Dogue

Col, thanks a lot.

I just ordered an inductance meter as well as some different gauge magnet wire. Also currently working on several 1.75" long drivers experimentally...

Well, there's your problem. Four hundred turns of wire is going to give you a fairly big inductance. I've just checked & established that 30AWG is about 0.25mm...so if you halved the wire diameter size (0.125mm), you'd need approximately 200 turns to get the same DC resistance - but that amount of turns is still perhaps a little too much.

From recollection, with my sewing machine bobbin, I used about 140 turns of 0.15mm (approx 34AWG), which yielded a DC resistance of about 4.2 ohms (which is fine for my particular output chip) & an inductance of 1mH.

You really only have two options...

1. Stick with your 30AWG but you must make big coils (eg rectangluar & with the longest bit at least 60mm), that way you get to nail your amp's required DC resistance fine yet witout too many turns.

2. Buy some thinner wire!

Hank: Thanks again.

Bigger coils are not a problem at all -- in fact, if I can get one to work well, it would be preferred. My initial large octave drivers were 7" (178 mm) long. And obviously one driver would be logistically easier to manage than 12 individual small drivers.

I'm sure I overrated the importance of 8 ohms for my amp -- I'm pretty sure it could handle quite a bit lower DC resistance.

Now, I've been using about 60 turns of 30 AWG for the large 7" (178 mm) drivers to reach 8 ohms. So, if I wanted to aim for 150 turns, should I use a larger diameter wire? If I'm still aiming for 8 ohms with 150 turns, it seems 27 AWG (0.36 mm) works out. Although, again, I'm not sure how the inductance of this coil is calculated.

(edit:) Upon your suggestion, I looked up a bit re: relationship between DC resistance and impedance. If I'm aiming for an 8 ohm impedance, then should I aim for roughly 6 ohm DC resistance? In which case, even slightly thicker wire would be appropriate?

col:

You're right; I presumed there was something that I wasn't considering in prioritizing DC resistance. And since I think my amp will handle lower DC resistance, it should be possible to arrange the series/parallel groups more equally.

I did just a brief test yesterday, sending signal into one of those modified audio transformers. As I mentioned, this was the best result I've gotten yet with any of my drivers. Although, as I discovered, it's very low DC resistance, measured at 1.5 ohms on my multimeter (although the package says something like 0.7 +/- 20%, and given the following series measured resistance I'm confused to why a single transformer read 1.5 ohms -- I checked several, and I got the same reading for each). I then connected 5 in series, and the measured DC resistance was the expected 3.5 ohms. However, the output was less satisfactory than the single driver. I suppose this makes perfect sense somehow, which seems to be the current which you explain: I presume the 5 in series are received 1/5 as much current as the single driver?

IMHO, you're focusing on the least important aspect of making a driver (the DC resistance!).

Re the sewing machine bobbin...what wire diameter are you using?

The problem you're going to have is, unless you know the inductance & the total winding count of your own particular coils, then you really are going to be in the dark - & walking a (very) solitary & uncertain path. You can put the coils in any combination you wish, so long as the DC resistance matches whatever your final power amp wants to see *but* it may not sustain - what's absolutely key with sustainers, is not so much the DC resistance (that bit merely placates the needs of your power amp chip), but the delicate balance of number of driver windings (a bit like the three bears...this one has too many, this one not enough this one is just right!), strength of magnet (again three bears!), type of core etc.

If you read through this thread you'll keep coming up with posts that wire gauge X is the most suitable - however such recommendations apply only to coils of a similar design. For example, many on here have established through trial & error, that circa 0.20mm diameter copper wire works best with a single driver breaching all six strings (I've seEn it said often on here that 0.2mm diameter wire is the only size that works ....this IMHO is quite incorrect *unless* we are all making drivers of exactly the same style/dimesions). theredfore 0.2mm wire gauge is the one to go for *if* (& only if) you are making a driver of similar proportions as the one most talked about on here.

