Welcome to stripboard heaven! Here you'll find all the projects I used to build my DIY Modular Synthesizer. I'm using the 'Kosmo' size standard but I also build Eurorack sized modules. All layouts are made by myself and verified to work. The schematics they are based on come from all over the internet. If you're on a PC or MAC, there's a complete MENU in the sidebar. For mobile devices the menu is in the black 'Move to...' bar below this text.
This is a bit of a weird one among filters and because it uses Vactrols it doesn't sound like any other filter. It has some percussive qualities too if you feed the CV IN with an LFO.
After not having built anything for two months I decided to get going again and this filter seemed like a nice one to try. It's a very simple design and thus easy to build. Because it's such a small stripboard and because it works on a dual 12V powersupply I made the layout more Eurorack friendly. I have not tested this circuit on a dual 15V powersupply but I think it will work just as well on 15V.
This project is more of an experimental one than one where the end product behaves in a way we expect from VCFs. What I mean is that the audio from this filter sounds different to all other filters and the CV IN behaves differently too. When I put an AD (envelope generator) signal on the CV IN, the resonance only comes up as the AD signal fades out, the opposite of how other filters behave and when I try to use a negative AD signal by putting it through the Dual Voltage Processor's Attenuverter, it doesn't give the expected result. Not even when I apply an offset voltage to the AD signal. So this is an experimenters dream. It works, but not like you expect. Just know that before you proceed to build it. ^____^
VACTROLS:
This is a Skull and Circuits design and you can find the original information by clicking here.
This is a 12dB LowPass filter, so a two pole instead of the usual four pole filters This means the frequency rolloff won't be as steep as other (4 pole) filters.
The Vactrols are something you'll need to make yourself. Well, you can buy them ready made but I advise to make them yourself because new ones can be hard to find and expensive and the ones from China will be fakes, I guarantee it. In the information link above is a video that shows you how to make your Vactrols step by step. I used two 5mm green LEDs (you need two Vactrols) and two LDRs no. 3P (or maybe 39) from a set with 10 pieces of 5 different types of LDR from eBay. They come in 5 little plastic bags and they are all numbered. I used the ones from the bag numbered 3P. I had used those before on the Lopass Gate and they react reasonable fast to light changes and go way down in resistance. The way your filter will turn out will be different from my filter. There are so many variables here that filters may turn out to be alike but never quite the same. This is mostly due of course to the Vactrols and the way you set the three trimmers. I tried two different types of LEDs in my Vactrols. First I tried bright white LEDs but with those I needed to turn most of the trimmers completely clockwise or counterclockwise to get the filter to behave correctly. The second time I used normal red LEDs (5mm) and they worked better. That's what I used in this build.
Later on I bought the kit from Skull & Circuits and in that kit I used Vactrols made from green LEDs which I found work even better than red ones. (The kit built filter sounds the same as the stripboard one.)
Now I could use the trimmers more accurately to get the right sound. So I advise to use either red or green 5mm LEDs for you Vactrols. The LEDs in the filter itself, mounted on the front panel, do not shine very bright in my module. I used two 3mm yellow LEDs and they only come on when the trim-potmeters are set to extreme positions. I can get them to shine brightly when I turn the trimmers a certain way but then the filter doesn't work properly. Again this is one of those things that will be different with your filter because of component tolerances and differences in the Vactrols etc.
Just out of interest, here are some measurements I took of my DIY Vactrol (out of circuit) with red LED:
OFF resistance = >200MOhm. ON resistance with a +/-5Vpp signal = about 10K, with a 0/10Vpp signal = about 5K. This is with a 1K resistor in series with the 5mm red LED of the Vactrol.
Of course these values differ when the Vactrol is in circuit because there you can set offset voltage etc. which alters the resistance. With the green LEDs, in the kit I built later on, I could get even lower resistance down to 200 Ohm!
NOTE: This module pairs extremely well with the Voltage Processor and the Lopass Gate module. If you create a beat with an LFO into the CV IN of the filter but first put the CV through the voltage processor you can accurately control the sound and if you then send the audio through the Lopass Gate you can turn your beat into a sort of galloping beat. Connect a signal from the same LFO used for the VCF to CV1 of the Lopass Gate and use a faster LFO signal on CV 2 of the Lopass Gate. Used in this way the Vactrol VCF can be used to make some cool Techno kickdrum sounds.
