Erica Synths EDU DIY Kick Drum User manual

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 ……………………..…………………………………………………."2!

BILL OF MATERIALS ……….……………………………………………………….…..…… "3!

POWERING YOUR BREADBOARD …………….…………………………………………… "6!

CIRCUIT DESIGN CLOSE-UP …………………………………………………………..…… "7

COMPONENTS & CONCEPTS APPENDIX ………………………………………………. "33!

TOOLS APPENDIX …………………………………………………………………………… "46!

MODULE ASSEMBLY APPENDIX …………………………………………………………. "48!

SOLDERING APPENDIX …………………………………………………………………….. "59!

Note that there is no breadboard included in this kit! You will also need a pack of jumper wires

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

shop.

1

THE KICK DRUM

I love old-school analog drum machines. But if you look at the other DIY kits in this series,

you’ll notice that I haven’t tackled any kind of percussion before. This is mainly because

percussion circuits are quite complex and dense. They mash a ton of diﬀerent functional

blocks – oscillators, envelopes, VCAs, ﬁlters etc. – into super eﬃcient little packages.%!

And they achieve that by cutting corners left and right, in sometimes surprising and

unintuitive ways. Which makes them even less approachable. So I decided to cut my

teeth on simpler single-purpose circuits ﬁrst. Now that I’ve covered all of the essentials

though, I felt it’s time to give percussion a proper go. So this circuit is my take on a

classic analog kick drum.!

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!

!

1M"x2

470k"x1!

120k"x1

100k x5!

47k"x2!

39k x1!

33k"x2!

14k"x1!

10k"x2

2k x1!

1k"x3!

10Ω"x2!

!

A bunch of capacitors. The speciﬁc values (which are

printed onto their bodies) are!

!

47µF (electrolytic) x2!

220nF (foil) x1!

100nF (ceramic) x6!

15n (foil)"""x3!

10nF (foil) x1!

3n3 (foil) x1!

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Some diodes. The speciﬁc model names (which are

printed onto their bodies) are!

1N4148 (signal)"x7!

1N5819 (schottky)"x2!

!

A couple of transistors. The speciﬁc model names

(which are printed onto their bodies) are!

BC558 (PNP) x2!

BC548/547 (NPN)"x3!

!

A handful of potentiometers. Their speciﬁc values

(which may be encoded & printed onto their bodies) are!

1M (B105) x1!

250k (B254) x1!

100k (B104) x2!

100k (A104) x1!

50k (B503)"x1!

10k (B103) x1!

!

A few jack sockets. The speciﬁc models (which you

can identify by their color) are!

Switched mono (black)"x4!

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A couple chips. Their speciﬁc models (which are

printed onto their bodies) are!

TL072 (dual op amp) x2!

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

end. !

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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

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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. !

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

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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

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mind.

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KICK DRUM BASICS

To make our lives a little easier, let’s start by broadly thinking about the functional blocks

we’ll need for a synthesized kick drum. First up: some sort of oscillator. This is a no-

brainer, since we have to have a sound source to work with.!

A typical oscillator will give us a static waveform like a sawtooth, square, triangle or sine

that just keeps going indeﬁnitely. Since a kick drum is percussive in nature though, we will

have to somehow shape that waveform’s volume curve into a quick burst.!

Of course we don’t want this to happen just once – but every time we trigger our circuit

with a gate signal coming from a sequencer or an LFO. Also, we’ll probably want to

emulate the initial punch that you’d expect from a real drum. Traditionally, you’d do that

by manipulating the pitch alongside the volume, starting oﬀ quite high, and then very

quickly dropping towards a steady base frequency.!

Alright, this should be everything we need. So let’s think about how we could implement

these blocks in our circuit. In an ideal scenario, we’d want to use a full blown VCO as our

oscillator, so we can precisely manipulate its pitch via a control voltage coming from an

envelope generator. !

Next, we’d need to add a proper VCA and yet another envelope generator, which, in

combination, would allow us to shape the signal’s volume curve into the aforementioned

quick burst. Now, the problem with this approach is that each of these circuits is plenty

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complex on its own. Throw four of them together, and you’ve got quite the behemoth of a

schematic.!

This would not only make our kick a pain to build – but also relatively expensive. So we

should probably think of an alternate approach. Thankfully, the people working at Roland

in the seventies were faced with the same issue, and they came up with a couple really

clever shortcuts and hacks that we can borrow.!

