Synthesizer Schematics

This page contains some circuits used in my synthesizer. Some simple ones are designed from scratch, but most of them are - at least for some sub-circuits - based on existing designs that can be found on the net. The circuits are available for download as drawings as well as KiCad projects. The KiCad projects sometimes also contain a PCB-layout. However you really should not use that PCB layout to manufacture a PCB! Those are routed in a way that can be set up with through hole components on a strip-board. If you want to get some PCBs manufactured I highly recommend to redo the layout and use SMD components instead (Unless you choose really small footprints (0603 or smaller) those are even easier to solder than through hole ones).

• Clock and LFO
  • Simple NE555 based clock, with integrated clock divider
• Instruments
  • Claves
  • Metallic Hi-Hat
  • Cymbal and metallic Hi-Hat
  • Hi-Hat and Snare
  • Kick Drum
  • Toms
  • 8 Channel Reverse Avalanche Drone
  • Thomas Henry 555 VCO
• Sequencers
  • 4 or 8 Step Trigger Sequencer
  • 8 (pseudo 16) Step CV Sequencer
  • 16 Step Euclidean Trigger Sequencer
  • 8 and 16 Step Trigger Sequencer with lots of blinky Buttons
• Effects and Filters
  • VCF - Voltage Controlled Filter
  • Spring Reverb Tank Driver
• Auxiliary Modules
  • Output Mixer with included Low Pass Filter and indicator LEDs
  • Attack (Decay) Sustain Release (ADSR) Envelope Generator
  • VCA - Classic Transistor-Based Voltage Controlled Amplifier
  • Buffered Multiple
• Case
  • Case

--------------- CLOCK AND LFO ---------------

Simple NE555 based clock, with integrated clock divider

No clock, no beat. This is probably the most important module unless you love never ending drone sounds and hate dance music. To generate the clock signal it uses a NE555 as main oscillator. Furthermore, it comes with an integrated clock divider that uses two CD4017 decade counters to divide the clock signal to 1/2 and 1/4 with the first CD4017 and then further divides it to 1/8 and 1/16 with the second CD4016. (Dividing the clock two times more would also be possible with the counting range of the counters by simply moving the reset signal to pin 9 instead, so each counter counts up to eight, and adding some more diodes) The supply voltage is +5V and it generates a +5V clock signal. The convention used for all modules listed here is that gate signals are at +5V, however, depending on the exact versions of the ICs you are using it could easily be modified for some other voltage.

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--------------- INSTRUMENTS ---------------

Claves

For the basics of a nice track typically a kick drum, a snare, and a hi-hat together with some baseline is all you need. However it will get far more interesting by adding some more instruments. And since I really like electronic music with Latin inspired rhythms (e.g. Monika Kruse or Mathias Kaden and so on) we of course need lots of percussive instruments. This instrument is inspired by the Roland TR-808 claves circuit. The original circuit has two T-bridge oscillators in parallel, with one of them having an op-amp behind it, that feeds back into the T-bridge to keep the oscillation alive for a longer time. The idea here is that the longer one is the main oscillator, and the shorter one adds the noise at the start of the sound when the wood is hit. In my design I added the feedback path to both of the oscillators and made it tunable by a pot to allow for more /other sounds. The resistor connecting the two caps in the T-bridge to GND is replaced by a pot, so the pitch of both oscillators is tunable. Finally a transistor is added in parallel to the pot to allow to control the pitch of each oscillator by a separate CV. In total this results in pretty interesting instrument that can be used as a clave but also can produce a wide range of percussive sounds and can even be used to generate a lead. Adding a bit of distortion to the sound transforms it to something more rim shot like, which is also pretty nice and can be achieved by just adding an additional transistor and few resistors a cap and a diode. In combination with a pot you can tune in between the clave and rim shot sound (see 2nd schematic)

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Metallic Hi-Hat

Metallic hi-hats are extremely important!!!11 (also cymbals). If everybody is already jumping around on the dance floor like wild and you want to further escalate the set probably a metallic hi-hat is a good choice. The hi-hat (as many other designs you can find on the net) uses a CD40106 hex Schmitt-trigger inverter to generate six triangular waves at different frequencies (all of them are tunable by trim pots in my design but you could also replace those with fixed resistors that tune the oscillators to a useful range (in my design the frequency range is about 400 Hz to 1000 Hz with the frequencies of the single oscillators more or less equally spaced, but it really does not matter a lot). This is what generates the metallic sound. Basically what makes a sound metallic is that there are several fixed frequencies in parallel (in this case 6). If the frequencies are high it sound like kicking against a thin sheet of metal, if the frequencies are lower it sound more like a thick walled metallic bell or sometimes also like cutting a thick rod of steel with a saw. Also wooden sounds can be generated somewhat similar, however as far as I could figure it out typically the envelope needs to decay faster (only a few oscillations) and one to three oscillators are enough. (see e.g. the claves schematic). In addition to the metallic oscillators there is a white noise oscillator, which is the same as for the non-metallic hi-hat or the snare drum. Finally the signal gets filtered through a resonant filter that allows to add some more sharpness to the sound. The basic schematic has only one input (trigger). In addition to this you can add two smaller circuits to add CV for the decay of the hi-hat as well as a second trigger (with a shorter or longer envelope). I've configured my instrument in a way, that I have one open hi-hat with the decay controllable by CV and then a second closed hi-hat that can generate really short tzzz sounds that I really like.

