Goal of project:

Generate mechanical vibrations on a string (such as a guitar, violin etc), the frequency of which can be controlled on my computer. The DSP part of this is easy, but my practical electronics background is weak, so that is what my questions are about.

Solution:

I decided that the easiest way would be to stimulate the string electromagnetically by building a 'reverse' guitar pickup. I reason that a guitar pickup creates an AC current when the string oscillates in the region of an inductor, so using reciprocity I should be able to create an oscillation on a string if I supply an AC current to a near by inductor.

In lieu of an inductor, I ripped a transformer out of an old boom box, slapped some neodymium magnets on the top (in axis with the coils), and attached the two power-supply side leads to the output of a stereo amplifier. Using this method (and some tones generated in matlab) I was able to excite low amplitude vibrations on the strings of a guitar and viola.

Two main questions:

1: So far I am using a transformer as an inductor, is there a way to optimize the magnetic output fluctuations by utilizing both coils? It occurred to me that maybe I should short the leads that are not attached to my amplifier. I would have played with this more, had the next question not come up:

2: How to I predict the load, seen my by amplifier, due to the transformer? I don't want to destroy the amp which, if I understand correctly, would happen if the transformer does not provide enough resistance. The only markings on the transformer are:

on top
LEE ON
LU43-21A14

on bottom
00-50

I had done a 'proof of concept' type of test using the coil and magnet from a loudspeaker, but the loudspeaker is rated at 8 ohms, so I was not too worried about destroying my amp. In my excitement I did not stop to consider the impedance of the transformer, so I need to slow down and understand the electronics a little better before I destroy something.

Any help is appreciated,
Erik

 

if the transformer has a magnetic shunt, magnetic short across the core shaped like an E, remove it, that will allow more of the field to escape the core. you will find that the "guitar string" will viberate more at a particular frequency, dedtermined by the length and tension of the string, and harmonics (multiples) of that frequency.

 

if the transformer has a magnetic shunt, magnetic short across the core shaped like an E, remove it, that will allow more of the field to escape the core. you will find that the "guitar string" will viberate more at a particular frequency, dedtermined by the length and tension of the string, and harmonics (multiples) of that frequency.

Thank you for your reply.

I have stripped off everything from the transformer that seems like it would come off without a hammer or other invasive tool.
I am not sure if what remains would count a shunt, but the remaining metal does wrap around two sides of the core, and looks like it shorts the core in a sense. This piece does not look like it wants to come off, perhaps I am not clever enough.

Regarding the physics of the strings, I have no problem there. I intend to drive the strings at the fundamental and several partials, and I am simultaneously monitoring the spectral density in of the generated tones to ensure that I am driving at the correct frequency. I have not figured out a way to phase lock the driving, because right now getting the hardware to work is my first priority.

 

use two coils, one to excite the string, and the other to pick up the vibrations. a pc running one of several audio analysis programs for the sound card should help display the results. search for soundblaster audio spectrum analyser.

 

use two coils, one to excite the string, and the other to pick up the vibrations. a pc running one of several audio analysis programs for the sound card should help display the results. search for soundblaster audio spectrum analyser.

Thank you for your response.

I do not have any problems regarding the production and analysis of the signals that I want, it is very straight forward to do both of those in matlab. What I am having problems with is on the hardware side, specifically that I don't want to fry my amplifier by loading it with a transformer that may not be enough impedance for the amplifier. The core of my problem is how to match an amplifier to the driving inductor (in this case a transformer), not how to produce and analyze the signal digitally.

 

What is the DC resistance of the transformer. It should probably be about 6 ohms if you amp is expecting an 8 ohm speaker.

 

What is the DC resistance of the transformer. It should probably be about 6 ohms if you amp is expecting an 8 ohm speaker.

Thank you, that is helpful. I will have to wait until I get home from the holiday break because my tools are all in a different state.

But, a follow up to your answer:

Is matching the DC resistance of a transformer with the nominal output of the amplifier sufficient? I suppose another way of asking this, is the magnitude of the frequency response of a typical transformer flat in the range that I am interested in, roughly 50 to ~10kHz? I intend to use fairly clean harmonic tones, so I can imagine the scenario in which the DC resistance could be quite different from the impedance seen using a 500 Hz tone.

Thanks again

 

Yes, that is true, but unavoidable I think. You can also measure the inductance it should account for the difference between the 6 ohms and 8 ohms or about .3 Mh. If your amp has current feedback it will account for it, if not you can probably fix it with base treble. The problem may be that it has really high inductance then it will be hard to drive.