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BRIDGED-T OSCILLATOR

Basically, they found a way to smash a pseudo-VCO, VCA and envelope generator into a

single block made from just a handful of components. The basis for that block is this little

circuit consisting of just one op amp, two resistors and two capacitors. Together, they

form a strange sine wave oscillator that needs to be kickstarted by a voltage pulse

to actually oscillate. And even then, it won’t continue oscillating (like other oscillators

do), but quickly drop in volume and eventually die out completely. Which isn’t great for

most scenarios – but ideal for ours! !

So let’s look at how this works. We’ll assume that the voltage at the op amp’s non-

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inverting input quickly jumps from 0 to 1 V. Now, to reach a state of balance, the op amp

will try to push the voltage at its inverting input up to 1 V as well. For that, it’ll increase its

output voltage to 1 V. Initially, this will work just ﬁne, since that voltage pushes straight

through the two capacitors and reaches the inverting input that way.!

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

4

change all values by double clicking on components.

9

But because there is a resistor to ground between the two caps, we’ll see current drain

out from the ﬁrst capacitor. Which means that the voltage applied to the second cap (and

subsequently, the inverting input) will drop. To compensate, the op amp will raise its

output voltage. But as it does that, even more current is squeezed out of the ﬁrst

cap, forcing the op amp to push even harder.!

And this would continue until the op amp runs into the upper supply voltage – if it weren’t

for the bridge resistor between output and inverting input. Because as the op amp

pushes harder and harder, that resistor allows a small current to charge up the

second cap from the other side.!

At some point, this process will add more voltage on the left as we lose on the right,

causing the whole mechanism to kick into reverse gear. Now, the op amp will start

dropping its output voltage to try and course correct. Only problem is that this will pull

current out of the ﬁrst cap, and subsequently up from ground, increasing the voltage

between the two caps and also at the inverting input. Which forces the op amp to reduce

its output voltage even further. !

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!

Eventually, we’ll again reach a tipping point where the whole mechanism reverses. Only

this time, it’ll be at a slightly lower output voltage. That’s because with every charging

and discharging cycle, we lose a bit of the momentum we initially put in. The circuit

behaves kinda like a pendulum that way.!

And so the output it produces is a sine wave swinging around the non-inverting input

voltage with a steadily decreasing amplitude. Which should already sound pretty kick

drum-ish – if the frequency is low enough. To make sure that it is, we have to choose the

right combination of capacitor and resistor values. This is a little tricky, since all of

those values inﬂuence both frequency and decay at the same time.!

In the original 808 bass drum schematic, Roland use a 1M bridge, a ~51k resistance to

ground and two 15nF capacitors, giving them a 50 Hz oscillation frequency with a quick,

but still discernible decay. Let’s use those values as a starting point.!

To be able to actually trigger the sound, we’ll then add a quick and dirty gate input.

Because the oscillator needs enough room to swing, we’ll have to divide the gate voltage

down. For that, we’ll simply insert a 100k/14k voltage divider at the non-inverting input. !

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To test this out, I recommend that you set the circuit up on a breadboard and then

connect an LFO, sequencer or similar to the gate input. You can listen to the output

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using any kind of headphones.!

You should get a quick, low-frequency bump when the gate goes high. Great! Though you

might also notice a second bump when the gate goes low. What’s up with that? Well, as

we saw earlier, the oscillation is triggered when the voltage at the non-inverting input

changes and the op amp struggles to charge (or discharge) the second cap to that

voltage. So it makes sense that we get a second bump when the input voltage drops.!

Note that I had to switch the 51k resistance to ground with a 47k, since your kit does not contain

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a 51k resistor.

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GATE-TO-TRIGGER CONVERTER

How do we ﬁx this? Simple: we’ll shorten the gate so much that the two separate

kick hits blend into one and the same. For that, we’ll set up a little circuit known as a

gate-to-trigger converter. It consists of two distinct sections. First, a high pass ﬁlter with a

pretty steep cutoﬀ frequency. And second, an op amp-based comparator. Here’s how it

works. !

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When we apply a gate signal to the input capacitor, we get a voltage spike on the other

side that quickly dies down as the cap charges through the 39k resistor to ground. For

the split second this spike crosses its threshold voltage, the comparator will then

push out an extremely short 12 V gate – also called a trigger. (Those 12 V are then

scaled down to around 1.4 V by the voltage divider at our kick’s trigger input.)!