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Hi-Hat and Snare

This is one of the simplest instruments you can build, but still pretty useful. Just take a npn-transistor with a unconnected collector, which will produce white noise. Add an envelope to this and the hi-hat is done. The envelope is also generated by a super simple circuit which is basically a capacitor that gets loaded when triggered and then discharges through a resistor resulting in a falling ramp (see this nice page about the DR-110 envelope generator for a detailed explanation. The decay time can be regulated by a pot. Depending on the values of the caps that are used for the noise generator as well as the filter at the output you can change the pitch and choose in between more hi-hat or more snare like sounds. This is basically based on the build here on youtube but I added a gate-buffer to the circuit. Without the buffer the sound is really dim when triggered with 5V. The envelope generator is basically the same as for the metallic hi-hat which means that you can use the same add-on circuits to make the decay time voltage controllable, or add a second trigger for open / closed hi hat.

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Cymbal and metallic Hi-Hat

Again, as already the metallic hi-hats cymbals are extremely important to escalate further. The hi-hat circuit is very similar to the other metallic hi-hat (CD40106 hex Schmitt-trigger oscillators) as source for the metallic sound, then some filtering and a simple swing type VCA. The cymbal circuit is also very similar however with one difference. It filters the signal from the oscillators through two different resonators resulting in two metallic noises (one very high like a really thin sheet of metal, the other a bit more clunky). Then both of these sounds are mixed together via 3 swing type VCAs with different decay times resulting in a sound that is a bit more massive at the beginning and has a long tail that slowly fades out. Both instruments is based on the Roland TR-808 cymbal and hi-hat circuit. The only difference is that there are some more pots to control the resonant frequencies for both filters and I've added a decay CV input for the hi-hat. I would recommend to build both circuits and put them into a single module, since then you only have to build the noise-source once and compared to the cymbal the additional parts for the Hi-Hat are not that much.

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Kick Drum

Since probably the most important thing - if you want to dance hard - is a kick drum you definitely need one. This schematic is based on a Youtube Video by Moritz Klein and I really like it, since there are a lot of things you can tweak to change the sound of the drum. I only did one single change to the original schematic which is adding a small cap after the trigger converter since I did not like the crackling sound at the begin of the kick. As far as I remember also some resistor values might be slightly changed, but nothing to important. For any more detail just go and watch the video.

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Toms

Only a kick drum, with a snare at every second beat and a high-hat in between typically sounds a bit as if there is missing something. To get a more epic sound it is typically a good idea to fill in the gaps with some type of bass sound. Depending on the genre this bass can be anything from a techno rumble, over a psytrance 16th note bass line to a bass guitar or whatever you like. One option that is also very common - especially in house music - is to put some toms there. The tom schematic is - again - a bridged T oscillator that is tuned a lot lower than the claves but a a bit higher compared to the kick. The attack and pitch are voltage controlled. Furthermore, as in the claves circuit there is a transistor distortion stage added and a pot that allows to dial in a clean tom or a fully distorted version.

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8 Channel Reverse Avalanche Drone

If you are into DIY modular synths then you probably have heard of Sam and his channel Look Mum No Computer. If not you really should click on the link. One of his most popular builds (besides the Furby Organ of course) are huge drone synthesizer that use simple oscillators based on the reverse avalanche effect to generate saw-tooth waveforms. The problem I had with his original circuit is, that I could not get it running with +12V as VCC which is the upper voltage used in my synth, since the breakdown voltage of the 2N3904 was too high. I tried some other transistors with a smaller breakdown voltage and the one that worked best for me was the BC337. Since the output is pretty quiet and high impedance and I want to keep all of my instruments somewhere around +-5V there is an op-amp at the output to amplify the signal. The oscillators are grouped into two groups of four. The tone knob lets you low-pass filter the sound. You can also use a switched jack at one of the outputs then there are either two outputs with four oscillators each or one output with all of them. What I really like is running the drone through a VCA and then slowly going from continuous drone sound to short pulses and finally placing the kick in between as an intro.

