[sdiy] Magnetic String Actuation

Tue Nov 5 18:53:34 CET 2024

You could also check out the Korg acoustic synth thingy:

https://www.youtube.com/watch?v=KMRmpc1cC7c&ab_channel=Reverb

https://www.gearnews.com/superbooth-23-korg-berlin-to-show-new-prototype-acoustic-synthesis_phase-5/

On Tue, Nov 5, 2024 at 2:07 AM Spiros Makris via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Well, it has been 2 years(!) since my last update. Time flies fast!
> I have not abandoned the project, but I do move really slowly due to my
> lack of electromagnetics knowledge and woodwork skills. I made a spring
> reverb driver following the circuits on eliot sound products (which worked
> like a charm), and after observing the construction of the tank actuators I
> couldn't help but fall back into the string actuation rabbit hole; I pulled
> out the Chinese electromagnets, and went over the MRP papers yet again;
> it's amazing how many things you miss when you're clueless on a topic.
>
> So I knew the chinese magnets I have do *something* but I was at a loss
> when it came to driving power into it, picking up the signal, etc.
> Things I've learned so far:
> 1. The MRP uses a 5Ohm electromagnet wound with 28AWG and a total radius
> of 20mm or so (actual coil radius seems like 10mm; they don't have any
> mechanical drawings on the supplier's website). Its cost is 67$ per piece,
> which is too much for my project, so on to other solutions, I guess.
> 2. The low impedance is important to make the task of driving power into
> it easier. The magnetic field is only dependent on current and turns; thus
> a thinner wire will dissipate more power and require greater voltage for
> the same current and turns. This is quite obvious I guess, but I hadn't
> understood the implications  until now.
> 3. The cheap chinese magnets I can easily source don't come close to 8
> ohms. The small ones I have are 70ohm, rated at "1W". They come with the
> suggestion to use them at 1W but at 50% duty cycle, so I guess their real
> rating is even smaller. Other options could handle more power but are
> substantially larger; I don't seem to need that much for now.
> 4. My DIY coil does work if you put some effort into finding the sweet
> spot, but I finally figured out why. Using a simple rod (screw) as a core
> makes the magnetic field lines spread, and combined with the miniscule
> cross section posed by the string, the resulting force is minimal. The kind
> of "E core" those electromagnets use (and MRP also uses) make the field
> lines start and end on the same side of the magnet, making the field much
> more concentrated, producing better results. This was pointed out by a
> fellow synth-diy member, but I was too clueless to understand at the time.
> 5. The spring reverb actuator uses a horseshoe core with a coil wrapped on
> one of the legs. A tiny magnet sits between the spring and the mounting
> point, making the produced force strong enough to induce movement. Again,
> this was also pointed out by another member, but it didn't click until I
> saw it in person. A similar design that sits around a string might be
> better suited than the cylindrical electromagnets, and possibly have less
> EMI emissions since less field lines exit the assembly.
> 6. Using a +15V supply you can drive just enough current on the 70 Ohm
> coil, assuming some headroom loss on the driver stage. Based on experiments
> run thus far, this should be plenty to induce movement reliably, so now I
> need to figure out a good way of driving the coil.
> 7. The circuits by eliot sound products aimed at the spring reverb can
> actually drive it (I tried), but are optimised for the much different
> current-impedance requirements of the reverb tank. Furthermore, they don't
> offer the possibility for a trimmable DC offset out of the box, which is
> essential for the application (the field must be biased). MRP presents
> a power amplifier as well as a smart approach for a bipolar to unipolar
> voltage converter, but the parts cost is not negligible.
> 8. My DIY coil is a horrible actuator but actually works fine as a single
> string pickup. So does one of the actuator electromagnets I got from china.
> Getting rid of the EMI is another story, but MRP mentions a cancellation
> approach which might actually work (measure the current going through the
> magnet and subtract it from the pickup. it cannot account for core
> saturation and hysteresis, but it's better than nothing.)
> 9. All examples I've seen so far use a single actuator and bias the field
