Tuesday, 14 January 2020

Synthesizer Build part-18: A REALLY GOOD AS3340 VCO DESIGN!!

The answer to my DIY VCO prayers. Easy to build, easy to tune and all the extra's like Synchronization, Frequency-Modulation and PWM.

This VCO is easy to build and can actually be tuned easily too. Before I found this, I used the datasheet VCO schematic for the AS3340 using the stripboard design from the LookMumNoComputer website. I could never get that VCO in tune over a wide range of octaves and I couldn't get it to play really deep tones either. (I'm sure I was doing something wrong with that VCO. I never heared anyone else complaining about it.) This 'new' design however changes all that! After looking through all sorts of VCO schematics, like the ones on the Electric Druid site, I decided to go for the Digisound 80 design but I left out the triangle- to sinewave converter because I already have one of those build in. (Click here for the Converter article) I can tell you, these VCO's (I build three) sound soooo much better than the Datasheet VCO. Of course it's the same waveforms but the range is so much bigger (0.1Hz to 50kHz!) and tuning this VCO is a breeze! And having two of them side by side I can slightly detune one and get that frequency beating effect. As a first time synth builder and having been into modular synths for only 6 months (at the time of writing this article) this was a real revelation for me. You can even use this VCO as an LFO, a Low Frequency Oscillator, because it goes down to 0.1Hz. If you're looking for a good AS3340 VCO to build, I think this is it. It certainly is perfect for my synthesizer DIY project.

Here's the new version of the schematic with all the opamp buffer stages drawn in. All the outputs are buffered and the same with the PWM input. So a total of 4 buffer stages are used here, all housed in one IC, the TL074. (There's a link at the bottom of this article to the original text and schematic.) I did not include any de-coupling caps in this schematic because I don't use them. If you have a normal linear dual power supply there should be no need for de-coupling but I have included them in the stripboard layout.

(Last revised: 14-Feb.-2020 Added numbering to opamps and extra text. 23-March-2020 Emphasized connections with black dots and added remark about R4.)

The picture below shows the stripboard layout that I made and used for my build. This layout is verified and the placing of the buffer stages follows the numbering on the schematic. For the Octaves control I recommend you use a 100K potmeter with a middle notch. You know the ones they use for Balance control on amplifiers with a little click you can feel when you reach the center point. This is very useful to easily re-tune the VCO after you've been using the Octaves control knob. I put in a 47K resistor for R21 which is the pull down resistor for the squarewave output. It originally was a 10K resistor because the CEM3340 was used and that will work fine too. (Use 10K if you're using a CEM chip) It's stated in the datasheet for the AS3340 that it needs to be 51K but in reality it doesn't matter at all. For the current limiting resistor (R23) I put in a 1K. This is necessary because we will connect it to negative 15 Volt. It says to use a 910 Ohm in the schematics but I always play it safe and use a 1K resistor. I know it's overkill but meh... it works fine. If you're going to use this VCO with a dual 12 Volt power supply then use a 680 Ohm resistor for R23. (On the stripboard layout R23 is the 1K resistor going from pin 3 of the AS3340 to the negative 15 Volt rail.) These resistors are needed for both the CEM3340 and the AS3340 ICs. The potmeter for High Frequency Tracking or Linearity can be a normal trimpot, not a multi-turn one. The influence it has is minimal. But you must use multi-turn trimpots for A and B on the layout, otherwise tuning will become very difficult. I used metal film resistors with 1% tolerance in those places where it matters and this is good enough. The 100K CV input resistors should all be measured and matched so they all have the same resistance value. This makes it easier when you connect different CV sources to those inputs, they will be in tune straight away. I was surprised that the two 100K potmeters I used in the panel for Octaves and for Fine Tune give exactly the range that is stated in the original description. Octaves is plus and minus 5 octaves and Fine is plus and minus half an octave. I'm not used to things actually working out as originally described in DIY projects. It's usually either a bit off or way off but the Digisound 80 designs are really good and spot on.
What's not really spot on is the range of the Pulse Width Modulation potmeter. There's a bit of throw left over at the beginning and the end. In other words 0% PWM starts at about one sixth in, counter-clockwise and 100% ends about a fourth before the end of the clockwise throw. You could solve this problem and spread the PWM over the entire potmeter throw by using resistors at pins 1 and 3. I tried to do this and experimented with different resistors with as a result that my TL074 blew up. Literally with a loud bang and magic smoke. I must have created a short circuit somewhere but I couldn't find it. Luckily I had a reserve VCO print all ready so I wired that one up and now everything is back to normal. This was a nice opportunity to install a jack socket with internal switch for the PWM so I could do away with the switch. This works much better. The AS3340 was fine after this incident. They are quite robust chips. Even putting them in the socket the wrong way around doesn't destroy this chip. (They call that 'burning in the chip', LOL) I actually did this once, by accident, and had it powered on for 20 seconds before I noticed. The chip had gotten really warm but it suffered no damage. The through hole package can be deceptive because it has a little circle on it which can be mistaken for the indicator for pin 1 but it isn't.
Don't place this VCO directly over the power supply in your modular set-up. If it gets influenced by the heat from the voltage regulators too much it can de-tune a bit but I think this is true of almost all VCO's. The AS3340 has internal temperature compensation but you shouldn't tempt it.

