Percussion Karplus Strong Inspired

In Uncategorized on July 25, 2011 at 11:52 am

Ka-tish Ka-tish Ka-tish go the synthesized drums as part of your eChuck music circuit.  How did you do that with a NAND gate?  Why, you used the percussion circuit shown above – easy!

*** Edit:  Please note that this circuit works with the HEF4011EP surplus NAND chip, and not with the CD4011BCN new chips.  We found that out the hard way when some readers could duplicate the circuit easily and others could not at all.  You can purchase the HEF4011EP chips from Electronics Goldmine, or at least that’s where I got my chips.  This is an occasional issue with linear CMOS and I wish I had been enough of a perfectionist to see it ahead of time, but at least we know now.  ***

Here’s how I dreamed it up myself, though I’d be surprised if it was not an already discovered circuit (remember the “re” in “reInventor”).  Nothing new under the sun as they say, right?  Well I was working on a circuit design for the Lunetta Challenge at and I wanted something fun and uniqe for the next subcircuit.  So as I lay down to sleep I imagineered the above circuit as a super simple linear CMOS version of the famous Karplus Strong algorithm.

The what?  The Karplus Strong algorithm, first published in 1983 by professors Karplus and Strong, it is a simple algorithm involving a summing amp, a delay line, and a filter in a feedback loop.  You apply a burst signal such as a pulse or a burst of noise or somesuch to the summing amp and the circuit’s feedback loop rings out percussive or stringed instrument types of sounds.

I had worked with Karplus Strong for years and even sold a bunch of modular synthesizer boards to electro-music friends, so I had become quite familiar with it.  Well, I reasoned, if the NAND gate was behaving like linear CMOS then it could be like the summing amp.  Then for the delay and filter I used an RC feedback circuit to one input, and added an inverting opamp type circuit with feedback and input resistors to the other input for the gain control of the summing amp.

The whole thing was all smooshed together around one NAND gate, and I had no idea if it would work anything like a Karplus Strong loop, but I was willing to give it a try.  This is an example of the creativity process that I hope you enjoy designing with as well.  Take some ideas, sort of squeeze them together into a simplified version of a thingamajig and try it!  You can’t be afraid to fail when it comes to reInventing the wheel.

So anyway I didn’t have a noise burst or even a pulse, just a square wave, so I added an input capacitor to give me edge pulses at the input from the square wave.  I fired it up and gently adjusted the volume pot – tada!  Percussion emerged from my weird little circuit.

It was chirpy Ka-tish sounding synthesized percussion, and certainly not the very best kind of synthesized percussion, but lo and behold it was indeed percussion from a single NAND gate and a few passive components.  Well I wanted something a bit more interesting than just the same sound repeated over and over so I mangled up the input with an extra resistor so that two digital signals were mixing at the input and I ended up with a nice little percussion rhythm.

OK, sold you on this circuit?  You wanna build it?  Yes!  Of course you do, so I’d better tell you the values of the components.  Start with 100k resistors in all three places and 0.1uF capacitors in both places.  Then try doubling or halving the various components to see what kind of sounds you get.  Go to a wider range of values for even more variation.  Also I suggest using a 100k pot in all three resistor positions for full experimental adjustability.

I’m not giving you exact values for the circuit I ended up with because the input is more complex than the circuit shown and also I can’t see all of the color codes with these 44 year old eyes, lol.  Also my meter broke and I haven’t made it a priority to get a new one so I cannot easily measure the components that I used.  Plus I’m lazy like a lot of creative people, heh.  Well I do get industrious from time to time but not today.

Which, as a side note, I’d like to mention that these little eChucK / Lunetta circuits are so simple that you don’t even need to use test equipment other than amplified speakers to listen to the signals – if you know what you are doing that is.  By all means use all the gear you have if you do have it, I do not.  Designing music circuits blindfolded?  As Richord Pryor joked in the movie Stir Crazy, “That’s right we bad, that’s right we bad!”

