[sdiy] Magnetic String Actuation

Mr&MrsAccount hbissell at wowway.com
Wed Sep 7 20:20:28 CEST 2022

I'd think a speaker is highly inductive. The coil is wound close to a magnetic core, it moves through a small gap.  I don't know how much power you need, but an audio amplifier should certainly 
handle that.   Try winding a coil around some iron core and try it. It might be good to use something like the core material from an old transformer (laminated metal) or maybe a bunch of nails 
bundled together.   You could aim for an impedance in the 4-8ohm range.   
There is a commercial product called "sustainiac" which is in the form factor of a guitar pickup. For starters I'd play with just a single string and then refine it from there. 

-----Original Message-----

From: Spiros <synth-diy at synth-diy.org>
To: synth-diy <synth-diy at synth-diy.org>
Date: Wednesday, 7 September 2022 2:02 PM EDT
Subject: [sdiy] Magnetic String Actuation

 Hello, list, 
I am interested in experimenting with magnetic string actuation. That is, use an electromagnet to induce vibration to metallic(?) strings; the opposite of what a guitar pickup does typically. I have seen this concept realised in a couple of different applications: 
1. Magnetic Resonator Piano is an electronically augmented piano that can do cool stuff such as real crescendos, electromagnetically dump strings, produce overtones etc. Every sound produced comes from the strings themselves. 
2. Ebow is an electromagnetic actuator for the electric guitar. Place it close to the strings and they will start vibrating, similar to the way they do when using a bow. 
3. Moog Guitar contains some sort of actuator and suitable circuitry to make it infinitely sustain notes, apply active damping and more. 
I bet there are more if we start digging, but these are some examples I can think of off the top of my head. Unfortunately, I don't have enough to go on here and decide on what kind of hardware I need to produce this effect. The magnetic resonator piano researchers have published a couple of papers that go over the fundamental equations involved. Still, unless you can really wrap your head around them (I can't) it's not enough to get you going. 
I obviously need some electromagnet, either off the shelf or custom wound. Then I need an amplifier that can drive enough power into it, in the correct frequency range.  
Aliexpress is full of various electromagnets. How could I decide on a couple that might be promising and give them a try?  
What about output amplifiers? I know the basics of driving speaker coils, but my guess is a strongly inductive load like an electromagnet is going to be a different story. 
I've been thinking about this for the past 10 years and still didn't manage to figure it out. Any kind of info or advice you could share is invaluable! 
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