Erica Synths EDU DIY Wavefolder Manuel utilisateur

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

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

and two 9 V batteries with clips. These things are cheap & easy to ﬁnd in your local electronics

shop.

THE 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. That’s why I came up with this super simple BJT-based double wavefolder.!

THE 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. !

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!

51k"x1!

22k"x1!

20k x4!

10k"x4

1k x5!

10 Ω"x2 !

A bunch of capacitors. The speciﬁc values (which are

printed onto their bodies) are!

47 µF (electrolytic) x2!

1 µF (1J63/foil) x1!

100 nF (104/ceramic) x4!

2.2 nF (2n2J100/foil) x2

Some diodes. The speciﬁc model names (which are

printed onto their bodies) are!

SB140 (schottky)"x2!

Please note that these could also be a diﬀerent model (e.g. 1N5818).

A couple of transistors. The speciﬁc model names

(which are printed onto their bodies) are!

BC548 (NPN) x4!

BC558 (PNP) x2!

One regular potentiometer. The speciﬁc value (which

may be encoded & printed onto its body) is!

10k (B103) x1!

A trimmer potentiometer. The speciﬁc value (which is

encoded & printed on top) is!

10k (W103) x1!

A few jack sockets. The speciﬁc models (which you

can identify by their color) are!

Switched mono (black)"x3!

A chip. The speciﬁc model (which is printed onto its

body) is!

TL074 (quad op amp) x1!

You will also ﬁnd a few sockets that are only relevant when assembling the module in the

end.

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. But what if you’re just starting out, and you’d like to use batteries instead?

Thankfully, that’s totally doable. You just need to connect two 9 V batteries to your

breadboard like shown here. For this, you should use 9 V battery clips, which are

cheap & widely available in every electronics shop.!

By connecting the batteries like this, the row on the left side labeled + becomes your

positive rail, the row on the right side labeled +becomes your negative rail, and both rows

labeled –become your ground rails. !

Please make sure you disconnect the batteries from your breadboard when you

make changes to the circuit! Otherwise you run the risk of damaging components.!

Since all circuits in this manual were designed for a 12 V power supply, we assume that to be

the default. Everything will still work roughly the same with 9 V, though.

This is a bit awkward because breadboards weren’t really made with dual supply voltages in

mind.

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.!

As you can see, everything above the folding threshold is folded inward – so that the tip

of the original triangle now points down. In practice, this would create a bunch of

additional overtones that give the resulting output an almost metallic feel.!

Of course not all types of waveforms are equally suited as an input signal here. You won’t

gain anything by folding a square wave, for example – on the contrary, you’ll just lose a bit

of volume.!

Try it with a sawtooth wave, though, and you’ll notice that if you set the folding threshold

just right, the result is actually a perfect triangle wave. !

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.!

Now, if you’ve worked with NPN transistors before, you might say „hold on – how is this a

wavefolder? It’s just a bog-standard ampliﬁer!“ And you would be right. What’s worse: it

doesn’t really amplify anything, since it has a gain of -1. So all it does is invert the input

signal while keeping the volume the same. !

Then why do I call this a wavefolder? Because it is if you abuse it! To understand why and

how, let’s dissect this circuit step by step. Functionally, we can split it into two sections:

the resistor up top and the transistor and resistor below. Let’s start with the lower section.!

These two components in conjunction serve one simple

purpose. They’re meant to convert voltage into

current. More precisely: the voltage we apply to the

base into a current ﬂowing through transistor and

resistor. Okay, but what is the relation between those

two variables? !

Well, for the transistor on its own, the relation between

base voltage and collector current is exponential.

Conversely for the resistor, the relation between the

voltage across it and the current ﬂowing through it is

strictly linear. !

Read more about resistors and transistors in the components & concepts appendix (page

28/37).

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. So for a given base

voltage, the current ﬂowing through transistor and

resistor will be (almost) the same as the current

ﬂowing through just the resistor with the base

voltage applied across it. I say „almost“ because

activating the transistor eats up a small but ﬁxed

amount of current –#so it will always be a little bit lower.!

Now let’s look at the upper part of our ampliﬁer/

wavefolder – which is just a single resistor connected to

the positive rail. That resistor serves the exact

opposite purpose: convert current into voltage. It

works like this: the more current we pull through the

resistor, the more the voltage below it will drop.!

If we put this together with the lower half now, it should become clear why the voltage at

the collector almost mirrors the voltage at the base. Because for every volt added to the

base voltage, we push an additional 50 μA through the bottom resistor. !

Which, in turn, means that we have to pull that current through the top resistor. And

because that resistor is of the same value as the one at the bottom, the voltage

below it will drop by exactly 1 V. There are two caveats here, though. First, the absolute

collector voltage will be a small but ﬁxed amount higher than the base voltage. !

You can try this chapter’s circuit in a simulator. I’ve already set it up for you right here: https://

tinyurl.com/2hl93mas –#you can change all values by double clicking on components.

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.!

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