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 breadboard1, 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 ………………………………………………………………………. 2
BILL OF MATERIALS ………………………………………………………………………… 3
POWERING YOUR BREADBOARD …………………………………………………….…. 5
CIRCUIT DESIGN CLOSE-UP ……………………………………………………………… 6
COMPONENTS & CONCEPTS APPENDIX ………………………………………………. 24
TOOLS APPENDIX …………………………………………………………………………… 37
MODULE ASSEMBLY APPENDIX …………………………………………………………. 40
SOLDERING APPENDIX …………………………………………………………………….. 52

1Note that there’s 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.

1
THE ENVELOPE
Envelope generators are one of the most important sources of control voltage in any
modular synthesizer. Though they might not generate audio signals themselves, many
iconic sounds like the squelchy acid bass line, snappy techno kick drum and swelling
dream pop pad wouldn’t be possible if it weren’t for envelope generators. So I knew I
needed one for my DIY modular – which is why I created this wily little fake-ADSR.
Why fake? Because though it does send out a four-staged signal (attack/decay/sustain/
release), the duration of decay and release are tied together, controllable only via the
same knob. To make up for this limitation, I decided to include both a looping function
and an inverted output as extra goodies.

2
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

100k x10
47k x1
10k x1
1k x2
470 x1
100 x1
10 x2

A few capacitors. The specific values (which are

printed onto their bodies) are

47 µF (electrolytic) x2
1 µf (1J/film) x2
100 nF (104/ceramic) x8

Some diodes. The specific model names (which are

printed onto their bodies) are

SB140 (schottky) x2
1N4148 (signal) x6

A transistor. The specific model name (which is printed

onto its body) is

BC548 (NPN) x1

3
 A few regular potentiometers. The specific values
(which may be encoded & printed onto their bodies) are

1M (A105) x2
100k (A104) x1

A switch. The specific model (which you can identify by

the number of connectors on its underside) is

Single pole, double throw x1

An LED (light emitting diode). The specific model

(which you can identify by measuring the body’s width)
is

3mm (red) x1

A bunch of jack sockets. The specific models (which

you can identify by their color) are

Switched mono (black) x3

A couple of chips. The specific models (which are

printed onto their bodies) are

TL072 (dual op amp) x3

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

4
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 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.2 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.

2This is a bit awkward because breadboards weren’t really made with dual supply voltages in
mind.

5
ENVELOPE BASICS
If you’re new to modular synths (or even synths in general), you might ask: what’s an
envelope generator – and what do we use it for? Simple. We’ve only got two hands, best
case. So there’s a hard limit to how many of our synth’s knobs we can turn and
parameters we can tweak at the same time. Also, those hands are generally sluggish,
sloppy and imprecise.
Wouldn’t it be much better if we had some virtual, programmable robot hand to help us
out? That would do what we tell it to do, reliably and precisely, whenever we want it to? If
your answer is yes, then you might be a control freak. Also, you’d probably want an
envelope generator. So let’s build one!
But before we can do that, we have to understand what an envelope does, exactly. How
does it tweak parameters on another module? Well, when designing oscillators, filters
and amplifiers, you’ll notice that people go to great lengths in order to make them
voltage controllable. This comes in handy for us. Because if we want a filter module to
open up, for example, we can simply send a high level voltage into its CV (control voltage)
input. Now of course, if we only want to open the filter (and keep it open), we could send
in a fixed 12 V signal and be done with it. That’s not all that useful, though. Normally,
you’d want the filter to close at some point, too. So why not simply use a square wave
oscillator here?

6
THE SIMPLEST ENVELOPE
Since a square wave is really just an oscillation between
a high- and a low level voltage, this would indeed cause
our filter to open up and close down. Even better – it
would do so rhythmically as the oscillator cycles
through its phases. And while this does work, it has two
severe limitations. First: the opening and closing
movements are always instant, with no way of making
them any more gradual. This might be what you want in
some contexts, but often, you’d need something less
abrupt. And second: We can’t change the rhythmic
pattern – all we get is a constant staccato.
So what can we do about that? Well, the most obvious thing is to try and make the rising
and falling edges less steep. In envelope terms, we’d say that we want to slow down
the attack and extend the release. And while this may sound complicated, it’s actually
anything but.

All we need are two components: a resistor and a capacitor, set up like this.3 Now, if
you’ve worked with analog filters before, this setup should look strikingly familiar. That’s
because this is really just a bog-standard passive low pass filter. And what it does to
our square wave is exactly what we said we’re after: it takes the rising and falling edges
and makes them less steep.
Here’s how it works. Once the voltage at the input switches from low to high, a current
will be forced through the resistor and into the capacitor, slowly filling it up. As the
capacitor is being charged, the voltage at the output slowly rises until the cap is
completely filled up, and the input- and output voltages align. Then, when the input
voltage swings low, the whole process reverses. Now the capacitor will push its contents
through the resistor and into the input, so to speak. This happens because the voltage on
the right is much higher than the voltage on the left. As the capacitor empties out slowly,

3 Read more about resistors and capacitors in the components & concepts appendix (page
26/27).

7
the two voltages align again. As you can see, this will turn our square wave input into
something like a very basic attack-release envelope.4
But before we can try this, we’ll have to think about appropriate values for the capacitor
and resistor. Since we are dealing with a very, very slow square wave oscillation at the
input, they’ll need to be pretty big. Otherwise, the effect will be so minimal that we won’t
be able to tell the difference.
Also, we’ll probably want to adjust the effect’s intensity. For that, we’ve essentially got
two options here: we could change either the capacitor- or resistor value. But since the
former can only be done by hand, and switching components is not the most user-
friendly strategy, we’re going to replace the fixed resistor with a potentiometer, set
up as a variable resistor.5 This way, we can adjust the resistance (and thereby the
steepness of our envelope’s rising and falling edges) on the fly. A 1M pot should give us a
decent enough range here.

To properly test this circuit, you’ll need a square wave LFO6 and a module with a CV input
like a VCF. Send in the LFO via the right-hand socket, while connecting the other one to
your filter. By turning the potentiometer’s knob, you should be able to dial in a more or
less intense effect. Great! The only problem with this is that the attack- and release
phases are not adjustable independently. Changing one will always also change the
other.

