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Erica Synths mki x es.EDU Hi-Hat Manual

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

TABLE OF CONTENTS

CIRCUIT SCHEMATIC .................................................................................... 2
BILL OF MATERIALS ..................................................................................... 3
POWERING YOUR BREADBOARD ................................................................... 6
CIRCUIT DESIGN CLOSE-UP .......................................................................... 7
COMPONENTS & CONCEPTS APPENDIX ....................................................... 32
TOOLS APPENDIX ....................................................................................... 45
MODULE ASSEMBLY APPENDIX ................................................................... 47
SOLDERING APPENDIX ................................................................................ 61
Note that there is no breadboard included in this kit! You will also need a pack of jumper wires
1
and two 9 V batteries with clips. These things are cheap & easy to find in your local electronics
shop.
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Summary of Contents for Erica Synths mki x es.EDU Hi-Hat

  • 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

    HI-HAT Synthesized, analog hi-hats are pretty fascinating. That’s because emulating any kind of cymbal using an analog circuit is tough, since the sound a real cymbal produces is not quite pure noise – but also not really harmonic. Still, a couple classic drum machines like the Roland TR-606 and 808 took their best shot at it, with quite strange sounding results that I personally really like.

  • Page 3: 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 820k x1 470k x1...
  • Page 4 A bunch of capacitors. The specific values (which are printed onto their bodies) are 47µF (electrolytic) 1µF (foil) 470n (ceramic) 100nF (ceramic) x13 10nF (ceramic) 2n2 (ceramic) 1n (ceramic) 330pF (ceramic) 100pF (ceramic) Some diodes. The specific model names (which are printed onto their bodies) are 1N4148 (signal) 1N5819 (schottky) x2...
  • Page 5 A handful of potentiometers. Their specific values (which may be encoded & printed onto their bodies) are 250k (B254) x1 100k (B104) x1 10k (B103) A few jack sockets. The specific models (which you can identify by their color) are Switched mono (black) x5 A couple chips.

