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last updated March 19th, 2010
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Magnetic Drive Coil

August 2, 2010

For a long time I have wanted an E-Bow, but I'm just too cheap to go and buy one. Still, I'd like to experiment with the idea.
So, I did a search on "DIY E-Bow". The first practical information I found was this diagram from ---------------->
At least one experimenter who tried it says the stock circuit does not work, but I don't remember what his "fixes" were. In fact, I have not yet tried to build it. However, a fellow member of the SAWStudio User Group is experimenting with an instrument that would work on a similar principle to the E-Bow, and is trying to come up with a drive coil to make the string move. I thought he should try the idea suggested in this schematic, but he thought he did not have the skills to "make" the coil, although he has tried some interesting experiments involving a nail and magnet wire.

Naturally I had the irresistible impulse to prove that anyone could do this, so I proceeded to apply my ten opposable thumbs to the task. A quick dig through my junk pile yielded this NOS audio transformer made by Philmore. I'm not sure the windings are the right kind, but I was more interested in testing the idea on something that I was less likely to need later. This particular transformer's primary and secondary impedances are 20k and 1k, respectively. It is not an output transformer, as recommended in the schematic, but the overall construction is much the same.

The first step in the modification is to remove the shell. I was able to bend the ears apart with my fingers, and then it was easy to pull the coil and core from the shell.

Next you can see the frame is separated from the "guts" of the transformer.

Removing the first "E" lamination is a bit tricky. There is a layer of wax between laminations, and the laminations are packed quite tightly into the plastic coil form. I had to pry up the first layer a bit befor I could pry it loose (for all of this I used a very small flat-bladed screwdriver). For the most part, I was able to pry loose the rest of the layers by inserting the tip of the screwdriver between the end of the coil form and the lamination.

Here we have one each of the "E" and "I" laminations. For each layer, the side of an "I" lamination touches the tips of the "E" lamination. For each successive layer, the laminations switch places to that the layers fully interleave.

Once all the laminations have been removed...

you can see the coil assembly all by its lonesome. The primary and secondary windings are wound on a plastic coil form, as you can see here.

Now we start putting just the "E" laminations back into/onto the coil form, only this time they are all facing the same way, pointing down into the core (in this application the wires come out of the top instead of the bottom).

Looking at the bottom, you can see the tips of the "E" laminations. This is the end that will face the string, which is why we wanted the lamination tips pointing away from the wires.

Above you can see top and bottom views of the finished drive coil.

Last, and most definitely least, here are the leftovers. The "I" laminations are not needed at all. I could not quite manage to squeeze the last "E" lamination into the coil form.

That's about it for how to make the drive coil for a home-made "E-Bow". Maybe one day I'll get around to making the rest of the thing, but for now I hope this helps anyone else who is trying this little bit of DIY.

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