4You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right
here: https://tinyurl.com/y7ea7j3a – you can change all values by double clicking on components.
5 Read more about potentiometers in the components & concepts appendix (page 30).
6 A clock module or sequencer with a gate output will also work.

8
PASSIVE A/R ENVELOPE
So how can we separate the two? It’s actually really easy. All we need are two diodes and
another 1M potentiometer.

Here’s how this works. Diodes are basically one-way streets for electricity.7 So by
putting two of them in parallel, facing in opposite directions, we are taking a two-
way street and splitting it into two one-way streets. Where before, our capacitor was
charged and discharged through the same resistor, now each phase gets their own.
So when the input signal swings high, a current will flow through the top diode – and only
that diode – through the top potentiometer and into the capacitor. During the low phase,
the current will take the other path. This means that one potentiometer now controls the
attack – and the other controls the release.8

If you connect your LFO and VCF like before, you should be able to sculpt the filter
movement much more freely by adjusting attack and release independently. So what
we’ve built here is an ultra-simple passive attack-release envelope generator. Why
passive? Because it does not include any form of amplification. It’s getting all of its

7 Read more about diodes in the components & concepts appendix (page 28).
8You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right
here: https://tinyurl.com/y7xguhoo – you can change all values by double clicking on
components.

9
power from the square wave LFO. Is this a problem? That very much depends on what
your goals & context are. In this current form, the envelope can only properly function if
two external conditions are met: the circuit triggering it (in our case, the LFO) needs to be
able to provide enough current. And the circuit we’re controlling (in our case, the filter)
needs to draw as little current as possible from our envelope.
Why? Let’s imagine we put a big resistor between our LFO and the envelope. This way,
we are severely limiting the amount of current flowing into our circuit. And that means that
even if we dial our attack and release pots all the way down, charging and discharging
our capacitor will not be instant – as we’d expect –, but instead would take a while.
On the other side, imagine our filter was also using the envelope to drive an LED. LEDs, if
you don’t know, are basically just diodes that light up when a current flows through them.
Since they’re pretty power-hungry, our filter’s LED would eat up most of the current
coming through the attack pot, preventing our capacitor from ever really being charged
up. And that would severely restrict our envelope’s range. Now granted, this is a worst
case scenario. Well-designed modules should always have high-current outputs and
low- to no-current inputs. Which, ironically, is a standard our envelope here does not
live up to at all. It eats up our oscillator’s signal, while providing barely anything for the
filter.

10
ACTIVE A/R ENVELOPE
Thankfully, fixing that is really straightforward. We’ll simply set up op-amp buffers both
at the in- and output.9 Buffers measure a voltage and provide an identical copy at their
output, while being able to supply a decent amount of current.10 Because of that, it no
longer really matters how much current the input can provide — and how much the next
circuit eats up.

Cool! But why the 1K resistor at the output, then? And what’s all this additional stuff at
the input? Well unfortunately, there are other worst case scenarios we need to consider.
The first one of which being a classic user error. Imagine that user plugs our envelope’s
output into some other module’s output by accident. If that other module also uses a
buffer there, we’d basically create a short circuit, since buffers can not only source (i.e.
send out), but also sink (i.e. absorb) plenty of current. So by placing a 1k resistor before
the output socket, we make sure that in this scenario, the maximum amount of
current flowing is limited. Saving our op amp from a potential early grave.
Okay, easy — so now, let’s tackle the additional op amp on the left. What kind of problem
does it fix? Well, while we did make sure that our circuit gets enough current, we haven’t
thought about the voltage we’re feeding it yet. We should, though, because that voltage
will determine the voltage range across which our envelope is operating.
Think of it this way: if all our envelope does is take the input signal and make the
rising and falling edges less steep, then the maximum „height“ of the resulting curve
is completely determined by that input signal. Why’s that a problem? Because if the
input signal would, for example, just swing between 0 and 1 V, that curve would be really
small. And a smaller curve means a reduced range of effect. When controlling our filter,
for example, this small curve would barely be able to move the cutoff point. So basically,
our envelope would behave very differently depending on what kind of circuit we use to

9You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right
here: https://tinyurl.com/ycz4qc3b – you can change all values by double clicking on
components.
10 Read more about op amps and buffers in the components & concepts appendix (page 33).

11
drive it. And for me, that would get very annoying very fast. But since it’s very easy to
eliminate this kind of external dependency, we’ll get rid of it.
To do that, we use another op amp, which we set up in the comparator configuration. A
comparator, if you don’t know, basically just looks at an input voltage, compares
that input voltage to a reference voltage, and then tells us which one is higher. How
does it tells us? By either pushing its output voltage up to the positive or pulling it down
to the negative supply rail. So in our case, that would be either + or –12 V.11 Here’s how
this particular setup works in detail.
I’ve set up a voltage divider to get our reference voltage.
A 100k/47k combination gives us approximately 3.8 V to
work with. So whenever our input voltage is higher than
that, the comparator’s output will jump to +12 V. And if
it’s lower, it drops down to –12 V. Why did I choose that
exact threshold? To be honest, mostly just because I
had packs of 100k and 47k resistors lying on my table
when I was testing this. But I still feel that 3.8 V is a
decent value here. It’s low enough so that any
sequencer should be able to trigger the comparator, but
definitely high enough to prevent it from firing randomly
because of electromagnetic interference.
Okay, so now our envelope will always get the same 12 V to work with – as long as our
input signal passes the 3.8 V threshold. But what about the comparator’s low state
output? Once the input drops below the threshold, it will swing down to –12 V. This is not
ideal, because traditionally, the base line for an envelope’s output is supposed to be
0 V. Which is why we’ll put a diode, followed by a 100k resistor to ground, between our
comparator’s output and the buffer’s input.

Here’s what that does. Whenever the comparator is pushing out 12 V, the diode conducts
and we also get about 12 V at the buffer’s input. But once the voltage turns negative, the
diode will block. Normally, the buffer’s input would now be undefined (or „floating“). But
since we have a 100k resistor to ground there, that input gets pulled down to 0 V instead.

11I keep talking about +/– 12 V as the positive/negative rail voltages because I use a +/– 12 V
power supply myself. You can substitute this with +/– 9 V all throughout the manual if you’re using
batteries.