Here's the layout. Download it by clicking on it to enlarge and then right click and save as. Then you can zoom in on the components. It's a reasonably high resolution image. I didn't put in the input jacks for the sync inputs or the output jacks for the wave forms. It's already spaghetti junction and that would make it even worse. I assume you know how to hook up jack sockets. All potmeters are frontal view with shaft facing you. I have recently added 100nF decoupling capacitors directly between the IC's and 22µF electrolytic capacitors on the power rails, because this came up on Facebook. These are not included in the schematic drawing but they are in the original schematic in the PDF linked below. (You can use any value for the electrolytic caps between 10µF and a 100µF as long as they are 25V or over.) This is the version that uses a switch to go between internal Pulse Width Modulation or external PWM.

(Last revised: 06-March-2020: Added 100nF decoupling caps near IC's and 22µF electrolytics on the power rails.)

To make things easier, here's a close-up of just the stripboard:

Here's the wiring diagram for the VCO using an internally switched jack-socket to go between external- and internal Pulse Width Modulation. As you can see the 47K resistor has moved from the print to the wiper of the PWM potmeter or the switch connection of the PWM jack-socket, which ever you prefer to use. This is the version I built for VCO's two and three in my synthesizer.:

And here's a close-up of the print for this version:

Bill of Materials:

Here's a look at the finished stripboard. I soldered on a little copper eye to make mounting the print on the particular panel I made easier, but there's room enough left on the print to drill a few holes to mount it however you like. Make sure the copper traces are cut so no contact is made with the bolt and nut etc.:

Here are the two identical VCOs side by side in my synth:

You can see in the picture that I have CV-OUT jacks on the VCO's. These are simply in parallel over the 1V/Oct. input jack so I can connect more VCO's to one 1V/Oct. signal. In other words, both VCO's receive the 1V/Oct signal from the Doepfer A-190-3 MIDI to CV converter. The Doepfer A-190-3 is the one module that I bought because I didn't trust myself to build one of these and I wanted the interface between the keyboard and the synthesizer to be absolutely fool-proof and reliable and it was certainly worth the €130 I paid for it. It adds a Portamento (or Glide) function to the synth and it has extra outputs for the modulation and pitch-bend wheels on the keyboard that you can connect to CV-2 for instance to get pitch-bend. I'm waiting for more patch cables that I ordered so I can hook these functions up. It also has a USB input so I can connect the synth to my computer.

And finally a look at the waves this oscillator puts out. All nice clean waves as may be expected from the AS3340 chip but the ringing issue in the downward slope of the squarewave, which I mention in article 2 of this build series, is still there even in this design. Although it is significantly less prominent. This ringing must be common to this chip or something. Anyway, it's not audible so no real problem. I thought the zener diode over the squarewave output resistor might help to eliminate this problem but it has no influence but you can see from the pictures below that there are only a few spikes and only on the lowest notes. The picture below shows the ringing at note C1. Only 3 spikes! They only occur on the downward slope of the squarewave and they have a frequency of 28kHz so well above human hearing capabilities.

