Synthesizer Build part-57: X-4046 VCO by THOMAS HENRY.

A fantastic sounding VCO with 5 waveform outputs and an amazing hard sync sound! Quite easy to build too. This is a Kosmo project, not Eurorack. At least not this particular article. But this VCO will run fine on a dual 12V powersupply.

PCBs for this VCO are in stock again, with newly designed boards.

Finally a new VCO project on my website. These are always the most popular projects as I can see in the data I get from Google. And this is a really nice one too. 

It's a favorite among Psy-Trance and Techno producers! I'm reliably informed by a top Dutch psytrance producer that the hard sync and FM functions of this VCO are better than that of the Nord Lead. He regards this VCO as his secret weapon creating unique sounds not heard on other albums (see demo below).

It has no less than five waveform outputs. The usual ones: Square/Pulse (with PWM), Sine wave, Triangle wave and Sawtooth wave and then there's the Rampoid wave. This is a mixture of the Triangle and the Sawtooth waves and there's a potmeter to go between the two which makes for some really cool wave shapes. See the scope pictures below.

One little side note though, this VCO is known for being difficult with tuning. It'll tune fine but the tracking over the octaves is not very precise. It's good enough to make the VCO useable of course but you will spend some time tuning and it won't be perfect. Just so you know!

[Edit: Dec 2025: I just want to add to that last remark that I just finished building this VCO using all PCB's and using a Texas Instruments CD4046 and I got it in tune over 5 octaves with +/-3 cents so that ain't half bad. I used just the V/Oct trim and the fine tune pot on the panel to get back on the exact notes and if the high octaves were a bit off I used the HF-trim, which on this VCO actually works. I also built a 4046 Super VCO with the suboscillator from the VCO Maximus and that one tuned very well too.]

When I was researching the VCF-1 filter (project 56) I found the 'Birth Of A Synth' website with all of Thomas Henry's projects on it and in the list was this VCO. I came across this design before and I always wanted to build it because the TH VCO-555 was also such a good design but also because of this VCO's famous Hard Sync sound.

The finished module.

The 'Birth of a synth' website also lists another VCO, the VCO-1 but if you look at the schematic it's just a scaled down version of the X4046 so I went with this design.

SCHEMATIC:

Below is the schematic for this VCO. The CD4046 is an interesting chip to use for a synthesizer VCO. It has an onboard voltage controlled oscillator and two types of phase comparators. The IC has been used in some very cool Eurorack modules too, among them the Wiard Wogglebug.

The 4046 produces 2 halfs of a sawtooth wave coming out of pins 6 and 7 depending on the state of the VCO output of the chip (which is a squarewave). When it's high, pin7 is ramping. When it's low pin 6 is ramping. So instead of picking the signal from the output of the chip, Thomas Henry picked them straight from pins 6 and 7. These are then combined to form a composite sawtooth wave and that's why we have to set a trimmer to match them up to eachother to form one wave. The triangle wave is made by inverting one of the sawtooth halfs and applying an offset voltage to it to glue them together to form a triangle wave. This has the side effect of creating those spikes you see at the top and bottom of the triangle wave. Once we have the triangle wave it isn't too difficult to turn it into a sinewave by using the tried and tested transistor pair converter method. The squarewave is made by sending that sinewave into an opamp comparator which flips polarity everytime the sinewave goes over a threshold voltage set by the PWM opamp. Varying this voltage produces a change in the dutycycle of the squarewave.

The opamp used in the exponential converter, with the inputs of the VCO, is again an interesting one, the LF442. This is a modernised version of the LM1458. It has the same input characteristics of the LM1458 but only draws one tenth of the current. In addition the well matched high voltage JFET input devices of the LF442 reduce the input bias and offset currents by a factor of 10.000 over the LM1458. This ensures very low voltage drift and it also has very low equivalent input noise voltage for a low power amplifier. Seems like a good choise then ^___^

New LF442's are very difficult to find and I struck it lucky when my local electronics shop still had two of them in stock but as an alternative, the 4558 will do nicely or the TL072. They all work fine in this VCO. I tested them myself.

Here are the main features of the X-4046 VCO:

Exponential control and modulation.

Linear modulation.

Five unique waveform outputs: triangle, sawtooth,pulse with pulse width modulation, sine and variable rampoid. All waves are roughly 10Vpp through zero. (+/-5V)

Frequency range goes al the way from roughly one cycle every 24 seconds (41.7mHz) upto around 16.4kHz. so this VCO is also usable as a LFO.

And as the article in 'Birth of a Synth' states; one of the finest hard sync effects ever heard from a VCO.

The original schematic has the circuit running on a dual 15V powersupply but I changed it to dual 12V power for this project.

THIS VCO WILL RUN ON BOTH +/-12V OR +/-15V. I first built my VCO to work on +/-15V but I also tested it on +/-12V and it works just as well. Even the tracking wasn't much different when I switched to +/-12V so because most people build for 12V (Eurorack and Kosmo) I revised the schematic. Some waveforms were a bit lower in amplitude though. Most notably the sinewave which dropped quite a bit so for that reason R18 and R19 going from Q4 and Q5 to the opamp were changed from 10K to 15K. Also the output for the pulse wave needed to be increased so I changed those output divider resistors to 2K2 for both.  The changes are made in the schematic and layouts.

Schematic:

LAYOUTS:

Below are the layouts for this project. As always they are verified. I used them for my build and I can tell you it worked flawlessly right from the get go. Not a single mistake! All I had to do was trim the waveforms into the right shape and the VCO was up and running. Oh and tune it for octave tracking of course. I have adapted the layout for use with +/-12V.

Here's the wiring diagram. We have 7 potmeters, 10 in- and output sockets and a toggle switch to wire up. It took me an afternoon and the next morning to get it done. I used 1M potmeters instead of 100K for all but the Frequency Coarse and Fine controls. The value of the panel potmeters makes no difference except the 'Skew' potmeter. That one should be 500K or higher. (I also used a 1M for that one) but I later used a 100K in my Eurorack version and that works fine too.

