It's taken almost three years and has seen a number of prototype revisions, but David Levi's Magnetic Cello is finally getting ready for its first production run. Looking like a minimalist version of the acoustic instrument on which it is based, the instrument features a single resistive ribbon on the neck to alter the pitch of a note and a huge, visible coil at the bridge end. Instead of a horsehair bow, the player uses a magnetic rod to induce a voltage in the coil, which is then transformed into a frequency and fed through to the unit's voltage-controlled amplifier. The bow has a small switch at the thumb position which allows the player to select virtual strings.

The Magnetovore Magnetic Cello is loosely based on the Theremin Cello developed by Leon Theremin himself and first appearing on stage in 1932. The first version of Levi's creation began to take shape in his junior year of high school and was a plywood outline of an acoustic cello with a guitar neck sporting an individual ribbon for each virtual string. A 555 timer and a coil from a crossover system filled with smaller coils ticked the electronic component boxes.


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The second version was the first to draw away from the traditional cello body shape, and featured a dedicated VCO circuit, a neck made from dowel sliced in half, a guitar pickup for a coil and just one ribbon. The player was required to twist the neck to select different strings, but this proved somewhat cumbersome so was dropped for the next prototype, which had a body made from an old cabinet and a second ribbon running down the back of the neck to select string tones.

This method also proved awkward, so the four-ribbon setup from the first instrument made a reappearance to get the Magnetic Cello at least playable, and just in time for the project to go viral. This version also proved to be the make or break prototype.

Had the project not gained quite as much attention as it did, Levi told us that he likely would have moved onto other things. Instead, he decided to make the Magnetic Cello into a truly playable instrument.

The string selection problem was finally solved by mounting a system of switches in the magnetic bow, which allowed the player to select a string tone with the roll of a thumb. Improved playability wasn't quite enough to satisfy the electrical engineering student though, and Levi took his invention to some friendly cellists at Cal Poly to ask for opinions and suggestions.

"They gave me suggestions that made their way into V.6, the latest version," Levi told us. "In V.6 I focused on expressiveness of the instrument and ease of manufacture (for example, not using one-off parts from the local thrift store). After talking to an Electrical Engineering professor on campus, I was able to come up with the flat and wide copper coil that is quite visible. I was also more careful with the woodworking; this version actually has the neck become slightly thicker going down, giving cellist the ability to feel how far down they are."

"The instrument is technically very simple. The bow switches select a tuning for the instrument, and pressing down on the resistive ribbon in different places along its length changes its output voltage. This voltage is fed into the VCO, which turns that voltage into a frequency. That frequency is fed through a voltage controlled amplifier. The voltage controlled amplifier is controlled by the voltage induced in the coil when the magnetic bow is moved."

Levi has now teamed up with Andrea Kizyma and Fred Wilby to head for a limited first production run by August 2012. Around 20 Magnetic Cellos are going to be made, and the team already has a number of musicians lined up to grab the first few out of the shop.

"I have an idea for a price, but I don't want to commit until I've finished building everything," said Levi. "Right now, I'm saying 'half the price of a standard acoustic cello' - which is somewhere between $600 and $900."

Custom modifications can be designed into the instrument in consultation with the build team, and more tonal expression can be achieved by routing the output signal through effects units or sound processors.

Source: David Levi

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