12
And with this, we’ve now forced the envelope’s operating voltage range to always be
between 0 and 12 V. Great! To see if that actually works, let’s try it on the breadboard.
Make sure that the TL072-chips (which house two standard op amps each) are set
up exactly as shown here – if you reverse the power connections, they will heat up
and die!

If you now go ahead and test this, everything should be working pretty much like before –
though the range of filter movement will be significantly increased.

13
AR VS. ADSR ENVELOPES

And while we could leave it there, I’d rather put in a bit more effort in order to give us finer
control over the envelope curve’s shape. On the left side here, I’ve drawn up what our
current circuit is capable of producing: a simple attack-release-curve. On the other side,
we have a more complex attack-decay-sustain-release curve. What’s the difference? Well,
while both curves have an attack and a release phase, the one on the right adds a
decay phase and the ability to set a specific sustain level. The idea here is this. If the
sustain is set to a lower value than the envelope’s peak, we get a drop after the attack.
This is the decay phase. Once that’s through, the curve settles on the set sustain level
while the input signal stays high. From here, we enter the release phase once the input
swings low.
What’s the benefit of this added complexity? Simple: we can produce a wider variety of
sounds. Personally, I really like short, plucky, percussive hits – and also gliding acid bass
lines. Both of them are not really doable with a simple attack-release envelope.
Now, turning the circuit we have into a proper ADSR envelope is somewhat out of scope
for this project. Still, with very little extra effort, we can can build something that
approximates it.

14
THE PSEUDO-ADSR ENVELOPE

Here’s how that would work. As you can see, I’ve basically just copied our comparator
and pasted it down at the bottom. Both the new and the original one get the input signal,
and they share the same reference voltage, but I’ve placed a high-pass filter before the
top one. That high-pass would normally turn a square wave cycle into two short voltage
spikes.
First a positive one, when the input transitions from low
to high – and then a negative one when it drops from
high to low. Now, since we’re not interested in the
negative spike, and it could cause our comparator to
glitch out under certain circumstances, I’ve decided to
eliminate it using another diode. Whenever the voltage
at the comparator’s input (i.e. after the 1 µF capacitor)
tries to go negative, the diode will open and thereby
neutralize it.12
Okay, but what do we need the positive spike for?
Simple. By feeding this positive spike into our
comparator, we get a quick 12 V burst right when
the envelope is triggered. So with a fast attack, the
envelope’s curve will always start at the peak level.

12You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right
here: https://tinyurl.com/yc2c8u5h – you can change all values by double clicking on
components.

15
Why’s that important? To answer this, we’ll first have to talk about the other comparator.
Since it doesn’t have a high-pass at its input, it will simply behave like the comparator in
our previous iteration. Whenever the input voltage is above 3.8 V, we’ll get a constant
12 V at its output. The diode afterwards serves the same purpose as the one up top – it
blocks the comparator’s low state.
After this, I’ve set up another potentiometer as a variable voltage divider. This allows us to
take the 12 V during the comparator’s high phase and freely scale them to any value
between those 12 and 0 V. Whatever voltage we dial in here will be our sustain level.
Why? Because the 100k resistor at the input buffer doesn’t connect straight to
ground like before, but rather to our sustain level voltage. If that sounds confusing,
let’s break it down step by step.
Our input starts out low. This means that both our
comparators’ outputs sit at –12 V. But because of the
two diodes, this doesn’t propagate, and so our buffer’s
input gets pulled down to 0 V through the 100k resistor
and potentiometer. Giving us 0 V at the envelope’s
output as well. Next, let’s assume that the input goes
high. This will do two things: we’ll get a voltage spike
after our high pass, which gets converted into a short
12 V burst by the top comparator. Simultaneously, the
other comparator pushes out a constant 12 V that get
scaled down by our sustain potentiometer.
Let’s assume we’ve set it to about 50 %. This means that at the buffer’s input, we’ve got
our 12 V burst coming from the top, and a constant 6 V coming from the bottom
comparator. Since there’s no resistor in the top path, but a 100k in the bottom one, the
burst will „win“ and push the overall voltage at the buffer’s input up to about 12 V.
Our buffer – being a buffer – will copy those 12 V and push them through the attack
potentiometer. Again assuming that we’ve dialed in a fast attack, there will be a pretty low
resistance in its path, allowing the capacitor to be charged up to about 12 V before the
burst is over. So at this point, our envelope’s output sits somewhere around 12 V – its
peak value. But because the burst is a burst, it’ll quickly die down and the top
comparator’s output will drop to –12 V. So suddenly, with the top diode blocking, the
only voltage applied to the buffer’s input is our sustain level: 6 V. This means that our
buffer’s output will drop from around 12 to those 6 V, allowing our capacitor to partially
discharge through the release path. Because remember – the burst charged it up to
around 12 V.
Once the voltage at the capacitor has dropped to the sustain level, it will stabilize, giving
us a constant 6 V at the envelope’s output. Until the input signal swings low, our buffer’s
output drops to 0 V, and the capacitor is allowed to complete its discharging process. The
result is an output curve with 4 distinct phases: attack, decay, sustain and release. Now
as you might have noticed, there are two rather big caveats here. First, both in the
decay- and the release phase, the capacitor discharges through the same

16
potentiometer. This means that you can’t control those two phases individually –
changing one will also change the other. So we can never have a curve with a long decay
and super short release.
Second, the decay phase is directly dependent on the set attack. Why’s that? Simple.
If we dial in a slow attack, the capacitor will not be charged up to the peak level during
the short initial burst. Maybe it won’t even reach the set sustain level, let alone surpass it.

In effect, we basically skip the decay phase, as we are never dropping down to the
sustain level. Now granted, a proper ADSR envelope shouldn’t have these two problems.
But for how simple our circuit is, and how few components it uses, I think we can book
this as a worthwhile tradeoff. So let’s set this up on the breadboard!

Try playing around with the new sustain knob, and test how it interacts with the other two.
You should be able to dial in a pretty wide range of filter movements now. Great! But it
might be nice if we had some visual indication of the voltage our envelope is sending out,
right?