Stripboard only view:

I had some difficulty in placing the matched transistor pair Q4 and Q5 near to the opamp they need to be connected to, so I had to use some jump wires for that. The jumpwires are not shown in the layout. Instead I have marked the places where they need to go with 2 orange circles with the number 5 meaning this point needs to be connected to pin 5 of IC-4 and 2 yellow circles marked with the number 6 which needs to connect to pin 6 of IC-4. I used shielded wires and I connected the outer braiding of the wires to the ground strip underneath IC-4 (strip X) and these points are also marked with green circles with numbers in them. (Only ground the wires at one end)

However, you don't have to use shielded wires. Normal jump wires will work fine too. I just played it safe because the wires pass right over IC-1 but you can save yourself the trouble.

Here's a look at all the cuts and wirebridges that need to be put in place before you start putting in the components. There are 45 wirebridges to solder in:

A close-up of where the two jumpwires need to go. This image doesn't show the whole stripboard just a zoomed-in bit to show where the wires must go.:

Cuts only view, seen from the component side. As always, mark the cuts on the component side first with a waterproof Sharpie or Edding400 and then put a pin through the marked holes and mark them again on the copper side. Then cut the copper strips at the marked places. That way you have the least chance of making mistakes.

And here's the Bill of Materials. For the PTC I used the same one as I used on the 555-VCO. See project 37. That article has links to the webshop where I got them from They are 3300ppm instead of 3500ppm but that 200ppm difference you can ignore. It works just fine. If you don't have a 2K PTC then just put in a 2K resistor instead. 

BOM has been updates as per 14-5-2026. I noticed there were a few items missing.

BEST CHIPS TO USE:

It is mentioned in the article that certain 4046 chips are better for tuning than others. I used a Texas Instruments CD4046. It was good enough for me but if you want the best chip for this circuit the ones to get are: National CD4046, Fairchild CD4046 or the Motorola MC14046. This last one is the best one you can get. They are pretty difficult to find though so you'll probably end up doing as I did a go with the Texas Instruments CD4046 and take the tuning difficulties for granted. You'll still end up with a fantastic and very useable VCO. Actually, the stripboard version took a bit of time to tune but I later made eurorack PCBs for this VCO and they tuned just fine with Texas Instrument IC's. You can have the occasional bad chip in a batch though, so if you struggle to get it tuned change out the 4046 for another and try again. 

I've had some feedback that the Philips HEF4046B chips work very well in this VCO and they're easy to source in some parts of the world.

THE BUILD PROCESS:

As I mentioned before I had to use jump wires on the stripboard and because the wires pass right over, or near, the CD4046 I chose to used shielded wires. I connected the shielding to the bottom right ground strip (X). The outside shielding of the wires must only be grounded at one end. At first I used unshielded wires and actually it will work just as well so you don't have to used shielded wires. I just played it safe.

The transistor pairs need to be matched because one of the pairs makes up the current mirror for the 1V/Octave tracking and the other pair determins the shape of the sinewave. It's a classic triangle to sinewave converter design. I matched them by measuring the Hfe on my multimeter and choosing two that have the same value. You really should use the Ian Fritz method though. See the TB-303 filter project for an in depth explanation of how that works.

Just like in the 555 VCO, Thomas Henry uses a 2K PTC for temperature compensation. Luckily I still had a few left so I didn't need to order any. I recently ordered ten more because they are out of production. So when the shops are out of stock, that's it. No more PTC's. At least, not these ones.

After I had finished making the stripboard, I made the front panel and put in all the potmeters and sockets. I made a special mounting bracket for the stripboard out of plexi glass. I took a small strip of it and bent it at one end in an L shape, using a heat-gun. then I glued small squares of plexiglass to the top and bottom ends so the stripboard could sit inbetween them. Then I hot glued the stripboard to the bracket. It works very well. Here's a front and back view picture to illustrate:

 

Here are some more pictures from the build process:

I had already started putting in some components before I remembered to take a picture of the stripboard with just the wirebridges.

Stripboard finished but chips not yet mounted in their sockets. In this picture you can see the jumpwires and because this was in the testing phase they are normal not shielded wires. I later put in shielded to see if there was any difference but there wasn't so you can just use normal jump wires.

Everything ready for wiring up. That took me an afternoon plus the next morning. All the socket grounds are connected together through one copper wire which then connects to ground on the stripboard.

My faceplate design. Just white acryllic marker on black powdercoated aluminium, sealed with a clear lacquer coating, which is why it's so reflective :)

The finished VCO undergoing testing. I have a special power output on the side of my synth that I can use to test new modules. Very handy to have :)

CALIBRATING THE WAVEFORMS:

Calibrating the waveforms of this VCO is really straight forward.

For the different waveforms you just adjust the trimmers until the waveforms look good to you. Set the triangle wave first. Use the trimmers to connect the two halfs of the waveform. After that you can set the sinewave. Triangle connect does exactly what it says, it connects the upward slope to the downward slope. Very straight forward to set. One trimmer is for the upward slope and the other for the downward slope. You'll see frequency spikes appear on the top and bottom of the triangle wave. This is normal. They are so high in frequency that they can't be heard so ignore them.

You need to set the offset voltage for the Triangle and Sawtooth waves so that the zero Volt line goes nicely through the middle.

The Triangle wave is made up of two sawtooth waves put together so the Ramp or Sawtooth wave is half a Triangle wave and therefore double the frequency of the other waveforms. (One octave higher)

TUNING THE VCO:

Tuning the V/Octave tracking of the VCO is a straight forward process of going between high and low notes. I always use the C2 and C5 notes. 

Use the Triangle wave for tuning.

- Press key C2 on your keyboard and use the front panel coarse and fine tune potmeters to tune to C2.

- Now press C5 and see if you are higher or lower than C5.

- If you're lower, trim the V/Oct up a little bit. Don't make big changes and remember if you tuned up or down.

- Now both notes will be higher so press C2 again and use the front pots to tune to C2 and then press C5 again and see if you're closer to it.

Repeat this process until both C2 and C5 are in tune.

If all is well the notes inbetween shouldn't be too far off either. You'll always be a few cents off on C3 and C4 but that's normal with this VCO.

If the notes in between are way off then you can bet your CD4046 is to blame. I tried altering all the components in the tuning circuit but nothing influenced it except capacitor C3 and the CD4046.