LAYOUTS:
Here is the layout I made for this filter. As always the layout is verified. You can see it's small enough for a Eurorack module and I incorporated a Eurorack power connector to make life easier for those of you using that format. I myself didn't use the connector in my build but just soldered the power cable straight to the print like I always do. All potmeters viewed from the front.
IMPORTANT:
Stripboard only view below. Note that the copper strip between pins 3 and 12 of the IC is NOT CUT! Both pins are connected to eachother underneath the chip. Also do not forget the cut at hole B-29 otherwise, when you close the Cutoff potmeter, you'll create a short circuit between +12V and ground through the wiper of the potmeter which will produce smoke, I guarantee it ^^ :
Strangely enough I found that I needed to reverse the wiring of the Cutoff potmeter for it to work the right way around. I left it the way it is presented on the schematic, in the layout so you can decide for yourself whether you need to do that or not.
Bill of materials:
TRIMMING THE CIRCUIT:
Trimming this circuit is a matter of trial and error. You need to set the trimmers in such a way that you get a good deal of self oscillation when the Resonance potmeter is set almost completely open and you need to get the right range for the Cutoff potmeter. I can not give you a procedure to help you do this. You will have to figure it out yourself but it's pretty straight forward. Just put a squarewave or rampwave signal on the audio input and connect an LFO signal (+/-5Vpp) to the CV input and then connect the output to your VCA. Changing R7 from a 47K resistor to a 100K trimmer helped a lot in getting this filter trimmed. It was suggested in the text of the original Skull and Circuits article about this VCF and I also saw they implemented it on the PCB so that's why I put in a trimmer for R7 in the layout. It might be a good thing to use multiturn trimmers instead of the single turn ones I use in the layout. I used normal trimmers on my board but it can be fiddly to set these correctly. I might replace mine with multiturn ones too. You might think it would be a good idea to put an attenuation potmeter on the audio input but I tried that using the passive attenuator on my mixer but it didn't help at all. The filter only worked if I had the potmeter fully open so there's no need for an audio input level potmeter. As you can see in the schematic, the audio is attenuated by a factor of almost 18 times by the 1K and 56 Ohm resistor voltage divider right at the audio input. Then it is boosted up again in the output opamp.
The whole upper portion of the schematic dealing with the Cutoff and CV input goes straight to the LEDs of the Vactrols so you can see that he whole filter hangs on these Vactrols. The Offset and Range trimmers control the Vactrol LEDs and determin when the LEDs turn on and how sensitive they are.
The LEDs over the opamp go on when the signal is clipping. You should try and trim the potmeters so these LEDs turn just about on when the filter is in full resonance and cut off is 3 quarters closed. So when the filter is really working its hardest these LEDs flicker in time with the CV input voltage. These LEDs should not be burning at full brightness in normal use!!
Schematic drawing:
Here are some pictures from the build proces. In these pictures the third trimmer is not yet put in. I tested it first with R7 as a normal 47K resistor. I later changed it for a 100K trimmer.
The stripboard:
Stripboard mounted behind the panel not yet wired up:
Video of the first test. You can see here that connecting an AD (or Envelope) signal to CV IN results in the opposite reaction to what normally happens. Using a negative or inverted AD signal doesn't fix this issue. Not even with a positive offset voltage applied. That just proves that this is a very quircky filter and all the more interesting for it ^__^ I've heard better results than mine from other people who built this VCF, so it depends a lot on how your vactrols turn out.
If you have any questions or remarks you can put them in the comments below or go to the Facebook Page for this website where we have an awesome little community of cool DIY synthesizer enthusiasts that are more than willing to help you out with any problems you might encounter.
An amazing sounding digital oscillator with 16 waveforms and a sub-oscillator with 8 waveforms that spans 4 octaves. All but one of the parameters of this VCDO can be changed with external control voltages including the Bitcrush option which does not have a CV input in the original Electric Druid schematic but it has one in this build. Only the Glide function doesn't have a CV input.
Warning: this is a big project. It needs two stripboards of the size I normally use (24x56) and my layout is definitely not Eurorack friendly. Everything needs to be shielded to prevent glitches in the main oscillator. Even the wiring of the control potmeters must be shielded.