> either with a permanent magnet or with some DC bias current through the
> actuator. Since you cannot produce a pushing force, it is critical to have
> some biasing so that the produced force is actually the same shape as the
> input AC signal. I think it is possible to have two magnets placed on
> opposite sides of the spring, operated on the positive and negative half
> cycles respectively. This would get rid of the DC biasing requirement,
> effectively reducing the power dissipation and doubling the potential power
> output of the assembly, while keeping the actuator power rating (and thus,
> its size) the same. Based on my understanding so far, the absolute polarity
> of the field does not matter, as long as only one of the actuators is
> magnetised at any given time the force will change directions. This might
> simplify the driver design and get rid of some signal conditioning stages.
> The horseshoe core approach used with a coil wrapped around each of the
> legs seems ideal for this.
>
> I was weighing my options moving forward; I find the solution of
> 1.50$/piece from China very appealing, but it's hardly optimal. Wrapping 20
> meters of wire around a core is very easy (that will bring me to 8ohms with
> 32AWG), but getting an off the shelf core of just the right shape is very
> hard, and probably more than 1.5$. I found a ferromagnetic 3D printer
> filament, which might do the trick and allow me to experiment with other
> configurations that might be better suited (i.e. horseshoe core). That's
> still a bit involved so it will have to wait a few months.
>
> I figured I will insist on the off the shelf parts, to get the ball
> rolling and arrive at a functional prototype as soon as possible. I am
> playing around with some simple class A BJT amplifier that will use the
> pickup as collector resistance. This will allow me to have a bias current
> through the pickup, and is very easy to breadboard for evaluation. The
> horrible power efficiency is not that important in my case, since I'm
> looking at 4-5 magnets (as opposed to dozens of them used in other more
> substantial projects). I will replicate my DIY coil design to make single
> string pickups, since it seems to do the trick just fine, and deal with
> that part later (if ever). The woodworking part seems daunting, especially
> the tuning pegs. I've looked at the various options and I think wooden
> tuning pegs used in instruments like the oud would work in my case and are
> simple to install on a wooden plank.
>
>
> On Wed, 14 Sept 2022 at 00:52, Spiros Makris <spirosmakris92 at gmail.com>
> wrote:
>
>> I got a random 24V/5kg magnet to try out. It's this one, for reference:
>> [image: image.png]
>> It actually works! I tried a few different things it seems like a good
>> starting point for what I want to achieve.
>> I first tried driving the magnet with a signal from my VCO, without any
>> feedback. I didn't even have to tune as precisely as we discussed earlier -
>> just being in the vicinity and getting the position right resulted in very
>> obvious sympathetic action. Sweeping the frequency up produced various
>> harmonics by the string. I didn't get to try more complex sounds yet, but
>> this is very promising.
>> I added feedback from the pickups to the magnet, through my eurorack.
>> Sure enough, there was sustain as long as I got the position right. This is
>> tricky; moving up and down a string with your left hand means the length is
>> changing and thus the position you need to be with your right hand to stay
>> on that specific harmonic. Since the feedback is coming from all the
>> strings, it's not possible to treat it entirely as an ebow, since moving to
>> a new string while the old one is still vibrating is messing up your
>> feedback and gets in the way of the new vibration building up. There is
>> some sympathetic action here, though, meaning that you can take advantage
>> of a string that is still sustaining to kickstart the next one. Pretty neat.
>> It was mentioned that it's not easy to start the vibration from the idle
>> state, and I found this to be true. I thought, most positive feedback
>> circuits are kickstarted by their inherent noise, so maybe I should just
>> add some noise to the mix and see what happens. For some reason, my amp
>> didn't like it and acted a bit crazy at high levels; eventually I figured
>> out high pass noise is ok and blasted it through. The effect is like a very
>> soft bow, and I think it's less dependent on the position you choose.