If you can't get the VCO in tune before you're at the end of the throw of your V/Oct. trimmer, you can lower the capacitance of C3 (10nF). Put in a cap of around 8.5nF. 

I tested different CD4046 chips and I found they have different characteristics over the octaves. 

I made a graph to illustrate this:

The red line represents the curve I got with a random CD4046 and the purple curve are the results I got with another CD4046, both from Texas Instruments. You see, you have to try a few chips if you can't get it in tune correctly. The purple curve represents a good result for this VCO.

Changing the value of the 390 Ohm resistor won't help with tuning at all. Believe me, I tried that.

Lowering the value of R15 (10K) won't help either. One other cause can be if your transistor pair isn't matched. That would upset the exponential converter too.

The make of your 4046 chip is important. Look at the text under the Bill of Materials for a sum up of the best chips for this circuit. Motorola works best, Texas Instruments is said not to work so good but I found that this depends on the individual chip, as I mentioned before. The TI chips in my VCOs work fine and I've got them in tune over 5 octaves with just a few cents between them but a few chips in my batch were unusable.

These are the test results Thomas Henry himself got when tuning his VCO to track over the octaves:

Source: Birth of a Synth website.

OSCILLOSCOPE SCREENSHOTS:

Here are the standard waveforms. The spikes you see on the triangle wave are a characteristic of the VCO as I mentioned in the schematic section. They are very fast and way beyond the human hearing range (80kHz or over) so not audible what so ever.

When I looked at the sawtooth wave I saw it had a bit of a wobble on the oscilloscope. However this changed to rock solid once I started playing the keyboard. Consequent versions I built of this VCO using PCB's all had this wobble but it goes away once you start playing the VCO with a keyboard.

Here are some screenshots of triangle and sawtooth waves in the Hard Sync function for which this VCO is (rightly) well known. It sounds awesome!

Here's a look at a recently finished project, the 4046 SUPER VCO with the suboscillator from project 69:

DEMO VIDEOS:

Here's a test video showing the VCO in action through the Thomas Henry State Variable filter of the previous project.

Here's a video showing how top psy-trance producer Jake Jakaan (Alienatic) uses this VCO to make his signature awesome alien sounds, making full use of the Hard Sync and FM functions. He's using one of the 4046 Super VCO's I built for him but it's the same VCO as in this project.

Here's the video where someone is building this VCO on small pieces of stripboard connected straight to the panel by means of the potmeters.

Okay that's if for this project.

If you have any questions or remarks please comment below or post them in the special Facebook Group for this website.

PLEASE CONSIDER DONATING.

If you find this content helpful, please consider donating to keep this website in the air and to contribute to future projects. There's a Ko-Fi donation button underneath the main menu if you're on a PC or Mac. Otherwise use this PayPal link to cut out the middle man. Thank you very much for your help!

Synthesizer Extra's No.4: QUADRUPLE OFFSET BOARD for 3340 VCO.

Before I made Bi-Polar versions of the 3340VCO using 2 capacitors and an offset trimmer, I had the idea of using offset voltage on all outputs with an extra stripboard. In this article I explain how that would work. You can use this as a stand alone module of course and use panel potmeters instead of trimmers to set the offset voltage. It's up to you :)

The one thing that always bothered me about the Digisound 80 VCO is that the outputs signals are unipolar. They're 0 to +10 Volt peak-to-peak and that is not very compatible with the rest of the builds on this website. Because this Digisound 80 VCO is the most popular project on this website, I thought I would design a little stripboard that gives you 4 offset options to turn all the signals of that VCO into more useful bi-polar signals at -5/+5Vpp.

There are more elegant ways of doing this perhaps but this project is meant more for people who are beginning in the DIY synth hobby and who are building the Digisound-80 VCO as their first VCO and they want bi-polar signals from that VCO. If you're one of those people you can build this project to solve that problem. It's a very easy to build and cheap project.

Btw, the Digisound 80.6 Lowpass filter works well with the Digisound VCO because it has a 1µF capacitor on the input that shaves off the offset voltage But I don't recommend capacitors on the VCO outputs because they can also act as filters.

OPTIONS:

You don't have to build this module into the 3340 VCO (project 18) if you don't have room. You can put this stripboard behind a small panel with just 4 input sockets and 4 output sockets and attach that next to the VCO. That way you have a choise of either using the outputs straight from the VCO at 0 to +10V or to patch them through this offset board and get -5/+5V output signals. That way you can also use the offset module for other things like LFO's if you want to. 

You can even 'normal' the VCO outputs to the socket-switch lugs of the offset input sockets and save yourself the trouble of having to use patch cables.

You can even add bi-polar LEDs on the outputs so you have a visual indication of the Voltage they output. As you see, if you want, you can really go mad with this project but I leave that up to you. 

Anyway, we also have the Dual Voltage Processor project to cover that functionality and it has extra options too so maybe it's better to keep this project simple.

I put in four offset stages eventhough three will be enough for the 3340 VCO so you can use the other for something else.  

SCHEMATIC:

It's a very simple design. Just 4 dual opamps, in this case TL072's (but you can use other ones if you wish as long as the pinouts are the same) each with an offset trimmer that allows you to give a negative 5V offset to the signals coming from the VCO and so turn them into bi-polar signals.  I choose to give every stage its own offset trimmer so you can set them all differently should you need to, but in principal you could feed all four opamps with the voltage coming from one trimmer and so have them all produce the same offset. That's simpler but not preferable I think so I went with four trimmers.

The schematic below shows two of the four offset circuits that are on the stripboard but they are all the same.

If the output voltages of the VCO waveshapes are 8.2V instead of 10V and you want to crank it up to produce 10Vpp (-5/+5V) waveforms then change the 100K feedback resistor over pins 6 and 7 to a 130K. That will give just enough gain to get the right output voltage. 

LAYOUT:

Below is the layout for this project. 

(Layout has been updated on 9th of March 2024. Previous version had 2 little mistakes in it.)