17
THE STATUS LED
Thankfully, there’s a quick and easy way to achieve this: by implementing a simple status
LED, which we’ll drive using our envelope’s output. This way, the LED will tell us what
voltage level the envelope is currently pushing out out by shining more or less bright. So
far, so simple. But there are two slight problems with this. First, LEDs are (as I said before)
pretty power hungry. And though our op amp buffer is capable of providing enough
current to light one up, this would pretty much completely occupy it. Second, LEDs are
super quick to burn out if we push too much current through them. So we need to make
sure we provide enough, but not too much current for our status LED. Doing this is
much easier than it might sound, though. We only need three components in total: our
LED, an NPN transistor and a small resistor.13
Here’s how this works. Whenever our buffer sends out a
non-zero voltage, a tiny current will flow into the
transistor’s base. This will cause that transistor to open
up, resulting in a much bigger current flowing from the
positive rail through the current limiting resistor and then
our LED, lighting it up.14
The nifty thing about this setup is that the higher the
voltage coming from our buffer, the more the
transistor will open up – and the brighter the LED
will get. All while the 470 Ω resistor prevents the current
from increasing too much. Note that it’s not really the op
amp that’s providing the power to light up the LED here
– instead, we’re getting it pretty much straight from the
power supply.

13 Read more about transistors in the components & concepts appendix (page 35).
14You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right
here: https://tinyurl.com/y7gr9hnj – you can change all values by double clicking on components.

18
When adding this to your breadboard, make sure you double (or even triple) check all
connections – it’s just too easy to accidentally burn an LED.

19
THE INVERTED OUTPUT
At the beginning of this manual, I promised two extra goodies to make up for our
envelope’s shortcomings. One of which was an inverted output. But before we can add
one, we’ll have to clarify what an inverted output is, exactly. If we take the name purely at
face value, we might imagine the signal coming from our envelope’s inverted output to
look like this.

So where our regular output would start off at a 0 V baseline and go up from there, the
inverted output would drop below that baseline instead. And though this is totally doable
(and is sometimes done this way in other modules), I think it’d be much more useful if the
output looked like this instead.

It’s the same core principle, but the baseline is now our high level voltage. Meaning that
the curve is no longer dipping into the negatives, but rather „peaking“ at ground level.
This way, it’ll do the exact opposite of what we’ve seen before when controlling a VCF
with it. The filter will be wide open by default, and then close down as the envelope is
cycling through its phases. Cool! But how do we actually implement this?

20
 Turns out it’s doable quite easily with another op amp,
which we’ll set up as an inverting buffer.15 But instead of
connecting the non-inverting input to ground – as you
would normally do –, we give it a constant 6 V from a
simple 50 % voltage divider.
To understand how this works, I find it very helpful
to imagine that the voltage we apply to the non-
inverting input is the mid-axis against which we are
inverting (or mirroring) the signal. If that mid-axis is
0 V, then an input of 12 V is going to give us an output
of –12 V. But if the mid-axis is 6 V, then the same input
will give us an output of 0 V instead. And conversely, a 0
V input results in a 12 V output.16
Since our envelope is operating squarely in the range
between 0 and 12 V, the output curve will simply be
flipped on its head while staying within that range.
Which is exactly what we said we’re after. So let’s add
this to our breadboard and see how we fare.

Try driving your VCF with the newly added inverted output. You should be able to verify
that it now starts off open and then closes down as the envelope cycles through its
phases.

15Read more about inverting buffers/amplifiers in the components & concepts appendix (page
34).
16You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right
here: https://tinyurl.com/y9rrt8pp – you can change all values by double clicking on components.

21
LOOPING THE ENVELOPE
The second extra goodie I promised was a loop-mode for our envelope. Here’s how we’ll
implement that. You can turn pretty much any regular envelope into a looping envelope by
using this little circuit.

You just patch it in between output (on the right) and input (on the left) – and the envelope
will re-trigger continuously. To make that happen, we’re configuring yet another op amp
as a Schmitt trigger inverter.17 You can think of that like a watchdog which constantly
monitors the envelope’s output voltage. Whenever that output drops to 0 V, our
watchdog will re-trigger the envelope and then sit back and wait until it’s time to
strike again.18
This works because a Schmitt trigger inverter has two
thresholds against which it compares the voltage we
apply to its input. The three resistors up top (or rather
the relation between them) set those thresholds. In our
case, the bottom threshold is set at 0 V, while the top
threshold is placed at around 8 V. So when the input
drops to 0 V, the op amp’s output will jump to 12 V.
Then once the input rises above the 8 V line, the output
will drop down to –12 V.
Now, you might’ve noticed the diode and 100k pulldown-resistor after our op amp’s
output. Why do we need these? Don’t we already block any negative voltages coming
from our two input comparators? Yes, we do – but for the loop mode, we’ll have to
circumvent them. This is because we need to make sure that our Schmitt trigger inverter
doesn’t get stuck. Because as we know, it will only change states if the envelope’s output
rises above 8 V. If we use our envelope’s regular input, the maximum output voltage
depends on the set attack and sustain levels. Dialing in a slow attack and low sustain
level might cause the voltage curve to peak at less than 8 V – which will break the loop by
not triggering the inverter. The solution to this problem is connecting the inverter’s output

17 Read more about Schmitt trigger inverters in the components & concepts appendix (page 28).
18You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right
here: https://tinyurl.com/y7wh2o6u – you can change all values by double clicking on
components.

22
directly to the input buffer – ideally through a simple switch that allows us to toggle
between the regular mode and our new loop-mode.

By circumventing the input comparators, the relation between our Schmitt trigger
inverter’s state and the envelope’s output voltage becomes a lot simpler. Because
now, there’s no envelope setting that can prevent its output from peaking at a value below
8 V. The inverter will simply keep pushing out a high level voltage until we cross that
threshold.
Great! But before we try this on the breadboard, there’s another small thing I snuck in
here: the 100 Ω resistor after our input buffer. This is really more of an added bit of polish.
It ensures that when our attack- or release-pots are dialed down all the way,
(dis-)charging the capacitor isn’t instant. If we leave this out, we might get an ugly
clicking-noise from our filter, for example – simply because the change in cutoff frequency
is so abrupt.

And with that, our envelope is done. If you now want to make your creation permanent,
dig out the panel and PCB from the kit, heat up your soldering iron and get to building!
You can find more information on how to populate the board & how to solder in the
enclosed appendix.

