Erica Synths EDU DIY Wavefolder Manual
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Erica Synths EDU DIY Wavefolder Manual

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INTRO |
Hey there, thanks for buying this DIY kit! We - Erica Synths and Moritz Klein - have
developed it with one specific goal in mind: teaching people with little to no prior
experience how to design analog synthesizer circuits from scratch. So what you'll find in
the box is not simply meant to be soldered together and then disappear in your rack.
Instead, we want to take you through the circuit design process step by step, explaining
every choice we've made and how it impacts the finished module. For that, we strongly
suggest you follow along on a breadboard , which is a non-permanent circuit prototyping
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tool that allows you to experiment and play around with your components. To help you
with this, we've included suggested breadboard layouts in select chapters.
In addition to this, you can also play around with most of the chapter's circuits in a circuit
simulator called CircuitJS. CircuitJS runs in your browser. You'll find weblinks in the
footnotes which will direct you to an instance that already has example circuits set up for
you. We strongly encourage you to fiddle with the component values and general
structure of those circuits to get a better understanding of the concepts we're laying out.
Generally, this manual is intended to be read and worked through front to back, but there
were a few things we felt should go into a dedicated appendix. These are general
vignettes on electronic components & concepts, tools, and the process of putting the
module together once you're done experimenting. Don't hesitate to check in there
whenever you think you're missing an important piece of information. Most importantly
though: have fun!

TABLE OF CONTENTS

CIRCUIT SCHEMATIC | WAVEFOLDER ............................................................. 2
CIRCUIT SCHEMATIC | SAW-TO-TRI CONVERTER ............................................ 3
BILL OF MATERIALS ..................................................................................... 4
POWERING YOUR BREADBOARD ................................................................... 6
CIRCUIT DESIGN CLOSE-UP .......................................................................... 7
COMPONENTS & CONCEPTS APPENDIX ....................................................... 28
TOOLS APPENDIX ....................................................................................... 41
MODULE ASSEMBLY APPENDIX ................................................................... 44
SOLDERING APPENDIX ................................................................................ 53
Note that there is no breadboard included in this kit! You will also need a pack of jumper wires
1
and two 9 V batteries with clips. These things are cheap & easy to find in your local electronics
shop.
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  • Page 1: Table Of Contents

    INTRO | Hey there, thanks for buying this DIY kit! We – Erica Synths and Moritz Klein – have developed it with one specific goal in mind: teaching people with little to no prior experience how to design analog synthesizer circuits from scratch. So what you’ll find in the box is not simply meant to be soldered together and then disappear in your rack.

  • Page 2: Circuit Schematic | Wavefolder

    WAVEFOLDER Even though wavefolding is more of a west coast synthesis concept, it’s a great addition to the otherwise very east coast mki x es.edu DIY system. Why? Because it can provide a ton of unusual timbres for your patches: from a slight dash of brightness to harsh metallic drones.

  • Page 3: Circuit Schematic | Saw-To-Tri Converter

    SAW-TO-TRI Of course we’ll need a suitable signal to fold – which, traditionally, has been either a sine or a triangle wave. And since the mki x es.edu VCO provides neither of those, we needed to do some extra work here. So I designed this super efficient little sawtooth-to-triangle converter that uses the same basic working principle as the main wavefolding circuit.

  • Page 4: Bill Of Materials

    BILL OF MATERIALS Before we start, please check if your kit contains all of the necessary components. In addition to a PCB, panel and power cable, your box should also contain: An array of resistors. The specific values (in ohms, which you should check for with a multimeter) are 220k x1 100k x3...
  • Page 5 A couple of transistors. The specific model names (which are printed onto their bodies) are BC548 (NPN) BC558 (PNP) One regular potentiometer. The specific value (which may be encoded & printed onto its body) is 10k (B103) A trimmer potentiometer. The specific value (which is encoded &...

  • Page 6: Powering Your Breadboard

    POWERING YOUR BREADBOARD Before we can start building, you’ll need to find a way of providing your breadboard with power. Ideally, you’d use a dual 12 V power supply for this. Dual power supplies are great – and if you want to get serious about synth design, you should invest in one at some point.