Below is the layout with just the cuts and wirebridges, seen from the component side. As ever, mark the cuts on the component side with a waterproof Sharpie. Then put a pin through the marked holes and mark them again on the copper side. Then cut the strips with a sharp hand held 6 or 7mm drill bit.

If you decide to build this into the Digisound VCO module, then it may be better to do away with the eurorack connector and simply use wire connections for the power. That way you can make the stripboard more compact too. The lower 4 copper strips are not used either so you can cut those off too or use the space to house the standoffs to connect the stripboard to the rest of the VCO.

I did not actually build this project myself but I know it should absolutely work the way it's presented here so that should not be a problem. I built so many offset circuits while I've been doing this hobby that I can dream them.

Here's the Bill of Materials for this project. Also order four 100nF ceramic bypass capacitors. I forgot to put them in this list.

TUNING THE CIRCUIT:

You need an oscilloscope to set the offset trimmers to the right value. Make sure you set the oscilloscope to DC when measuring otherwise the scope won't show offset voltage. Remember offset voltage is a DC voltage.

Connect the signal(s) from the VCO output to the input(s) of this stripboard and then connect the scope probe to the output(s) and set the offset voltage so that the signal displays the same amplitude on the positive side as the negative side of the zero Volt line. In other words, set it so the zero Volt line cuts nicely through the middle of the signal. That's it. 

Okay, that's all I have to say on this little extra project. I thought it might come in handy because the Digisound-80 VCO is really a very cool VCO and now you can make the signals more compatible with the rest of the projects on this website.

I hope it comes in handy.

If you have any questions about this project feel free to comment below or on the special Facebook Group for this website.

Synthesizer Build part-37: THOMAS HENRY VCO-555.

One of the best sounding analog VCO's you can build, with 4 waveforms. It has excellent 1V/Oct. tracking. With verified layout. I also made a layout specifically for EuroRack with a stripboard that is cut in half and folded over. You can find that further down the article. This VCO project has very rapidly become one of the most popular on my website together with the 3340 VCO.

There are PCBs available for this VCO. See 'PCB Service'

This VCO is a brilliant design by Thomas Henry and he worked on it for a long time. He calls it his best design to date and it sure is that! VCO's of different designs, can sound different from eachother despite them producing the same basic waveforms. This difference is not really that noticeable if you listen to just the basic wave outputs. It starts to become more noticeable when you start playing with the Synchronization and the Frequency Modulation inputs. That's where this VCO really shines and I think that's why it is so popular in the modular synthesizer world. With the Hard Sync and FM functions the VCO-555 produces a very full sound, rich in harmonic content and very musical sounding. And it tracks very well over the octaves. Plus it's very stable with the PTC installed for temperature compensation. You'll find out once you build this module and start experimenting with it. It's the VCO of choise for many hardened Modular Synthesizer aficionados. (You should really have at least two of these in your setup.). The FM function of this VCO in combination with the Yamaha filter in Hipass mode is a favourite of top Dutch psy-trance producer Jake Jakaan (Alienatic). He says it's better than anything you can buy.

This is a medium difficulty project. Not one I would recommend for beginners and certainly not as a first project. You need to have a reasonable knowledge of electronics for this one and you can not do without an oscilloscope. Please read the entire article before you start building so you are aware of things you need to look out for. Be extremely accurate in copying the layout. Check and double check and mark the components you soldered in on a paper printout of the layout.

ABOUT THE CIRCUIT:

I wasn't going to post the schematic here and instead linked to it on the Electro Music forum but then the forum had occasional accessibility problems so here's the circuit. (The link to Electro Music also has the original parts list if you scroll down.) 

For the schematic below, just click on the image to enlarge it. Then right-click and 'save as...' Save it to your hard-drive. Then you can zoom in on it. In the lower part there are three resistors named R26. I think Thomas made a little mistake there:

The VCO uses no exotic chips. There's only two TL074's, an LM13700 and a TLC555. You can also use the ICM7555 (which is what I used) or the LMC555. Do not use a normal NE555 for this, you need the CMOS version. Although I'm told it will work with the normal NE555 these chips consume a lot more power and they can have oscillation problems under certain conditions (see my 81 LED chaser article.) The normal NE555 shorts V+ to ground for an instant as part of its normal operating routine and as you can imagine this creates extra noise on the powerrails. In circuits where noise is an issue (like VCO's) you definitely want to use the CMOS version.  

Instead of the LM13700 you can also use the LM13600. I tested this myself and there's absolutely no difference between them in this circuit.

I didn't put in the bypass capacitors for the chips. I have a bad habbit of leaving those out but you should include them. Solder them in over the powerrails on the stripboard as near to the chips as possible. The connections are shown at the bottom of the schematic. You can use small ceramic 100nF caps for this. You can solder in the two 10µF electrolytic caps over the power rails anywhere on the stripboard too. One from plus to ground (- to gnd) and one from ground to minus (+ to ground). The exact position of the electrolytic caps on the stripboard doesn't matter. Put them where you have room. Note these caps are not included in the Bill of Materials below.

For temperature compensation the circuit uses a PTC Thermistor which I guess is the only exotic component in this VCO. PTC stands for Positive Temperature Coefficient, meaning that when the temperature goes up the resistance also goes up. This temperature dependent resistor has a nominal value of 2K. However, it's not necessary to use a Thermistor. You can get away with just using a 2K resistor. It'll just mean that it isn't as stable as it can be, but many people use this VCO without the Thermistor. If it does go out of tune you can easily adjust the Frequency Fine Control and set it right. It is handy to have a hacked tuner like the JOYO tuner attached to the VCO to keep an eye on the tuning.

Capacitor C4 (2200pF or 2.2nF)  is the timing capacitor for the oscillator and therefore it should be a non-ceramic type like a Polystyrene or a Polyester or Silver-Mica type capacitor for temperature stability.

The VCO doesn't have an extra CV input because actually the Exponential FM input has that function. If you look at the schematic you'll see it is connected to the same pin as the 1V/Oct input and it has an attenuator too. The Frequency Coarse and Fine tune are on that same pin too. 

