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Yosefu 20th May 2013 02:22 PM

Hex sustainer thingy
 
Hi you all!

I'm a poor guitarist who once got interested in the field of microtonality so I could starve as badly as Harry Partch did. With this purpose I built myself a fretless guitar with which I'm partially happy, but I miss some polyphonic sustain. Something like a fretless Moog guitar would make me happy, but the price is really out of my range.
So being a total noob in electronics I'm starting to build a hex pickup and sustainer, with pacience :)

However I'm trying first with only one string to see if it works. I built a Little Gem amp and it seems to work. I feed it with the signal from the normal pickup. Then I took the coil out of a 8ohm speaker and feeded it with the signal coming out of the Little Gem. It seems to be working since if I hold it close to the pickup it makes this high pitched sound of feedback gone crazy. But it doesn't make the string vibrate.

I've tried putting a nail with a magnet through the hole of the coil and then have it close to the string, but no magic seems to be happening. I feel like I might be missing something important because of my noobility in electronics, but I believe it has more to do with the driver coil than with the amp.

Anyone of you has tried something similar in the past?

Many thanks in advance :P

tubelab.com 20th May 2013 03:56 PM

I have been experimenting along the same lines. The speaker coil is where your problem lies. ou need to concentrate the magnetic field at a point right under the string.

I am not familiar with the little gem, but I am assuming that it is some kind of low powered solid state amp. If so, try this:

The impedance of a guitar pickup is quite high. 10,000 to 100,00 ohms. Your small amp was made to drive an 8 ohm speaker. I used the output transformer from a small tube amp, hooked up in reverse to boost the 8 ohm output of a chip amp up to a higher impedance, which I fed into a second ordnary single coil guitar pickup. This works quite well, but it lights up all 6 strings at once.

I got a pair of guitar pickup winding kits from Stewart Macdonald with the idea of winding 6 seperate coils on each, but haven't had time to work on it yet.

I may try to use 6 piezo pickups for the "normal" pickup, but they don't work very well in reverse.

chrispenycate 20th May 2013 08:42 PM

There is/was a toy designed to do just that called the Ebow; I have no idea about price or anything. But you're not going to get enough magnetic field out of a voice coil to vibrate strings, though apparently it does produce enough to be picked up on the pickups. Which means that any field powerful enough to move strings is just going to swamp magnetic pickups totally. You could try for piezo or electrostatic (no, your fingers are bound to induce electrostatic hum or go the opposite direction and use acoustic energy to vibrate the strings (build an actual small speaker into the body of the guitar; though how to make it vibrate just one string without damping it right down I can't offhand work out.

Or if you built something where the string slotted into a gap in a driven electromanet, so the field was contained and focused, like a speaker magnet, but varied with coil current? Would too much of it travel through the strings themselves? Sort of a cross between a tape head (in record mode) and a moving iron speaker, but tight. Or a loop you have to thread the string through, so the field is contained? Trouble is, as I imagine them allthese possibilities make it much less easy to play the instrument.

I did the six separate coils pickup back in the seventies, and without a lot of electronics driving antiphase signals and the like, the separation is not excellent.

Yosefu 20th May 2013 11:02 PM

Thank you for answering!

I placed the coil back in the speaker (now de-coned and not making any sound) and a small nut on top of it. Having it very close to a low E string I could get infinite sustain... on the second and third harmonics. Maybe I placed the input pickup very close to the string so it dampened the fundamental.

Also, the Little Gem amp outputs 1/2W, I don't know if that's short for the purpose. Anyway, tomorrow I'll try with the Ebow schematics, they might be better suited for this.

Tubelab:
" The impedance of a guitar pickup is quite high. 10,000 to 100,00 ohms. Your small amp was made to drive an 8 ohm speaker. I used the output transformer from a small tube amp, hooked up in reverse to boost the 8 ohm output of a chip amp up to a higher impedance, which I fed into a second ordnary single coil guitar pickup. This works quite well, but it lights up all 6 strings at once. "

I'm not sure I understood as I don't understand yet the relation between impedance and pickups very well, but that sounds really interesting. You mean all of them 6 vibrating at the same time?
That'd be perfect for me since I dampen the open strings with a cloth, so it could be the polyphonic sustain I need without having to go into hex hell!

So (forgive my electronics illiteracy) you are "translating" the signal from the chip amp to a higher impedance and feeding it into a normal 10k pickup? Haven't seen anything like that. Could it harm the pickup?

Chrispenycate:
"But you're not going to get enough magnetic field out of a voice coil to vibrate strings, though apparently it does produce enough to be picked up on the pickups."

I wouldn't mind spacing them a lot or using a piezo for input.

"build an actual small speaker into the body of the guitar; though how to make it vibrate just one string without damping it right down I can't offhand work out."

I think that's what the Sustainiac Model C does, but it only sustains one or two notes which quickly go into harlmonics. Clean polyphonic sustain is more what I'm after, doesn't need to be endless, but enough to overcome the limitations of a fretless.

" Or if you built something where the string slotted into a gap in a driven electromanet, so the field was contained and focused, like a speaker magnet, but varied with coil current? Would too much of it travel through the strings themselves? Sort of a cross between a tape head (in record mode) and a moving iron speaker, but tight. Or a loop you have to thread the string through, so the field is contained? Trouble is, as I imagine them allthese possibilities make it much less easy to play the instrument."

That's an idea I find beautiful, I don't know if I could make it work, though. I tried something similar this evening but in a very primitive shape, without much success. I'll keep experimenting :)

Andrew Eckhardt 20th May 2013 11:35 PM

I've heard of such systems and have wanted to fiddle with it but haven't done much research and have built nothing related, so I'm hardly an authority, and here are my first thoughts on your experience.

