The Karplus Strong eChucK DIY Project

[Inventor]

Hello and welcome to the Karplus Strong Modular Synthesizer project thread. I've once again rewritten this first post to greet you and welcome you to the project.

The latest design files are attached below. There is currently no text documentation. The first run of boards was sold out many moons ago, and the project kind of fizzled out but has regained an interest recently. To address the many requests for boards, wmonk plans to probably make a run of boards, sell as many as are requested, and give the remainder to mosc for later distribution in the electro-music store. All proceeds from the sale of these boards will go to support electro-music!

I would like to thank the many people (over two dozen) who have contributed to the project with their ideas and creativity. This is a real community project. The thread has grown into a 22 page monster at this point and it tells an interesting tale of our efforts in working together. And I wasn't always the ringleader, if I recall correctly Kevin Kissinger took over in my absence and through tedious and thorough efforts he was able to solve the transistor mystery! So thank you Kevin!

Here's to everyone who designs, builds, maintains, and performs using our community developed electro-music circuit boards - we are all one big family and we love and accept one another unconditionally. So stop procrastinating and get to work on your very own electro-music build or project, and may the electromotive force be with you!

Les

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KS11sch.pdf
Description:	Karplus Strong Schematic	Download (listen)
Filename:	KS11sch.pdf
Filesize:	39.34 KB
Downloaded:	3033 Time(s)

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KS11bom.txt
Description:	Karplus Strong Parts List (Bill of Materials)	Download (listen)
Filename:	KS11bom.txt
Filesize:	3.63 KB
Downloaded:	2007 Time(s)

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KS11pcb.pdf
Description:	KS11 Silkscreen	Download (listen)
Filename:	KS11pcb.pdf
Filesize:	36.68 KB
Downloaded:	2173 Time(s)

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[Inventor]

For anyone who would like to experiment with the circuit in software form, I am posting the ChucK program that I wrote along with an audio file captured from it. You can change the parameters in the file for different sounds.

If it turns out that the Karplus-Strong circuit actually works in hardware form, we may be able to use this program to prototype the circuit's component ranges. For example, I understand that K-S will generate percussion but I'm not quite sure how to do that at this point. Perhaps playing around with the program's parameters will help to figure that out.

Les

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BSLDemo2.mp3
Description:
Filesize:	1.01 MB
Viewed:	3315 Time(s)
▶ Play    ⇩ Download

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BooleanSequencerLunetta2.ck
Description:		Download (listen)
Filename:	BooleanSequencerLunetta2.ck
Filesize:	937 Bytes
Downloaded:	1601 Time(s)

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[Inventor]

I made some progress today on the BSG's Karplus-Strong Circuit. It doesn't work yet but most of the subcircuits do work properly. I think I just need to get the biasing and lop gain correct. Below is a photo of the test circuit.

Also for your review, from Thomas Peter (wmonk) here are two links to Karplus-Strong circuits created wit modular synthesizers:

http://vimeo.com/2651547

http://vimeo.com/1952077

The fact that these modulars do the job so well is encouraging for this little DIY project.

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KS_Test1.jpg
Description:	The first test circuit
Filesize:	330.2 KB
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This image has been reduced to fit the page. Click on it to enlarge.

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[jeff-o]

That's an impressive idea! I'll have to play with the software when I get a chance. Regarding everything leading up to the K-S filter, could that be replaced by a microcontroller? That would simplify the hardware, and allow you to change parameters more easily.

[Inventor]

yes, jeff-o, a microcontroller could do the job nicely, it's just that then you'd have an even more custom chip because you'd have to program it... but that could be a good thing too, depends. I choose to try analog first but of course digital is also a good way to go.

Les

[jeff-o]

[Inventor]

if you like miniaturization better, then you may very well prefer a digital implementation. For that I'd choose the dsPIC chips although there are several alternatives. I can help you get started with dsPICs if you are not familiar, but I'm no expert.

I'm not using the clock driver IC because I don't have one. I just have two MN3209 chips. I made the clock driver from a 555 circuit with frequency control input of 2 octaves and a D Flip Flop to create a 50% duty cycle plus inverted output. It works great.

The main problem at this time is that I totally ignored the bias and gain setup of the feedback loop and that part needs to be redesigned. More updates later and good luck with your choice of implementation.

Les

[jeff-o]

Ah, cool. I didn't think you had the driver, so I was wondering what you used to drive it. I also read in the data sheet that the MN BBD device needs a 14/15 VCC input, did you use a resistor divider for that?

[Inventor]

Yep, 1k and 14k.

[jeff-o]

[Inventor]

Now that I think of it, I better put a 0.1 uF cap on that divider too, huh?

[jeff-o]

[Inventor]

Yes, well it's often a good idea to put a cap in a reference voltage divider. Now I'm thinking that I'd better buffer it with a spare opamp too. That companion chip you bought is looking better all the time, but I'll make do with what I have for now.

[jeff-o]

[Inventor]

I played around some more with the BSG ChucK simulator this morning. I adjusted the loop filter cutoff frequency and the delay line duration within the limits of the hardware design. It was fun to discover percussion sounds coming from that Karplus-Strong thing, plus it made some spacey sounding twangy noises at times. It's nice to have something in software that at least attempts to model the circuit.

Les

[jeff-o]

[BobTheDog]

Les,

Are those chopsticks in that pic, top right?

[Inventor]

[Inventor]

I've realized some things about the circuit that i need to correct, notes as follows.

1. The speaker is a high impedance speaker, so it doesn't make sense to drive it with an LM386. I'll just drive it with an opamp.

2. The counter will be a CD4040, not a CD4017. What is needed here is a binary counter not a decoded counter.

3. Lots of little details that I keep darting between in my thoughts, making notes of them. As I understand it all more I'll report.

Les

[blue hell]

[Inventor]

Yes it does Jan, it's fun to share your work but without some kind of interactions it all goes flat and boring so I appreciate it.

Here is the product of my efforts recently after some much needed sleep. The image below shows the 555 oscillator that sets the tempo, the 4040 counter, the LEDIL logic, and the outputs of SUM, R2R, and NOTE.

The clock and reset are bussed so that multiple cards can be synchronized to create separate voices in a song. The available voices are basically lead guitar, bass guitar, and drums so you can make rock and roll with three cards.

Les

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BSG_schematic1.jpg
Description:	BSG schematic, close up of BS circuit
Filesize:	76.38 KB
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This image has been reduced to fit the page. Click on it to enlarge.

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[Dan Lavin]

[Inventor]

Cool antman, i'll keep posting here with progress.

Les

[seraph]

[BobTheDog]

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Chopsticks.jpg
Description:	Look like chopsticks to me
Filesize:	15.04 KB
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