Here is an amazing Falstad simulation of this circuit made by Fabian Kempe: -- CLICK HERE --

ABOUT THE TRANSISTORS:

The two PNP transistors Q2 and Q3 need to be matched. In earlier builds I matched them using the Hfe transistor tester on my multimeter and this seemed to be good enough. However you really should use the Ian Fritz methode (see article 66 the TB-303 filter, for more info). That's what I used for later versions and the PCB versions I built.

The transistors need to be thermally connected to eachother on the stripboard. Look at the pictures below to see how I did this. For the first VCO I built I covered them in Heatsink Compound and bent some thin copper sheet around the bodies to keep them together. For the second VCO, I just glued them together with some super-glue. That'll work fine too. On the layout below, the transistors are mounted in such a way that you can bend them towards eachother so the flat surfaces connect to eachother. You can then bend the thermistor legs so that the body of the PTC rests on top of the transistors and then glue it in place. Once the glue is dry you can cover the PTC with a little bit of Heatsink Compound if you wish.

 

SOME OF THE FEATURES OF THE VCO:

The VCO has four waveforms: Sine-, Ramp-, Triangle- and Square/Pulsewave. 

All the waves have an amplitude of +/-5V so 10V peak-to-peak.

It's got a Linear and an Exponential FM input, one Hard Sync input and a 1V/Octave input naturally for the keyboard. 

Frequency range is roughly from 0,1Hz to 28.000Hz (28kHz).

The FM inputs have attenuators. If you connect a signal to the Exponential FM input the pitch of the oscillator will change, with it being connected to the same input that also drives the 1 Volt per Octave control voltage. The linear FM input has its own circuitry and it has a capacitor on the input, blocking any DC voltages so it behaves differently than the Exponential FM. 

There are trimpotmeters for Volt per Octave tuning (100 Ohm), High Frequency tracking, Ramp Wave connection (this makes sure the ramp wave has a smooth slope. If it is set wrong the ramp wave will have a step in it at the zero Volt level. There are two trimmers for the Sinewave. One for roundness and one for symmetry. The roundness trimmer will also change the amplitude of the Sinewave a little.

You will need an oscilloscope to set these parameters correctly, but a cheap 20 dollar one from eBay will do fine. Make sure you set it to DC when measuring.

At first I used multiturn trimmer potmeters for all but the 1V/Octave trimmer but I have changed that because I found it very tedious to tune the VCO with a multiturn trimmer for HF tracking. It's not necessary for HF trim.. I only use multiturns for the Sawtooth step and Sinewave symmetry and really only to save space on the stripboard. After due consideration I do advise to use a multiturn for V/Octave trimmer, for tuning.

When I built this module I had set all the trim-pots in the middle position before I soldered them in and when I started the module up, everything was perfect except for the tuning. Even the Sinewave was perfectly symmetrical right from the get go. So was the Rampwave :) Btw, the Rampwave is the reverse from what the 3340 VCO produces. It goes straight up and then slopes down. So I guess that is called a Sawtooth wave officially. I always get Saw and Ramp mixed up anyway so we'll keep caling it a Ramp wave ^___^

PULSEWIDTH MODULATION:

There's a potmeter for the Pulse Width Modulation which goes from 21% to 75% if you use the 330K resistor (R47) as seen on the schematic. I changed that resistor to 190K and now the Pulse Width goes all the way from 1.7% to 95%. You can also put in a 200K trimmer with a 47K resistor in series so you can set the range you want manually. If you don't have a 190K resistor, use a 180K or 200K, just the closest you have or combine two resistors in series to make up the right value. The layout below uses the 190K resistor instead of the 330K of the original schematic.

Grounding troubles:

Make sure the potmeter for External Pulsewidth Modulation has a ground wire that connects straight to the Ext. PWM input socket. I had problems with it, in so far that I couldn't turn the external signal off completely by closing the potmeter. There was still some external modulation going on. It turned out that the cause was that the input socket was only grounded through the metal of the front panel. When I soldered in a wire from the socket ground to pin 1 of the potmeter the problem was solved. So you can see that good grounding is very important! This particular VCO is very sensitive to grounding issues so make sure you get that right. Do not rely on the metal front panel alone for grounding of the sockets.

Btw, you can still use the internal PWM potmeter when you're using external Pulse Width Modulation. 

TUNING:

Before I started tuning, I set the 'Frequency Coarse' potmeter in the 11 o'clock position (the pen stripe I mentioned in the text above) to get in the right octave, and the 'Frequency Fine' adjust was set to the 12 o'clock position. Let the VCO warm up for about 15 minutes before you proceed.

Tuning the VCO is just a matter of playing a low C note like C2 and a high one like C5 and turning the trimmer for 1V/Octave and checking it against a good tuner or tuning app on your smartphone. The trimmer is just a 100 Ohm one and I used a normal type for this, not a multiturn trimmer, and it works but I do recommend a multiturn one. It's a matter of tuning the C notes and seeing if the higher note is a bit lower or higher than it should be and compare it with the low note. If the one is too high and the other too low and the middle note is spot on then you have to turn the HF Tracking trimmer a tiny bit and also the 1V/Oct. trimmer. In the tuning proces you mainly use the 1V/Oct. and the HF Tracking trimmers and you use the Fine Tune potmeter on the panel to put it back on a C note. Changing the 1V/Oct. potmeter also influences the tracking so it's a delicate balancing act. Once you get it right it'll track marvellously over a wide range of octaves. I was impressed. It tracked even better than the Digisound-80 VCO and I didn't even had the Thermistor installed at first, but it was a lot more difficult than tuning the Digisound-80 VCO. I must admit though that I had trouble getting the lowest octave in tune. Octaves 2 up to 5 would be tracking beautifully but octave 1 was a bit high. To get that lowest octave in tune you'll have to use some trial and error. But you can do it. Tiny movements on the V/oct trimmer are the way to go.

After I installed the Thermistor and re-tuned it, the VCO was rock solid with temperature changes.

I had my VCO in tune over 4 octaves in a timespan of about 10 to 15 minutes. Before I installed the Thermistor the VCO would go out of tune after a while because of temperature changes but after I put the PTC in it stayed in tune beautifully. It can still be off a little when you first switch on but a slight re-adjustment with the fine tune potmeter and it's all back in track.