It is possible to build a Eurorack friendly version of this VCDO on stripboard and the circuit will work fine on a dual 12 Volt powersupply but you can NOT cut these prints in half and connect them together like in other projects. You will need to make your own layout and much more compact than mine is now. However there is a smaller size layout available on the EB Facebook group files section. Check the link below the layout images under the heading 'Eurorack Layout' half way down this article.
ABOUT THE BUILD PROCESS AND PROBLEMS ENCOUNTERED.
Okay, not to put you off or anything but this is the most difficult build on this entire website so if you're a beginner or you don't have the right tools, especially a fine tipped soldering iron and good soldering skills plus a good oscilloscope, then please do not attempt this. By saying this I just want to avoid disappointment. As an alternative I would advise you to just order the Klang Stadt PCB and Panel from Frequency Central. That one is in the Eurorack format which mine won't fit in to.
I foolishly started out building this module like I did with all previous ones. Just build it up on stripboard, make a panel and then wire it up and test it. Well this turned out to be too light-hearted an approach because the main oscillator sounded like a scratchy vinyl record. I couldn't get it to sound right so I wrote to Tom Wiltshire (Electric Druid) about this problem and here's what he said about it
<Quote>
"It happened a bit on the first version of the PCB I did for the Frequency Central "Waverider" module. Eventually we tracked it down to the outputs being to close to the CV inputs. Keeping tracks and wires to those apart as much as possible helps a lot. On revision 2 of the Waverider PCB I routed the inputs on one side of the PCB, and the outputs on the other and that solved the problem. It boils down to noise getting into the CV inputs. That can come from many sources but from the chips own outputs was a big cause for us."
<End quote.>
So I started out fresh and re-built the entire module. This took me three days including designing a new layout. I now decided to shield everything and to keep the controls and the CV inputs on separate boards. This turned out to be the solution because it now works like it should. Instead of wires I used pinheaders to connect the two boards together. This would be the shortest route for the signals with less chance of noise getting into the connections. I had a blank single side copper clad circuitboard in my stock so I decided to cut this board to the same size as the stripboards and mount it inbetween the two stripboards and then connect it to ground. I cut holes in it for the pinheaders to go through and soldered upstanding copper edges around the holes to provide extra shielding for the pins. (The upstanding copper edges are a bit overkill and if you build this module, you don't really have to replicate that.)
Here's a picture of how the boards eventually fit together. I stuck 4 transparent rubber feet on top of eachother and put it between the shield board and board two to act as spacers to prevent the upstanding copper edges around the pinheaders from touching the copper traces of board two. I also put some gaffa tape on the edges to insulate them.
SCHEMATIC:
[EDIT: As of Oct. 2023 the chip is available on back order.]
You can order the chip from Electric Druid. On that same page you will also find a link to the Datasheet with the schematics for this project. Download it and print it out.
NOTE: On the schematic, in the PDF, you will see one CV input circuit marked as "Spare CV Amount". On the page with the processor it is marked as 'Unused CV'. I don't know why it's marked like that but this is the Sub Oscillator CV Amount circuit which eventually goes into pin 18 of the processor chip. Just so you know.
If you're going to breadboard this circuit before building it up properly, don't leave any pins of the processor chip floating and make sure all the CV inputs get the voltages they need, otherwise it won't work properly.
LAYOUTS:
Below are the layouts for this project. They are all verified as usual. Not only by me but I had confirmation from a number of people who built this module successfully using the layouts below.
I'm going to give you a whole collection of layouts detailing the different steps. The Bitcrush section of this module was designed by Mike Desira, whom most of you will know from a lot of synthesizer DIY related Facebook groups. He has been a great help to me, not only in this but many other projects too. On my panel I have a Bitcrush switch to go between internal and external sources. I still had that in there from the first version so I thought I might aswell use it again. I personally prefer it to mixing the Bitcrush signals together. The layout shows both versions for the Bitcrush option, without switch and in dotted lines with switch. Mike did away with the switch in his design. Instead you do need to open the internal Bitcrush potmeter for the CV potmeter to work. The signals are mixed together in an opamp and inverted.
