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Diy optical pickup for guitar -- is it possible?
sorveltaja:
The phototransistor- and 4049 amp board is ready for drilling:
Hopefully there isn't any errors, as I checked the layout multiple times. The size of the board is 72 x 68 mm. I could have made it smaller, but then I would have to (again)cram the parts in.
Besides that, I probably should have made it even bigger, as it's all analog circuitry. "The bigger and uglier it is, the better it sounds"
But seriously, as there are six 4049-based amplifiers so close together, their inputs and outputs should have some distance between them.
When doing the previous breadboard testing, they don't seem to do the hard clipping, as the 386-based amps did.
sorveltaja:
New amp board:
I tested all the six sections, and they seem to work, as expected. As usual, square wave was fed to the ir-led of the pickup(from the earlier mechanical setup), and the phototransistor was connected to the circuits input:
There is one thing, that bothers me about that 4049-chip, though. While testing the sections one at a time(while others were not connected), it did hog constantly about 50mA(at 9V), and got warm.
Bit like on the breadboard, where I forgot to connect unused inputs to the ground, but not that bad.
From that came this shady logic conclusion, that the inputs might act like weathercocks, that aren't tied to anything during the storm. So they rotate at forever increasing speed, until they get glowing red, and therefore get very hot.
But enough of that. I took a look of other 4049-based circuits on the net(there are lots of them, as that chip was/is used in some commercial, and in many diy guitar fuzz pedals), and at least some of them have 1M resistor between inputs(that are in use) and ground.
Not a big deal, so I drilled holes for the 1M resistors, and soldered them to the circuit. But they didn't help at all. Am I missing something important here?
I have tinkered with other cmos-ic's(mostly for guitar effect purpose, some counters, and pll) before, but never had problems with excess power consumption/heat.
Let it be said, that I read somewhere, that this chip(4049), when used as an amplifier, has a notorious reputation of having wildly different current consumption, even between individual chips from the same manufacturer.
I have two spare chips, and will probably test, how they compare.
But in the end, if the circuit works, regardless of current consumption and warming, it's okay by me, as I never intended to rely on batteries to use it. I'm more worried of how the chip lasts, being warm all the time it is used.
Also, one thing to test is to connect all the pickups to the circuit(to'occupy' all the inputs), to see if it makes difference.
sorveltaja:
Audio test using the new amp board and schmitt triggers, again without any effects:
testi_3.mp3.zip
It seems, that changing the amp board was a step to the right direction. There is very little noise, when I tested it with the cables, and practically none of that noise is strong enough to get through the schmitt triggers.
Also less distortion(although some of it could add nice texture to the sound), as the 4049 doesn't clip the signal as hard as 386 amps did.
There is one downside, though. When plucking a single string, or playing solo, the outputs are very weak/lacking(or outright unpleasant). Don't know why that is. But when several strings are plucked(as chords), somehow the outputs 'morph' together, having a lot more powerful, and usable output.
What comes to the power consumption of the 4049 chip, I tested another one, and it acts just the same. I guess that's just the way it works in the current circuitry.
As the power supply has a lot more beef(24V/1500mA), than needed, I can live with it.
So next thing to do, is to fit the new circuitry to that external enclosure, that I made earlier, and some wiring.
sorveltaja:
I've been eagerly waiting to get to the point, where the circuitry is worth putting inside the external box, and here we are:
No smoke or sparks, when I shortly tested all the six sections, they seem to be working just fine. Also no extra noises on the outputs.
That certainly doesn't mean, that the project is finished. It just means, that all the(well most of it) wire hassle is now out of the way.
Lots of aspects to test, including 'stealth' covers for the pickups.
Also to see, if I can make sense about that 'soloing'(playing single string) -problem mentioned earlier. There might be some strange phase cancelling stuff going on, that makes plucked single strings to sound so weak. When looking at individual string outputs on the scope, they are rectangular, and should sound, like fuzz does.
There is now less crosstalk between the strings/pickups/sections, when compared to 386-amps, but it still exists, and that alone could cause phase anomalies between the strings.
Almost like the next strings(that are muted) pickup(s) 'loot' most of the plucked single strings signal somehow.
Once I get into it, I'll make a diagram, which hopefully makes babbling above less fuzzier(pun intended).
After all, now it's good time to focus on testing/expanding/exploring the systems usability further.
sorveltaja:
It seems, that the 'weak single string' -problem has something to do with the way I used to mix them together. So far, 10k resistor network like this, is what I've used:
I have a sneaky suspicion, that mixing sources, which have exact same output levels(like schmitt triggers in use) is the cause for that 'cancelling' -anomaly.
More proper audio mixing circuit is on my(ever growing) to-do list.
When the outputs are listened one by one(not mixed together), they sound a lot more solid, and the way they should.
So I guess it could be possible to use each output for individual purposes.
One example, of what I mean by that, is, that one(string's) output could be panned to the left of the stereo field, while other is panned to the right.
Another example is to drive one output to echo effect, while other one goes to some kind of cmos-based device(like up- or down counter).
Schmitt trigger circuit, that I use, has probably too low output levels to drive cmos-level devices, as it uses ~1,8V supply to lower the input thresholds. I haven't tested it yet, but I think that very mild amplification of the outputs should be enough to bring them back to cmos-level, if needed.
After all, there are six outputs to boot for what ever effect combinations. I could say, that it's like some form of an analog synthesiser, but naah, I'm not going to say it.
One thing, that might increase the sustain of the thinnest strings a bit further, would be to use Darlington phototransistors. But as they should be "side looker" -ones, there are only few types of those available.
Some of them are thicker(2,5mm), which makes them quite difficult to fit in, or require whole new mechanical setup, when compared to slimmer 1,5mm ones.
Anyways, it would be interesting to test them(1,5mm Darlington ones) with current electrical setup. Sourcing them seems to be bit difficult, though. But again, we'll see.
They aren't that necessary, but could add some 'kick' to the project.
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