23
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. To make these less abstract, we can use a
common beginner’s metaphor and compare the flow of electrons to the flow of water
through a pipe.

In that metaphor, resistance would be the width of a pipe. The wider it is, the more water
can travel through it at once, and the easier it is to push a set amount from one end to the
other. Current would then describe the flow, while voltage would describe the pressure
pushing the water through the pipe. You can probably see how all three properties are
interlinked: more voltage increases the current, while more resistance to that voltage
in turn decreases the current.

24
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. You just need to connect two 9 V
batteries 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 positive terminal of the left battery becomes your
+9 V, while the negative terminal of the right is now your –9 V, and the other two combine
to become your new ground.19 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.

19If you're struggling with setting this up, you can watch me do it here: https://youtu.be/
XpMZoR3fgd0?t=742

25
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.
Resistors have two distinctive properties: linearity
and symmetry. Linearity, in this context, means that for
a doubling in voltage, the current flowing will double as
well. Symmetry means that the direction of flow doesn’t
matter – resistors work the same either way.
On a real-life resistor, you’ll notice that its value is not
printed on the outside – like it is with other components.
Instead, it is indicated by colored stripes20 – along with
the resistor’s tolerance rating. In addition to that, the
resistor itself is also colored. Sometimes, depending on
who made the resistor, this will be an additional
tolerance indicator.
For the resistors in this kit, a yellow body tells you that
the actual resistance value might be ±5 % off. A dark
blue body indicates ±1 % tolerance. Some kits will also
contain light blue ± 0.1% resistors to avoid the need for
manual resistor matching.
While in the long run, learning all these color codes will
be quite helpful, you can also simply use a multimeter to
determine a resistor’s value.

20For a detailed breakdown, look up resistor color coding. There are also calculation tools
available.

26
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. The maximum
size of the balloon is determined by the capacitor’s
capacitance, which we measure in farad (F). There are
quite a few different types of capacitors: electrolytic,
foil, ceramic, tantalum etc. They all have their unique
properties and ideal usage scenarios – but the most
important distinction is if they are polarized or not.
You shouldn’t use polarized capacitors against their polarization (applying a negative
voltage to their positive terminal and vice versa) – so they’re out for most audio-related
uses like AC coupling, high- & low-pass filters etc.
Unlike resistors, capacitors have their capacitance value printed onto their casing,
sometimes together with a maximum operating voltage. Be extra careful here! That
voltage rating is important. Your capacitors can actually explode if you exceed it! So they
should be able to withstand the maximum voltage used in your circuit. If they're rated
higher – even better, since it will increase their lifespan. No worries though: the capacitors
in this kit are carefully chosen to work properly in this circuit.
Ceramic capacitors usually come in disk- or pillow-like
cases, are non-polarized and typically encode their
capacitance value.21 Annoyingly, they rarely indicate
their voltage rating – so you’ll have to note it down
when buying them.
Film capacitors come in rectangular, boxy cases, are
non-polarized and sometimes, but not always, directly
indicate their capacitance value and their voltage rating
without any form of encoding.22
Electrolytic capacitors can be identified by their cylinder
shape and silver top, and they usually directly indicate
their capacitance value and their voltage rating. They
are polarized – so make sure you put them into your
circuit in the correct orientation.

21For a detailed breakdown, look up ceramic capacitor value code. There are also calculation
tools available.
22 If yours do encode their values, same idea applies here – look up film capacitor value code.

27
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.
There are a few quirks here, though. For one, the diode
will only open up if the pushing force is strong enough.
Generally, people say that’s 0.7 V, but in reality, it’s
usually a bit lower. Also, diodes don’t open up abruptly
– they start conducting even at much lower voltages,
although just slightly.
There are a lot of different diode types: Zener, Schottky,
rectifier, small signal etc. They all have their unique
properties and ideal usage scenarios – but usually, a
generic 1N4148 small signal diode will get the job done.

SCHMITT TRIGGER INVERTERS

You can think of a Schmitt trigger inverter as two
separate things. On the left, there’s a sensor that
measures the pressure inside an attached pipe. On
the right, there is a water pump. This pump’s
operation is controlled by the sensor. Whenever the
pressure probed by this sensor is below a certain
threshold, the pump will be working. If the pressure is
above a second threshold, the pump won’t be working.
Here’s a quick graph to visualize that. The squiggly line
represents the voltage at the input, while the dotted line
shows the voltage at the output. So every time we cross
the upper threshold on our way up, and the lower one
on our way down, the output changes its state. One
thing that’s very important to keep in mind: no current
flows into the sensor! It’s really just sensing the voltage
without affecting it.

28
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. Attached to it is a narrow pipe,
representing R1, followed by another wide pipe. Then at
the bottom, there’s another narrow pipe, representing
R2, where water can exit the pipe system. Finally,
imagine we’ve set up a sensor measuring the voltage in
the right hand pipe.
First, think about what would happen if R2 was
completely sealed off. Our sensor would tell us that the
pressure on the right side is exactly the same as the
pressure on the left. Because the pushing force has
nowhere else to go.
On the other hand, imagine R2 would just be a wide
opening. Then the pressure on the right would be 0,
because it’d all escape through that opening. But what
happens if R2 is neither completely closed off nor wide
open? Then the pressure would be retained to varying
degrees, depending on the narrowness of the two
resistor paths.
If pipe R1 is wide and pipe R2 is narrow, most of the
pressure will be retained. But if it’s the reverse, the
pressure level will be only a tiny fraction. And if R1 and
R2 are identical, the pressure will be exactly half of
what we send in.

