I found myself setting up a simple bass patch on various occasions. Since I hate doing the same task over and over again, I immediately start to ponder a way to automate things. An thus was born the Big Bass Machine, a module made to rattle your speaker cones to pulp.

The basic premise of the module is fairly simple. A sine wave oscillator through a VCA with a simplified envelop to shape the sound. If it doesn’t sound rather boring you’re not envisioning it well. But with some added special sauce, it does gets a bit more interesting. A pure sine is a rather dull thing on its own. In a perfect world it’s just a single frequency. Most analog oscillators produce a sine wave by approximation, which result in some additional overtones still being present. Building a real voltage controlled sine wave oscillator is not impossible, it’s just much harder.

In my humble opinion, the overtones generated by the triangle to sine converter bring some life to the sound, especially in the low frequencies. A solid sine based fundamental, sparsely sprinkled with harmonics of all kind makes for a very useful bass or sub-bass sound. You might have noticed the psychoacoustic effects ( big word right there ) of this kind of saturation when dabbling with low frequencies. If our brain is presented with the harmonics of a sound, it will dutifully fill in the missing fundamental to a certain extend. One can use this knowledge to make a bass more prominent without the need to push the levels up, just add a slight touch of saturation and presto. The effect can even give the idea of a sub-bass when the speakers/amp aren’t up to the task of fully reproducing those frequencies. It’s cunning trickery, almost magic even, but still science, bitches.

Drop the bass

So, let’s take that idea and run width it. Let’s add harmonics! What harmonics actually are I leave to wikipedia to explain cause I don’t feel like it and I assume a bit of knowledge on your part. Harmonics come in 2 forms, even and uneven. If you take the fundamental and add all uneven harmonics you will get a square wave in the end. We can abuse the sine wave convertor to overdrive it to a square wave. It’s not the same as actually adding up the different harmonic components one by one though, for from it even, but it’s a simple and manageable way to add harmonics to a signal. If you add the even harmonics you would get a triangle wave. So again, the tri-sine-square wave shaper makes up a nice saturator which we can use to toy with various harmonics.

The big bass machine

So, I decided to stop at 2. For the 4th harmonics I decided to invert the waveform because the peaks kept adding up and surpassed the available headroom, also when inverted it drew nicer pictures on the scope and that in itself is reason enough. You need to embrace the small things in life and pretty waveforms on a scope are just one of those.

So, all said and done, I ended up with 1 triangle oscillator with 1 octave up and 2 octaves up. Sum all that and push it through a sine wave shaper which can be overdriven to the point of total mayhem. It is starting to look like something useful by now. As an afterthought I added an EQ section, because more bass is better, right. Something simple that gives a bit of control over the low, high frequencies balance. Reading up on EQ circuits I came across a rarely used circuit called a tilt EQ. Never knew it existed, but life is full of surprises like that. It basically tilts the frequency spectrum around a fixed point. So when you add bass, you cut high end, and the other way around. The reason it is rarely used is simple, tilting the whole damn audio spectrum is hardly ever a good fit for the job at hand. However in this case I felt it would be a useful tool with a small footprint that could add some extra sonic possibilities to the module, so I went with it. One knob to rule them all and all that.

So far I’ve only written about the actual sound generation. I’ve added no less than 3 vca’s to the thing. One for the actual sound and one for the 2nd and 4th harmonics, so these volumes can be modulated over time. I took the easy way out by just outsourcing the modulation sources to other modules, things were getting complex enough as it is and y’all have a shitton of other fancy modules anyways. Add a CV input jack and attenuator and call it a day, that kind of thinking. The main envelope generator took a bit more doing. It’s still not a full ADSR though, but it’s getting close. It’s actually a modified decay circuit which make sustained notes possible. It’s done by letting the decay go from the initial level to the sustain level, then when the gate ends, it’s released until it hits ground level. Decay and release have the same speed, we just pause it a while at the sustain level before continuing. It’s not a fully featured envelope, but it’s about the envelope shape one needs for a bass sound.

The more nerdy stuff

The oscillator section isn’t all that interesting. It’s the same design as the ones in the kick drum module. A low component count triangle oscillator which can track 1/v oct. It won’t track well over a wide range though, but you will be able to squeeze a few octaves out of it with reasonable stability.

