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INTRO |
Hey there, thanks for buying this DIY kit! We - Erica Synths and Moritz Klein - have
developed it with one specific goal in mind: teaching people with little to no prior
experience how to design analog synthesizer circuits from scratch. So what you'll find in
the box is not simply meant to be soldered together and then disappear in your rack.
Instead, we want to take you through the circuit design process step by step, explaining
every choice we've made and how it impacts the finished module. For that, we strongly
suggest you follow along using LABOR , which is an all-in-one circuit prototyping tool
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that allows you to experiment and play around with your components in a non-permanent
way. To help you with this, we've included suggested breadboard layouts in select
chapters.
In addition to this, you can also experiment with some 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 ..................................................................................... 4
USAGE WITH MKI x ES LABOR ....................................................................... 7
CIRCUIT DESIGN CLOSE-UP .......................................................................... 9
COMPONENTS & CONCEPTS APPENDIX ....................................................... 46
TOOLS APPENDIX ....................................................................................... 59
MODULE ASSEMBLY APPENDIX ................................................................... 61
SOLDERING APPENDIX ................................................................................ 73
TROUBLESHOOTING APPENDIX .................................................. COMING SOON
You can also use a standard breadboard, but this will require you to get a little creative when
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adapting the suggested layouts. You'll also need to do some additional engineering to get the
different supply voltages.
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Summary of Contents for Erica Synths mki x es.EDU FM Drum

  • Page 1: Table Of Contents

    INTRO | Hey there, thanks for buying this DIY kit! We – Erica Synths and Moritz Klein – have developed it with one specific goal in mind: teaching people with little to no prior experience how to design analog synthesizer circuits from scratch. So what you’ll find in the box is not simply meant to be soldered together and then disappear in your rack.
  • Page 2: Circuit Schematic

    FM DRUM These are the schematics for Roland’s TR-909, an iconic drum machine from the mid-1980s. I’ve been staring at this absolute maze of components for a long time, trying to figure out how I can save a vintage 909 I bought a while ago. While it worked just fine initially, the kick, snare and toms quickly started to misbehave.
  • Page 3 Instead of fixing my machine, I ended up creating something new: an FM drum circuit that combines the punchy character of the 909 with the metallic textures of FM synthesis.
  • Page 4: Bill Of Materials

    BILL OF MATERIALS Before we start, please check if your kit contains all of the necessary components. In addition to a PCB, panel and power cable, your box should also contain: An array of resistors. The specific values (in ohms, which you should check for with a multimeter) are x4
...
  • Page 5 A bunch of capacitors. The specific values (which are printed onto their bodies) are 47 uF 1 uF x1
 470n 100 nF 15 nF 10 nF 5.6 nF x1
 2.2 nF 470p 330p Some diodes. The specific model names (which are printed onto their bodies) are 1N4148 (signal) x18...
  • Page 6 A handful of potentiometers. Their specific values (which may be encoded & printed onto their bodies) are 1M (B105) x1
 500k (A504) x1 250k (B254) x1 100k (A104) x2 100k (B104) x1 A few jack sockets. The specific models (which you can identify by their color) are Switched mono (black) x5 A couple chips.
  • Page 7: Usage With Mki X Es Labor

    USAGE WITH MKI x ES LABOR We recommend that you follow this guide using an MKI x ES LABOR prototyping board. LABOR comes equipped with everything you need for testing the circuits we lay out: a standard 830 tie point breadboard, an integrated dual power supply with over current protection, a manual gate/trigger/envelope generator, an LFO, a variable CV source, an output amplifier, and a modular interfacing section where you can insert all of your interfacing components like potentiometers, jack sockets, and switches.
  • Page 8 Sometimes, this guide will ask you to use external gear like sequencers or LFOs to send CV, audio signals, triggers or gates into your circuit. With Labor, there’s no need for extra equipment – just use the built-in oscillator (audio/LFO), CV source or manual gate/trigger/ envelope generator.
  • Page 9: Circuit Design Close-Up