At note C3 there's only one spike left and after that it is completely clean. Maybe it adds to the character of the sound though. Who knows ^___^

Squarewave. You can see that the ringing is not even visible once you zoom out on the oscilloscope image:

Here's the ramp wave:

And this is the triangle wave:

Just for fun, here's a Triangle and a Ramp wave after being mixed together and after it's been through the dual Korg MS20 filter. You can see the high frequency resonance, produced by the filter, on parts of the wave form:

You can see that the output voltages are all around the 10 Volt except for the squarewave which is 13.4 Volt. I recently received a batch of 10 Volt Zener Diodes from China and I have soldered those in over the squarewave output and now all signal output are at the same 10Vpp level. Perfect!

This VCO has 3 trimpots for tuning but we're only going to use 2 because the High Frequency Tracking or Linearity potmeter is not really effective for the lower octaves. So we leave that in the middle position. I have developed a tuning procedure of my own that is very simple and will get this VCO in tune over many octaves in less than 15 minutes.
If you don't have a useful tuner for calibration purposes but you do have a smart-phone then I recommend you download the 'Universal Tuner' app by Dmitry Pogrebnyak. It's available in the Google Play store for free. Of course any tuning app that displays frequency and notes will do. There's plenty to choose from.
Before we start tuning, turn the Coarse Tune or Octaves potmeter off with the switch on the panel and set the Fine-Tune potmeter in the middle position. In the original text the wire connected to the wiper of the Fine Tune potmeter is de-soldered, but I recommend just leaving it in the middle position. Take a little screwdriver and turn trimpot A up about 3 quarters of the way.  Now go to trimpot B and turn that down about 3 quarters of the way (it's not necessary to be accurate with this and it also doesn't matter which way you turn them. It's just for setting a start position.)

- Launch the Universal Tuner app. or the tuner of your preference.
- Now press key C5 on your keyboard and turn Trimpot A until note C5 is in tune on your tuner.
- Now press key C2 on your keyboard and turn Trimpot B until C2  is in tune.
- C5 will now be out of tune again so press key C5 and retune it with Trimpot A.
- Now C2 will be out of tune again so press C2 and retune it with Trimpot B.
- Repeat these steps over and over until the VCO is in tune.
- You'll notice that you will need to turn the potmeters less and less to reach the C notes. After a few cycles of tuning they will be spot on their respective C notes.
- If you find that you need to turn the trimmers more and more to reach the C notes then switch potmeters and use A for C2 and B for C5

We tune with the Octaves control switched off to prevent variations in resistance from de-tuning our VCO. This keeps the VCO nice and stable but if you want it to be in tune with both control potmeters in the middle position then you can re-tune the oscillator after the first tuning session but now with the Octaves control switched on. It's up to you. :)

Be precise with the final tuning. Check the exact frequencies of the C notes. The app I mentioned will display the note graphically and it shows the frequency. You can get it in tune to at least 1/10th of a Hertz although in my experience you don't have to go more than one figure behind the comma.

If you're running this VCO on +/-12V and you have trouble tuning it, change R4 from 200K to 270K. Someone kindly reported having trouble tuning this VCO on 12V and reported this as the solution in the comments below, so I thought I'd include it here. That's why the comments are so useful. If you come across problems like this please report it down below and we'll find a solution.
I've also had a comment on Facebook about the V3340 chip not holding tracking when used in this circuit. I have no further details on that, but just so you know. It's recommended you use either the AS3340 or the CEM3340 chip.