That said, what's been generally established on here, is that (for the main) a driver with about 150 windings & an inductance of about 1mH (maybe a little more) is about right. Therefore for my part, where I've been making single string drivers, to keep the winding count circa 150 turns & the inductance about 1mH, I have to use thinner wire than most (about 0.12mm-0.15mm)...your mileage will certainly vary.

Therefore in summary...if you're making your own coils towards a different end goal, then you're going to need to find the wire gauge that suits your own situation (ie that satisfies the aforementioned characteristics). You'll be on forever & a day with the trial & error route...so I strongly I suggest you invest in a cheap inductance meter from China (mine cost a mere £14 delivered to my door from the far east! http://www.tinyurl.com/ompaon )

Thanks again.

Wow, yes, somewhere along the way I got so focused on the impedance that I neglected the other characteristics...which would probably explain why I haven't had as much success as I'd like. I've previously made some single string drivers that have worked, but I guess it was probably just luck due to core size, etc. At some point I forgot about the 150 turns and instead started using a pickup-winding calculator to determine how many turns to make the impedance 8 ohms.

Some of the single string drivers (which were the most successful ones I've built) ended up being 120-160 turns, 30 AWG. The sewing bobbin I've tried I think I ended up with about 400 (!) turns. I did however make two other sewing bobbin drivers of around 150 turns, 3.5 ohms, which I haven't yet tested. The large drivers I used about 60 turns.

I've been using 30 AWG for everything.

My ideal driver would be about 7" long. Is there a good way of calculating wire size to satisfy the goals of turns, inductance, and impedance?

I don't have a problem with making 12 individual single unison drivers. The main thing I'm trying to avoid is have each one powered by its own amplifier. Is this reasonable?

It's reasonable....but what's going to be key here is the coil characteristics & how you connect them all together. For example an inductance figure of 1mH (circa 150 windings) & a DC resistance of 8 ohms seems to be the ballpark for a general purpose driver ...the inductance figure is derived (in the main) from the number of windings your driver coil has.

Therefore if you want to connect say 12 drivers in series, you'd want to shoot for 1/12th of those figures per coil (therefore each individual coil should be about 0.66 ohms & 83uH!). You'd need to experiment with differing coil gauge as you'd be beating a trailblazer's path there! My gut feeling is that a coil of 83uh, that the wire would need to be very thin to be able to get 150 turns - so thin, I reckon it'd not handle the current needed to get a higher frequency type string 'excited' sufficiently.

I'd also have to assume that trying to span/excite so much string real estate with one 'virtual coil' (12 mini coils in series) would need a fairly chunky driving amplifier.

if you don't need individual control per string (as per the EMPP), then I'd go with your 12 drivers connected in parallel or a combination of series/parallel (You'd still need to be careful with your coil characteristics, but they'd be more do-able vs pure 'series' connection)

Well, I tried a couple more designs. One was a thin steel core, 4" long, wired with 30 AWG to 8 Ohms, solidified with PVA glue, with a ceramic block magnet attached to the blade.

Another was like you suggested, wire wound around a plastic sewing bobbin, again potted with PVA glue, with a bolt in the middle of the bobbin and a neodymium magnet attached to the bolt.

Both worked okay, not great, although using the glue eliminated the direct sound I had been getting with my earlier attempts.

However, neither was nearly as good as the Radio Shack Miniature Audio Output Transformer, I cores removed and E cores aligned, with a neodymium magnet attached. This driver is by far the best of those that I've tried thus far.

So...thinking aloud, would it at all be reasonable to wire up 12 of these transformers in a combination of series and parallel like:

7 in series

3 in series

2 in series

and those three groups connected in parallel?

With 8 ohms per transformer, that combination seems to yield a total impedance of 8.1951 ohms. Of course, this doesn't consider inductance.

EDIT: I had incorrectly assumed that the transformers had an impedance of 8 ohms. In fact, after measuring, they have an impedance of 1.5 ohms each! So, how does that effect what I should do?