What can be really helpful with tuning is to use a sequencer to play a string of C notes from low to high in a slow tempo. That way you can easily hear how the tracking tunes or de-tunes the VCO over the Octaves as you turn the trimmers. (Just an idea.) I did it by hand though using the AiryWare Tuner app on my smartphone.

12V vs 15V:

At the time of writing this article I have not tried this circuit on a dual 12 Volt powersupply yet. However there are some notes about this on the Electro-Music forum stating that for 12V you need to change these resistors:  

R13 = 2K This is the 3K resistor in series with the Square- or Pulsewave output from pin 14 of IC4. 

R27 = 22K This is the 39K resistor in series with the Sinewave Roundness trimpot to pin 16 of the LM13700.

R33 = 137K This is the 100K resistor over pins 6 and 7 of IC4. 

That last one is a bit of a weird value for a resistor but the resistor values don't have to be spot on so you can also just put in a resistor closest to that value. It determins the gain of that opamp so a few K's more or less won't be a big deal. The circuit is quite forgiving anyway.

As for the Pulse Width Modulation resistor (R47). I already changed it from 330K to 190K and for 12V operation I guess it'll have to be changed to a lower value still. You'll have to do some experimenting with that to get it to your own liking. My advise would be to use a 200K trimmer with a 47K resistor in series and solder that in temporarily, set it so the pulse duty cycle goes from 1% to 100% or closest to that, de-solder it again carefully and measure the resistance and then put in a resistor of the measured value to replace the trimmer.

Edit: There is now a Eurorack compatible layout down below which has the resistor changes already implemented. 

I found one more change that needs to be done if you have trouble getting the ramp wave to connect in the middle on +/-12V. In that case change the 1M resistor R49 for a 560K or place a second 1M in parallel over the resistor. This change can even be necessary if you run it on +/-15V.

SYNCHRONIZATION:

The Hard Sync function works like a treat. If I connect a sawtooth signal from VCO-1 to the Hard Sync input of VCO-2 the second VCO will follow the tuning of the first VCO perfectly, even if that second VCO isn't tuned very well of itself. If you then force the second VCO out of tune by adjusting the Coarse Frequency control on VCO-2 you'll get some awesome distortion-like sounds that sound very musical. It totally blew my mind when I fed that through the Steiner-Parker filter. The VCO tries to stay in tune with VCO-1 and you can almost hear it struggling to do that. On the oscilloscope you can see the wave jumping in frequency as it tries to stay in tune and it does stay mostly within the main note played on VCO-1. But hearing the VCO struggle to do that is so awesome sounding. Unfortunately I didn't film this in the demo video below because when I filmed it I only had one VCO built, but you're going to get some great results if you build more then one Thomas Henry VCO-555.

I might make a new demo video soon, but in the mean time you can get an idea of the Hard Sync function by listening to the Fonitronik video I posted below my own demo video. 

LAYOUTS:

Okay, below here is the layout I made for this VCO. The first VCO I built was made with a different layout. That layout was published in this article before and is still visible on the LookMumNoComputer Forum, but it had the transistors and the thermistor quite far away from each-other and it also had some jump-wires. I have since made a new layout and built a second VCO with the new layout to verify it and luckily it all worked first time. Then I built a third one. The first one is now in use as a stand-alone signal generator on my workbench in combination with a MFOS LFO. Numbers two and three are in my synth as the main VCO's. The layout has also been verified by at least 10 people who gave me feedback that they built the VCO successfully. So here is the new and verified layout. Don't forget the 220nF capacitor between the linear FM input socket and its potmeter. 

And if you use this on a dual 12V powersupply you need to change the 1M resistor going from position O-35 to V-35 (from the wiper of the Ramp Connect trimmer to pin 6 of IC4) to a 560K or solder an extra 1M resistor in parallel over that resistor to lower the resistance to 500K. This will make sure the rampwave will connect at the zero volt line, using the trimmer.

Wiring diagram:

(Last revised at: 17-Jan.-2021: Made cosmetic changes to layout and changed two trimmers from multi-turn to single turn (also updated in BOM. 25-jan.-2021: Slight cosmetic changes. 24-Aug-2021: Slight cosmetic changes, removed color coding from resistors to make values more ledgible).

Here's the stripboard only view. Note the stripboard used is 56 holes wide (not 55) with 24 strips:

And here's an overview of the cuts that need to be made and the wirebridges that need to be put in. I'm giving you a component side view for the wirebridges. Also mark the cuts on the component side with a black felt pen. I always mark the cuts on the component side first and then stick a needle through the marked hole and mark and cut it on the copper side where the needle comes through. That's my procedure and it guarantees that all the cuts are made accurately and you can also see on the component side where the cuts underneath are located.

Here's the 'cuts only' view from the copper side:

ABOUT THE PTC THERMISTOR:

Here is a link to UK retailer Thonk. who has the PTC Thermistors listed. These are the ones I use. They are 3300ppm/°C instead of the desired 3500ppm/°C but it's close enough and will work fine. My VCO's stay in tune rock solid with these. These Tempco's have gone out of production in 2019. Luckily they just have the 2K version left but once they are gone they won't be restocked so get them while you can!!

I actually got 10 extra from them myself so I have a little stock for the future.

https://www.thonk.co.uk/shop/tempco-resistor-akaneohm/  

The US retailer link I had here earlier is no longer valid.

And here's a link to a good supplier in Germany, twice as expensive (€4 per PTC) but no VAT. These were sold out but they now have just under 1000 in stock. - CLICK HERE - 

There's also a 3500ppm/°C version from the UK Thonk retailer, (they are back in stock for the last time) but it is a bit bigger in size because it's a 3 Watt resistor. These are marked as obsolete and won't be restocked when sold out so they are now on sale.

(either of the two types will work fine): - CLICK HERE -

I bought eight of the Akaneohm 2K PTC thermistors from the first link (Thonk) in the UK and they work like a charm. They are also the ideal small size. The VCO is now rock solid on it's frequency.  (When I heard they went out of production I bought ten more so I'm set for the future ^___^)

Bill of Materials. Please note the decoupling caps and electrolytic caps you can see on the schematic bottom right, are not included in this BOM. 

