Author Topic: Cheapo guitar sustainer/feedbacker  (Read 1022 times)

Offline sorveltaja

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Cheapo guitar sustainer/feedbacker
« on: January 02, 2021, 09:01:29 PM »
Another audio effects project? really? But yeah, nothing else has appeared, after finishing the scalloped guitar, so it's time to open a new can of worms.

Bit of background about this one: As far as I know, there has been commercial sustainers in different forms available for a long time. Lots of diy-versions are on the net also.

So, what is sustainer, and what is it's purpose? It simply imitates really loud guitar amplifier, that produces acoustic feedback, making the guitar, and its strings to vibrate more wildly.

I guess, that most of us don't have a luxury of cranking the amp to ear-bleeding levels. That's when the sustainer enters the room. No loud amplifiers are required.

In general, there are two types of sustainers: most widely used ones, that look like a guitar pickup, and fit in the same cavity. They use electromagnetic means to excite the strings. Major trademark is, I guess, 'Sustainiac'.

Other one uses electro-acoustic way to achieve the results. A transducer is fastened to the guitar's headstock, making the guitar's neck vibrate, which then excites the strings. Trademark for that is 'Sustain-Man'.

Obviously, on both systems, there is a guitar pickup, that feeds the exciter circuit.

--
To get the project started, I'll focus on the latter, electro-acoustic one, as it's way more simpler to approach, since it doesn't require winding of exciter coil.

When I was adjusting the hexaphonic pickups in one of the previous project, I used a small speaker(8 ohms, 0.25watts) to make the strings vibrate, one by one.

That same speaker at this moment, on the right side:



Although not necessary, I reduced it's outer diameter a bit. Also the printed parts might not be necessary at all. Then I glued M8 nut to the center with hot glue. Next I added a neodymium magnet to it, so that it sticks nicely to one of the tuning machines:



Amplifier used is a Velleman's 2.5 watt one, that I had laying around:



For this purpose, 2.5 watts is way too much, and I ended using that amp at 3 volts, instead of 9-15 volts. Otherwise nasty rattlings appear, when it clips/establishes feedback loop.
Also I have no desire to fry that speaker.

--
In the end, I have tested that 'exciter' on the guitar, and yes, it seems to have something in it. Actually at the lower voltage supply level, the feedback is more pleasant.

Next thing to do, is to make some audio samples, then probably breadboard a 386-based amplifier for further testing. But we'll see.



Offline vtsteam

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Re: Cheapo guitar sustainer/feedbacker
« Reply #1 on: January 02, 2021, 11:03:46 PM »
Love your projects!  :coffee:
I love it when a Plan B comes together!
Steve
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Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #2 on: January 03, 2021, 08:55:56 PM »
First test audio sample:

http://www.elisanet.fi/kz1706/sustainer/Sustainer_test_1.mp3

I used an old Zoom multieffect pedal to add a bit of reverb and also a hint of compression.

Connections at the moment: guitar--> sustainer amp, and Zoom. So far the guitar signal is split between them, using a 1-in, 2-out stereo plug adapter.
Guitar pickups used are passive ones, and it would be a better idea to use an op-amp based, buffered splitter instead.

One thing that I've noticed so far, is that the feedback loops are kind of 'fixed', and therefore are bit too predictable for my liking. But then again, the source of vibration is in fixed position in the guitar's headstock.

Commercial units apparently have settings/switches for the fundamental, and harmonic frequencies of the sustainer. As far as I can guess, there is probably some sort of phase differences involved between the guitar pickups and the exciter coil, which then allows the player to choose, which one is desirable.

After all, I wonder, why those commercial ones don't have an effect loop options in them. At least, if there were such, they aren't much advertised, or perhaps I'm totally missing something.

What I mean by that, is simply adding some kind of subtle, pulsating/modulating element to the signal path, to possibly make it more lively.
I have some possible ideas, how to achieve thing like that, so testings ahead.   


 

Offline Brass_Machine

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Re: Cheapo guitar sustainer/feedbacker
« Reply #3 on: January 04, 2021, 01:54:19 PM »
Audio projects are very interesting. I have a few I will be attempting shortly...

Keep it up. Love the inspiration.

Eric
Science is fun.

We're all mad here. I'm mad. You're mad.

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #4 on: January 06, 2021, 07:48:06 PM »
So I've done some testings, using the zoom's effects between guitar and the sustainer. It (zoom)has several modulating effects, and for some reason, only the phase shifter appears to give a sort of effect, that I'm after. That phase shifter is quite lacking, but still.
Makes me wonder, does it really modulate/shift the phase of the signal, or is it just imitating. Shouldn't be too difficult to check that out with function generator and scope.

I tried to record the audio results, but guitar's signal was very weak, due to the lack of the buffer for the pickups.
As always, I have a habit of tinkering with everything else first, and only then consider building something, that is an essential basic block, when starting project like this   

Anyways, if there is something about involving the phase shifter with the sustainer, I have an analog one, that I built as a past project. It needs an enclosure, and some wiring, to get it in more usable form, though.
 