29
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. This wiper makes contact with the
track and also has a connector. It can be moved to any
position on the track. Now, the resistance value
between the two track connectors is always going to
stay exactly the same. That’s why it's used to identify a
potentiometer: as a 10k, 20k, 100k etc. But if you look
at the resistance between either of those connectors
and the wiper connector, you’ll find that this is
completely dependent on the wiper’s position.
The logic here is really simple: the closer the wiper is
to a track connector, the lower the resistance is
going to be between the two. So if the wiper is dead
in the middle, you’ll have 50 % of the total resistance
between each track connector and the wiper.
From here, you can move it in either direction and thereby shift the ratio between the two
resistances to be whatever you want it to be. By now, you might be able to see how that
relates to our voltage divider. If we send our input signal to connector 1 while grounding
connector 3, we can pick up our output signal from the wiper. Then by turning the
potentiometer’s knob, we can adjust the voltage level from 0 to the input voltage – and
anything in between.
In these kits, you will encounter different types of
potentiometers. First, there’s the regular, full-size variant
with a long shaft on top. These are used to implement
user-facing controls on the module’s panel and they
usually – but not always – indicate their value directly on
their casing. Sometimes, they’ll use a similar encoding
strategy as capacitors, though.23
Second, we’ve got the trimmer potentiometer, which is
usually much smaller and doesn’t sport a shaft on top.
Instead, these have a small screw head which is
supposed to be used for one-time set-and-forget
calibrations. Trimmers usually encode their value.

23 Look up potentiometer value code for a detailed breakdown.

30
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.

Next, we’ll bring in a resistor after the coupling point, going straight to ground. This acts
like a kind of equalizing valve. Now imagine we apply a steady 5 V from one side. Then
on the other side, we’ll read 0 V after a short amount of time. Why? Because we’re
pushing water into the balloon with a constant force, causing it to stretch into the other
side, displacing some water. If we didn’t have the equalizing valve there, we’d simply raise
the pressure. But since we do have it, the excess water can drain out of the system. Until
the pressure is neutralized, and no water is actively flowing anymore.
Okay, so now imagine that the voltage on the left hand side starts oscillating, let’s say
between 4 V and 6 V. When we start to go below 5 V, the balloon will begin contracting,
basically pulling the water to the left. This will create a negative voltage level in the right
hand pipe – like as if you’re sucking on a straw, making the voltage there drop below 0 V.
Then, once the pressure on the other side rises above 5 V, the balloon will inflate and
stretch out again, pushing water to the right. And the pressure in the right hand pipe will
go positive, making the voltage rise above 0 V. We’ve re-centered our oscillation
around the 0 V line. Okay, but what about the resistor? If current can escape through it,
doesn’t that mess with our oscillation? Well, technically yes, but practically, we’re
choosing a narrow enough pipe to make the effect on quick pressure changes negligible!

31
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. No current flows into the op amps
inputs – that’s why we say their input impedance is very high. Near infinite, actually.
Okay, but why are there two of them?
The key here is that op amps are essentially differential
amplifiers. This means that they only amplify the
difference between their two inputs – not each of them
individually. If that sounds confusing, let’s check out a
quick example. So we’ll imagine that one sensor –
called the non-inverting input – is reading 8 V from
somewhere. The other sensor – called the inverting
input – reads 5 V. Then, as a first step, the op amp will
subtract the inverting input’s value from the non-
inverting input’s value. Leaving us with a result of 3.
(Because 8 minus 5 is 3.) This result then gets
multiplied by a very large number – called the op
amp’s gain. Finally, the op amp will try to push out a
voltage that corresponds to that multiplication’s result.
But of course, the op amp is limited here by the voltages that we supply it with. If we give
it –12 V as a minimum and +12 V as a maximum, the highest it can go will be +12 V. So in
our example, even though the result of that multiplication would be huge, the op amp will
simply push out 12 V here and call it a day.
The handy thing though about op amp outputs is that they draw their power directly from
the power source. This means that they can supply lots of current while keeping the
voltage stable. That’s why we say an op amp has a very low output impedance.

32
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. This creates what we
call a negative feedback loop. Think of it this way. We apply a specific voltage level to
the non-inverting input – let’s say 5 V.
Before the op amp starts processing the voltages at its
inputs, the output will be switched off. This means that
output and inverting input sit at 0 V at first. So then,
the op amp will subtract 0 from 5 and multiply the result
by its gain. Finally, it will try and increase its output
voltage to match the calculation’s outcome.
But as it’s pushing up that output voltage, the voltage
at the inverting input will be raised simultaneously.
So the difference between the two inputs is shrinking
down. Initially, this doesn’t matter much because the
gain is so large. As the voltage at the inverting input
gets closer to 5 V though, the difference will shrink so
much that in relation, the gain suddenly isn’t so large
anymore.
Then, the output will stabilize at a voltage level that is
a tiny bit below 5 V, so that the difference between the
two inputs multiplied by the huge gain gives us exactly
that voltage slightly below 5 V. And this process simply
loops forever, keeping everything stable through
negative feedback. Now if the voltage at the non-
inverting input changes, that feedback loop would
ensure that the output voltage is always following. So
that’s why this configuration works as a buffer: the
output is simply following the input.
How about amplifying a signal though? To do that, we’ll
have to turn our buffer into a proper non-inverting
amplifier. We can do that by replacing the straight
connection between inverting input and output with a
voltage divider, forcing the op amp to work harder.
Here’s how that works. Say we feed our non-inverting
input a voltage of 5 V. Now, the output needs to push
out 10 V in order to get the voltage at the inverting
input up to 5 V. We call this setup a non-inverting

33
 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. The real action, then, is happening at the
inverting input. Here, we first send in our waveform
through a resistor. Then, the inverting input is connected
to the op amp’s output through another resistor of the
same value.
How does this work? Well, let’s assume that we’re applying a steady voltage of 5 V on the
left. Then, as we already know, the op amp will subtract the inverting input’s voltage from
the non-inverting input’s voltage, leaving us with a result of –5 V. Multiply that by the huge
internal gain, and the op amp will try to massively decrease the voltage at its output.
But as it’s doing that, an increasingly larger current will flow through both resistors and
into the output. Now, as long as the pushing voltage on the left is stronger than the pulling
voltage on the right, some potential (e.g. a non-zero voltage) will remain at the inverting
input. Once the output reaches about –5 V though, we'll enter a state of balance. Since
both resistors are of the same value, the pushing force on the left is fighting the exact
same resistance as the pulling force on the right. So all of the current being pushed
through one resistor is instantly being pulled through the other.
And that means that the voltage at the inverting input will be lowered to about 0 V,
allowing our op-amp to settle on the current output voltage level. So while we read 5 V on
the left, we’ll now read a stable –5 V at the op amp’s output. Congrats – we’ve built an
inverting buffer! If we want to turn it into a proper amplifier, we’ll simply have to
change the relation between the two resistances. By doing this, we can either increase
(if you increase the right-hand resistor’s value) or reduce (if you increase the left-hand
resistor’s value) the gain to our heart’s content.