    909-STYLE TRIANGLE VCO To understand how the FM Drum circuit works, we’ll retrace my steps and start by isolating the core concepts of the TR-909’s design. First up: the oscillators. Compared to the TR-808, Roland decided to step up their game here. Because where the 808 used bridged-T oscillators –...
  • Page 10 This way, the voltage on the left will stay at half of the CV, while the voltage on the right will steadily drop. This keeps going until the integrator’s output crosses the schmitt trigger inverter’s lower input threshold, causing it to latch into the high state. This enables the NPN transistor, allowing current to flow from the integrator’s inverting input to ground.
  • Page 11 Which, conveniently, is exactly as much as is flowing in from the CV. Causing the integrator’s output to rise at the same rate it was previously dropping at. Until it hits the schmitt trigger inverter’s upper input threshold, and the whole cycle repeats. At the integrator’s output, this creates a triangle oscillation whose frequency is determined by the CV.
  • Page 12 TRI TO SINE WAVESHAPER Since triangle waves sound a little too buzzy and bright compared to an actual drum hit, Roland needed a way to remove the harmonics above the base pitch. But instead of a low pass filter, they decided to go with a dead simple waveshaper. Which uses just a couple resistors and diodes to turn their triangle wave into a somewhat dirty sine wave.
  • Page 13 Here’s how you can try this for yourself. For our triangle wave, the amount of scaling is just right: the waveshaper should give you a pretty decent sine wave on the oscilloscope that’s noticeably less buzzy. 
 You can try this chapter’s circuit in a circuit simulator. I’ve already set it up for you right here: https://tinyurl.com/24p8j6hg – you can change all values by double clicking on components.
  • Page 14 CRUDE VCA There’s only one problem with it: it’s not percussive at all. This is one of the key advantages the TR-808’s bridged-T oscillators had – they produced sine waves with gradually decreasing amplitudes. To mimic this, we’ll have to set up a proper VCA and use it to modulate our sine wave’s amplitude.
  • Page 15 And since the emitter current depends on the base current, we can control the output signal’s gain by changing the base voltage. And that’s it: we've got a VCA! Of course, since most input signals will be way too loud to keep the transistor in saturation, we'll also add a strong voltage divider at the VCA’s input.
  • Page 16 forces the op amp to counteract, giving us an elevated output voltage when it should actually be neutral. How do we fix this? Easy – we neutralize the backwards current flow by connecting our control voltage to the emitter via a big resistance. Now, the issue here is that the exact amount of backwards current varies quite heavily between different transistors –...
  • Page 17 To center your sine wave, you’ll need to view it on your oscilloscope and then fiddle with the trimmer until it’s properly aligned with the 0 V line. Please note that this will only work properly if your op amp causes the VCA to have a positive DC offset in the first place.
  • Page 18 DECAY ENVELOPE Great! But our circuit still doesn’t sound like a drum. To get there, we’ll need to add an envelope generator to drive our VCA and turn the static sine wave into a quick, percussive burst.c The 909’s envelope generators are mostly just a diode, a capacitor, a potentiometer and a small resistor.
  • Page 19 To test this, we’ll use LABOR’s manual gate, which has a dedicated setting for generating triggers. If you now push the button, you should get a percussive sine wave hit.
  • Page 20 GATE TO TRIGGER CONVERTER Sounds like a decent starting point to me! But before we can refine it, we’ll need to talk about triggers some more. That’s because unlike LABOR, most other gear sends out gates, not triggers. So to make our circuit compatible, we’ll want to add a gate-to-trigger converter.
  • Page 21 To test this, connect a sequencer to both the gate input and the accent CV input. (Alternatively, you can also use LABOR’s EG OUT socket and the CV SOURCE.) You should be able to trigger the circuit and set its output volume simultaneously.
  • Page 22 VCO RESET While our circuit does work decently well, you probably noticed that the sound’s transient (or attack) changes pretty dramatically between hits. We can also see this on the oscilloscope: the start of each hit looks different every time. That’s because our VCO is always oscillating and could be at any point in its wavecycle when the trigger hits.
  • Page 23 If you set this up and test it with your gate/CV sequence, all the individual hits should be much more consistent.
  • Page 24: Pitch Envelope