Here's a schematic drawing of Mike's solution. On the right is the version with a switch to go between internal or external bitcrush as indicated with dotted lines on the layout wiring diagram.
EDIT: Until the 8th of June 2021 I had a second inverter stage in the Bitcrush circuit but this turned out to be a mistake so I have corrected the circuit and all the relevant layouts. So if you built this VCDO before the 8th of June 2021 and your Bitcrush section isn't working, check it against the new layouts and make the necessary changes. It's a matter of changing two resistors and a wirebridge to bypass that second opamp. The second opamp must be properly connected to ground, which it is in the layouts.
The de-coupling capacitors are mostly not included in the layout. I soldered those straight to the pins of the IC's on the copper side. So get yourself some small ceramic 100nF caps and solder them on, on the copper side, to pins 4 and 8 of the TL072's. The processor chip has a 100nF cap in the layout already.
I also put 100nF caps over all the inputs of the processor chip. With the previous version of this board, when I was troubleshooting it, I soldered 100nF caps over the inputs and this seemed to improve the input signals a lot. Before I did that the signals had little spikes on them when viewed on my oscilloscope and after I put in the caps these spikes were gone. So I used this idea in this version, eventhough these caps are not in the Electric Druid schematic. They are included in the bill of materials. The de-coupling caps, soldered directly onto the chips on the copper side, are not included in the bill of materials.
Here is the wiring diagram for this project. The wires coming from the wipers of the top right five potmeters to pins 1, 2, 3, 7 and 8 of the processor must be shielded wires. The outer shielding must be connected to ground but only on one side. If you connect both sides to ground there could be current flow in the outer shielding and that would create a lot of hum! (Ground loops) The easiest solution is to connect the outer braiding of each wire to the grounded pin of the respective potmeter it's connected to.
(Last revised: 9-June-2021 Removed second inverter stage from the Bitcrush section and tied off the second opamp to ground. 12-June-2021: Reversed polarity of top capacitor (+5V to Gnd) )
The two stripboards are mounted with the copper sides towards eachother. The pinheaders are soldered straight to the copper sides of the stripboards. In between them is mounted the blank single-sided copperclad circuitboard (shield board) with holes for the pinheaders to stick through. It will help with soldering on the female pinheader sockets if you bend the pins 90 degrees. Makes it easier to solder them in place. But be careful, they are fragile.
Drill two holes at the bottom of this shield board, at the same distance as the mounting holes of the two stripboards, for the M3 mounting bolts to go through. Make sure the copper side of the shield-print is facing the copper strips of stripboard two and facing away from the main board with the processor on it. We will mount the shield board with the non-copper side touching the copper strips of the Main Board and the copper-side towards the copper strips of board two and we don't want any short circuits :)
Here's the main board. You can leave out the 10K resistor over the output of the 7905 voltage regulator. This resistor is there because some regulators only work well if they are presented with a load on the output but the circuit itself is enough of a load usually:
(Last revised: 23-April-2021: Corrected mistake with C2 (330pF) which went to pin 3 instead of pin 1 of IC-1 like it should. 9-June-2021: Removed colour-code striping from all resistors for clarity. 12-June-2021: Reversed polarity of top capacitor (+5V to Gnd))
Board Two with the CV Inputs on them and the Bitcrush circuit by Mike Desira. (Updated corrected version):
(Last revised: 8-June-2021: took out 2nd inverter stage for Bitcrush option.)
In the layout of board two you can see 10 Schottky Diodes. These, together with the 4K7 resistors, are there to protect the inputs of the processor chip from voltages that exceed the +/-5 Volt limit for CV voltages.
The two voltage regulators are the big TO220 packages and they don't need heatsinks. The current going through them is so low they won't even warm up. You can also use the smaller 'L' types that look like a little transistor but I used the big ones because that's what I had in stock. If you use the big ones, make sure the backsides don't touch eachother, otherwise you'll get a short circuit.
Here's an overview of the cuts, wirebridges and the positions of the pinheaders. I used a double row of pinheaders to make sure I got good contact and to make sure it doesn't get loose. Mark each cut on the component side of the print with a felt pen or a Sharpy. Then stick a needle through the marked hole and mark it again on the copper side. Then make the cuts. This is the most accurate way to do it and prevents errors.