34
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. Here’s how we distinguish between its three
legs. There’s a collector, a base and an emitter.24 All
three serve a specific purpose, and the basic idea is
that you control the current flow between collector and
emitter by applying a small voltage25 to the base. The
relation is simple: more base voltage equals more
collector current. Drop it down to 0 V and the
transistor will be completely closed off. Sounds simple –
but there are four important quirks to this.

First, the relation between base voltage and collector current is exponential. Second,
unlike a resistor, a BJT is not symmetrical – so we can’t really reverse the direction of the

24Please note that the pinout shown here only applies for the BC series of transistors. Others, like
the 2N series, allocate their pins differently.
25The voltage is measured between base and emitter. So „a small voltage“ effectively means a
small voltage difference between base and emitter!

35
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.
Now, for the PNP transistor, all of the above applies, too – except for two little details.
Unlike with the NPN, the PNP transistor decreases its collector current when the
voltage at its base increases26. So you have to bring the base voltage below the emitter
to open the transistor up. Also, that collector current flows out of, not into the collector!

26 Again, the voltage is measured between base and emitter.

36
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. It can measure a bunch of different
things: voltage, current, resistance, continuity. Some
have additional capabilities, allowing you to check
capacitance, oscillation frequency or the forward
voltage drop of a diode.
When shopping for one, you’ll probably notice that there
are really expensive models boasting about being TRUE
RMS multimeters. For our purposes, this is really kind of
irrelevant, so don’t feel bad about going for a cheap
model!
Using a multimeter is actually really straightforward. Simply attach two probes to your
device – the one with a black cable traditionally plugs into the middle, while the red one
goes into the right connector. Next, find whatever you want to measure and select the
corresponding mode setting.
In some cases, it doesn’t matter which probe you
connect to which component leg or point in your circuit.
This is true for testing resistors, non-polarized
capacitors (foil/film, ceramic, teflon, glass etc.),
continuity27 or AC voltage.
In others, you’ll have to be careful about which probe
you connect where. For testing the forward voltage drop
of a diode, for example, the multimeter tries to push a
current from the red to the black probe. Here, you’ll
have to make sure the diode is oriented correctly, so
that it doesn’t block that current from flowing. For
testing a DC voltage, you want to make sure the black
probe is connected to ground, while you use the red
one to actually take your measurement.

27 Just a fancy word for saying that two points are electrically connected.

37
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.
To start using your scope, simply attach a probe to one
of the channel inputs. These probes usually have two
connectors on the other end: a big one that you operate
by pulling the top part back – and a smaller one, which
is usually a standard alligator clip. The latter needs to be
connected to your circuit’s ground rail, while you probe
your oscillation with the former. Now what the
oscilloscope will do is monitor the voltage between
the two connectors over time and draw it onto the
screen as a graph. Here, the x-axis is showing time,
while the y-axis is showing voltage. You can use the
device’s scaling controls to zoom in on a specific part of
your waveform.
Usually, digital oscilloscopes will also tell you a couple useful things about the signal
you’re currently viewing: minimum/maximum voltage level, oscillation frequency, signal
offset. Some even offer a spectrum analyzer, which can be useful to check the
frequencies contained in your signal.

38
BUILD GUIDE

39
MODULE ASSEMBLY APPENDIX
Before we start building, let’s take a look at the complete mki x es.edu Envelope sche-
matics (see next page) 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. Denominations help identify each component
on the PCB, which is particularly useful during calibration, modification or troubleshooting.

XS1 is the Gate input jack socket, XS2 is the envelope signal output jack socket and XS3
is the inverted envelope signal output jack socket – these are the very same we’ve
already been using on the breadboard for interfacing with other devices. In our designs,
we use eurorack standard 3,5mm jack sockets (part number WQP-PJ301M-12).

XP1 is a standard eurorack power connector. It’s a 2x5 male pin header with a key (the
black plastic shroud around the pins) to prevent accidental reverse polarity power supply
connection. This is necessary because connecting the power incorrectly will permanently
damage the module.

VD2 and VD3 are schottky diodes that double-secure the reverse polarity power supply
protection. Diodes pass current only in one direction. Because the anode of VD2 is con-
nected to +12 V on our power header, it’ll only conduct if the connector is plugged in cor-
rectly. If a negative voltage is accidentally applied to the anode of VD2, it closes, and no
current passes through. The same goes for VD3, which is connected to -12 V. Because
schottky diodes have a low forward voltage drop, they are the most efficient choice for
applications like this.

Next, we have two 10 Ohm resistors (R5 and R6) on the + and – 12 V rails, with decou-
pling (or bypass-) capacitors C2 – C5. These capacitors serve as energy reservoirs that
keep the module’s internal supply voltages stable in case there are any fluctuations in the
power supply of the entire modular system. In combination with R5 and R6, the large 47
microfarad pair (C2 and C3) compensates for low frequency fluctuations, while C4 and C5
filter out radio frequencies, high frequency spikes from switching power supplies and
quick spikes created by other modules. Often another component – a ferrite bead – is
used instead of a 10 Ohm resistor and there’s no clear consensus among electronic
designers which works best, but generally for analogue modules that work mostly in the
audio frequency range (as opposed to digital ones that use microcontrollers running at 8
MHz frequencies and above), resistors are considered to be superior.

Another advantage of 10 Ohm resistors is that they will act like slow “fuses” in case
there’s an accidental short circuit somewhere on the PCB, or an integrated circuit (IC) is
inserted backwards into a DIP socket. The resistor will get hot, begin smoking and finally
break the connection. Even though they aren't really fuses, just having them there as fuse
substitutes is pretty useful - you’d rather lose a cent on a destroyed resistor than a few
euros on destroyed ICs.

40
Capacitors C6 – C13 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. If the
input voltage drops, then these capacitors will be able to bridge the gap to keep the volt-
age at the IC stable. And vice-versa - if the voltage increases, then they’ll be able to
absorb the excess energy trying to flow through to the IC, which again keeps the voltage
stable. Typically, 0.1 uF capacitors are used for this purpose.

41
42
Before you start soldering, we highly recommend printing out the following part place-
ment 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.