    PITCH ENVELOPE Unfortunately, our drum still sounds pretty flat. It’s definitely missing that characteristic 909 punch. To add it, Roland used a simple pitch envelope to modulate the VCO’s frequency. Which is much easier to implement and more reliable than it was in the 808’s bridged-t architecture.
  • Page 25 Also, we’ll set up another diode after the envelope’s output – so that it only affects the total mix if the envelope is higher than the tune CV. This way, we will always drop down to the set base pitch (and not lower). For the trigger, we’ll use the unaccented one again, since the pitch bend is integral for the output to sound like a drum.
  • Page 26 By playing with the tune decay and tune depth potentiometers, you should be able to get sounds that are way more 909-ish already. If you tune the VCO really low, you can get some decent kicks out of our circuit.
  • Page 27 IMPACT DISTORTION Still, the sound is missing the 909’s sharp attack. There are multiple things that Roland did to add it – and I’ve cherry picked two of them. We’ll start with an enhancement to the triangle to sine waveshaper (which Roland kept exclusive to the toms). Its purpose is to distort the sine wave momentarily when the trigger hits, adding high frequency content and extra volume for an aggressive impact.
  • Page 28 For the trigger, I think it makes sense to use the accented version here. This way, unaccented hits stay a bit more mellow, while accented ones become more harsh. Because the effect is subtle, you can do a before/after comparison by removing the diode before the 5n6 capacitor while testing the circuit.
  • Page 29 ADDED CLICK For me, the distortion adds some much needed complexity to the sound’s transient, making it much more characterful. Though I’d still like to add a little more spice to it. The 909’s kick injects white noise mixed with the trigger pulse into the output to achieve this. I’m not keen on adding a dedicated noise generator, though.
  • Page 30 Since the effect is, again, pretty subtle, you can do another before/after comparison by removing the new diode while testing the circuit.
  • Page 31 FREQUENCY MODULATION At this point, our circuit is essentially a hybrid between the 909’s kick and toms. This would’ve been enough exploration for me to go and fix my machine. But I wasn’t done yet: I started thinking about ways to expand on the 909’s sounds. And one technique that’s absent from all of Rolands vintage drum machines is Frequency Modulation.
  • Page 32 First, we’ll use a significantly bigger capacitor in our integrator. This shifts the oscillator’s frequency range downwards. This is desirable because it generally sounds more interesting if the modulator’s frequency is lower than that of the carrier – at least in my opinion.
  • Page 33 Also, we’ll add a simple switch that lets us choose between applying the modulator’s output or ground – so we’re able to use the circuit in regular 909 or FM mode. To prevent the carrier from crashing into either supply rail when the input CV goes negative, we also add a diode at the carrier’s CV input.
  • Page 34 XOR PULSE But while the circuit is already pretty versatile, there is still some untapped potential here: So far, we’re only using our VCOs’ triangle outputs. They also generate square waves, though! And square waves, as I’ve learned by studying the TR-606 and TR-808 schematics, are great for synthesizing analog hi-hats and cymbals.
  • Page 35 Coincidentally, this signal is identical to what you would get from a ring modulator. Which is why XOR gates are often used as a cheap substitute for true ring modulation. (The Korg MS-20 is one famous example.) To implement an XOR gate, we only need 4 diodes, 2 resistors and a PNP transistor.
  • Page 36 As expected, you should get a decently chaotic output from our XOR circuit. Try adjusting the frequency relationship between the two oscillators to find some interesting sweet spots.
  • Page 37: High Pass Filter

    HIGH PASS FILTER The XOR gate does add some strange metallic overtones, which sound great. But for proper cymbal sounds, the signal has way too much low-end. So we’ll remove it with a simple sallen-key high pass filter. You can check the Hi-Hat module’s manual for an in- depth explanation, but here’s the basic gist.
  • Page 38 If you try this, the output should sound a lot thinner (and more cymbal-like) than the unfiltered signal.
  • Page 39 SINE/PULSE SWITCH Okay, so now we’ll need to turn this static cymbal noise into a percussive hit. Thankfully, we can repurpose our existing VCA and decay envelope for this. All we need to do is set up a simple SPDT switch before the VCA’s input. Then, we apply our sine wave output to one side of the switch –...
  • Page 40 To test this, first set the VCA’s input to pulse wave and turn the FM mode off. By tuning the two oscillators to different intervals, you can get a decent variety of cymbal- and hihat sounds. Cool – but what if you also turn on the FM mode? The difference is subtle, but you should get a different flavor of cymbal.
  • Page 41 DECAY CV With 4 different modes (single oscillator sine, FM sine, dual oscillator pulse, and FM pulse) our circuit now gives us plenty of manual sound design options. It’s a little light on voltage control, though. So let’s do something about that! First, I’d like to add a CV input for the decay envelope.
  • Page 42 You should now be able to adjust the output’s tail by applying different CV levels to the new DECAY CV input. Try it in the pulse wave mode as well, for simulating opening and closing hi-hats!
  • Page 43 TUNE CV Besides a decay CV input, it would also be great to have a tune CV input that controls both oscillators’ frequencies at once – so we can play some simple melodies and bass lines. Good thing our circuit uses proper VCOs! Because this allows us to simply mix our existing control voltages for VCO 1 and 2 with an external control voltage.
  • Page 44 It works like this. The resistor ensures that a small current flows through the diode at all times. And when a current flows through a diode, there will be a diode drop across it. So for 0 V external CV, we’ll get around -500 mV at the PNPs’ bases. Which should then give us about 0 V at their emitters.
  • Page 45 Try sending a CV sequence into the new TUNE CV input. By playing with the individual steps’ CV values in combination with the TUNE CV INT potentiometer, you should be able to dial in some basic melodies. Great! And with this, our FM Drum is complete. Once you’re done experimenting, dig out the panel and PCB from the kit, heat up your soldering iron and get to building.
  • Page 46: Components & Concepts Appendix