  • Page 6: Powering Your Breadboard

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

  • Page 7: Circuit Design Close-Up

    HI-HAT BASICS To understand how it works, we’ll start by identifying the different functional blocks that make up a classic analog hi-hat. First up: some sort of noise source. There are designs that go for simple white noise here, but I think this sounds a little basic and overly artificial.
  • Page 8 After that, we’ll have to shape the result into a quick burst – or a somewhat longer one, depending on whether the hi-hat should currently be open or closed. For that, we’ll use an amplifier which we’ll control using an envelope generator. Finally, we’ll apply another round of highpass filtering to get rid of any remaining low end in the signal.
  • Page 9 INVERTER OSCILLATOR Our best bet here is probably a schmitt trigger inverter-based solution, since it consists of just three parts per oscillator: the inverter, a capacitor, and a resistor. If you’ve built my DIY VCO, you’ll mostly understand how this works. Still, here’s a quick recap.
  • Page 10 Until we hit the lower input threshold, the inverter latches into the low input state, and the whole process repeats. At the inverter’s output, this gives us a square wave whose frequency depends on the size of both resistor and capacitor: increasing either will decrease the frequency, cause it takes longer to charge and drain the cap.
  • Page 11 To try this out, here’s how you could set it up on the breadboard. If you listen to the output through headphones, you should hear a dissonant, atonal swarm of bees. Just as expected! But it doesn’t really sound like a cymbal yet. One problem is the low end, but the high end is also a little too intense.
  • Page 12 BANDPASS FILTER For that, we’ll chain a simple passive highpass and a simple passive low pass together. If you need a refresher on how these work, I recommend reading the manual for my DIY VCF kit. By selecting the right capacitor and resistor values, we can carve out the rough frequency band that is suitable for hi-hats.
  • Page 13 First, we ground the op amp’s non-inverting input. Then, we take our highpass and we connect it to the inverting input. This might look a bit confusing, because we previously picked up the filter’s output from after the capacitor, not the resistor. But in an inverting op amp setup like this, the inverting input acts as something we call a virtual ground node.
  • Page 14 If you listen to this through headphones, the signal should be pretty loud. Great! Note that since your kit does not contain a .22 nF capacitor, I swapped it for a .33 nF capacitor.
  • Page 15 RESONANT BANDPASS Next, we’ll want to add some resonance to make the sound more sharp and biting. To get there, we only need to add two components: a capacitor and a resistor. If we insert them into the feedback path like this, then our filter will strongly emphasize the low pass stage’s cutoff...
  • Page 16 manual control for the tone here, we’ll insert a 1k potentiometer between the resistor and ground. This should allow us to move the cutoff from 7 kHz all the way down to 4 kHz. If you listen to this, it should sound much more sharp and aggressive. Cool! You can try this chapter’s circuit in a simulator.
  • Page 17 DISTORTION/VOLUME MODULATION With the filtering and resonance sorted, I’d now like to add some crunch to our processed signal. And while we could use a dedicated distortion stage for this, there’s a way more efficient solution. In Roland’s 808 and 606 cymbal and hi-hat sections, they used something they dubbed the „swing type VCA“...
  • Page 18 Alright, but what about the diode between the 33k resistor and the collector? Well, there’s one small issue with this setup. If the control voltage is 0 and there is no diode there, current will flow into the base, out of the collector and towards that low voltage node. And as the input signal oscillates, so will that current flow.
  • Page 19 To properly test this, you’ll need to send some form of control voltage into the VCA’s CV input (on the left). Once you do, you should hear the VCA open up and push out a pretty distorted version of the filtered signal. Great! You can try this chapter’s circuits in a simulator.
  • Page 20 ENVELOPE/GATE-TO-TRIGGER So that’s the VCA/distortion block down. Next up, we’ll want to add a snappy envelope generator to drive our VCA and give us a quick, percussive hi-hat hit. For that, we can re- use the design I came up with for my kick drum circuit. The basic version of that design consists of just five components: a diode, a capacitor, a potentiometer, a resistor and an NPN transistor.
  • Page 21 This little circuit takes in a gate signal (from a sequencer or an LFO) and transforms it into a super short voltage pulse. For that, it combines a highpass filter with an op amp-based comparator. That highpass transforms the gate into a quickly falling voltage curve – and the comparator then heavily distorts that curve into a pulse.
  • Page 22 HIGHPASS FILTER Still, our hi-hat is sounding a little too chunky for my taste. That’s because our VCA didn’t just add a little fizzy distortion – but also plenty of low end. So let’s add another highpass filter to complete our signal chain. We’ll start off...
  • Page 23 And while this does get rid of the low-end more effectively, I think it could honestly use a bit more bite.
  • Page 24 RESONANT HIGHPASS To fix this, we’ll simply make our highpass filter resonant. That way, we emphasize the frequencies around the cutoff point, which should hopefully give us a little more of that metallic ringing. Alright, but how do we make our setup resonant? Easy: we just have to connect the first 47k resistor to the op amp’s output.
  • Page 25 If you test this, it should sound pretty nice and metallic. Great! Next, I’d suggest that you try to control the accent level with a sequencer and tweak the tone & decay levels a bit. I’d say that we now have a perfectly fine hi-hat circuit. But there are two extra features that I’d like to add: a way to open the hi-hat –...
  • Page 26 OPEN HI-HAT Let’s start with the former. In Roland’s 606 and 808 designs, they basically set up the open hi-hat as an additional voice – meaning that they introduced another trigger input and another envelope generator to get the job done. For our circuit, I don’t think we need to go through all that, to be honest.
  • Page 27 To trigger the open hat, connect the new CV input to your sequencer’s velocity or pitch output. If you’re interested, try removing the 470k resistor and listen for the difference in the sound’s tail end. You can try this chapter’s circuit in a simulator. I’ve already set it up for you right here. You can change all values by double clicking on components.
  • Page 28 TUNE CONTROL Alright, so this leaves only the tune control – which is a little tricky, because ideally, we want to shift the frequencies of all six oscillators up or down by the same amount at the same time. Unfortunately, those frequencies are determined by the capacitor and resistor values in each individual oscillator core.
  • Page 29 Next, we take the result and apply it to the 40106’s positive supply pin. This way, the transistor will give the chip just enough current so that the supply voltage stays locked to the variable voltage divider’s output. If you now turn the tune knob, you should be able to increase the pitches for all six oscillators.
  • Page 30 TUNE CV But why stop at manual tune control? Since we’re already using a voltage to set the oscillators’ frequencies, why not go the extra mile and also add a tune CV input? All we have to do to make that happen is somehow combine an external voltage with the one coming from our variable voltage divider –...
  • Page 31 So we need to make sure that we never drop it lower than that. How do we pull this off? Simple: by raising the pull-down voltage we set after the diode. Right now, it’s fixed at 0 V. But if we replace this 100k to ground with a voltage divider that produces roundabout 3 V and connect the NPN’s base to it, then the voltage at that node can never drop below those 3 V.