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Thomas Henry 555 VCO

I really love basslines. And this is the module by Thomas Henry that I use to make them. The original post from Thomas Henry can be found here: Thomas Henry 555 VCO at electro-music.com. There are lots of good descriptions out there on how to build it so there is no point in writing another one. What I really like about it is that it tracks the V/Oct really nicely and also if you have two of them you can sync them which results in some really crazy sound. However I rarely use any oszillator other than the saw tooth (ramp) or pulse, so if you dont need the sync there are probably some schematics out there that are easier to build.

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--------------- SEQUENCERS ---------------

4 or 8 Step Trigger Sequencer

You can never have enough sequencers. Especially trigger sequencers. Also you definitely need some that can be configured completely free and some that use euclidean patterns. This one is especially useful if you want to make some psytrance style basslines which typically go something like this (k=kick, b=bassline): kbbb-kbbb-... or k_bb-k_bb-... The design is basically just a decade counter that counts up to 4. If you want a 8 step sequencer instead just move the reset signal to pin 9 instead and wire up the additional switches in the same way as the first ones. Also if you want to make this a CV sequencer just replace the switches with pots and remove the and gate. The gate buffer is not necessary but it prevents the clock input from rising over VCC if some higher voltage is connected for some reason.

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8 (pseudo 16) Step CV Sequencer

Lots of electronic music tracks have some kind of pattern that repeats for three bars and then the fourth bar is different. This could be realized using a 16 step sequencer where you set up the first 12 pots in a repeating pattern of length four and then the pots 13-16 with a different pattern. However this would be annoying, since you need to exactly tune them the same and also because it would be a waste of resources for all the pots and take up a lot of space. The idea of this design is to get kind of a pseudo 16 step sequencer that can cover that special case but still is compact and not a full sized 16 step sequencer. The bad news is that you have to cheat a bit since the design uses a micro-controller and therefore isn't completely analog any more. If you're ok with that then this is how it works: There are four switches, each switch selects if the first four or the second four steps are selected. Then the sequencer goes from switch one to four always playing the steps that are selected. If the switches are on off on off you end up with a regular 8 step sequencer. If they are off off off on you end up with the pseudo 16 one described above. and off off off off as well as on on on on result in a 4 step sequencer. Switch 5 is used to revert the direction. The additional pot can be used to reduce the total length if you want to set up some kind of poly-rhythm by tuning the length to something that isn't a power of 2. The gate buffer isn't needed but it prevents the micro-controller from blowing up and releasing blue smoke if you accidentally plug in some signal that is above 5V to the clock for some stupid reason. If you think you do not need this buffer you need to change the code, since it expects an inverted signal for the clock.

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16 Step Euclidean Trigger Sequencer

The nice thing about euclidean rhythms is, that they always sound good, no matter what you dial in. So if you want to be sure to not mess up your set when performing after having a few drinks or whatever relying on euclidean sequencers is a good option. The first time I heard about this concept was at the 25C3 (see recording at media.ccc.de) and I directly liked the concept (see also the paper by Godfried Toussaint - The Euclidean Algorithm Generates Traditional Musical Rhythms. From there it only took me 15 year to finally start and realize a project based on them^^. The result is a 16 step sequencer with 4 channels. The fill (number of beats per 16 steps) and start of the sequence for each channel can be selected by a rotary encoder with integrated pushbutton. The current pattern gets displayed on a Neopixels strip. I've chosen to use through hole Neopixels which are a bit more expensive, however they better fit into the style of the other modules. There are some similar builds out there that use circular 16 Neopixels boards for the display, but I think a sequence has to go from left to right and not around a circle (but probably you can have some different opinion here without being completely wrong).

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8 and 16 Step Trigger Sequencer with lots of blinky Buttons

There ar lots of drum sequencer out there that use some NeoTrellis like interface, including some modules in eurorack format as well. This is just another one, that nobody asked for. The idea for this build is to have a simple trigger sequencer to use for the basic percussive instruments like kick, snare and hi-hats for a basic 4-to-the-floor rhythm. Since the snare typically only comes on every second kick we need at least 8 steps. Furthermore, it should also be able to play polymeters as well as some longer sequences with 16 steps. This is one of the tracks I really like which has an additional sound at the last 16th note before the end of the barMarco Carola - Avalanche. Another thing that's really important for me is that you do not have to dive into some menus when programming the sequences, especially when you want to perform live. Therefore this sequencer only has four sequences as there are also rows of buttons. Pressing a button toggles the trigger at this step on/off. Pressing a button for >1 second reduces the length of the sequence to the length where the button has been pressed and allows setting up polymeters. Pressing the first button in a row for > 1 sec however will turn to a different mode. In the second mode the first 4 LEDs/buttons count the current 4-step-interval, and the second 4 LEDs/buttons the current step inside the interval. This way you can directly turn a trigger on off at any time and also directly can see all the sequence settings at any time. Again the gate buffer isn't needed and just there to protect the ATmega if you accidentally plug in a 12V Signal to the clock in.