  • Page 7: Circuit Design Close-Up

    WAVEFOLDING BASICS Before we can start designing our circuit, we’ll first need to make sure we understand what wavefolding actually is. Thankfully, that term is pretty hard to misinterpret. A wavefolder quite literally takes a waveform and folds it in on itself. Here’s what that would look like with a triangle wave as the input signal.
  • Page 8 THE SIMPLEST WAVEFOLDER Okay, but how do we make this happen? The circuit will probably be quite complex, right? Yeah .. not really. All we need are three components: two same-value resistors and an NPN transistor. If we set them up like this and send an oscillation into the transistor’s base, we can pick up a folded version at the collector.
  • Page 9 So when the components are paired up, we can see that their behaviors are clashing: while the transistor wants to ramp up its current throughput exponentially, the resistor only allows for a linear increase. This means that the resistor is the bottleneck in our system: it throttles the transistor.
  • Page 10 This is because, as we said earlier, the transistor eats up a bit of current while active. But second, and more importantly, this mechanism only works as long as the transistor can pull enough current into its collector.
  • Page 11 TRANSISTOR SATURATION Let’s look at our circuit’s behavior using a quick graph. We’ll imagine we steadily increase the base voltage (V ) from -12 V all the way up to +12 V. In the region between -12 V and around 450 mV, everything works as expected: the collector voltage (V ) drops in sync with the base voltage.
  • Page 12 This means that we have two distinct regions in our graph above: one where the collector voltage is roughly the inverse of the base voltage – and one where it’s roughly the same. So if we send an oscillation into the transistor’s base, every part of the wave that crosses the 450 mV threshold will be folded over in the output.
  • Page 13 SAW-TO-TRIANGLE BASICS If you’ve used a sawtooth wave as your input signal, you probably noticed that at maximum volume, we almost fold it into a triangle! Since wavefolding generally sounds really good when applied to triangle waves (and because the mki x es.edu VCO does not have a triangle output), it makes sense to take a short detour here and talk about turning our circuit into a proper sawtooth-to-triangle converter.
  • Page 14 Interestingly, this doesn’t result in a voltage relation of 1:(-)2 across the board. Instead, we get a 1:-2 relation in the non-saturated region, while the saturated region sticks with the 1:1 relation. But that’s not all. As you can see in the graph, the folding threshold also drops noticeably, now sitting at about -3.5 V.
  • Page 15 After you’ve set this up, send a sawtooth wave into the jack socket, while monitoring the collector voltage on your oscilloscope. Next, play with the trimmer until the gap between the rising and falling slopes disappears.
  • Page 16 PROCESSED & BUFFERED TRIANGLE Got a good-looking triangle out of this? Great! Next, try in- and decreasing the input signal’s frequency. You’ll probably notice that your triangle suddenly doesn’t look so good anymore. That’s because of slight shifts in the sawtooth wave’s vertical offset across the full frequency range.
  • Page 17 And here’s what the whole thing put together looks like. Note that the buffer transistor will add a bit of a negative offset to the input sawtooth – so once you’ve set this up, you’ll have to fiddle with the trimmer again to get the best results. For the op-amp, we’ll be using a TL074 IC, which is four op-amps in a single chip.
  • Page 18 PNP WAVEFOLDER Now that we’ve got a decent triangle wave, let’s get back to our wavefolder. The problem with our design so far is that it only affects the bottom of the output waveform. In a traditional wavefolder, you’d want to fold both the bottom and the top, though. To get there, we’ll first check out an alternate version of our circuit that folds just the top of the output waveform.
  • Page 19 In the space between +12 V and -450 mV, the collector voltage rises as the base voltage drops. And once they meet, they both start dropping together. So if we use our triangle wave as an input here, we should get an output with a folded top. To be able to vary the volume of the input, we’ll also set up a simple, temporary attenuator using the 10k potentiometer from your kit.
  • Page 20 TOP/BOTTOM WAVEFOLDER Now the question is: how do we combine the NPN- and PNP folding circuits into one? Lucky for us, it’s actually pretty straightforward. All we have to do is wire them together like shown here: base connects to base and collector connects to collector. If we then send in our triangle wave on the left, we can pick up a top- and bottom-folded output on the right.
  • Page 21 If you again check on the voltage at the two transistors’ collectors with your oscilloscope, you should see both the top and bottom of the input triangle get folded over. Perfect! So how do we listen to this now? Unfortunately, we can’t just attach an output socket to the transistors’...
  • Page 22 BUFFERED WAVEFOLDER To get there, let’s first clean up the existing schematic while keeping the circuit itself the same. We’ll move the two transistors directly on top of each other. Then, we’ll pull both collector resistors over to the right. This leaves us with these two vertical paths on and between which current flows.
  • Page 23 This approach has two benefits. First: we undo the inversion that the wavefolding stage applies to the input signal. So the output will be in phase with the input again. And second, the new 10k resistor gets to work double duty: it’s providing the extra current for our two transistors –...
  • Page 24 INPUT STAGE Great! But since using our existing attenuator feels pretty clunky and imprecise, we should come up with a slightly more elaborate solution. For that, it makes sense to restrict the input volume’s range somewhat. If the intensity knob is dialed all the way down, you’d expect the input signal to just pass through the circuit unaffected instead of going completely silent.
  • Page 25 DOUBLE WAVEFOLDER Cool – but a single wavefolding stage sounds a bit too tame for my taste. So let’s fold the folded signal again! For that, we’ll simply add another wavefolding stage after the first one’s output buffer. Then, we’ll have to adjust the input attenuator’s range. That’s because we’ve previously made sure that the input signal doesn’t cross the folding thresholds a second time.
  • Page 26 2.5x WAVEFOLDER Now, a double wavefolder is nice, but I’d still like to add a bit more zing. For that, we just have to increase the gain of the first folding stage’s output buffer. Because if it’s more than 1, the signal will get folded one additional time for free. Why is that? Because coming into the second stage, the signal will now wiggle across the thresholds one additional time.
  • Page 27 Once you’ve set this up, check if the signal does pass through unchanged at minimum intensity. Then, see what happens as you dial it up. You should get additional spikes in the output that make it sound even more harsh and metallic. And with this, our wavefolder is done.

