I recently read a lot about guitar sustainers, for those who don't know what are they - it is basically an electromagnet with an amplifier. It takes an input from pickup, amplifies it and then generates the magnetic field that vibrates the string.

I have the question - if we need magnetic field that changes corresponding to specific frequency in order to vibrate the string - why would we put a permanent magnet underneath the core? Wouldn't it be pointless, or even destructive? What's the point of it hanging around there? How does it help?

I probably missing some basic laws here, but can't find out what it is...

The picture below from one of many patents on this kind of invention(http://www.google.co.in/patents/US5932827).

Other patent I found also mention permanent magnets, but none of them describe in clear language what is the point of it.

enter image description here

  • \$\begingroup\$ Where did that diagram come from? It might help the community if they could read the document that diagram is copied from. \$\endgroup\$
    – gbulmer
    Aug 26, 2014 at 0:56
  • \$\begingroup\$ That looks useful. Part of the stackexchange aim is to build a wiki of good, clear questions, which stand alone without the comments. So it would be helpful to update your question with that information. (and even delete your comment) \$\endgroup\$
    – gbulmer
    Aug 26, 2014 at 1:01
  • \$\begingroup\$ Here's another place where this diagram surfaces. [Courtesy of google image search.] \$\endgroup\$ Aug 26, 2014 at 2:50

1 Answer 1


You do not need permanent magnet here indeed(well, in theory).

Though using only coil can be problematic since magnetic field from it is very modest, you would need to place the coil very close to the string to vibrate it and it is not practical.

Permanent magnet helps simplify the coil, by changing the magnetic field generated by coil you would change the magnetic field of permanent magnet, therefore vibrating the string. With this setup you can make coil suitable for amplifier(8 ohms, ~1 Henry for example) to work with.


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