This is the Digisound-80 VCO. The answer to my DIY VCO prayers. Easy to build, easy to tune and all the extra's like Synchronization, Frequency-Modulation and PWM.
Not only is this VCO easy to build, it 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 think the fact that he left out the HF tracking had something to do with that. This 'new' design however changed 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
because I already have one of those build in as a separate module. I can tell you, these VCO's (I built five so far) 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 this Digisound design isn't even that different from the Datasheet design. Except for the extra trimmer, the Hard Sync options and a few resistor value changes, but this makes all the difference in the world. As a first time synth builder and having been into modular synths for only 6 months (at the time of writing this article) this VCO 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. You can also use a TL084. (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.
The picture below shows the stripboard layout that I made and used for my build(s). 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 center detent. 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 practice 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. Use a good quality polystyrene or polyester or silver mica type capacitor for C7 (1nF). This is the frequency determining capacitor and must be stable with temperature changes.
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. According to the datasheets for both the CEM3340 and the AS3340 the chip can be fed with a voltage between +10 and +18 Volt and -4.5 to -18V. I wouldn't run it on any lower voltage than +12 Volt though.
Further down this article, in the 'Tuning' section, I mention that if you experience problems with tuning while running this VCO at 12V, you can make R4 bigger. Use a 270K or even a 300K resistor instead of the 200K. Some people experienced problems because trimpot-A was at its end before the VCO was in tune. Making R4 bigger will prevent that.
When running this on dual 12V you must also change C7 from 1nF to 0.5nF or 500pF. Otherwise you'll only get very low frequencies out of this VCO. This is not in the Datasheet but it has been established by feedback from many users who built this VCO for Eurorack.
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 although the Octaves control is not linear, at least mine wasn't but maybe that is due to the potmeters with center dent I used. 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. This differs a bit with each build, due to potmeter tolerances. The latest VCO I built only had a bit of throw left on the clockwise side, the rest was okay. 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 this only really works for changes in room temperature. If you put it over a heat source like a power supply it will most definitely de-tune. Of course other components around the chip will also warm up and add to the de-tuning of the VCO when influenced by the heat from the powersupply.
Here's the layout. I didn't put in the input jacks for the sync inputs or the output jacks for the wave forms and CV-OUT. 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, three and four in my synthesizer. My personal advise would be to use this layout for your build:
And here's a close-up of the print for this version:
Bill of Materials.
(07-June-2020 Revised version. Numbering now follows numbering on schematic.)
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. In this picture you can also see the annoying little circle at the bottom of the AS3340 chip. Do NOT mistake this for the pin-1 indicator, and put the chip in the wrong way as I once did!! (Although the chip survived, they don't like it.):
Here are the two identical VCOs side by side in my synth. You can see a 'Tuner' and 'Sync Out' output, which I added lately. More on that in the 'Synchronizing' section below:
About the CV-OUT connector:
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 daisy-chain more VCO's to one 1V/Oct. signal so that all VCO's receive the 1V/Oct signal from the Doepfer A-190-3 MIDI to CV converter. This is not included in the stripboard layouts but you can see it in the schematic drawing. Do not use the CV-OUT as a CV-IN because it shares a 100K resistor with the 1V/Oct in and will detune your VCO. If you use the
Dual Buffered Multiple described on this website, then you don't have to include this CV OUT and you can spread the 1V/Oct. signal over all VCO's with the Multiple. But I do advise to include it. If you daisy-chain your oscillators like this you keep the Buffered Multiple free for other functions and you can daisy-chain upto 8 oscillators of this design before you'll get a slight drop in voltage in the 1V/Octave signal.
Synchronizing multiple VCO's:
I recently added two more outputs. One is parallel over the squarewave output socket and is used to connect the VCO to the negative Hard Sync of one of the other VCO's, so I can keep the Squarewave output free for normal use. The other output is in parallel over the triangle wave output and is used to connect the hacked Joyo tuner to the VCO, also to keep the Triangle output free for normal use. These outputs are not on any of the pictures or on any of the layouts but you can easily add them if you feel you need them. I find them very useful. If you want to synchronize multiple VCO's then just take a squarewave out from the main VCO into the Negative Hard Sync input of the second VCO. Take a squarewave out from the second VCO and connect it to the Negative Hard Sync of the third VCO etc. Now if you turn the Frequency Control of VCO-1 all the other VCO's will stay synchronized with the first one. Now you see why the extra outputs are so handy to have.