  • Page 28: Components & Concepts Appendix

    COMPONENTS & CONCEPTS APPENDIX In this section, we’ll take a closer look at the components and elemental circuit design concepts we’re using to build our module. Check these whenever the main manual moves a bit too fast for you! THE BASICS: RESISTANCE, VOLTAGE, CURRENT There are three main properties we’re interested in when talking about electronic circuits: resistance, voltage and current.
  • Page 29 USING TWO 9 V BATTERIES AS A DUAL POWER SUPPLY Dual power supplies are great – and if you want to get serious about synth design, you should invest in one at some point. But what if you’re just starting out, and you’d like to use batteries instead? Thankfully that’s totally doable.
  • Page 30 RESISTORS While a conductive wire is like a very big pipe where lots of water can pass through, a resistor is like a narrow pipe that restricts the amount of water that can flow. The narrowness of that pipe is equivalent to the resistance value, measured in ohms (Ω). The higher that value, the tighter the pipe.
  • Page 31 CAPACITORS A capacitor is a bit like a balloon that you can attach to the open end of a pipe. If there’s some pressure in the pipe, the balloon will fill up with water until the pressure equalizes. (Since the balloon needs some space to expand into, both of the capacitor’s legs need to be connected to points in your circuit.) Then, should the pressure in the pipe drop, the balloon releases the water it stored into the pipe.
  • Page 32 DIODES Diodes are basically like one-way valves. Current can only pass through in one direction – from anode to cathode. That direction is indicated by the arrow in the diode symbol and by a black stripe on the diode’s casing. So any current trying to move in the opposite direction is blocked from flowing.
  • Page 33 VOLTAGE DIVIDERS A voltage divider is really just two resistors set up like this: input on the left, output on the right. If R1 and R2 are of the same value, the output voltage will be half of what the input voltage is. How does it work? Let’s use our analogy again: so we have a pipe on the left, where water is being pushed to the right with a specific amount of force.
  • Page 34 POTENTIOMETERS Potentiometers can be used as variable resistors that you control by turning a knob. But, and that’s the handy part, they can also be set up as variable voltage dividers. To see how that works, let’s imagine we open one up. Inside, we would find two things: a round track of resistive material with connectors on both ends plus what’s called a wiper.
  • Page 35 AC COUPLING What is AC coupling – and how does it work? Imagine two adjacent pipes with a balloon between them. Now, no water can get from one pipe into the other, since it’s blocked by the balloon. But, and that’s the kicker, water from one side can still push into the other by bending and stretching the balloon, causing a flow by displacement.
  • Page 36 OP AMPS Op amps might seem intimidating at first, but they’re actually quite easy to understand and use. The basic concept is this: every op amp has two inputs and one output. Think of those inputs like voltage sensors. You can attach them to any point in your circuit and they will detect the voltage there without interfering.
  • Page 37 OP AMP BUFFERS/AMPLIFIERS Buffering, in the world of electronics, means that we provide a perfect copy of a voltage without interfering with that voltage in the process. With an op amp-based buffer, the buffering process itself works like this. We use the non-inverting input to probe a voltage, while the inverting input connects straight to the op amp’s output.
  • Page 38 amplifier because the output signal is in phase with the input. For an inverting buffer/amplifier, the input signal is no longer applied to the non-inverting input. Instead, that input is tied directly to ground. So it’ll just sit at 0 V the entire time.
  • Page 39 BIPOLAR JUNCTION TRANSISTORS Bipolar junction transistors (or BJTs for short) come in two flavors: NPN and PNP. This refers to how the device is built internally and how it’ll behave in a circuit. Apart from that, they look pretty much identical: a small black half-cylinder with three legs. Let’s take a look at the more commonly used NPN variant first.
  • Page 40 collector current. (At least not without some unwanted side effects.) Third, also unlike a resistor, a BJT is not a linear device. Meaning that a change in collector voltage will not affect the collector current. And fourth, the collector current is affected by the transistor’s temperature! The more it heats up, the more current will flow.

