Help with Rolling my own electromagnet!

[K3mHtH]

Hey everyone,

I'd need to vibrate a guitar string with an electro magnet (it's a bit more complicated than that, but I won't confuse things). My plan was to use an off-the-shelf audio amplifier chip and feed it a sine or square wave from a function generator, the frequency matching the string's tuned frequency. I'd like an electro magnet that has an 8 ohm impedance so it plays nice with the audio amplifier and I want to wind this electromagnet myself.

Lets say I really wanted to drive this thing with a 3-5W amplifier, what gauge magnet wire would I be looking at using?  and is there a way to calculate the # of turns required before I start winding to achieve 8 ohms?

I'd also like to know what I use for the metal cylinder in the middle, is there an ideal metal for this? Heck, is there an ideal shape for this magnet too?

[HAL-42b]

Here is one calculator: http://www.daycounter.com/Calculators/Coil-Physical-Properties-Calculator.phtml

The best material for the slug is steel. There are some types of steel which are better than others but it doesn't matter in your application.

The most efficient shape is a cylinder.

You need to make sure it reciprocates without rubbing the coil from the inside. The best method of making this without friction is a flexure parallelogram. A flexure really has zero friction, quite ingenious mechanism.

[obiwanjacobi]

What are you trying to accomplish? You know a normal guitar pickup can also be used in reverse, right? That won't be 8 ohm more like a couple of k to tens of k.

Also, this may be of interest: guitar sustainer.

[2c]

[K3mHtH]

What are you trying to accomplish? You know a normal guitar pickup can also be used in reverse, right? That won't be 8 ohm more like a couple of k to tens of k.

Also, this may be of interest: guitar sustainer.

[2c]

Thanks, I'm trying to build a similar device. I tried to use a pickup in reverse as you described, but it didn't work - maybe because the pole pieces are magnets not steel? Either way, I'd still like to make my own electro magnet on this (single or multi pole) so I can get the impedance right.

Just to clarify from reading the other replies, I'm trying to vibrate the string without touching it or physically vibrating anything else.

[uncle_bob]

Hi

Be careful of what you use for core material. Check it with a magnet first. There are indeed some alloys that have really rotten magnet properties.

If you wind a solenoid (bunch of turns around a straight core) that s a pretty long magnetic path. You will have field over a fairly large area. That may or may not matter in your application.

Your coil has both resistance and inductance. You are running at audio frequencies so the inductance is not a massive issue. I would aim for  a resistance on the coil *below* 8 ohms to allow for some inductance. You will have eddy current losses in a simple core, so the net result is going to be a bit exciting to try to calculate exactly.

As you sit there counting an enormous number of turns. Consider that 4 ohm output amps would require half the number of turns ....

Bob

[pmbrunelle]

My plan was to use an off-the-shelf audio amplifier chip and feed it a sine or square wave from a function generator, the frequency matching the string's tuned frequency.

If you force a string to vibrate with an external sine wave excitation, the string will vibrate at the frequency of the function generator; it will not vibrate at its own natural frequency. And if the function generator frequency is poorly matched to the natural frequency, the string won't vibrate much. The response away from resonance depends on the Q of the string.

If you are building an electronically-played guitar, then you need to pluck the string with a current impulse, like a musician does with a pick. Then the string will vibrate at its natural frequency.

If you want to build a sustainer, then I would just amplify the pickup signal and send it to the electromagnet. Essentially, this would be some controlled amount of positive feedback.

If you wind a solenoid (bunch of turns around a straight core) that s a pretty long magnetic path. You will have field over a fairly large area. That may or may not matter in your application.

Your coil has both resistance and inductance. You are running at audio frequencies so the inductance is not a massive issue. I would aim for  a resistance on the coil *below* 8 ohms to allow for some inductance. You will have eddy current losses in a simple core, so the net result is going to be a bit exciting to try to calculate exactly.

As you sit there counting an enormous number of turns. Consider that 4 ohm output amps would require half the number of turns ....

I'm thinking gapped toroid with the string in the gap. This may also increase isolation between the strings. I wonder if you could get some DC current through the strings if that would help the sensitivity.

As for the impedance/number of turns issue, a transformer may help things in that department.

[uncle_bob]

I'm thinking gapped toroid with the string in the gap. This may also increase isolation between the strings. I wonder if you could get some DC current through the strings if that would help the sensitivity.

As for the impedance/number of turns issue, a transformer may help things in that department.

Hi

Toroids are a real pain to put a bunch of turns on. Cutting a gap in one that is useful at audio is equally a hassle. Both things can be done and yes it will have some advantages. A pot core construction is another way to go that is a lot easier to wind. (Chuck the a straight bobbin up in the drill and away things go ... turns count ... ahhh ... fill it with wire)

If you go with a transformer to couple to a very low impedance driver,  you are going to need one that is designed for a very low input and output impedance. Things like winding resistance and stray inductance are your enemies when using one designed for a higher set of impedances. You also probably will have to derate the power rating of the transformer due to core saturation (ampere turns issues).

Bob