2 Full-wave rectifiers make the oscillator section complete. The first 2 op-amps make up the rectifier. The 3th opamp applies an offset and has a 2-times gain. The trimmers make it possible to finely define the offset ( you should use multiple turn trimmers here ). Since the 3th frequency multiplier is derived from the 2nd one, it’s important the signal has no DC offset, hence the trimmer. If you plan on building this, it’s kind of vital to have a scope to make sure these offsets are correctly set, weird things happen otherwise. Not that weird things happening are necessarily a bad thing, but correctly calibrating the offset is impossible without some form of visualisation.

The whole envelope thing is a bit more interesting. Q2 and Q14 act as a variable resistor according to the output of the velocity sample and hold. I added a little switch with an offset so the velocity input can be used with a signal ranging from 0 to 10V. If you use a Beatstep pro velocity output or sone midi to cv module, you’ll need this. Or between +5/-5V so it can be used with various modulation sources which use bi-polar signals, like LFO’s, random voltage generators or even noise. It features the same sample and hold circuit as in the metal-o-tronII and Can I kick it, that way you can use velocity inputs which vary over time without changing the note velocity mid-note. The decay implementation is the same as in the kick module. But instead of draining the timing cap (c3) to ground, we keep it at the sustain voltage as long as the gate signal is present. The moment the gate signal ends, the timing cap can continue to drain to ground. So described it all seems simple enough, but it took some time getting it working right. Transistors aren’t the ideal variable resistors so they took a bit of convincing to behave.

*small addendum: A decay circuit is a rather simple thing, but at the heart of any sort of envelope generator, understanding it can be quite handy to tweak things. We squeeze a capacitor between 2 opamps (c3) and add a diode to prevent the current to reverse. Now when the first opamp goes up (u1A) it will quickly charge the capacitor, so a small pulse is enough to do the job, then it goes back down again. The diode prevents the charge present in the capacitor to drain, so it will stay high. U1D simply works as a buffer and poses little to no load on the capacitor, so it stays high. When we now add a resistor to ground, we provide a path for the capacitor to drain. Depending on how much resistance there is, the capacitor will drain faster or slower. So add a potentiometer and you can vary the time it takes for the capacitor to drain. And there you have your decay circuit.

*small addendum to the small addendum: The size c3 can be depends on how fast it can and has to be charged. The opamp should be able to deliver the current for it. So a 100uF capacitor won’t be able to charge fast enough on the limited current the opamp can deliver. 10uF is about as high as you can go to charge the capacitor in an acceptable timeframe.

Putting it all together is straightforward and it makes for a nice module. A bit of a one trick pony though, but that’s not necessarily a bad thing. I added a few extra tweaks to make the module a bit more versatile in a full modular environment. Like a separate output of the envelop generator, a possibility to bypass the VCA so it can be used as an oscillator module, various CV controls over decay/release, sustain level and an external audio in.


Big schematic on a small picture. click for pdf version.

Buy one?

I’m not planning, as of writing, to release this module commercially, but I also didn’t wanted to simply throw away the schematics, hence the post. I do use the one in my rack regularly I like it, but it’s a big module with a substantial component count ( nothing fancy though, but still ). It would get fairly expensive to sell kits and I didn’t felt it would be worth the price I would have to ask to keep the module commercially valuable. As a fully factory finished product it could be doable, but as a through hole kit the numbers just turn out the wrong way. Maybe I do sell the PCB/Panel set only if there are enough people interested. Who knows.

Build one?

Big Ass Disclaimer: The following files are offered without any form of support. Which means, I won’t help you with ordering PCB’s, buying and selecting components, troubleshooting builds or whatever other questions you have about this particular project. Therefor, you should only try to build it if you are confident enough in your capabilities at debugging this kind of thing by yourself. If you find any flaws in the design, or have a great idea for useful modifications, it would be appreciated if you let me know, and I’ll update the page accordingly. All files you need to build this thing can be downloaded via the links below (schematics, Gerber files and BOM).

You can do with the above information whatever you god damn please. I would not appreciate if you take all my work and release this module as a commercial product though, but I probably can’t do much about that so I’m not going to loose sleep over it. In case you would wonder, if you build one and sell the excess PCB’s (already build or otherwise) that’s all fine by me.

May you enjoy the soldering fumes.