Percussion – Metal-o-tron

This one is a re-build of one of my first ‘designs’, if you can call them that way. Nevertheless, It’s a percussive module designed for all kinds of metallic sounds; cymbals, hihats, gong, bells, plate hits, cowbells, trash cans, … you name it. Since I’ve build the first one I’ve been using it in every song since. It’s actually a very useful and versatile percussion module. At that time however I didn’t really had the habit of writing down schematics, I just soldered things together until it worked. Yet I wanted to revisit this module again because I could use a second one and I also wanted to iron out some of the quirks my initial build had as well as add a few features I regret not having on the first module. So I kinda started over, this time taking notes though.

The concept for this module is similar to the Synbal, a metallic percussion generator from the eighties. It uses a XOR logic gate to mix a bunch of square waves together and generate a metallic noise-ish kind of sound, then put that through a filter and finally through a VCA. In my first build I used opamps to generate the square waves, this time I used a 40106 hex inverter to build a simple square wave oscillators. I don’t think it makes much of a difference though. Most importantly, it’s simple, it works and requires few components. 

Now, I did want to get a bit more out of the design compared to the Synbal, so just like in this article ,I made the oscillator frequencies variable, feeding all 4 to a XOR logic gate. You can look up the truth table and work out how frequencies will work against each other, or you could just believe me when I say you’ll end up with a complex sound not unlike a badly tuned AM radio. While you can get a sort of digital noise going from these 4 oscillators alone I’ve also added a ‘real’ noise generator based on an unconnected NPN transistor. A technique used in plenty of noise generators (like the 808). You can use whatever NPN sounds good to you, just put in whatever you have lying around and see what works.

After mixing the square wave mess with the noise I needed a filter. Dunno why but I’ve spend the better part of 2 weeks building a multi-mode filter. But persistence is key as they say and after a while I got a working filter based around a Coolaudio V2164 IC ending up with a LP/BP/HP and a bunch of resonance. yay for that and it even has CV controllable resonance. 

I had build the VCA before my adventures in trying-to-get-that-damn-filter-to-work-world. Thus I was unaware I would have a spare VCA in the V2164. that I could have used instead of doing it the transistor-way making the circuits needlessly complex. But alas, I’m way too lazy to redo the whole VCA at this point when I have one that works just fine. 

The last part of the puzzle was building an AR envelop generator to control the VCA & filter. I wanted CV control on the decay length to allow for more variance in hihat patterns. Turned out not all that difficult although it changed the decay from logarithmic to linear. I’ve pondered the implications of it for a while and then decided on letting time tell if one is better then the other.

Audio demo

I’ve recorded a small audio file while finishing up. It’s not much as I just could trigger it on a steady beat and couldn’t use the various CV inputs. I’ll try to make a more interesting demo if I find some time but this will already gives an idea of what this unit can do. 


C11 & C32 : These are the timing caps for the decay time, If you want real short decays or really long ones you might want to change these.

C14: This is added to dampen the ‘attack’ a bit as it gets real ‘clicky’ otherwise. If things aren’t snappy enough for you, lower this value or omit it. 

Q1 : This is the unconnected NPN transistor, use whatever you have around.

C9,C10,R24,R28 : This is a passive low-pass and hi-pass combination. You’ll need the hi-pass one to compensate the DC offset from the oscillators and logic gate. In this design the hi-pass cutoff frequency is extremely low ( at 20Hz ) so it just functions to cancels the DC offset.  The low-pass cutoff frequency is about 22kHz. Feel free to play around with these values.

RV15,RV16,RV10,RV9: These trimmers are put in place to attenuate the CV voltage to the range the V2164 likes. You should put them so you have approx 0v to 2,5v on the CV input of the V2164 as a starting point. Adjust to taste.