In the meantime, I've been working on a single EM driver to cover an octave of piano strings. My attempt is a simplified, generalized version of the "Electromagnetically Prepared Piano" (http://ccrma.stanford.edu/~sbacker/empp/).

Hmm...I had high hopes in 'porting' the principles used in the Electro Magnetically Prepared piano over to a guitar sustainer - but in my (albeit brief) experimentations...it wasn't very successful.

I reckon they have the success they have, by chucking a fair old chunk of 'power' at each string...because they don't have to worry about EMI (on account the piano sound is 'acoustic' & not needing to use electric pickups like an electric guitar).

IMHO, you'd be better off making a single driver more akin to the many variants on here but in a scaled down version for one string. (I suggest a sewing machine plastic thread bobbin, like these http://www.brother-usa.com/usaimages/Acces...Large/sa155.jpg, winding your copper around...about 150 turns of 0.15mm wire, dip it all in molten wax then let cool to solidify the windings , find an old drill bit that fits into the hole snug, cut the drill bit to length & pop it in the hole - now place a 3mm-5mm diameter rare earth magnet on the end of the cut drill bit - voila a single string driver!)

One thing though....you shouldn't hear any sound from your driver at all - if you can, it suggest the wire windings are moving &/or the magnets....everything needs to be totally solid/rigid.

Thanks for the reply.

In a way, porting the electromagnetically prepared piano to the guitar would be akin to Nicolas Collins "backwards electric guitar," in which he sends audio to the guitar strings. I suppose the key would be to have enough distance between the driver and the pickups.

In the original version of the electromagnetically prepared piano, each of the 12 drivers is powered by a 7-watt amplifier. In my experiments, with both the large octave driver and the small unison driver, I've used a 15-watt amp. Basically you want to give it as much power as you can without the driver overheating. Using the headphone jack output from my Macbook with the 15-watt amp, I generally had the Macbook volume all the way up and the 15-watt amp volume about 1/2 to 2/3 of the way up.

I thought that I shouldn't be hearing any sound from the drivers themselves. This was basically the reason I ended up making four octave-long drivers -- trying to wind each one tighter. But since the last and best one still makes some audible sound, I doubt my ability to make it any better under that design. With the single unison driver, however, perhaps because it's circular and thus easier for me to wrap the wire tightly, I don't really hear any sound from the driver itself.

Also, I haven't yet potted them in wax -- I've tried some epoxies and plasti-dip, but I don't think they've done enough to solidify the windings.

I don't have a problem with making 12 individual single unison drivers. The main thing I'm trying to avoid is have each one powered by its own amplifier. Is this reasonable?

So my 12 parallel ebows are in construction but so far all seems to be going OK.

In the meantime, I've been working on a single EM driver to cover an octave of piano strings. My attempt is a simplified, generalized version of the "Electromagnetically Prepared Piano" (http://ccrma.stanford.edu/~sbacker/empp/).

Whereas in the original, 12 drivers are used, each with its own audio input and amplifier, I've attempted to use one larger driver, with only one amplifier.

It works alright, although it is pretty quiet, and I think too much sound comes directly from the driver. I've actually made four versions now, with varying degrees of success, each using four ceramic bar magnets, each 1.75" x 0.5" by 0.5", glued end to end, and 30 AWG magnet wire wrapped to 8 ohms.

I've also made one small driver, the width of one unison, for comparison purposes. I made this driver using a 0.5" diameter x 0.25" high circular neodymium magnet, again with 30 AWG wire wrapped to 8 ohms.

The small driver seems to be clearer and louder, and with less noticeable audio coming directly from the driver. I suppose this is due to several things:

-The circular magnet is easier to wrap wire tightly against.

-The magnetic field is more focused

-and others

Supposing I use one amplifier, what would be suggested ways of making the output as loud and clear as possible?

Just thinking aloud, would any of these be advisable?

-12 small EM drivers with circular neodymium core, each wired to 8 ohms; there are three groups of drivers, with each driver within the group wired in series; then the three groups are wired in parallel

-12 small EM drivers with circular neodymium core, each wired to 2/3 ohms, connected in series

-other solution?

