Taking this wise words of A Tribe Called Quest into account I thought it was about time for a kick drum module. I already have a few drum modules, but the lack of a decent kick drum was a bass-drum size hole in my setup. So I set out on another design journey and got to work, full of dreams of a kick-ass-kick module.
Turned out, bass drums are ‘kinda’ tricky to do. Most analog drumcomputers use some form of a bridged T network. It’s a cool little trick engineers have used to make drumcomputers affordable and compact. While the circuit was originally designed to be used as a sine wave oscillator circuit, if you chose the right component values you could make it so that it half-asses the whole oscillating business. You could excite it with a small pulse and it would start oscillating briefly and gradually die out. The frequency at which the oscillation occurs is determined by the circuit’s component values. It comes down to a low part count, no fancy components and a nice short sine burst with a natural sounding decay. It’s at the base of the famous TR-808 (kick, toms, snares, … ) and tons of other drum computers. In fact, it’s pretty hard to find an analogue drumcomputer which doesn’t use this type of circuits in one form or another.
It looks ideal, but there are some issues. The circuit is tuned by design and choice of resistor/capacitor values. The frequency at which it resonates can be made, more or less, user adjustable, but the range stays limited and the pitch is intimately linked with the decay time of the circuit. Adapting the circuit to change one, also changes the other. In short, everything interacts with everything else. That’s why you don’t find lots of controles on these old analog drum machines either, there’s simply little you can control in the circuit. Get a bit out of the sweet spot and it either doesn’t oscillate anymore, dies out too quickly, or doesn’t stop oscillating at all. Neither option being a desirable outcome.
The frequent use of the bridge-T network is due to it’s low part count and as a result, price. You don’t need a VCA, or filter, nor any expensive components. It has a small footprint as well so you can cramp a whole bunch of percussive sound generators in a small box and keep it affordable, it’s great. However, I don’t want to cram several sounds into a single module, I want a single drum voice able to generate a wide range of sounds. And if that’s your design goal, then the bridge-T idea doesn’t seems that great anymore. Also, the world has enough 808 clones already.
One of the things I do when I get a new synth is attempting to synthesise a kick drum. It’s one of those little quirks I have. The first step when doing so is trying to coax something resembling a sine wave from the machine, since most synths don’t offer a sine wave at the oscillator section, the filter is often the ‘to go to’ place for this. Then, an envelop generator to make the pitch drop quickly and another envelop to shape the sound. A lot of fiddling and tweaking later you’ll end up with something resembling a kick drum from a Kraftwerk album and quite a decent feel for the filter and envelopes of the synth at hand.
So I started looking for a sine wave generator as a base for my sound. I searched the lands of google far and wide but came up empty handed. There are some usable signal generator IC’s, but availability of these is a bit spotty, so that was a no go. Seems like building a ‘usable’ voltage controlled sine wave oscillator isn’t a simple task. Not that it can’t be done, you could, for example, make a self oscillating filter to get a sine, but the part count is high and it’s pretty complex to get it more or less right. So the next best thing is a triangle wave oscillator paired with a sine wave shaper. I had all the necessary elements in the VCO so that seemed easy enough. I wasn’t very keen on using the 3340 though as it’s way too over-speced for what I needed, and also pretty pricey. I have no real need for precise 1V/OCT tracking over a wide range in this application, nor for saw/pulse waves or osc syncing. In the end, I ended up settling with an OTA triangle core design which seemed to tick most of the boxes and is a lot cheaper to build.
Since the chosen oscillator design has a reasonably small footprint I decided to add 2 of them, because why not. The LM13700 has 2 OTA’s so that’s prefect and it’s just duplicating the circuit. From using my FML module I know you can get a wide range of timbres, even with just 2 oscillators, when FM modulating one with the other and it’s pretty easy to implement once you already have the oscillators. And why not adding the same option to the second oscillator, effectively making cross-modulation possible. I seldom have heard cross-modulation produce something else then pure and utter chaos, sweet sweet chaos.