You’ll notice that there’s also a diode pointing up from ground at the op amp’s non-

inverting input. This is necessary because some op amps will glitch out if they read a very

negative input voltage in this scenario. This is an issue because once the gate goes low,

we’d normally see a big negative spike as the capacitor discharges. The diode mitigates

that by allowing the cap to discharge instantly.!

Great! There’s just one more issue left to solve with this setup. During the comparator’s

low state, it’ll set its output to -12 V – simply because that’s what we give it as its low

supply voltage. This is not ideal. Because remember: our oscillator will swing around the

voltage we apply to the non-inverting input. For audio signals, that voltage should be 0 V.

So how do we ﬁx this? Simple: by putting a diode between the gate-to-trigger

converter and the kick’s trigger input. This way, the trigger can pass through, but the

comparator’s low state is blocked – and the voltage gets pinned to ground level via the

voltage divider instead.!

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

6

change all values by double clicking on components.

13

If you now feed your gate signal into the gate-to-trigger converter, you should get a single

kick hit instead of two per gate.!

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VARIABLE KICK PITCH

Next, let’s try and mess with the oscillator’s pitch. As I mentioned before, this is a little

tricky, since all of the component values in the feedback path inﬂuence both pitch and

decay at the same time. That said, varying the resistance to ground is probably our

best bet, since it aﬀects the pitch much more noticeably than the decay. So we’ll

replace the 51k resistor with a 250k potentiometer. !

This by itself would allow us to vary the pitch over a pretty wide range. Now, for our kick

drum, it makes sense to restrict that range to the lower frequency spectrum. To do that,

we just have to put a baseline resistance in series with our potentiometer. With a 10k, the

highest possible pitch is around 110 Hz, which is bordering on tom territory.!

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!

By playing with the potentiometer, you should now be able to set the base pitch. Great!!

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

7

change all values by double clicking on components. Also, note that you can adjust the pitch

range to your liking by changing the baseline resistance and potentiometer value.

15

VARIABLE DECAY

With the pitch sorted, let’s talk about the decay. Right now, it’s a little quick for my taste,

so I’d like to make it a good deal longer. Unfortunately, this will be a bit more involved

than changing the pitch. That’s because in order to keep the oscillation going for

longer, we either need to reduce the amount of momentum we lose during each

wavecycle – or ﬁnd a way to add some back in.!

To try and do the former, we’d need to increase the value of the bridge resistor. Because

as a rule of thumb, the relation between bridge and resistance to ground determines how

much momentum we lose per wavecycle. If they’re close to the same value, you won’t get

any oscillation. If they’re orders of magnitude apart, you get a decently long tail. !

Only trouble is that a) increasing the bridge resistance will also lower the pitch and b)

there’s a hard upper limit for the maximum decay we can squeeze out of this. Since

we’re already pretty close to that upper limit (and it’d be great if the pitch stayed the

same), I think we should skip this approach and focus on the other one: adding

momentum back in. To do that, we’ll set up an inverting buﬀer and feed it the kick’s

output. Then, we’ll take the buﬀer’s output and connect it to the node between our two

capacitors through a big resistor. !

8

!

Here’s how it works. We’ll imagine that we’ve just triggered the kick, which means that the

output voltage is rising. Like before, this will push current out of the ﬁrst capacitor. But

unlike before, we also actively pull current out via this newly added, second path. That’s

You can try this chapter’s circuits in a simulator. I’ve already set them up for you right here. You

8

can change all values by double clicking on components.

16

because the inverting buﬀer’s output voltage will always be the inverse of the kick’s

output voltage. !

So as the voltage pushing against the ﬁrst cap increases, the voltage pulling at it

from the other side decreases in lockstep. Which, in turn, decreases the voltage at the

inverting input, forcing the kick to push even harder. !

And of course, the same idea applies in reverse when the kick’s output drops below 0 V.

Then, the inverting buﬀer will push against the caps, exaggerating the downward motion

of the kick. We’re basically applying positive feedback to the system. Which, with the

resistor values speciﬁed above, will keep the oscillation going indeﬁnitely without altering

the pitch.!