There's an extra 2K resistor included if you want to put a 2K in, instead of the 2K PTC Thermistor. I advise to order a batch of 100 2N3906 transistors so you can easily find a matched pair. Again, there are two R26's in the BOM but they are both valid resistors, they just got the same number by mistake.

--- EURORACK LAYOUT ---

I made a second layout for stripboard that is 59 holes wide. That can be cut in two halves and folded over to get a smaller footprint and it requires 14 jump wires to connect the necessary copper strips together. It has the resistor changes for operating this VCO on dual 12 Volt already done. I have not built or tested this VCO on 12 Volt but I'm assured it will work fine. As you can read in the comments below I already had confirmation that this layout works like it should but if you have any feedback you think could benefit others then please do share it in the comments below.

Again, this stripboard is 59 holes wide so the standard 24 by 56 hole stripboards will be too small.

Here it is:

Wiring Diagram:

Stripboard only. Once you finished the build and tested the VCO fold the stripboard over with the copper sides facing eachother and glue a little plastic spacer between them with hot glue so they can never touch. (Hot glue works well because it can be removed should you need to solder something.) 

Then you can use the L-Bracket to mount the stripboard onto your panel. One bracket will be strong enough, the wiring will help keep everything in place. Do note that this VCO will have a considerable depth and won't fit into some Eurorack cases if you mount the board at a 90° angle to the panel. It's better to have it parallel with the panel in such a way that you can easily remove it so you can get at all the trimmers for tuning. Or maybe have some holes in the panel to stick a little screwdriver through. I leave that up to you.

Change the 1M resistor going from position O-35 to V-35 (from the wiper of the Ramp Connect trimmer to pin 6 of IC4) to a 560K or solder an extra 1M resistor in parallel over that resistor to lower the resistance to 500K. This will make sure the rampwave will connect at the zero volt line, using the trimmer.

The cuts that need to be made seen from the Copper Side. Naturally, the six cuts that are directly next to the edge of the stripboard cutting line don't need to be made, but I left them in to make it clear where the jumpwires need to be placed.

Of course, instead of jump wires you can also use Pin-Headers like in the Wavetable Oscillator project. You can get extra long ones or push the male pins further through the plastic holders to make them stick out more so they make good contact with the  female strips.

This is the layout for the copper side. So use this as a guide and cut where indicated.

The next layout shows the cuts as seen from the component side!!! (always a good idea to mark the cuts on the component side). So only use this one for marking purposes. 

Bill of Materials. Like in the other BOM there are two resistors designated R26. This is a little numbering mistake but both resistors are needed:

DEMO VIDEO:

Here's a demonstration video, demo-ing the waveforms and especially the Exponential FM option. I put it through the Steiner-Parker filter and I compare it with the Digisound-80 VCO. That comparison is not entirely fair because the DS-80 has no Exponential FM input, only a Linear one. Although, I suppose you could use the normal CV input as am FM input. That should be the equivalent of exponential FM but I haven't tried that. Btw, I forgot to mention the Steiner-Parker filter has a slow Triangle wave on the CV input which accounts for the 'Wah' sound you can hear. This video was made before I altered the Pulse Width Modulation so here you only hear it going from 25% to 75%.

For some odd reason my YouTube embedded videos don't show up on mobile devices so here's the link to this demo video in case it's not visible underneath. - CLICK HERE - 

Here's an other video (by Fonitronik) with a very cool demonstration of this VCO. If you look closely you can see that the Coarse potmeter on this VCO is also set to the 11 o'clock position to hit the right octave (confirming that it is the exact same VCO circuit). There is some reverb on the signal in this video so it sounds a bit fuller than the clean audio you get from this VCO. This video also demonstrates the awesome Hard Sync function:

https://www.youtube.com/watch?v=wnXtOu3ljA8

Here are some pictures from the build proces. I always start by making the cuts and then I put in all the wire bridges. You can see the cuts marked in black on the component side. There are 33 wire bridges to put in. They differ a bit from the layout because since the first build I made some cosmetic changes to the layout:

Here you can see how I bent the two transistors Q2 and Q3 towards eachother and then thermally connected them together with some thermal heat-sink compound and some thin copper. I left some extra copper on there which I intended to use to mount the thermistor to but I decided to put that on top of the transistors, so I later cut the extra copper off.

As you can see from the pictures below it is quite an easy build. Just over 40 resistors, 4 IC's and some other components. If you work methodically you should be able to easily copy this design and have yourself a fantastic VCO for a fraction of the price they cost new.

In the picture below you can see progress of the third VCO-555 I'm building. Here I used super glue to connect the two matched transistors together and I neatly bent the legs so it all fits in place nicely.

Here's a look at the finished product: In the picture below the thermistor is not yet installed. There's a 2K resistor in that position.

Here's the Thermistor installed on top of the transistors, covered with heatsink compound:

Here's a look at the panel I made for it. On the right you can see a 1V/Oct. output socket. It is connected in parallel over the 1V/Oct. input without any buffering. It's just a wire connection. I use that to 'daisy-chain' all my VCO's together and so keep the Dual Buffered Multiple free for other things. This feature is also included in the Digisound-80 VCO in article 18. Do not use this output as a CV input because it has no resistor in series. So that wouldn't work and could even damage your MIDI to CV converter.

Here's how it's installed in my synth. A Digisound-80 VCO flanked on both sides by a Thomas Henry VCO (the second and third TH VCO-555 I built.). Note the fine tune buttons. It's the first time I used knobs with a number scale on them, like the ones LMNC uses only smaller, and they fit very well here. I used big knobs for the Frequency Coarse potmeters and they leave only the numbers of the decal visible. I somehow like that, but that is just a personal consideration and may change in time:

ADDING LEDs:

Lately I installed some 3mm LEDs in the front panel to have a visual indication that the VCO gets proper power. I had some trouble earlier with dodgy contacts on my powerbus system so I thought this would be a good idea to detect any trouble. The LEDs each have a 15K current limiting resistor and they are connected straight to the +15V and the -15V on the stripboard. I made the current limiting resistors a high value to make sure they wouldn't pull much current and with a 15K resistor each LED pulls 0,882mA. So less than 1 milli amp at 15 Volt. The Cathode of the positive LED is connected to the Anode of the negative LED and from there goes a wire to ground on the board. The plus and minus are connected through the resistors to the power rails on the stripboard.

