Pickup with a closed magnetic circuit

[madzub]

Imagine you have a saddle and nut made of magnetic steel, and you have a stack of magnets connecting nut to bridge, say under the fretboard. So you've got a closed magnetic circuit, in that one pole of the stack of magnets goes thru the nut to the string, then along the string to the saddle then back into the other pole of the stack of magnets.

So if you put a coil around, say, the saddle, or the strings, or the stack of magnets, then what sort of a signal would you get?

And has it been done?

I'll try to test this out but first let's hear what the theory would predict. Would it pick up all the vibrations along the string, including those behind the fret being pressed, or only the vibrations close to the coil?

Obviously this would be unconventional, but not necessarily impractical to implement using thin neodymium cylinders (a stack under each string), if the results were worthwhile.

[Antigua]

You get an output from a pickup coil with the magnetic field through the coil increases and decreases, causing a current to move back and forth in the coil, but the arrangement you're describing would cause little to no variation in the magnetic density though the coil. It's optimal for the coil to be placed as it typically is, where the guitar string comes and goes away from the coil, in a back and forth manner. I think those tiny neodymium magnets might make some new pickup ideas possible, but you wouldn't get anything useful from this type of arrangement.

[kmensik]

I remember seeing this principle in some electronics hobby magazine or book. You only need to electricaly connect the bridge as one pole and the nut as the other pole, ie run a wire from the conductive nut to some electronics cavity as a hot and ground the bridge as a cold terminal. Then place a magnet anywhere between the strings and the return wire. You will get a very low signal likely with a lot of noise from the big loop antena. Maybe connect it to a walkman instead of the tape head, you have a low Z pickup.

[bbsailor]

I remember seeing this principle in some electronics hobby magazine or book. You only need to electricaly connect the bridge as one pole and the nut as the other pole, ie run a wire from the conductive nut to some electronics cavity as a hot and ground the bridge as a cold terminal. Then place a magnet anywhere between the strings and the return wire. You will get a very low signal likely with a lot of noise from the big loop antena. Maybe connect it to a walkman instead of the tape head, you have a low Z pickup.

If you read this post title “String based current induced pickup shared idea” in the Pickup Theory sub forum you will see that the nut should be the common ground end and the hot string ends should be behind the bridge. I have used non metal bridge inserts to keep the strings isolated and used a way to insulate the string end from making conductive contact with a metal tail piece. By using a very low resistance ground return from the common nut ground end you now have a way to attach the string to a 8 ohm to 10K or higher transformer to boost the induced voltage from the vibrating string in a magnetic field to a useable level. You can use miniature pots across six transformers wired in series or have independent string outputs. Based on my experiments, a longer magnetic field produces a higher output level and tonal variations depending on the location of the magnets from the end of the neck to the bridge area.

Making a metal nut with a good low resistance connection to the truss rod, and having a good conductive connection to the body end of the neck that extends to the bridge makes a practical way to using current neck components in a new way to have individual string outputs. Since the strings have a low resistance, any wire connections need to have one tenth the string resistance to avoid unnecessary resistive losses on this low impedance input circuit. Think of each string operating like the ribbon in a ribbon microphone.

Give it a try.

Joseph J. Rogowski