Since we do want that oscillation to die down over time though, we’ll need to dial back

the amount of feedback we apply. For that, we’ll simply add a control that allows us to

reduce the inverting buﬀer’s gain. That gain, in case you don’t know, is set by the relation

between the input and the feedback resistance.!

If the feedback resistance is smaller than the input resistance, the gain drops below 1

(and vice versa). So we could simply replace that feedback resistance with a

potentiometer. Unfortunately, there is a small issue with this idea: the relation

between the inverting buﬀer’s gain and the amount of decay we get is exponential.

So as we’d change the resistance linearly, the decay would change exponentially.!

To counteract this, we could switch our linear pot for a reverse logarithmic one, which

would cancel out the exponential behavior. Unfortunately, that type of pot can be diﬃcult

to ﬁnd. Lucky for us, we can fake a reverse log potentiometer if we simply put a big

linear pot in parallel with the original 47k resistor. Because of the way those

resistances will interact, the end result mimics a reverse log curve quite convincingly.!

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!

If you test this, you’ll notice that dialing in the decay goes from short to long very

smoothly. What’s even better is that the kick doesn’t go into full drone mode at the

maximum setting. That’s because two resistances in parallel will always net you a lower

total resistance than either individually.!

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BASIC PITCH CV

Alright, so that’s the decay down. Our kick is starting to take shape, but there’s one

essential element still missing: the initial punch you get from a pitch envelope. To get

there, we need to modify our design so that the kick’s pitch can be manipulated

using a voltage. But doing this cleanly would be quite involved.!

Thankfully, there’s another quick and dirty hack we can apply here. Remember how we

discovered that the resistance to ground between the two capacitors determines the

oscillation frequency? That was because the faster current can be pushed out of or pulled

into the ﬁrst capacitor, the faster the whole mechanism will swing. So if we had a

component whose resistance can be controlled with a voltage, we’d be golden.!

Unfortunately, that component doesn’t really exist – at

least not in an ideal form. So we’ll have to make do with

a compromised version. And the most widely

available compromised version is probably the good

old NPN transistor. On paper, it sounds like it does

exactly what we’re looking for: allowing more or less

current to ﬂow between its collector and emitter in

response to a voltage we apply to the base. In practice,

there are of course a ton of caveats to this (which we’ll

run into in a second). !

Still, it’s worth a shot – so let’s try and add an NPN

transistor going to ground in parallel with our existing

resistance. This way, we get the pitch we dial in using

the potentiometer if the transistor is completely

closed – and we assume that we can increase it

from there by applying a voltage to the transistor’s

base, opening it up.!

We shouldn’t do that directly though, because the path

between base and emitter basically turns into a short

circuit if the base voltage increases past 600-700 mV.

So we’ll add a 100k series resistor to protect our

transistor. !

9

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

9

change all values by double clicking on components.

19

To try this out, connect your sequencer’s CV output to the newly added input socket. By

dialing in diﬀerent CV levels per step, you should be able to manipulate the kick’s pitch.

Great, so that does indeed work! But what about the caveats I mentioned? Does this

mean you can plug in NPN transistors anywhere you need a voltage controlled

resistance?!

Let’s take a closer look at the actual waveform our oscillator now produces. For that,

you’ll need to monitor the output using an oscilloscope. When you do this, you’ll notice

that the clean sine wave we get without CV morphs into something like a rounded

sawtooth. What’s up with that? Well, we’re running into one of the major caveats of

using an NPN transistor as a makeshift voltage controlled resistor: asymmetry.

Here’s what I mean.!

Let’s imagine we isolate our transistor and apply 1 V to

the base via a 100k resistor, 1 V to the collector and 0 V

to the emitter. This will cause around 400 µA to ﬂow

between collector and emitter in what we call the

transistor’s forward active mode. !

Now, if we drop the collector voltage to -1 V, you’d

expect to see the same amount of current ﬂow in the

opposite direction, right? But in reality, that current

will be much, much smaller – just around 15 µA in

what we call the transistor’s reverse active mode. !

This explains why our sine wave gets morphed into a

rounded sawtooth: because the transistor speeds up

one oscillation phase considerably more than the other

by sinking more current than it sources. But since the

distortion is not too intense (at least for my taste), I think

we can write this oﬀ as an okay compromise.!

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Erica Synths EDU DIY Kick Drum User manual

Erica Synths EDU DIY Kick Drum User manual

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