Finally a look at some scope images of the VCO. The picture below shows the basic waves and the Duty Cycle of the squarewave with the Pulse Width potmeter fully counter clock-wise and then fully clock-wise. The somewhat limited range of the PWM was the only drawback of this VCO and it was naging me so I changed resistor R47 from a 330K to a 190K (after experimenting with a trimmer) and now the PWM has a nice range all the way from 1% to 95%. You can see the exact values in the image below. 

In the next picture we see the Sinewave FFT or Fast Fourier Transform at the top left. This shows the main peak in the middle at approx. 332Hz and then the harmonic frequencies as the peaks to the right of the middle. As you can see the harmonics are at least 30dB attenuated compared with the main wave so well suppressed.

The rest of the pictures show the waveforms being 'Hard Synced' by an other Thomas Henry VCO. Here you can see how it influences the different waveforms. (The picture at the bottom right shows the output of my VCA after the Hard Synced squarewaves have gone through the Steiner-Parker filter (not important)).

TIP: If you want to create a phat bass sound, use a sawtooth wave into a filter but add a sinewave from the same oscillator. The sinewave will pass through the filter practically un touched but it will add the bass rumble to your sawtooth filter sound. Modulate with a slow LFO for extra effect.

That's a tip from Anthony Marinelli, the synth guru on youtube :)

TROUBLESHOOTING TIPS:

I've had a few people who built this VCO and then it didn't work. Most of them eventually got it working though. Here's a little summation of the most common causes of trouble that I came across. This list can be used for any of the projects on this website:

- Forgetting a cut or a wire bridge. I think this must be the number one cause of the VCO not working.

- Grounding problems. This is something I experienced myself. If you rely on grounding the sockets through the metal front panel you are asking for trouble. It can cause the VCO to just not work. This Thomas Henry design is particularly sensitive to grounding errors I found. So make sure everything is grounded with wires. You can connect all the grounds of the sockets together by weaving a copper wire through them, soldering the connections and then connect it to a ground point on the stripboard with a single wire.

- Short circuits between strips. Solder whiskers. This is also a leading cause of modules not working. (This has actually happened to me and also a lot of other people posting problems in the Facebook group) A tiny bit of solder can be the cause of the short circuit or sometimes even bad etching of the stripboard itself so two strips are connected by remaining copper. 

This can cause all sorts of problems like the waveforms not looking right and the trimmers hardly working at all. Those are all symptoms that led back to simple shorts across the copper strips.

Take a sharp iron pin or small screwdriver and scrape the area between the copper strips to make sure there are no connections. Measure strips that are next to eachother for continuity before you start building. Also check the cuts you made in the strips and make sure the connection is really cut.

- Chips not working or bad/fake chips. Seems obvious but it has occurred. Make sure your chips are good and from a reputable source (in other words not fakes from AliExpress.)

I've had feedback where someone had static noise in the audio outputs. This turned out to be caused by a bad 7555 chip. You can also see in the comments below a comment about a bad TL074 chip being the cause of the rampwave not working properly. An other person had a TH VCO that slowly lost its tuning which was also down to a bad or fake opamp.

- Small bits of wire getting into potmeters and causing a short circuit. This is also something that happened to me and it is almost impossible to find. I discovered this once when I couldn't find a short circuit which I knew was there and at long last decided to put the full voltage of the powersupply on the short to see where it would start smoking (after first taking out the chips). A big flash came from inside one of my potmeters as the wire evaporated and that solved the problem. The potmeter wasn't even damaged.

- Powersupply issues. The VCO is not getting power or intermittent power on one or both of the power rails. Check your powerrails with an oscilloscope while in use and while you're at it, check the powercord or ribbon-cable to the VCO. 

- Bad solder joints. Bad soldering can be a difficult problem to spot. A wire or component might seem to be connected but that isn't always the case. When in doubt, re-flow your solder joints to make sure. Also make an effort to solder neat and tidy. Keep a magnifying glass handy and use it to check your soldering (I always do this too).  

Use thin solder (0.5 or 0.6mm) with rosin core and don't use too much. A good solder joint looks from the side like a christmas tree not like a snowball. (I also prefer to use lead solder 60% tin/40% lead). Make sure your soldering iron is at the right temperature and that is HOT! (about 370°C) Better too hot than too cool because with a hot iron you can solder more quickly and expose the component to heat for less time.

- Faulty patch cable. A no brainer right? It happens though. Use good quality patch-cables!

- Problems getting the VCO in tune over multiple octaves?? Maybe it isn't the VCO that's at fault but your source of the 1V/Octave CV signal. We had a case recently where the problem turned out to be the Beatstep Pro that was used to check the VCO tracking. If you use a Beatstep Pro, check it is set to the the Chromatic scale. So make sure your 1Volt/Octave signal is reliable and when in doubt, try other sources to see if the problem persists.  

Need to tune the V/Oct trimmer everytime you switch on and the Sine and Triangle waves are very low in volume?

Solution: Your transistors aren't matched well enough.

< this list will be updated as more causes and solutions come in >

There's an article on the MFOS website that deals with things you can do to improve 1V/Octave tracking. The article deals with filter tracking but these rules apply equally to VCO's. 

Here's a link to that article: --- CLICK HERE ---

Okay, that's another one done. 

For questions and other help you can use the comments below but I also advise to check out the EddyBergman Discussion and Help FaceBook group. You can also find the schematic of the VCO in the Files section of that group

PLEASE CONSIDER DONATING.

If you find this content helpful, please consider donating to keep this website in the air and to contribute to future projects. There's a Ko-Fi donation button underneath the main menu if you're on a PC or Mac. Otherwise use this PayPal link to cut out the middle man. Thank you very much for your help!

DISCLAIMER: The author of this article does not accept any responsability for the correct functioning of this, and any other, module/project on this website. What you build, you build at your own risk. All project layouts are thoroughly tested before publication, it's up to you to replicate them and the author can not be held responsable for any mistakes made.