LOL, just kidding!  As the legendary Jimi Hendrix said “Dont’ get mad nawawww…”  OK enough silly ramling, off you go – build your own homemade percussion circuit right now!  Go for it!

  1. Hi. Looks very nice what you’ve done here.

    So if i got that right, you put in a constant signal, and out comes a percussion sound? Or is it like you put in something like an arpeggio or sequenced notes of let’s say a sawwave and out comes some percussion that you might tweak with the resistors if you choose to replace them with pots?

    Is that right? I would like to build it up, because it looks so very simple.

    Do you maybe have some sound examples, how it sounds like?

  2. Ok,

    so i’ve built it up yesterday and i get no percussion sounds out of it :). But i get a very nice destortion. Esepcially on synth sounds its very interesting. So how did you get percusison out of it? Can you maybe tell me the exact values of your passive components. I tried some settings with pots instead of resistors, but no chance to get something like percussive sound out of that thing.

  3. You are the second person who has said this and that is no coincidence. I assure you that I did get a synthesized percussion-like sound out of the circuit, but it was a slightly different circuit than the one published. So what I will do is take some time right now and build this circuit *exactly* as shown, then see what happens.

    I do want to clarify that it is a synthesized percussion sound, not really a super realistic percussion, as I mentioned in the article. However, you would certainly notice the resemblance between this circuit’s sound and that of a percussion instrument. I will report back with my results soon.

  4. Oh yeah great. Thank you very much for checking this again. It’s absolutely clear for me that there will not come out a realistic percussive sound. I’m not in search of that. If i would need that i’ll use a sampler :). I’m lookinf for dirty, glitchy, distorted, synthetic sounds that are unique and never the same when you play them twice. So i think we talk nearly about the same percussive sounds :).

  5. Good, glad we’re on the same page. Well, I have good news and bad news. The good news is that I reproduced the circuit and it works just fine and I have correct values for you. The bad news is it doesn’t work for all NAND chips. It works for the HEF4011EP chips that I purchased as surplus, and does not work for the CD4011BCN chips that I purchased new. The exact values that worked for me are: C1=C2=0.1u, R1=R3=100k, and R2=49.9k.

    On the one hand I would like to apologize and beg forgiveness regarding the chip-specific nature of this circuit. I had once read about the possibility of chip-specific issues when dealing with linear CMOS so if i were as perfect a thinking machine as, say, Data of Star Trek, I should have seen this possibility and tested for it. On the other hand I am only human and therefore prone to such unfortunate oversights.

    Also I tried a NOR chip and it worked just fine as well, making a different sound, so that is something else for experimentation. I should also mention that I can’t quite read the HEF4011EP chip’s second to last letter, it might be an E or a B, not quite sure. I purchased the HEF chips in a grab bag from Electronics Goldmine, so I’m sure they have more if you need a source of them. What can i say? “Live and learn, mystery solved, and thank you very much for your comments – you were quite helpful!”

  6. Oh yeah. I’m glad that i was heplful. So i will try to get some of the HEF4011EP chips. Hope i’ll get some here in Germany. And thank you for the exact values. Seems that they are not far away from the originla circuit. Will try that at home. 🙂

  7. I built this and it works. Using a HEF4011BP by NXP. The circuit did seem a little tricky… if one veers off too far from the 0.1uF caps for example, there is a kind of background noise, and the percussive aspect gets lost. Other strange noises occur for me when I approach my finger near C2, but probably that’s my cheapo breadboard (too many projects going at once, and all my good ones were taken but I just had to try this!). Anyway, I’ve determined a good set of tweaks and this circuit is a keeper… thanks!

  8. Me again… another observation is that the circuit seems to be load-sensitive (or something… my electronics theory is not very strong). I am using 3 oscillators built on a 4093 and used 50K rez to join them to one point to go into the first 0.1uF cap. For fun, I added a tap on each output going to LEDs (through 1K’s), and that slowed the rate of the oscillators and affected the tone of the percussion sound. Hmmm. Anyway, if I ever get around finally to making my suitcase Lunetta, this circuit will be part of it.