  • Page 41: Tools Appendix

    TOOLS APPENDIX There are two types of tools that will help you tremendously while designing a circuit: multimeters and oscilloscopes. In this appendix, we’ll take a quick look at each of these and explore how to use them. MULTIMETERS Multimeters come in different shapes and sizes, but the most common type is probably the hand-held, battery powered variant.
  • Page 42 OSCILLOSCOPES While multimeters are fairly cheap and compact, oscilloscopes are usually somewhat pricey and bulky. If you’re willing to make the investment, they are a huge help with the troubleshooting process, though. Using one is, again, surprisingly straightforward – if you manage to work your way through the sometimes quite convoluted UI, especially on digital models.
  • Page 43 BUILD GUIDE...

  • Page 44: Module Assembly Appendix

    MODULE ASSEMBLY APPENDIX Before we start building, let’s take a look at the complete mki x es.edu Wavefolder schematics (see page 3) that were used for the final module’s design and PCB fabrication. Most components on the production schematics have denominations (a name – like R1, C1, VT1, VD1, etc.) and values next to them.
  • Page 45 Capacitors C5 and C6 are additional decoupling capacitors. If you inspect the PCB, you’ll see that these are placed as close to the power supply pins of the ICs as possible. For well-designed, larger PCBs you will find decoupling capacitors next to each IC. Like the others, their job is to simply compensate for any unwanted noise in the supply rails.
  • Page 47 Before you start soldering, we highly recommend printing out the following part placement diagrams with designators and values. Because some of our PCBs are rather densely populated, this will help you to avoid mistakes in the build process.
  • Page 48 Place the Mixer PCB in a PCB holder for soldering or simply on top of some spacers (I use two empty solder wire coils here). I usually start populating PCBs with lower, horizontally placed components. In this case, these are power protection diodes. Bend the diode leads and insert them in the rele- vant places...
  • Page 49 In order to save space on the PCB, some of our projects, including the Wavefolder, have vertically placed resistors. The next step is to place & solder those. All components on the PCB have both their value and denomination printed onto the silkscreen.
  • Page 50 Next up: insert & solder the electrolytic capacitors. Electrolytic capacitors are bipolar, and you need to mind their orientation. The positive lead of each electrolytic capacitor is longer, and there is a minus stripe on the side of the capacitor’s body to indicate the negative lead.
  • Page 51 Now, turn the PCB around and inspect Make sure your solder joints. components are soldered properly and there are no cold solder joints or accidental shorts. Clean the PCB to remove extra flux, if necessary. Insert the jack sockets and solder them. Then insert the potentiometer, but don’t solder it yet! Fit the front panel and make sure that the potentiometer shaft is...
  • Page 52 Congratulations! You have completed the assembly of the mki x es.edu Wavefolder module! Connect it to your eurorack power supply and switch it on. If there’s no "magic smoke”, it’s a good sign that your build was successful. Patch the sawtooth signal from the DIY.EDU VCO to the SAW-TRI input of the module and connect the OUT of the module to a mixer.

  • Page 53: Soldering Appendix

    SOLDERING APPENDIX If you’ve never soldered before – or if your skills have become rusty – it’s probably wise to check out some THT (through-hole technology) soldering tutorials on YouTube. The main thing you have to remember while soldering is that melted solder will flow towards higher temperature areas.
  • Page 54 DIY electronics is a great (and quite addictive) hobby, therefore we highly recommend you invest in good tools. In order to really enjoy soldering, you’ll need: A decent soldering station. Top-of-the-line soldering stations (brands like Weller) will cost 200€ and above, but cheaper alternatives around 50€...
  • Page 55 A solder suction pump. No matter how refined your soldering skills are, you will make mistakes. So when you’ll inevitably need to de-solder components, you will also need to remove any remaining solder from the solder pads in order to insert new components.

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