43
Place the Envelope 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 most of the resistors, switch-
ing diodes and the power protection
diodes. Bend the resistor leads and insert
them in the relevant places according to the
part placement diagram above. All compo-
nents on the PCB have both their value and
denomination printed onto the silkscreen. If
you are not sure about a resistor’s value, use
a multimeter to double-check. Next, insert the
diodes. Remember – when inserting the
diodes, orientation is critical! A thick
white stripe on the PCB indicates the cathode
of a diode – match it with the stripe on the
component. Flip the PCB over and solder all
components. Then, use pliers to cut off the
excess leads.

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 Next, insert the first DIP socket, hold it in
place and solder one of the pins. Continue
with the next DIP socket. Make sure the DIP
sockets are oriented correctly – the notch
on the socket should match the notch on the
PCB’s silkscreen. Now, turn the PCB around
and solder all remaining pins of the DIP sock-
ets. Then proceed with the ceramic capaci-
tors. Place the PCB in your PCB holder or on
spacers, insert the capacitors and solder
them like you did with the resistors & diodes
before. Now your PCB should look like this:

In order to save space on the PCB, some of our projects, including the dual VCA, have
vertically placed resistors. The next step is to place & solder those. Bend a resistor’s legs so
that its body is aligned with both legs and insert it in its designated spot. Then solder the longer
lead from the top side of the PCB to secure it in place, turn the PCB around and solder the
other lead from the bottom. You can insert several resistors at once. Once done with soldering,
use pliers to cut off excess leads.

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Once you are done with soldering all resis-
tors, your PCB should look like this:

Next, insert & solder the electrolytic

capacitors. Electrolytic capacitors are bipo-
lar, and you need to mind their orientation.
The positive lead of each electrolytic capaci-
tor is longer, and there is a minus stripe on
the side of the capacitor’s body to indicate
the negative lead. On our PCBs, the positive
pad for the capacitor has a square shape,
and the negative lead should go into the pad
next to the notch on the silkscreen.

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Next up: inserting & soldering the transis-
tor. Make sure you align the transistor with
the marked outline on the silkscreen – orien-
tation is critically important here. Also,
insert film capacitors and solder them.

Then complete the component side of the

Envelope PCB by soldering the PSU socket.
Make sure the orientation of the socket is as
shown in the picture below – the arrow point-
ing to the first pin is aligned with a notch on
the silkscreen. The key on the socket will be
facing inwards towards the PCB. Now your
PCB should look like this:

Now, turn the PCB around and inspect your

solder joints. Make sure all components
are soldered properly and there are no
cold solder joints or accidental shorts.
Clean the PCB to remove extra flux, if neces-
sary.

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Insert the jack sockets and solder them.

Insert the potentiometers, but don’t

solder them yet! Fit the front panel and
make sure that the potentiometer shafts are
aligned with the holes in the panel – and that
they’re able to rotate freely. Now, go ahead
and solder the potentiometers.

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The single/loop switch requires special
attention. There are two nuts for the switch
(they look identical to the jack socket nuts,
but the thread is different). Screw on one of
the nuts until it fixes itself on the bottom of the
thread.

Now, insert the switch in the relevant place

on the PCB, place the front panel, fix it with
few nuts on the jack sockets and solder the
switch.

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Insert the LED in the relevant place on the
PCB, but do not solder it, yet! Orientation of
the LED is important – check the silkscreen! A
notch on the silkscreen indicates the cath-
ode of the LED (a shorter lead next to a
notch on the LED) and the longer lead – the
anode of the LED – has to go into a hole
with square-like polygon on the PCB. Fit the
front panel again and fix it with 3 nuts on
jack sockets and one nut on the switch. Now,
solder the LED. We are almost done!

Now, insert the ICs into their respec-

tive DIP sockets. Mind the orientation of
the ICs – match the notch on each IC with the
one on its socket.

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Congratulations! You have completed the assembly of the mki x es.edu Envelope
module! It does not need any calibration and, if assembly is correct, it should work
straight away. 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. Flip the switch to the
LOOP mode and check if the LED is blinking. If it is, adjust Attack and Release settings
to see if turning the potentiometers clockwise decreases the LED blinking frequency.
If you consider making slowly evolving drone music, you can install an optional C11 (not
included in the kit), which will double the Attack and Release time. Enjoy!

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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. So you need to make sure you apply equal heat to
the component you are soldering and the solder pad on the PCB. The pad will typically
absorb more heat (especially ground-connected pads which have more thermal mass),
so keep your soldering iron closer to the pad on the PCB. It’s critically important to dial
in the right temperature on your soldering station. I found that about 320 °C is the opti-
mal temperature for most of parts, while for larger elements like potentiometers and
sockets, you may want to increase that temperature to 370 °C.

Here’s the recommended soldering sequence:

1 2 3 4

Heat part and Add Continue Let cool

pad 2 - 3 sec solder heating 1 -2 sec.

After you have completed soldering, inspect the solder joint:

Perfect Too much Not enough Cold Too much Short

solder solder joint heat

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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€ are often good enough. Make
sure your soldering station of choice comes
with multiple differently-sized soldering iron
tips. The most useful ones for DIY electronics
are flat, 2mm wide tips.

When heated up, the tips of soldering irons

tend to oxidize. As a result, solder won’t
stick to them, so you’ll need to clean your
tip frequently. Most soldering stations
come with a damp sponge for cleaning the
iron tips – but there are also professional
solder tip cleaners with golden curls (not
really gold, so not as expensive as it
sounds). These work much better because
they do not cool down the iron.

Solder wire with flux. I find 0,7mm solder

wire works best for DIY projects.

Some soldering flux paste or pen will be

useful as well.

Cutting pliers. Use them to cut off excess

component leads after soldering.

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 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.

Once you have finished soldering your

PCB, it’s recommended to remove
excess flux from the solder joints. A PCB
cleaner is the best way to go.

All of these tools can be found on major electronic components retailer websites, like
Mouser, Farnell and at your local electronics shops. As you work your way towards
more and more advanced projects, you’ll need to expand your skillset and your tool belt
– but the gratification will be much greater.

“Twenty years from now you will be more disappointed by the things that you didn't do
than by the ones you did do. Explore. Dream. Discover.”

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