MAIN BOARD, CUTS and WIREBRIDGES - COMPONENT SIDE:
BOARD TWO, CUTS and WIREBRIDGES - COMPONENT SIDE:
(Last revised: 17-March-2021: Corrected a cut at the powersupply pinheader. 9-June-2021: Tied off second opamp of Bitcrush section because that stage has been removed.)
And finally a view of the cuts and the position of the pinheaders seen from the copper side of the print where they actually need to be soldered on and, obviously, where the cuts need to be made.
MAIN BOARD, CUTS and PINHEADERS - COPPER SIDE:
BOARD TWO, CUTS and PINHEADERS - COPPER SIDE:
(Last revised: 17-March-2021: Corrected a cut at the powersupply pinheader.)
Bill of Materials:
Note: the pinheaders are not included in the bill of materials. You can order them in strips of 40 pins long. Make sure you get both the male and female versions and order ten strips of 40 pins. They cost pennies and are always handy to have in stock.
Here's the link to a listing on AliExpress that has the same ones I used. These are female ones but the male ones are also listed on the same page. I'm afraid I could only find the Dutch site version. Order some of both:
There is a smaller size layout available in the files section of the 'Eddy Bergman Projects Discussion and Help' Facebook page made by Markus Möbius. He used it to build his VCDO and based it on my layout but just made it more compact and he used single rows of pinheaders. His VCDO works fine. Look for the file name ED_VCDO.diy It's a DIYLC project file. The layout doesn't have any potmeter and socket connections so you have to reference it with mine to get that sorted out.
DEMO VIDEO:
Here's a demo video with a look at the functions and the different sounds/waveforms you can get from this digital oscillator. When you start mixing the two outputs together, you can get some awesome sounds. If you then put it through a filter it starts sounding really amazing. That's in the last bit of the video. Btw, the mixing together of the Main Oscillator and the Sub Oscillator is done with the 4 channel mixer/passive attenuator from article 17.
For some reason my YouTube embedded videos don't show up on mobile devices anymore. Please go to my YouTube channel if you can't see the video here.
Here's an other video, not by me, that I found on YouTube with a 12 minute demo of the Frequency Central Waverider module, which is the same as the module I built here. The subtitles are in what I think is Spanish though.
TIP: Try connecting the Triple Sloths (proj 61) to some of the CV inputs and a slow sample and hold on the V/Oct input and let it generate its own music.
PICTURES OF THE BUILD PROCESS:
Here are some shots I took whilst building the latest version of this module. In the top picture you can see the green ground wire soldered to the shield print. This is connected to the ground of the powersupply. You can also see I put some tape over the copper edges to insulate them electrically should they touch the copper of board two. If you use upstanding edges then look out not to make them too tall. If they are taller than the pinheaders they will touch the copper strips of Board Two and cause short circuits!. So make sure they are not too tall and use tape over them to be extra sure they can't cause short circuits.
I also made cuts in the corners of the upstanding edges so I can bend some of it out of the way when connecting the two boards together. They obstruct the view of the pinheaders making it difficult to align the two boards correctly. After connecting the two boards I can bend the copper back up and leave it like that.
In the next picture you can see the preparations I took before soldering on the pinheaders. I applied some solder inbetween all the holes and on the pins themselves too. This way I had only to touch them with the soldering iron and the solder would flow and connect them to the copper strip. Then I would add some more solder to make sure the connections were nice and stirdy and I checked the continuity between the strips to make sure there were no short circuits.
In the middle picture you can see the shield print as I call it, mounted inbetween the two stripboards. All the orange wires in the bottom picture are shielded wires. The outer braiding of these wires must be connected to ground but only at one end of the wire. If you connect both ends to ground you can get ground loops with all sorts of problems. I connected the shielding of the wires to the ground connection of the potmeters they are connected to. On the other side of the wires I cut off the outer copper braiding and put a little piece of shrink tubing on to prevent little individual wires from the outer braiding sticking out and contacting other components by accident. (You never know). The prints are connected to the panel by two copper L-Brackets with thin M3 bolts through them, 3 centimeters long. The order of mounting is as follows: Bolt goes through the main board first, then the shield print, then a plastic spacer, then the L-Bracket, then a plastic ring to prevent the L-Bracket from touching the copper of Board Two and then through Board Two. Then I put a ring and a nut on it.