    COMPONENTS & CONCEPTS APPENDIX In this section, we’ll take a closer look at the components and elemental circuit design concepts we’re using to build our module. Check these whenever the main manual moves a bit too fast for you! THE BASICS: RESISTANCE, VOLTAGE, CURRENT There are three main properties we’re interested in when talking about electronic circuits: resistance, voltage and current.
  • Page 47 USING TWO 9 V BATTERIES AS A DUAL POWER SUPPLY Dual power supplies are great – and if you want to get serious about synth design, you should invest in one at some point. But what if you’re just starting out, and you’d like to use batteries instead? Thankfully that’s totally doable.
  • Page 48 RESISTORS While a conductive wire is like a very big pipe where lots of water can pass through, a resistor is like a narrow pipe that restricts the amount of water that can flow. The narrowness of that pipe is equivalent to the resistance value, measured in ohms (Ω). The higher that value, the tighter the pipe.
  • Page 49 CAPACITORS A capacitor is a bit like a balloon that you can attach to the open end of a pipe. If there’s some pressure in the pipe, the balloon will fill up with water until the pressure equalizes. (Since the balloon needs some space to expand into, both of the capacitor’s legs need to be connected to points in your circuit.) Then, should the pressure in the pipe drop, the balloon releases the water it stored into the pipe.
  • Page 50 DIODES Diodes are basically like one-way valves. Current can only pass through in one direction – from anode to cathode. That direction is indicated by the arrow in the diode symbol and by a black stripe on the diode’s casing. So any current trying to move in the opposite direction is blocked from flowing.
  • Page 51: Voltage Dividers

    VOLTAGE DIVIDERS A voltage divider is really just two resistors set up like this: input on the left, output on the right. If R1 and R2 are of the same value, the output voltage will be half of what the input voltage is. How does it work? Let’s use our analogy again: so we have a pipe on the left, where water is being pushed to the right with a specific amount of force.
  • Page 52 POTENTIOMETERS Potentiometers can be used as variable resistors that you control by turning a knob. But, and that’s the handy part, they can also be set up as variable voltage dividers. To see how that works, let’s imagine we open one up. Inside, we would find two things: a round track of resistive material with connectors on both ends plus what’s called a wiper.
  • Page 53 AC COUPLING What is AC coupling – and how does it work? Imagine two adjacent pipes with a balloon between them. Now, no water can get from one pipe into the other, since it’s blocked by the balloon. But, and that’s the kicker, water from one side can still push into the other by bending and stretching the balloon, causing a flow by displacement.
  • Page 54 OP AMPS Op amps might seem intimidating at first, but they’re actually quite easy to understand and use. The basic concept is this: every op amp has two inputs and one output. Think of those inputs like voltage sensors. You can attach them to any point in your circuit and they will detect the voltage there without interfering.
  • Page 55 OP AMP BUFFERS/AMPLIFIERS Buffering, in the world of electronics, means that we provide a perfect copy of a voltage without interfering with that voltage in the process. With an op amp-based buffer, the buffering process itself works like this. We use the non-inverting input to probe a voltage, while the inverting input connects straight to the op amp’s output.
  • Page 56 amplifier because the output signal is in phase with the input. For an inverting buffer/amplifier, the input signal is no longer applied to the non-inverting input. Instead, that input is tied directly to ground. So it’ll just sit at 0 V the entire time.
  • Page 57 BIPOLAR JUNCTION TRANSISTORS Bipolar junction transistors (or BJTs for short) come in two flavors: NPN and PNP. This refers to how the device is built internally and how it’ll behave in a circuit. Apart from that, they look pretty much identical: a small black half-cylinder with three legs. Let’s take a look at the more commonly used NPN variant first.
  • Page 58 collector current. (At least not without some unwanted side effects.) Third, also unlike a resistor, a BJT is not a linear device. Meaning that a change in collector voltage will not affect the collector current. And fourth, the collector current is affected by the transistor’s temperature! The more it heats up, the more current will flow.
  • Page 59: Tools Appendix