Since we have a sine wave at the core of the design, there is little extra to be gained by actually adding a filter to it. Filtering sine waves is pretty much and exercise in futility as there is no harmonic content whatsoever. With this approach, control over the harmonic content is done by adding complexity to the signal rather than having an harmonically rich signal to begin with and subtracting the excess. I don’t know which ‘coast’ of synthesis this leans into, but who cares anyway.
Plan of attack
So now we need to think about getting the sound in shape. We want a fat kick, so no need to put it on a strict diet, a simple ‘decay’ will suffice. We’re after a percussive sound anyways.
The ‘performance’ features as found in the metal-o-tron have been very useful to me in the studio so I decided to add at least the ability to CV control velocity and decay. I’ve implemented both somewhat differently from the metal-o-tron trying to keep component count down. Also, the decay control on the metal-o-tron didn’t felt completely right as most of the action was on the far right side of the control potentiometer. It took some tweaking, breadboarding and a prototype or 2, but it ended up working better in the end, totally worth the effort.
Velocity control is still being sample&hold at every trigger and needs a +/-5V signal. Ideal for use with LFO’s to add some dynamics to the sound. Decay control works as one would expect, higher CV input equals longer decay, who would have thought.
Now, there was the question of ‘attack’. I needed a separate envelop generator to do the attack portion of the sound. Easy enough to implement as it didn’t needed any CV control. I know a lot of people like CV control on everything and anything, but you’ve got to make some compromises sometimes and I didn’t saw any added value in adding such CV control that would justify the extra component cost involved, not to mention the extra front panel room it would take up. It’s the age old design question, it’s not because you can do something, you should. For these kind of decisions I tend to rely on my personal experience when making music, not a very broad focus group though, but at least it’s not decided by the engineer ( euhm… anyways )
Let’s get dirty
So far there are 2 oscillators, cross-modulating, envelop shaping, yada yada. I still had one trick left. Playing around with the design I found out that the sine wave shaper I used could do more than just shape a triangle into a sine. In fact, the range where this happens is small, if the voltage going into the wave shaper isn’t high enough the triangle wave stays unaltered. If it’s too high the tops of the waveform gets flattened, turning it to a square wave. That’s basically a distortion effect right there. Now, I like overdriven shit, and I’m not too adverse adding a tad of drive to a kick drum. Or a whole shitton of drive for that matter.
So, I brought the control to the front panel, tweaked the values a bit so the pot sweeps through the whole spectrum of sounds the wave shaper can do, from triangle wave to sine wave to square wave. No need anymore to overload your mixer channels to get that aggressive kick sound, it now comes right out of the box if you want it to.
Before releasing it to the masses It has been through an extensive run at my place. I would say, Autechre would be proud, if you get all the modulating options going you can really create that post-rhythmic – euhmmm – ‘groove’. Both oscillators go well into LFO territory which makes the cross-modulation stuff spew out some rather interesting and unpredictable results. They also go high enough to make for some useful, non kick-drum percussive sounds.
It also tickles my party itch as it can set down a solid kick. You know, the ones that get you going in a more ‘lizard brain’ kind of way. Does it sound like a 909-808 ? Hell no, and that’s great. I have a few other boxes with knobs on in my studio which can do those sounds if I really need a x0x kick, plenty of options out there actually. It has it’s own character and that’s fine – why does that sounds like a psychologist analysing a drumcomputer.
Can I get one?
Links & resources
I’ve added this warning to the metal-o-tron as well, but this board is even denser. There are a lot of components on a small pcb, loads of vertically placed resistors ( routing was a fun job ), so if you’re new to soldering that might prove a challenge. In this case, it really makes putting things together a bit harder than is usually the case with my modules. There are components on both sides and you need to take that into account when assembling. Otherwise you’ll find yourself soldering trimmers with very little room to manoeuvre your soldering iron.
Also, you will need the extended toolkit. A multi-meter, some kind of scope and a way to measure frequency (most scopes can do that). That being said. There are no weird parts and calibration is straightforward.
Notes, remarks & corrections
None as of yet.