Here's a look at the panel I made for this module. The mix-up of colours of the knobs is intentionally done, I assure you =). The yellow is for control functions and the red knobs are for CV Input levels. Make sure when designing a panel, that you make the two wave selection potmeters, those connected to pins 3 and 7 of the processor chip, your main controls on the panel. Not the frequency controls as is usual with normal VCO's. This is not a normal VCO ^___^. I took my inspiration for designing the layout of the panel from the Klang Stadt version from Frequency Central.
Here's the Wavetable Oscillator next to my two Thomas Henry VCO's. A killer combination!
Here's how I prepared the potmeters on the panel before I connected them to the prints. I soldered in all the 1K resistors and 100nF capacitors and made all the ground connections for the potmeters when I still had access to them. Then I soldered in the wires with the prints laying loose on top of the panel starting with the shielded wires first because they are the bulkiest.
I made two L-Brackets from some thick copper sheeting I had lying around. You can or course use any metal to make your own L-Brackets or even buy them ready made. Make sure non of it contacts the prints' copper side. I used home made plastic rings to insulate the print from the L-Brackets.
Here's a picture of how I made the first version of this module with much too long wiring and unshielded prints. This did not work so don't copy this!!
TUNING THE VCDO:
Tuning was quite a difficult operation. This is mainly due to the fact that the oscillator quantizes the incoming 1V/Oct signal. But this does have the advantage that once you get the tuning right, it'll be rock solid over many octaves (I got it tuned over 7 octaves in about 15 minutes).
Because the tuning trimmers are a bit fiddly I put in two of them on advise from Mike Desira. One for normal tuning (20K multiturn) and one for fine tuning (2K multiturn). I also used the Offset control potmeter in the tuning process. This is a normal 20K potmeter, not a multiturn.
Before you start tuning, set the frequency potmeters on the panel in the 12 o'clock position.
It was a trial and error kinda process but, like I said, after about 15 minutes I had the VCDO tuned over 7 octaves. You just need to try this and develop a feel for it. I can't give you a procedure for tuning. Make sure you use all the potmeters in this process. The Offset, the tuning trimmers and also the main Frequency control on the panel (not the finetune one).
At one point I had it tuned and tracking nicely only the lowest few notes were way off. I used the offset trimmer and retuned until I also had those low notes in tune and then it suddenly was tracking fine over the 7 octaves of my M-Audio keyboard. You just have to experiment until you get it right.
This is an amazing sounding oscillator. It's got 16 different waveforms in the main oscillator and an other 8 with 4 different octaves in the sub oscillator. You can connect both to a mixer and mix the two signals together and you get some awesome results!! All parameters can be externally changed with control voltages, except for the Glide control. You don't really need CV control on that.
Here's an overview of the 16 waveforms from the main oscillator. The waves all merge and flow over into eachother so there's no stepping between waves so really you have a lot more waveforms inbetween these, but these are the 16 main waves:
And here are the 8 waveforms from the sub-oscillator. Each of these waves is present in 4 different octaves. (The last image on the bottom right is aan example of a Bitcrushed wave with Bitcrush set half way.) The waves from the sub-oscillator do not merge from one to the other. As you turn the Sub Oscillator knob you'll get a waveform at the lowest octave and as you turn more clockwise the octaves will go up in 4 steps until it reaches the 4th octave and then it switches to the next waveform, again at the lowest octave. Then the cycle repeats. So this all happens in definite steps.
You can find the original DIYLC Layout file in the 'Files' section of the EddyBergman FaceBook group.
Okay, that's it for now. All in all not so much a difficult build but a very labour intensive build number 40. I will take it easy with the building for the coming months now. I really need to build a new case because I already built 7 modules for which I have no space. I'm also out of a lot of vital components so I need to replenish my stock which will take some time.
If you have any questions, please put them in the comments below or post them in the special Facebook Group for this website. There are a lot of expert people there who can help you, or at least try :)
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.
A fun little project to make your modular synthesizer react to light. It has offset and level controls and CV smoothing plus an external LDR input.