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #5 on: January 17, 2021, 01:08:54 AM »
An update: I tested the phase shifter, that I previously built, but what a surprise, it didn't work anymore. I should have built an enclosure for it right after I finished it.

At this moment, I don't feel like debugging it, maybe later. I got derailed by other phasing stuff, but none of it worked in a way, that was expected.
One device, that works really well with the sustainer, is the super tone control, that I also built previously. The effective/usable feedback frequency range is rather narrow, though.

Today I was about to record a sample of it, but there are still plenty of factors to test, to refine it.

That super tone control doesn't have any modulating parts in it, so it would require an external circuitry. But I'll leave it as it is, for the use with vocoder.

Simply, what I'm after, is to emphasize certain harmonics, either by modulation, or by using an expression pedal. 

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #6 on: January 24, 2021, 06:43:25 PM »
Again, an update. I decided to build another, modified super tone control(stc). Instead of pot, it has ldr to control the frequency.

I had some leftover ldr-led optoisolators, but modulating the led appears to be quite tricky(with function generator), to find usable range for the sustainer. Apparently there are things like offset, and others, that need tweaking.

On the other hand, the automated modulation may not be that necessary, as the sustainer could be used with a pedal to control the frequency, and therefore, harmonics.

The guitar, that I mainly use for testings, is the scalloped one, that I finished as a previous project. It has a bolt-on maple neck, with very lightweight body(could be some form of balsa).
It resonates easier, than the other guitar, that has a glued maple neck with heavier, poplar body.

What comes to vibrating element, tactile transducer could be better, than the 8ohm/0,25W speaker, that I have used so far. Haven't yet looked, what the local suppliers have to offer.

I found out, that there is only one commercial sustainer, that has an effect loop option in it:



It doesn't seem to be that popular. Or perhaps it is, but at least there aren't videos on 'tube of anyone using its effect loop option.

--
Bit of rant:

Don't know why it is, but those who do demo videos of various commercial ones, in my ears, they all sound the same, no matter how talented the guitar player is. Dull, predictable harmonics.

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #7 on: January 26, 2021, 06:06:45 PM »
A simple test pedal:



It uses pairs of angle-irons, and M5 screws with locking nuts, to keep it from moving too freely.

Next thing is to test, how the led-ldr optoisolators could be attached to it, and also what kind of moving aperture could be used.
Super tone control uses dual ganged pot for frequency, so two pairs of led-ldr's are needed.

One might wonder, why not use ordinary pots instead of optoisolators. Good ones(used in wah-wah, and such pedals), that can stand the mechanical wear, tend to be rather pricey.

Anyways, after some drawing, this is perhaps the next step:



Obviously, the range of the action depends, how far or close the optoisolator and moving aperture are from the pedal's pivot point.

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #8 on: February 03, 2021, 09:25:18 PM »
Time for an off-topic update.

No process with the project, as I had to upgrade my old computer. Hell of a side project. I have done it many times in the past, but this time, when using newer hardware, it was pita to install Win7, that I like to use. I tried Win 8.1, but nah, I'll stick with seven as long as possible.

So it'll take some time to install essential softwares, like 3d modelling and such, plus all the settings/tweaks to get back to somewhat familiar environment.

But yeah, after that is sorted out, I'll be glad to get back to the main project.

Offline vtsteam

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Re: Cheapo guitar sustainer/feedbacker
« Reply #9 on: February 04, 2021, 11:22:45 AM »
Comp hardware switch always a major consumer of time!   :doh:
I love it when a Plan B comes together!
Steve
www.sredmond.com

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #10 on: February 06, 2021, 07:43:17 PM »
Steve, yeah it indeed is. Especially, when tweaking the user interface to suit. I admit being very picky about it, as everything unnecessary should be either removed or simplified to the core, in the name of utility.

At this point, almost all of it is accomplished.

--
What comes to the main project, I've been looking for a source for tactile transducers in my country. Only one, that I was able to find, is 58mm, 25W one. Pretty bulky for this purpose.

Other ones available are 'bass shakers'. Not sure how they would work as an exciter for the sustainer, as there are higher harmonics involved also.

Although my current setup with 2.5W amp and 8ohm 0,25W speaker works, the speaker, as it has a moving cone, makes it rather fragile for extented use.

So, next thing to do is to see what ebay has to offer. I have never used tactile transducers, so I'm not quite sure, what size, or power rating to look for.

In the spirit of this forum, I'd rather like to try and build my own version of tactile transducer. I haven't found much info on the net about it, or perhaps I used wrong words, when searching.   



Offline WeldingRod

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Re: Cheapo guitar sustainer/feedbacker
« Reply #11 on: February 06, 2021, 10:45:29 PM »
Laziest is copying the "buttkicker" brand bass shaker.  Round magnet in a slippery tube, set up where its magnetically centered.  A big winding either exerts an up or down force, depending on polarity.  Should be scalable to small size.

Look for the cross section halfway down.
https://thebuttkicker.com/company/

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Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #12 on: February 08, 2021, 08:19:19 PM »
WeldingRod, thanks for the tip. That's the principle, that I'm going to use as a starting point.