  • Page 32: 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 33 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 34 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 35 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 36 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 37 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 38 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 39 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 40 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 41 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 42 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 43 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 44 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 45: 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 46 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 47 BUILD GUIDE...

  • Page 48: Module Assembly Appendix

    MODULE ASSEMBLY APPENDIX Before we start building, let’s take a look at the complete mki x es.edu Hi-Hats schematics (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.
  • Page 49 Capacitors C17-C20 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-de- signed, 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 51 Before you start soldering, we highly recommend printing out the part placement diagrams with designators and values and follow step-by step instructions below. Hi-Hats PCB is one of most densely populated PCB in our DIY.EDU line, so, this will help you to avoid mistakes in the build process.
  • Page 52 Place the Hi-Hats 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 10 Ohm resistors, switching diodes and the power protection diodes.
  • Page 53 Then proceed with the ceramic capacitors. Because ceramic capacitors look very simi- lar, I recommend sorting them by values before you proceed with installing and solder- ing them. Yellow capacitors in the picture to the left are 470nF, 0,1uF, 10nF and 330pF, blue capacitors are 100pF, 1nF and 2,2nF.
  • Page 54 Then proceed with other capacitors. When completed, your PCB should look like this: Now, let’s proceed with resistors! All components on the PCB have both their value and denomination printed onto the silkscreen. Resistors are particularly tricky because they are color coded, and sometimes colors are difficult to distinguish. If you are not sure about a resistor’s value, use a multimeter to double-check.
  • Page 55 Let’s start with 100 k resistors, because we have plenty of those. Once you have soldered all 100 k resistors, your PCB should look like this: Then proceed with other resistors. Remem- ber – if you are not sure about resistor values, use the multimeter to check! When al resis- tors are installed, your Hi-Hats PCB should look like this:...
  • Page 56 Next, insert and solder transistors. There are PNP and NPN transistors in the kit, therefore before soldering them, I highly recommend to sort them. Make sure you install them in correct places and pay attention on the orientation of the transis- tors –...
  • Page 57 Also, populate the 1 uF film capacitor and 2x5 PSU socket. Make sure the orienta- tion of the socket is as shown in the picture below – the arrow pointing to the first pin is aligned with a notch on the silk- screen.
  • Page 58 Insert the top potentiometer and jack sockets, then fit the panel to align components, you just installed, and solder them. Insert other potentiometers, but don’t solder them yet! NB! There’s a replacement in the kit - Tune CV potentiometer should be 10k, while the BOM and silkscreen on the PCB says 100k.
  • Page 59 Now, insert the ICs into their respective DIP sockets. Mind the orientation of the ICs – match the notch on each IC with the one on its socket. Finally fit the Decay potentiometer knob and we are done!
  • Page 60 The module doesn’t need any calibration. Patch trigger signal (the gate output of your DIY.EDU Sequencer will work fine, but the Erica Synths Drum Sequencer is the best choice) to the input of the module and connect the output of the module to a mixer.

  • Page 61: 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 62 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 63 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.