Let me start by saying that this module needs to be connected to a VCO to work so it's not a stand-alone thing. This circuit outputs a Control Voltage and the height of that voltage is dependent on the brightness of the light falling on the LDR, the Light Dependent Resistor.
This was a little project I dreamed up myself and I designed the circuit too. It's quite a simple build. It consists of two opamps. The first one has the LDR input and the Offset control but it inverts the CV voltage. Then the second opamp inverts the CV voltage back to normal and feeds it to the output socket via a level potmeter. There are two 3mm LEDs on the output to indicate if there's a positive or negative voltage present and the brightness indicates the voltage level.
The Smooth switch is there to suppress the 50Hz or 100Hz hum you get from LED (or other) light fittings. They flicker so fast you can't see it but the LDR reacts to it. The switch simply puts a 10µF electrolytic capacitor over the LDR smoothing out the control voltage.
This circuit works on +/-12V but it will work equally well on +/-15V. It consumes very little current. On 12V the maximum current I measured was 5.6mA. 1.6mA of that is consumed by the LEDs if one of them is on. So you see, it hardly draws any current at all. It will also work on 9V although the CV voltage will be less high obviously. The offset feature needs a dual powersupply in order to transpose the CV voltage down so you'd need two 9V batteries to create a dual 9V powersource. I haven't tried it on 9V but I get questions about it but I don't see why it wouldn't work. You might need to adjust some of the resistor values or experiment a bit to get the best out of it. That's up to you.
There's a link to a 3D printed front panel for this project in the comments below.
HOW TO USE IT:
This circuit doesn't generate any sound itself. It just outputs a Control Voltage. You can connect the output Control Voltage to the CV input of a Voltage Controlled Oscillator (VCO) to get the Theremin effect. You can also put it through a Sample and Hold first and then into a VCO. In that way you will get stepped tones. An other option is to lead the CV through a Quantizer first, to get it to output true notes that adhere to the 1V/Oct standard and so try to make melodies with light. Of course you can also use the CV to affect the Cut-Off frequency of a filter. Your imagination is the limit :)
The CV voltage and therefore the tone will get higher as the light that shines on the LDR gets brighter, and lower when it gets darker. The exact way it reacts can be set very accurately using the offset control, without limiting the dynamic range of the notes. (The same idea as with version 2 of the 8 step sequencer).
The idea behind the external LDR input is to make it possible to bring the LDR to the light-source instead of having to shine lights on the panel itself (with the built in LDR). This makes it much more flexible. You can just take the LDR in your hand and point it at things and use it as an instrument. The whole circuit works so much better when you use this option. Try it!!
If you feel you need more output voltage then change the 100K feedback resistor over pins 6 and 7 of the IC. Double the value to double the gain. 147K should be enough to go really high but normally it shouldn't be necessary to change it. Obviously you are limited by the height of the power supply voltage but with 1V/Octave you really don't want your CV output to exceed 8 Volts.
You can experiment with light sources and where you put the external LDR and let your imagination run wild. You could build a whole light operated synthesizer with this project as a starting point. =)
LDR:
I can not give you a part number for the LDR to use in this circuit. I ordered a set of 5 different LDRs with 10 pieces of each, from eBay. The link to that has expired but here's an other one to a set of 70 with 7 different values. That will last you a long time. Handy for making Vactrols too to have this in stock:
I used the one marked 5537. That's a very fast reacting LDR. But to make sure, test them with the resistance meter on you multimeter and choose one that reacts fast to light changes.
Here's how you can make an external LDR that you can move around and point at light sources. Simply take a female jack socket like the ones you mount in panels and solder an LDR over the audio and ground contacts. Use some hot glue to make it nice and stirdy. Now you can connect it to a patch cable and connect the other end of the cable to the external LDR input. Flick the switch to 'Ext.' and you're in business. =)
External LDR. Just put some hot glue around the LDR leads to stiffen it up a bit. The external LDR worked really well when I tested it. Of course you can shorten the leads of the LDR or bend them at 90° so it can lay on a table and catch the light from above. It's all up to you.