Doepfer A-190-3V MIDI to CV:
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 3 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. It also has a Velocity output and a 'Learn' option on CV-4. And it also has a USB input so I can connect the synth to my computer. Btw, I got the A-190-3
V which is the Vintage edition which means the panel is black with white lettering to stay in keeping with the other panels in my synth. It's only 5 more euro's than the normal grey edition. I just made a 20cm high panel and cut a Eurorack sized hole in it and mounted the Doepfer in there. Then I made a special adapter cable to go from the Eurorack power system to the one I invented for my own synth. (See
powersupply article).
OCTAVE SWITCH:
With VCO number 4 I changed the Octave potmeter for an octave switch as an experiment. I used a 10 step rotary switch and I measured out a bunch of 10K resistors so I had 10 with the same resistance upto 10 Ohm accuracy. They were all 9K99. I soldered on the resistors in the way shown in the drawing below:
So you get 5 Octaves up and 4 Octaves down with a 10 step switch. If you want -5/+5 you'll need a 11 step switch and I think they are rare ^___^. This is more then enough though.
Now this works but it is not the case that when you turn the switch you land on the exact same note as the previous Octave. To try and address that problem I exchanged the 10K resistor connected to -15V for a 10K multiturn potmeter, with a 2K resistor in series with it, going to -15V. Now it's not possible to tune it so it is spot on but I did manage to tune it so that each octave I go up, I can turn the Fine Tune one stripe up on the dial (decal) and I'm in tune. So you switch one Octave higher, you turn Finetune to the next stripe on the dial and you're bang on. And the same but backwards for switching down the octaves. This works well enough for me :) This will really only work well if you also have a hacked
Joyo tuner connected to the VCO so you can see what you're doing.
To get this bang on the right note, you would need to experiment with the 3M3 resistor and try to buffer this potmeter and use really accurate resistors. So I wouldn't advise this switch solution, as it is presented here, for any serious project where everything needs to work perfectly.
Here's a picture of the switch in the panel:
The output waves:
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 outputs are at the same 10Vpp level. Perfect!
TUNING THE VCO:
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.
- Open up the 'Gain' potmeter on your VCA so you get permanent sound.
- 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.
Extra info for tuning on +/-12 Volt and using the V3340 chip:
As mentioned earlier, if you're running this VCO on +/-12V and you have trouble tuning it, change R4 from 200K to 270K or 300K. 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 personally use the negative Hard Sync input for syncing up two or more VCO's. I input a square- or ramp-wave from an other VCO into Neg H.S. and then they both react to pitch changes of the main VCO. The FM input is also very cool to use. 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. You can also input a Control Voltage from an LFO to get Vibrato or Tremelo effects.
Finally and by request, here's a list of individual notes and their corresponding voltages, should you want to tune the VCO without a keyboard, using an accurate voltmeter. Ignore the 'Expo Output' column. It is not relevant to this VCO:
Here's a picture of VCO number three installed in the second case of the Bergman-Berlin synthesizer :)
Btw, I recently built a version with the sinewave section included but can't get the sinewave to work using the original Digisound schematic with the CA3080 chip. Maybe my chips are Chinese fakes though, I don't know but I resorted to using a discrete transistor based triangle to sinewave converter. This VCO gives you a choise in output voltage for the Triangle and the Sinewaves between +/-5Vpp or 0 to 10Vpeak-to-peak. Any future Digisound VCO's I might make will be made using this panel design and using the triangle to sinewave converter I've built into this one. I'll put up schematics for that in the near future because it is a slightly different design than the one in the Triangle to Sinewave Converter I describe on this website in
chapter 3.
Here's a picture of VCO number four with the sinewave output. I have some more experimenting to do but there will be extra info on this website soon on adding the sinewave option:
The panel I made for VCO number four is 2 centimeter less wide than the other 3 which are 10 CM wide. I included an output labelled 'Tuner' to which you can connect the
JOYO Tuner after you hacked it. That is connected in parallel over the 10V Triangle output, soldered to the 5V/10V switch directly. (This is not included in any layout or schematic because it was a last minute addition to keep the normal outputs free.) As I mentioned earlier I later also added an extra output in parallel over the Squarewave output to connect the VCO to the negative Hard Sync of an other VCO so as to keep the regular output free for normal use.
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.
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>
