Magnetic Isolation for a Hexaphonic pickup

[Sweetfinger]

It may be useful to consider the shape of the polepieces on the Roland unit: Short blades that run perpendicular to the string. There is a reason(or reasons) they went with that design. Only one way to find out.

[David Schwab]

My problem might be from what Mike Sulzer mentioned, I'm just too far away from the strings. Magnetic fields drop off in intensity with r^2 right? So, the closer I can be to the main string, even if it puts me a little closer to the adjacent strings, the better off I am? A separation of 20 dB would be great, more is always better, but with some of the testing I've done that should be enough. I haven't measured the field above each coil, I don't have equipment to do that, but my signal is around 60 mV for each pair of coils.

The circuit might be bringing in some crosstalk, I'm gonna try and think of a good way to test that.

This is what I was thinking when I wondered it you were using too much gain. If you can get the pickups closer to the strings, and therefore get more signal into the pickups, you wouldn't need to amplify them as much, and the cross talk should be lower.

[Mike Sulzer]

It may be useful to consider the shape of the polepieces on the Roland unit: Short blades that run perpendicular to the string. There is a reason(or reasons) they went with that design. Only one way to find out.

To make it easier to locate the pickup to get signal from all strings and fit it in when little space is available. Turning the coils ninety degrees would give more signal and less cross talk, so those reasons are less important.

[David King]

Roland had to make a one-size-fits-all unit but it works.

[Mark Hammer]

As Peskywinnets aptly notes, hex/quad pickups are virtually always located as close to the bridge as possible because there is least risk of the string's lateral movement being sensed by an adjacent coil.

Of course it bears noting that such arrangements generally anticipate that the divided pickup will not be used as an audio signal source, but be used to drive another signal source. This is why they can tolerate being located in that position.

If the player wants to use the divided pickup as an audio signal source, and wants to get the full bandwidth of the string, then they generally have to tolerate leakage between coils such that the output of each coil or pair of coils is mostly from the string above them, not only from that string.

As for reducing interference between adjacent coils, one way to do that is by physically staggering the coils, like those 60's Japanese guitars that would have a diagonal cascade of 2-string pickups.

[jason lollar]

well there is more to it than just the steel- youll have to experiment with different grades of magnets, pole piece end shape and location and height of the steel compared to the pole- it all works together.

[kmensik]

The bone shape of the ferrite core is the reason of crosstalk. The magnetic flux lines do not go up in the centerline of the coil, they go from the top edge to the bottom edge. I tried to make one pickup like that. It is more sensitive to the string next to the edge of the top cylinder than to the string above it. Try it with a plain cylinder ferrite without enlarged top and bottom.

[bbsailor]

As Peskywinnets aptly notes, hex/quad pickups are virtually always located as close to the bridge as possible because there is least risk of the string's lateral movement being sensed by an adjacent coil.

Of course it bears noting that such arrangements generally anticipate that the divided pickup will not be used as an audio signal source, but be used to drive another signal source. This is why they can tolerate being located in that position.

If the player wants to use the divided pickup as an audio signal source, and wants to get the full bandwidth of the string, then they generally have to tolerate leakage between coils such that the output of each coil or pair of coils is mostly from the string above them, not only from that string.

As for reducing interference between adjacent coils, one way to do that is by physically staggering the coils, like those 60's Japanese guitars that would have a diagonal cascade of 2-string pickups.

Mark, and all interested,

The often maligned "moving coil" pickup where the string is used as the active conductive element in the generation of voltage in a relatively low impedance string loop in the 0.1 to 2 ohm range, offers total isolation between strings. In fact the strings need not be ferrous, only be a moving conductor in a magnetic field.

The average string being about 25 inches long allows about a full quarter string length or about 6 inches to be located between the bridge and fingerboard. The output level is mostly governed by the length of the string being in a magnetic field, then increasing the field strength governs output. Where you place the magnets also affects the harmonic level of each individual and isolated string's output.

If someone is building a guitar from scratch or wants to modify a design to try this, all that is needed is a good low resistance ground return to get a common ground/common end from one end of the strings, the nut end! Most truss rods are conductive and the ground return resistance should be a minimum of 10 times lower than the lowest string resistance/impedance to optimize the string output being converted to a workable output without too much resistive losses in the string loop. There are many ways to skin this cat. Isolated tailpieces inserts are commercially available. Tail pieces can be easily modified to keep the strings electrically isolated and connected to the string input loop. Different magnet locations, types, height and length under the string will have an impact of obtaining the right string balance.

Search the web on a thing called the "Stringamp" which is this same technique being applied to a volin or cello. Read about the perceived sound quality of the "Stringamp" directly using the signal generated by the individual strings vibrating in a magnetic field. There is not too much technical detail on the "Stringamp" web site but there must be a common ground return at the nut and the magnets must be mounted under the neck or in the body somewhere. Note the DIN-plug with a common ground and individual string outputs and a body mount mixer to balance individual string output.

What I described in "Moving coil pickups for the technically curious" is the physics of the "Stringamp" but also fits into this discussion as it relates to fully isolated and independent string output.

For those who venture into this area, thinking in terms of current source impedance, turns ratios, reflected impedance and load impedance are primary considerations.

Joseph Rogowski