Schematic drawing of the circuit:
LAYOUTS:
Here is the verified layout. As you can see you can build this on a very small piece of stripboard. Don't be fooled by the wiring of the Offset potmeter. It looks like it's wired the wrong way around but remember that the opamps are wired as inverters so the offset voltage is turned the right way up in the second stage. (All potmeters are viewed from the front with shaft facing you):
Stripboard only:
Bill of Materials:
TAKE NOTE OF THIS:
There are a few things you need to beware of when building this project. I built in the option to connect an external LDR to the panel instead of using the built-in LDR with a switch to choose between them. The socket for this External LDR input must be mounted in such a way that it is completely isolated from the panel if you are using a metal panel. The socket is connected directly to +12V, if it touched the panel it won't short out because there is still a 10K resistor between it and ground but it won't work as an input anymore. So connect the socket to some plastic and make a big enough hole in the panel to glue or screw the socket behind the opening without it touching the panel itself. (See pictures below to see how I mounted it to the panel.)
DO NOT CONNECT ANYTHING ELSE THAN AN LDR TO THE EXTERNAL LDR INPUT! It has +12V on it and may damage sensitive electronics. So make sure you label that input clearly! Maybe it's wise to use a different kind of connector for the Ext. LDR. That way you can't put in a patch cable by mistake.
The same caution must be taken with the LDR that is mounted in the panel itself. Make sure it makes no electrical contact with the panel when you glue it in place with hot-glue. Check it with a continuity meter after glueing it in place.
Here's a short video I made of the very first test I did. This thing can really make a VCO squeel!!:
Here are some pictures of the build proces and the end product:
In the picture below you can see how I mounted the 'External LDR' input socket. I used some white plastic and mounted the socket in that and then I drilled a hole in the panel, wide enough to take the socket with plenty of room around it, so it wouldn't touch the metal of the panel. Then I hot-glued it all in place and it works very well like this:
Here are some oscilloscope screenshots showing the maximum amplitude of the light pulses I got with testing. It was more than 20Vpp when I ran the module on +/-15V but you can turn that down with the level control.
Here's an image which shows the 100Hz hum you can get from flickering lights (the small ripple in the lower parts of the waves):
Here's the effect the 'Smooth' switch has on that. It not only gets rid of the ripple but also dampens the pulses. You can lower the value of the smooth capacitor a bit to 4,7µF but no lower than that otherwise the ripple won't be surpressed.
Okay, that's it for now. If you have any questions please put them in the comments below or visit the Facebook Group that was setup especially for this website.
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.
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.
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 Dutch psy-trance producer Jake Jakaan. 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...' 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 so 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 almost never do because I have a good powersupply and no noise or induction problems in the circuit. If you want to 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 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. I matched them using the Hfe transistor tester on my multimeter and this is good enough. When measuring Hfe, give the transistors time to cool off after you touched them because the Hfe value will change with temperature. Some will tell you that the transistors need to be matched on the Base Emitter Voltage (Vbe) and that is correct but I noticed that if you match them on Hfe, the other parameters will be pretty close too. Anyway it works fine this way.
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 but with Exponential FM the VCO pitch will change the moment you open the attenuator. Obviously you have to have some external way of influencing the pitch otherwise you couldn't, for instance, connect a sequencer to it.
There are trimpotmeters for one 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. I only use multiturns for the Sawtooth step and Sinewave symmetry and really only to save space on the stripboard.
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 I had the input socket just 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.
Btw, you can still use the internal PWM potmeter when you're using external Pulse Width Modulation. If you want to change that you'll need to install a switch in the PWM connection to the print. You can't use the Ext. PWM socket switch because that connection goes through a different value resistor.
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 fine. 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 but you can also use the Fine Tune potmeter on the panel if you're just off frequency. 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 upto 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.
After I installed the Thermistor and re-tuned, 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.)
12V vs 15V:
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.
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.
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!!
Here's an other link for the same item as the first link, this time from the United States. Last time I looked however, the website was offline. (404 error) : - CLICK HERE -
And here's a link to a supplier in Germany, twice as expensive (€2 per PTC) but no VAT. These were sold out but I believe they are now back in stock:. - CLICK HERE -
There's also a 3500ppm/°C version from the UK Thonk retailer, (they are back in stock) but it is a bit bigger in size. (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.
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.
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). 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:
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 print. The plus and minus are connected through the resistors to the power rails on the print.
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)).
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.
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
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.