  9. RingMad, I’m happy that you like the circuit – that’s what it’s all about and why i do this stuff. Yes, touching the circuit and loading CMOS, especially with LEDs, will affect it. That will be especially true of linear CMOS circuits which is what we have here. I hope you get around to building your suitcase Lunetta and happy mudic making!

  10. Oh that sounds all very interesting. I already ordered my HEF4011BP. Hopefully it’ll arrive soon and i can play around with it. I’ve already built upan atari punk console at the weekend and hoping to modify that circuit with this one to generate percussive noise. 🙂

  11. Now that’s a compliment. The Atari Punk Console is the stuff of legends. It’s a classic circuit that has inspired generations of electronic music hobbyists. To read that this little percussion circuit will be driven by such a creation is very satisfying indeed. Rock on!

  12. ok, so i’ve built it up finally and getting nice results. I did not try it withthe apc yet, but used the monotribe as signal input. And it works ine for me. But as i like to experiment with the stuff, i tried several combinations of pots and condensers to get different results. I’ve found one that can maybe be called something like an effect. Cause when you add pots instead of resistors, a lot more of sounbd possibilities open up with this schematic. It’s really cool to play around with it. I will try to set up more stages so like not only one NAND, but more together, parallel and daisy chained. One thing i would like you to ask. Would it be ok if i publish my version of the schematic on my blog? Needles to say that i will add a link to your website to it. So that it’s clear that you have made the basic schematic of the idea.

  13. Oh yes, by all means you are certainly welcome to expand the circuit and modify it however you like, then publish it on your blog. It’s nice of you to mention my contribution and also to link back to this blog. Sounds like a plan!

  14. Again, me. A warning for would-be builders of this cool circuit… the brand of the chip can be important. Yesterday I got a couple more HEF4011BP’s, but they don’t work with this circuit! I just get clicks and the pots have no effect. They’re Philips brand (and as straight NAND gates, they do work). I swap back in the NXP brand, and the circuit works.
    Also, any tips on reducing body proximity noise and hum? I put 100K pulldowns to ground the unused NAND gates, but that didn’t help. The “linear CMOS” stuff is beyond my comprehension.

  15. RingMad, thanks for your continued efforts on this circuit. Quite frankly I got annoyed at how many chips do not work with this circuit so I decided to try some things to make the other chips work. I ended up with three modifications that help make the CD4011BCN chip work: Dividing the supply, adjusting the feedback resistor, and dividing the output.

    Just use a 100k pot with it’s ends at power and ground, plus it’s center tap connected to the IC power pin. Then reduce the feedback resistor from 49.9k by putting a 100k resistor in parallel with it (or adjust your feedback pot) – that is, the feedback resistor associated with the input, not the RC filter feedback resistor. Also add a 1MEG pot to the output for help with isolation from cable and load capacitance.

    These three changes gave me some good percussion sounds and also a variety of other cool sounds. Basically I am thinking that what is happening here is that the CD series chips are “too good” in that their gain is higher so they need to be made “worse” with supply starving. The other changes are tweaks that you may not find necessary.

    Let’s play around with the circuit some more and publish another article with our findings. Again, I appreciate your involvement RingMad – community interaction is what this is all about, not the circuits really but the people! Enjoy.

  16. Ok, I got distracted by several other projects I have on breadboards… I already had the input feedback pot, which I had called “sharpness” or “resonance” (the other one I called “pitch”). As for the power starve, it didn’t really fix the problematic Philips HEF chip, but it does indeed do interesting things, so that’s a modification I will keep. I’m pretty happy with the circuit as I have it now, although I will probably try a few tweaks before finalizing… using really high clock rates, it can produce more of a droney noise instead of percussion, which can be interesting, and something I want to keep in it in order to make it more versatile for jamming. This circuit works quite nicely with your 4052-based spatializer!

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