[sdiy] Magnetic String Actuation

Mr&MrsAccount hbissell at wowway.com
Wed Sep 7 20:57:55 CEST 2022

Sine wave excitation would be good if you want to cause a particular resonance... Square wave could excite odd harmonics and might not be what you want (but it would probably be interesting... 
A Class-D amplifier will also be spewing some RF harmonics. Most should be damped by the output filters and the rest contained in the speaker (where you try not to spew external electromagnetic fields. 
In your case you want to spew a magnetic field. Your neighbors will hate you if they listen to AM radio... 

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

From: Spiros <synth-diy at synth-diy.org>
To: grant <grant at musictechnologiesgroup.com>
Cc: synth-diy <synth-diy at synth-diy.org>
Date: Wednesday, 7 September 2022 2:47 PM EDT
Subject: Re: [sdiy] Magnetic String Actuation

 String exciter was a good idea! I found this blog post https://frankpiesik.wordpress.com/2020/04/02/gregs-harp/ Unfortunately, the amplifier is not shown, but I think I can make something similar to that wound coil and give it a try. I can get a TPA3118 amplifier module locally, let's hope it will be enough to get at least some faint movement.
I'm not entirely sure how I should drive them, starting with a square wave on the string fundamental frequency makes sense?  
  On Wed, 7 Sept 2022 at 21:29, grant musictechnologiesgroup.com <grant at musictechnologiesgroup.com> wrote:   
There are several ebow "clones" out there. I know I've seen a few on Indiegogo and Kickstarter, but this is the only one I could find with a quick search: 
So maybe string exciter is the term you need. Also worth a listen is the famous Gizmo (the Gizmotron). It's being reissued. 
------ Original Message ------ 
From: "Spiros Makris via Synth-diy" <synth-diy at synth-diy.org> 
To: "synth-diy mailing list" <synth-diy at synth-diy.org> 
Sent: 9/7/2022 10:54:50 AM 
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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