It will usually be the case, when we start tuning, that the notes are too far apart rather than too close together and if you repeat the steps above and keep switching between C2 and C5 and using trimmer A for the high note and B for the low note, you will notice, as mentioned before,  that the notes get closer together and you'll have to turn the trimpots less and less to hit the right note. Eventually you will be spot on and the VCO will be in tune over at least 4 octaves. Be careful that you don't overshoot but you'll notice that soon enough if you have to turn the trimpots more and more to hit the right C note. In that case switch trimpots and use trimpot A for the low note and B for the high note.
You can of course use even higher octaves and other notes, like tune it between A2 and A7 for instance. I leave that up to you. I don't use C1 for tuning because it is so low my phone with the app has trouble tuning into it.
You'll get the hang of this tuning proces soon enough. It's really simple. It took me just 15 minutes after turning the VCO on to have it perfectly in tune, and when I say perfectly I mean perfectly! I was really chuffed about this ^____^

For those of you who want to go the scientific route and calibrate the VCO with a scope and an accurate voltage source and meter, I posted the original calibration procedure below here and it's also in the PDF I link to at the bottom of this article.
(click on text to enlarge)

Here's a little overview of features and technical data about this VCO:

Frequency range:                    0.1Hz - 50kHz
Most accurate freq range:          5Hz - 10kHz
Waveform amplitude:                0V to +10Vpp
Octave adjust control range:     +/- 5 Octaves
Frequency Fine control range: +/- 0.5 Octaves
+ and - Hard Sync
Soft Sync
FM input with level control
CV-2 input with level control
Pulse Width Modulation both internally controlled and externally controllable.
Extra CV inputs can easily be added by using 100K resistors connected to pin 15 of the VCO chip. Measure the resistance of the 100K CV input resistors and make sure they are all the same, that way anything you connect to the inputs will be in tune right away.
All outputs are protected and can be short-circuited continuously without damage to any components.

The positive Hard Sync doesn't obviously do anything but I think it needs a pulsed signal. I'm getting excellent results with the other inputs. Negative Hard Sync and Soft Sync all work fine and FM also gives great results. I can't describe how it sounds but if you build two of these and input one into the other and you turn the Octaves control back half an octave on the oscillator connected to the inputs, you're gonna get some great results. I have all three VCO's synchronized by having a squarewave from VCO-1 divided into two signals in the mixer and inputting them into the Soft Sync inputs of VCO's 2 and 3.

Okay, that's it for this one. I hope this is useful to you. After searching for a good VCO design using the AS3340 chip it was a real relief to see that this VCO performed so well and was so easy to build and tune too. I wish I found an article like this one when I first set out to build my first VCO but now I've written one of my own I really hope it will help out all those of you who are building their own synthesizer, maybe for the first time like me, and are looking for a good VCO design.

Here's a picture of VCO number three installed in the second case of the Bergman-Berlin synthesizer :)

If you found this article useful or if you have any questions, please leave your feedback in the comments below. I would really love to hear from you. Share this article with your friends and follow this blog to keep informed of new posts. There are buttons for sharing on social media right below here. It would help to maintain this website. If you want to support my work you can buy me a coffee. There's a button for that if you're on a PC or MAC under the menu in the sidebar.
Thank you for checking my website out and see you on the next one! :)

Here is a link to the original PDF file with all the text and schematics and tuning procedures for the Digisound 80 VCO: <click here to download or read the PDF file>


  1. Thank you for this project.
    Today was my first try with this VCO but I have failed :P
    I think I have some short on the circuit because my PSU gets hot. Can I use this schematics to power it with 12V? I am not sure which resistors should be smaller because is different to "stripboard schem"... Well, I will play with it tomorrow!
    Best for you!

    1. Oh sorry to hear it didn't work for you. I have had problems too with stripboards that had short circuits between the strips. That could be the problem. The circuit itself is good because I have build 3 of these now and they all work very well. If you want to use it with 12 Volt you need to change R23 to 680 Ohm. Good luck tomorrow. Let me know if you have any questions.

    2. The resistor you need to change is the 1K on the stripboard layout that goes from pin 3 of the AS3340 to -15 volt rail.

    3. Don't be sorry for my mistakes :) I am still learning and yours projects with schems are like a playground for me. I will let you know about my progress.

  2. This comment has been removed by the author.

  3. Eddy,

    First of all thank you very much for documenting your work like this. You and a few other folks have made getting into DIY synth stuff so much easier. I searched for quite awhile, but this is the first VCO build I found that looked like it hit the sweet spot between fully featured and too simple (I was aware of the Look Mum VCO, but the poor tracking is an automatic pass for me).

