driver has on the strings. Weak field, loose grip, poor transfer efficiency. I pointed out that to use this as a pickup, you needed some serious amplification because it cuts the number of turns down dramatically, which cuts down on the output. If you've found that a thinner coil works well, why not section a single coil pickup bobbin, putting a thin layer of driver windings up at the top of the pickup, and winding zillions of turns of thin stuff underneath? That lets you have both a driver and a real pickup on the same bobbin. It compromises the real pickup a bit, but you at least do get to have one. Here's another one you might look into. The logical progression of drivers for this is to few turns of thick wire to drive the mechanics. No surprise, that's what happens in the motors world too. You want low inductance so you can build ampere-turns of Magneto-motive force (MMF) fast without having the inductance require huge voltages to make the magnetic field change quickly. At the same time you want high ampere turns for intense magnetic fields. The logical conclusion of that is *one* turn of thick wire drive with a lot of current. That's probably not practical, but you could go to some fairly thick wire and high current if you used a step down transformer driven by the power amp and with the secondary driving a few-turns driver coil. More current, more agile, same driver grip on the strings.