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--------------- EFFECTS AND FILTERS ---------------

VCF - Voltage Controlled Filter

When using a classical oscillator the only wave shapes that come out of it are a typically a sine, a triangular wave, a saw tooth and a square wave. The sine and the triangle sound relatively boring. The saw tooth and the rectangular one sound a lot more "analog synth like" but they contain so many high frequencies that they hurt in the ears when played loud. That's why you typically want to low-pass them. During this process what is typically done as well is to amplify the cutoff frequency. Adding some voltage control to shift the cutoff frequency leads to a whole new range of sound that you can achieve. The schematic here is taken from the Youtube Video by Moritz Klein without any modification, and any details are as always explained extremely well in the linked video.

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Spring Reverb Tank Driver

A reverb can add so much atmosphere to any sound. Also the concept how a classical spring reverb works is just to cool to not build one. Basically its just a coil (as in a speaker) that is connected to a long fluffy spring. The coil then shakes the one end of the spring in the frequency of the audio put into it. On the other side of the spring there is a second coil which acts as a microphone that picks up the vibrations in the spring. Since the vibrations inside the spring get reflected at the ends to some amount they bounce forth and back a few times which results in the reverb sound everybody likes. I've put together the schematic based on Rod Elliotts discussion on spring reverb circuits and it works pretty well. However the feedback I got was way too strong and the tank immediately started to self oscillate as soon as the feedback was not turned to zero. That's why I've limited the feedback by the resistor in the feedback path so it really only starts self oscillating if you turn it up to the max.

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--------------- AUXILIARY MODULES ---------------

Output Mixer with included Low Pass Filter and indicator LEDs

Since you probably do not want to play only the drum or only the bassline at a time you need some circuit to mix everything together. This can be realized pretty straight forward with a summing amplifier (which is just a single op-amp with some resistors). However before sending the signal to the amplifier I wanted to add an additional step of filtering it. Sure, in most cases the signal - e.g. from the VCO - already gets filtered, but when you open up the filter all the high frequencies will get through and there might be lots of them e.g. in a saw tooth that really hurt in the ears. Therefore this circuit just filters everything above 15KHz with a 5th order butterworth filter. This might be a bit over engineered but anyway, it works. Also the output is connected to a Cinch-socket which is mounted at the side of the case so you can directly connect it to the main DJ-mixer or your stereo. In my setup I currently have four of these mixers each going to a different channel on the DJ-Mixer. Also there are some indicator LEDs that are really useful to see where a signal is going and can prevent getting too much confused by all the cables. Of course the mixer is not limited to four inputs. You can add as many additional inputs as you want.

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Attack (Decay) Sustain Release (ADSR) Envelope Generator

When a note is played the pitch can be mapped to some frequency. Depending on the instrument there are some different overtones that lead to the characteristic sound of each instrument. However, what changes the sound evenly drastic is how the envelope is shaped. E.g. the sound may abruptly be turned on, and then slowly fade out (like e.g. its the case for a bass guitar). Or the volume might slightly rise and then fade out again (e.g. like you can play on a violin). Of course you can also shape other things like with the envelope - like the pitch of a note, the filter pitch, and much more. In summary this means in any case you need some envelope generators. This schematic is taken from this Youtube Video by Moritz Klein without any modification, and any details are explained extremely well in the linked video.

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VCA - Classic Transistor-Based Voltage Controlled Amplifier

A Voltage controlled amplifier is also one of the basic build blocks every synth needs. Without it there will be only endless drone noises without any rhythm or modulation. Again the schematic is based on a Youtube Video by Moritz Klein without any modification, and any details are as always explained extremely well in the linked video.

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Buffered Multiple

Multiples are needed to split a signal into many. In most cases you can just solder together some mono-jacks and the multiple is done. However there are some cases where it makes sense to buffer the signal. One reason could be if there are some modules with a low input impedance. Then the voltage may drop and e.g. if the second signal path is used as CV for a VCO the sound may get detuned. Another reason where it makes a lot of sense to buffer the signal is for the clock. Imaging you have your clock signal going to many sequencers. Now during the performance you pull out a patch cable because you want to plug it into some other sequencer. If you accidentally short the signal while doing this a clock step might get lost and the beat will stumble. If the signal is buffered this cannot happen, and to prevent this was the main reason for me to build some of the buffered multiples. Each of the multiples has four outputs but I only put 3 on the front panel since there is not enough space.

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Case

Simple case build of 6mm plywood and painted black. It has two cinch sockets at the side, that internally connect to the 4 channel output mixers. Currently I have two of these cases in my setup and connect the 4 channels to the 4 channels of a Pioneer DJ-Mixer. One of the cases holds the "drum machine" with the kick, snare, hi-hats, cymbals, toms, claves,... and the other case is for the bassline, rumble and other additional sounds.

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