Thanks!

I've read a fair amount of this thread and also searched. Forgive me if a similar situation has already been addressed.

Basically, my goal is to run a plurality of DIY Ebows on somewhat of an acoustic zither. I'm trying to do this in the easiest possible way, and thus far have had acceptable results running a

telephone pickup coil with small neodymium magnet attached -> LM386-based amplifier -> small audio output transformer with modified cores and neodymium magnet attached

I want to run many of these (at least 12) close to each other. This presents in my mind two possible problems:

1) EMI from adjacent sustainers may adversely affect performance

2) Magnetic attraction between adjacent sustainers will be logistically annoying

From what I've heard, the original Ebow's coils were put in ferrite cups of some sort to minimize unwanted EMI. Would it be possible/advantageous to use EMI ferrite core filters (often intended for ridding EMI in wiring) over each pickup and over each driver? This would obviously bulk up the apparatus.

Suggestions? Thanks a lot.

I think you have rightly seen some of the problems. The thing to consider in shielding, especially over the top of the drivers is that the EM energy coming out of the driver is what makes the thing go! As a result, if it is contained a lot or completely, this will drop efficiency (requiring more power and putting you largely back where you were) or not work at all (as the EMF (force) will not be working on the strings either) if fully contained.

Crosstalk may well be an inevitable result of the multi-driver approach, however, you still may get a working result from such experiements.

In the ebow, there are steel (I believe) sheilds from the back and sides allowing the driver and pickup to be relatively close. However, if you put another beside it, the likely hood is the pickup from one will pick up the signal from the driver of the the other. This could be cured bu further distancing the pickups and drivers perhaps much as is done with a sustainer...the more drivers, the further the distance.

On a zither I imagine though, you have the whole string length to play with and perhaps such distances (the pickups on one end and the drivers on the other) may well be possible as no fretting is required.

If you get it working, you might consider some kind of damping mechanisim so that it could be played by releasing strings (taking off a felt damper perhaps as in an auto harp) and letting the sustainer "play" the string for you rather or in addition to plucking.

Just a few thoughts...while I'm about

pete

Pete, thanks a lot for the reply.

Yes, I do have basically the entire length of the string to work with, so I can space out the drivers and pickups. In initial experiments there haven't been any noticeable detrimental phase or other effects on having significant distance between the drivers and pickups.

Do I need to worry about pickups effecting adjacent pickups and drivers effecting adjacent drivers? I suppose this is the EMI issue.

Not necessarily this exact one, but this is the general type of ferrite core I had been considering trying out for EMI shielding:

http://www.radioshack.com/product/index.js...oductId=2103222

If I had a pole piece extending such that the magnetic field extended close enough to the string as possible, perhaps it would work. However, having not used one of these ferrite cores before, perhaps that wouldn't work, since perhaps it diminishes the magnetism irrespective of whether it is unwanted interference or not.

In fact, I'm now trying it on a piano, and so I can control the sustainers via the sustain pedal (appropriately enough) or individual keys.

I've read a fair amount of this thread and also searched. Forgive me if a similar situation has already been addressed.

Basically, my goal is to run a plurality of DIY Ebows on somewhat of an acoustic zither. I'm trying to do this in the easiest possible way, and thus far have had acceptable results running a

telephone pickup coil with small neodymium magnet attached -> LM386-based amplifier -> small audio output transformer with modified cores and neodymium magnet attached

I want to run many of these (at least 12) close to each other. This presents in my mind two possible problems:

1) EMI from adjacent sustainers may adversely affect performance

2) Magnetic attraction between adjacent sustainers will be logistically annoying

From what I've heard, the original Ebow's coils were put in ferrite cups of some sort to minimize unwanted EMI. Would it be possible/advantageous to use EMI ferrite core filters (often intended for ridding EMI in wiring) over each pickup and over each driver? This would obviously bulk up the apparatus.

Suggestions? Thanks a lot.