>> Mixing some feedback into this seems to help the oscillation start issue,
>> but maybe it's just my impression. Unfortunately because the pickups also
>> listen to the magnet's output (it's pretty loud, considering the guitar's
>> gain) I found limited use for it for now.
>> I tried impulses as well, which admittedly aren't coming through great in
>> an AC coupled amp, but still I do get a thump on the string and something
>> that resembles a very light, rounded pluck. Unfortunatelly the pickup can
>> catch this too, so the sound is not ideal. Faster looping envelopes also a
>> sort of sustain , so maybe that's another path to that effect. Turns out
>> the electromagnet doesn't like this, it got very warm after a few minutes
>> of thumps coming its way.
>> All in all I'm very impressed by the results, considering I just bought
>> random Chinese parts and threw them together in an afternoon. I'd like to
>> try a few different magnets to get a better high frequency response, but in
>> the context of a sympathetic string bank this might actually work just
>> fine. I'll try sending a melody through and see how it reacts to it.
>> Unfortunately I can't actuate all 7 strings; I'll need to make some wooden
>> prototype for this to work (and get more magnets and amplifiers). I'm
>> thinking of picking up the sound (not the feedback) with piezo pickups; do
>> I need an actual acoustic chamber for this to work or is a simple wooden
>> plank fine, to get things going?
>> I'll upload some samples later. This has been in my mind for 10 years
>> give or take and I can finally see a path to making it work thanks to
>> everyone that chimed in!
>>
>>
>>
>> On Tue, 13 Sept 2022 at 12:25, ulfur hansson via Synth-diy <
>> synth-diy at synth-diy.org> wrote:
>>
>>> you can also run current (signal) through the string itself - and place
>>> magnets close to thw string - be careful not to zap yourself though by
>>> accidentally touching the strings, also make sure the string gauge is heavy
>>> enough to handle the current :)
>>>
>>> sent from outer space
>>>
>>> > On 13 Sep 2022, at 09:06, René Schmitz <synth at schmitzbits.de> wrote:
>>> >
>>> > On 13.09.2022 08:26, Spiros Makris via Synth-diy wrote:
>>> >> Alright! Then it sounds like I'm on the right track since I got the
>>> cheapest electromagnets (2.5kg) I could find on aliexpress! I'm kinda
>>> itching to give this a go so I'll pay a visit to my local robotics shop and
>>> get another cheap one and give it a go sooner.
>>> >>
>>> >> Harry that's useful, I'll try the middle of the string length for my
>>> next experiment. While you are right, I do think my tuning was good enough
>>> to warrant some sympathetic action. It's true that the sustainer
>>> configuration will probably produce vibration more easily, since it cuts
>>> out any problems with tuning.
>>> >
>>> > I'm thinking, you should try to drive it at half the frequency of the
>>> natural frequency of the string, as with a simple plain electromagnet, and
>>> an non-magnetized string, the force is attractive regardless of the
>>> polarity of the current.
>>> >
>>> > So you get two peaks of force per cycle when you drive it with a sine
>>> wave.
>>> >
>>> > This is a <electroboom> full bridge rectifier </electroboom> action,
>>> which doubles the frequency.
>>> >
>>> >
>>> >> My goal is to create a sympathetic string bank and excite it with
>>> sound from synthesizers or other electric instruments (ie guitars, lutes
>>> etc). This effect is most common on instruments like the sitar, but all
>>> stringed instruments have some sympathetic effect going between the strings
>>> while you play them
>>> >>  I expect that by adding the right amount of positive feedback I can
>>> get a sort of sustain control, and self-oscillation when cranked up. My
>>> assumption here is that the actuator, string, amplifier and pickup
>>> essentially form an oscillator, with the string acting as a high Q
>>> resonator, akin to an LC tank.
>>> >
>>> >
>>> > An Ebow has a magnetic bias in the form of a pre-magnetized core (both
>>> in the pickup and the transducer),
>>> >
>>> > so that they can use the same frequency that they pick up. You might
>>> want to add a DC current to your magnet to simulate this, and be able to
>>> use feedback. Or stick a permanent magnet on your core.
>>> >
>>> >
>>> > Best,
>>> >
>>> >  René
>>> >
>>> >
>>> > --
>>> > synth at schmitzbits.de
>>> > http://schmitzbits.de
>>> >
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