    I built this to be powered by +/-12V. The R23 resistor change was the only change I made, but I was unable to get the oscillator tuned properly. No matter which trimmer (A or B) I used for C5 vs C2, which trimmer I set first, or where I set the trimmers to start, I'd eventually run out of travel on trimmer A before getting it all the way in tune. It would be at 100k and I'd still need some additional resistance that I couldn't get from it.

    To fix this, I replaced R4 with a 270k resistor and got it tuned up on the first try after that. Sounds excellent! I'm not sure if it's because I'm running this off +/-12V or it was something else that caused that. I did make my own stripboard/perfboard layout for this in order to get its size down, so it's possible I messed something else up in the process. It got a little convoluted at the size I was aiming for (10HP eurorack). But besides that tuning issue, everything seems to work as expected, and swapping the resistor out worked just fine as far as I can tell. Just wanted to mention that here in case someone else has the same issue. Thanks again for posting this!

    Pics of my messy build:


    1. Thank you so much for sharing your experience with this build. I'm sure is the 12V conversion that gave you trouble but loads of people ask me about running it on 12V. I'm glad you got it working in the end.

    2. I've made a note of your tuning problems and the solution you found, in the article. I'm sure other people will experience this problem so I'm glad we can offer them a solution. Thanks again for commenting about it!

  4. Hey
    I'm quite the beginner and building my own DIY synth as my High-school graduation project. I'm fascinated, by all the modules you actually built and want to build some of them on my own.
    But when it comes to capacitors I'm having some difficulties. Until now I've only been using cheap Ceramic caps but in your layouts some of them look different.
    I'm quite certain that the blue ones should be Electorlyte because they dont look symmetrical, indicating they are polarised. Also when you called for polystyrene ones I will try to get my hand on those. I know that they should be much more stable than other types and are used a lot in audio curcuits.
    But is it ok now for the other ones to use carbon disc ones? I heard that they can add a lot of noise, especially when they are under vibration.
    Also someone told me this:
    A pretty good rule of thumb that I think most people use and is reflected in vero layouts:
    Anything in pF-- ceramic disc
    Anything in nf-- Polyester film "greenies"
    1uF or over-- Electrolytics (although these will be specifically noted in vero layouts due to polarity).
    Should I go by this advice, and what did you use? And what colors stand for what type?
    Thanks for your help.

    1. Hi Leon, that rule of thumb is okay for 99% of the cases. I use a lot of ceramic caps in my builds and I never had any noise issues with them. Many of them are vintage ones, I have a large stock of components I got out of other circuitboards. The one issue with ceramic caps is that they are temperature sensitive and they can be prone to 'microphoning', they can react to vibration, like you already mentioned. You can go-ahead and use them for almost anything but in oscillators there's always one cap that is responsable for the frequency and that should always be a stable polystyrene or silver mica type capacitor. Otherwise your VCO will go out of tune with temperature differences. Also in filters, the caps responsable for the actual filtering will almost always be polystyrene or silver mica types, but it's always stated what you should use. Most caps in the pF range are used for filtering of power supplies or to stop opamps from oscillating and they can be ceramics, no problem. The blue ones are indeed always electrolithic caps and the little light blue stripe indicates the negative pole. I use both axial and radial electrolithic caps in my designs so the elongated ones are axial, but is doesn't matter what type you use as long as the value is correct. Electrolithic Caps directly on the power rails can be any value from 10 to 100µF. That doesn't really matter. In the layout software (DIYLC) I use the yellow symbol for ceramic caps for everything that is not polarized, so if you see a yellow capacitor in my layout it can be any type that is not polarized but if it is anything other than ceramic it will always be noted, usually on the layout itself and otherwise in the accompanying text. Okay, I hope this helped you out and if you come across anything you don't understand, just ask me. I'm always here to help out ^____^ Good luck with the build. I hope you'll enjoy it as much as I did.

    2. Hey
      Thank you very much. This really helps me a lot. I think I'm all prepared now.
      I will give you feedback, how the whole project turned out when I'm done.

    3. Excellent! I look forward to finding out how you experienced the build and what, if any, problems you ran into. It should be pretty straight forward though. And contact me if you have any questions. Good luck!