    TOOLS APPENDIX There are two types of tools that will help you tremendously while designing a circuit: multimeters and oscilloscopes. In this appendix, we’ll take a quick look at each of these and explore how to use them. MULTIMETERS Multimeters come in different shapes and sizes, but the most common type is probably the hand-held, battery powered variant.
  • Page 60 OSCILLOSCOPES While multimeters are fairly cheap and compact, oscilloscopes are usually somewhat pricey and bulky. If you’re willing to make the investment, they are a huge help with the troubleshooting process, though. Using one is, again, surprisingly straightforward – if you manage to work your way through the sometimes quite convoluted UI, especially on digital models.
  • Page 61 BUILD GUIDE...
  • Page 62: Module Assembly Appendix

    Before we start building, let’s take a look at the complete mki x es.edu FM Drum schematics (see next page) that were used for the final module’s design and PCB fabrication. Most components on the production schematics have denomi- nations (a name –...
  • Page 63 Capacitors C9, C10, C12, 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.
  • Page 65 Before you start soldering, we highly recommend printing out the part placement diagrams with designators and values and follow step-by step instructions below. As mentioned above, FM Drum is the most complex project in our DIY.EDU line, so, this will help you to avoid mistakes in the build process.
  • Page 66 Place the FM Drum PCB in a PCB holder for soldering or simply on top of some spac- ers (I use two empty solder wire coils here). I usually start populating PCBs with lower, horizontally placed components. In this case we have lot of resistors, so, for sake of clari- ty, let’s start with 100k and 10k resistors.
  • Page 67 Next, populate remaining resistors. Now, proceed with switching diodes and the power protection 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. Solder all diodes.
  • Page 68 Then proceed with the ceramic capaci- tors. Start with soldering 0,1uF 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, solder other ceramic and film capacitors.
  • Page 69 Then populate the trimpot and 2x5 PSU socket. Make sure the orientation of the socket is as shown in the picture below – the arrow pointing to the first pin is aligned with a notch on the silkscreen. The key on the socket will be facing outwards the PCB.
  • Page 70 The switches need special attention. Insert the switches in relevant places, but do not solder them, yet. Place the front panel, fix it with a potentiometer and jack socket nuts. Then tighten switch nuts, so that switches are pushed against the front panel.
  • Page 71 Finally fit the Tune potentiometer knobs and we are done!
  • Page 72 Congratulations! You have completed the assembly of the mki x es.edu FM Drum module! Connect it to your eurorack power supply and switch it on. If there’s no "magic smoke”, it’s a good sign that your build was successful. The module has one trimpot that needs to be adjusted. Patch trigger signal (the gate output of your DIY.EDU Sequencer will work fine, but the Erica Synths Drum Sequencer is the best...
  • Page 73: Soldering Appendix

    SOLDERING APPENDIX If you’ve never soldered before – or if your skills have become rusty – it’s probably wise to check out some THT (through-hole technology) soldering tutorials on YouTube. The main thing you have to remember while soldering is that melted solder will flow towards higher temperature areas.
  • Page 74 DIY electronics is a great (and quite addictive) hobby, therefore we highly recommend you invest in good tools. In order to really enjoy soldering, you’ll need: A decent soldering station. Top-of-the-line soldering stations (brands like Weller) will cost 200€ and above, but cheaper alternatives around 50€...
  • Page 75 A solder suction pump. No matter how refined your soldering skills are, you will make mistakes. So when you’ll inevitably need to de-solder components, you will also need to remove any remaining solder from the solder pads in order to insert new components.

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