First test coil:



Coil has 0.2mm wire, and to get approximately 8 ohms, about 15 meters of it was used. To get more practical view, I modeled the coil to fit to that size of bobbin.
Finished coil has resistance of ~9 ohms.     

That coil was used to test, how it works with 12x3 round neomagnet, that is glued behind the guitar's headstock. It showed signs of life, so next step is to concentrate to the 'tactile' part.

Basically it's like a solenoid, that is controlled by audio signal. The moving part (in this case neomagnet), while vibrating, needs to have something, that prevents it from rattling against the bobbin. Some kind of diaphragm. For that, I'm going to test the 1mm thick rubber sheet, that is under the coil in the above pic.

--
To get back to previous setup with a small speaker, it's probably the easiest, non-intrusive way to get acoustic feedback from the strings. At first I aimed to clean, undistorted signal to feed the exciter(speaker), but it would require a lot more power, and perhaps (too) big exciter.

So far, conclusion is, that small speaker works because the amp distorts heavily, creating overtones and harmonics. That particular amp was/is used with 3 volts, that is way below its 'normal' value. It alone produces plenty of distortion.

And no, it's not necessarily suitable for battery use, as it draws 60-70mA at 3 volts.

In the meantime, I think it would be fair to make raw samples, of how it responds to chords and single notes.   

   

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #13 on: February 14, 2021, 06:51:09 PM »
In the past week, I've been testing different size coils and magnets for the exciter, but none of them works like the small speaker. There seems to be all kinds of unwanted resonant peaks involved. Also it appears, that I'm not that good at making an efficient coil-magnet device.

As the speaker works at quite low amplifier supply voltage, I'll stick to it. Simpler it is, better it is.

Obviously the guitar itself has its own resonance frequencies, as well as that speaker. That's the reason, why I use super tone control between guitar and the sustainer.

Finally I got myself together, and recorded a sample, while turning the knobs. It's unedited, though:

http://www.elisanet.fi/kz1706/sustainer/Sustainer_test_2.mp3

Haven't done much with the previously mentioned pedal yet, so next on to it.

 

Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #14 on: February 24, 2021, 06:50:47 PM »
Time for an update. Finding usable range for the led/ldr-optoisolators(which control the frequency of the super tone control) isn't as easy as I thought.

Main problem is perhaps, that the freq. control for the stc was originally a dual 10k pot, and the ldr's, that I have, have resistance range from about 100 ohms to over 30M.
I guess that's the reason, why the effective range is so tiny, 10k vs 30M. I tested different resistor values in paraller with ldr, but it didn't have the effect, that I was looking for.

Next thing was to try the voltage divider. It's a bit better, but still the usable range is quite narrow. I'm not exactly sure, what other factors are involved.

But anyways, a block diagram of the current setup:



On the left is the pedal section, having 317-based adjustable regulator, that gives ~2.5V output. It controls the stc's optoisolators. Reason for that extra optoisolator, is that instead of six wires, only one coaxial cable is needed for the pedal connection.

As the usable range of the optoisolators is narrow, very little current is needed to drive the pedal's led(~20uA). I have no idea, why that is, but 9V battery could well be enough for it.
317's, that I have, seem to keep the voltage level constant, until the supply voltage drops below 5V.

On the right side is the 2.5W amp. I happened to find 8ohm 0.5W speaker, while going through the shelves. It works better than the 8ohm 0.25W one, that I used before. It's not much larger, but has more mass in it. Conclusion: the size of the speaker isn't critical. But small ones (~50mm dia.) are cheap, and appear to fit in the headstock, without sticking out too much.

Also the amplifier can be about anything, as long as the speaker can handle it. It really doesn't take tens of watts to make the guitar's strings 'sing'.

In the end, what comes to narrow range of the optoisos, there might be a workaround in a mechanical form.
When the pedal is moved up an down to block/unblock the led's light, the ratio of the movement... yeah, sounds easy, but we'll see, as I have a nasty habit making simple things complicated 


Offline sorveltaja

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Re: Cheapo guitar sustainer/feedbacker
« Reply #15 on: February 26, 2021, 07:49:12 PM »
I took another look at the pedal's ldr. I removed the 10M resistor, that was in previous block diagram. Then tested again with parallel resistor, and now it worked, as expected.
About 360k seems to be usable value in this case. I really need to make clearer block diagram of the current setup, to make sense of what I'm doing.

This is the whole mess:



and the pedal:



It appears to work, but there is a 'bump' in the frequency response, when rocking the pedal up and down, making it difficult to dial in certain frequencies, so it's not as linear, like on the wah pedals.
That is about the only bug so far. Yeah, I've tested the actual wah pedal(Dunlop Crybaby) in place of super tone control, but it has rather limited frequency range for this purpose.   

But babbling aside. First results using the pedal. I used another guitar, that has a commercial tremolo, which has a 'dive bomb' range, to see, what's the overall response:

http://www.elisanet.fi/kz1706/sustainer/Sustainer_pedal_test.mp3

There are many factors, that affect the end result. Just adjusting the super tone control settings offers plenty of options for fine tuning.