Since the string is a non-biased permeable material, the drive from the output coil, in just about any useful configuration, is mostly only able to attract the string. It will do this in large part twice for every complete fundamental cycle if a normal full wave drive output is used. The result will be mainly second harmonic drive right off the bat, which would seem to be a problem.

Maybe some other historical circuits have addressed this, but you might want to try something as simple as half wave drive to the strings. Making the drive more sinusoidal after half wave rectification, to minimize injecting "tone", while making the output track the peaks well in phase would obviously be more complicated but probably also helpful.

jcx 20th May 2013 11:54 PM

some seem to wind the coils on Alnico rod magnet - has some magnetic permiability to concentrate the coil's flux, and gives bias field

I would try iron core and small a neodymium magnet on the end, or even just nearby - since I have some but no Alnico

Andrew Eckhardt 21st May 2013 12:04 AM

Interesting idea. Even an idle bias current in the drive coils might do the same thing. Ideally there would be no additional static load on the strings by the system, but this is likely the easiest way to get something like a linear regeneration.

tubelab.com 21st May 2013 01:42 AM

Quote:

You mean all of them 6 vibrating at the same time?.....That'd be perfect for me since I dampen the open strings with a cloth,
Yes, my experiment did not have enough gain to start the vibrations without some initial input, but it will allow for some extended sustain. My experimets were just to see what would happen so I didn't put much time or thought into it. I did notice that the system tended to favor certain notes. A chord would sustain for a while, but within a few seconds one string would dominate and the others would die out. The effect is similar to playing a guitar with the amp cranked pretty loud and the headstock touching the speaker cabinet. It is good for lead runs with a piece of felt under the strings at the nut.

Quote:

you are "translating" the signal from the chip amp to a higher impedance and feeding it into a normal 10k pickup? Haven't seen anything like that. Could it harm the pickup?
Looking at this another way, the pickup is meant to sense the energy of a vibrating string and turn it into an electrical signal. A hot pickup will produce about a volt of signal when the strings are hit hard.

We want to use the pickup in reverse. We want to apply electrical energy to the pickup and have it exert physical force on the string. To do this we need to apply much more than one volt of electrical energy to the pickup.

A 1/2 watt amplifier produces about 2 volts of electrical energy. We need to step this up to a higher voltage with a small transformer. I measured about 35volts in my experiments.

A guitar pickup was never meant to have voltage applied to it, so there could be a possibility of damage, but if the pickup was well made nothing should happen. Of course, if you try this, don't use a rare, vintage, or expensive pickup. I used an old no-name single coil that has been in a box full of guitar parts for so long that I don't remember where it came from.

There is another way of doing this too. If you have a dead pickup, take it apart, remove all the very fine wire, and rewind it with something thicker and connect it directly to your little amp. I have not tried this yet, but I will someday. A typical pickup has about 5000 turns of #39 or 40 wire. I would try something like #30.

Search Youtube for guitar sustainer I remember seeing some videos about rewinding pickups for this purpose. There are lots of sustainer videos out there.

Quote:

Since the string is a non-biased permeable material, the drive from the output coil, in just about any useful configuration, is mostly only able to attract the string.
A guitar pickup is a coil of wire wound on a magnetic pole, so it is biased. Applying audio energy to the coil modulates the magnetic field so the attraction exerted on the string is varied. The magnetic field is somewhat weak so too much audio energy can overcome the field causing an increase in energy at the second harmonic.

Quote:

some seem to wind the coils on Alnico rod magnet - has some magnetic permiability to concentrate the coil's flux, and gives bias field.....
I would try iron core and small a neodymium magnet on the end, or even just nearby - since I have some but no Alnico
Stewart Macdonald sells Alnico pole pieces that are not magnetically charged when you get them. I found some tiny Neo magnets at Parts Express that fit on the end. My plan is to wrap a coil of wire over this combination and build one for each string. This will allow for a hex pickup for the sensing pickup for MIDI purposes and adjusting individual string gain levels on the drive pickup.

Quote:

Ideally there would be no additional static load on the strings by the system,
There are a few Neo pickups on the market. Their biggest complaint is that the magnetic field is strong enough to exert a constant pull on the strings which creates a damping effect.

Yosefu 22nd May 2013 03:04 PM

Quote:

A 1/2 watt amplifier produces about 2 volts of electrical energy. We need to step this up to a higher voltage with a small transformer. I measured about 35volts in my experiments.
Wow, that's quite a lot, if it was a loudspeaker that should be enough to drive the neighbors mad.

Actually I found this guy in Youtube:
diy sustainer pickup / six string ebow . - YouTube

It's a six string ebow using just a 9v battery and an 8 ohm driver pickup. In one of the chords I can hear the bass, a fifth, an octave above and a major third (I guess the G-B strings), so that should be enough since I play with only four fingers (the cons of not being a mutant or hindu godess).

As I said, I mute the open strings at the nut, so no problem about it being "uncontrolled".

So I did an experiment with the ebow circuit, a normal pickup and the same de-coned speaker that seemed to work earlier with the Little Gem. But it's not working.

First of all, I don't understand what a diode does with the - pole in parallel with the +9v input and the + pole to ground.

I connected the output of the circuit to a normal guitar amp for "debugging purposes". No sound was coming as long as the pin 2 of the LM386 was in parallel with ground (as in the schematics). Funny though, holding both input and output coils together wouldn't make that feedback bleeping sound, but instead... it would work as a radio. Well that was unexpected. I was like "how the **** is techno-music coming out of my amp??", it felt a bit paranormal but in a cheesy way.

bear 22nd May 2013 05:29 PM

strings need to be grounded or you get awful hum


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