Welcome to my website. Here I publish projects detailing all the modules I built for my DIY Modular Synthesizer. I build using the 'Kosmo' standard but also some Eurorack sized modules. All layouts are made by myself and verified to work. The schematics they are based on are sourced 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.
A very good working VCA and Mixer using the AS3080 IC. Very easy to build and with a Line Out option too.
Having built the VCF-1 and the X-4046 VCO designs by Thomas Henry I thought I might as well build the VCA-1 aswell. By coïncidence I needed a new VCA in my DIY synthesizer anyway because the transistor based VCA from way back in the beginning when I started this Modular journey is now starting to act up after working flawlessly for 3 years. It was time for a new and updated VCA and one with a little more options too. This project can also be found on the Birth of a Synth website where I got the others from too.
I built and tested this VCA to run a dual 12V powersupply so perfect for Eurorack too. It's designed for +/-15V but either voltage will work fine. No component changes needed.
If you're new to DIY synthesizer building and you're not sure about the function of a VCA and what it's for, then please go back to project 10 on this website and read the first bit of that article. That will explain it for you. Then come back here and build this one ;)
This VCA is not only a Voltage Controlled Amplifier. It's also a 3 channel mixer and it has a Line Out option (all in mono though). I did put in 3 inputs in the layout but I only used one. I didn't have the room for 3 because I need this VCA to replace my old one and that one is only 4 CM wide. You could easily add even more channels just by putting in more 100K resistors connected to pin 2 of the TL074, and having you inputs come in on those resistors.
This project was yet an other hole in one. It all worked flawlessly at the first try, just like the X-4046 VCO. I LOVE Thomas Henry designs. They just work so well.
Here's a look at the finished product on the test bench:
I made a few changes in the way I'm using this VCA as opposed to how Thomas Henry intended it to be used. TH had the ADSR coming in on an unattenuated input and an extra CV input with attenuation. I turned that around and I have given the ADSR a level control and the extra CV input has no level potmeter. I also put in an extra bi-coloured LED because I used a quad opamp instead of a dual plus a single opamp and just like with the State Variable Filter I had one opamp left over. I connected that to the ADSR input so we have a visual indication that the VCA is receiving an envelope signal and if that signal is positive or negative in Voltage. Actually a very useful feature to have.
SCHEMATIC:
Here's the schematic I used to make the layouts but like I said I made some changes in the type of opamps I used. I also only used one input instead of three. As you can see it's a very simple circuit and it works very well with the AS3080. Like I mentioned above, I used a TL074 for the opamps so the numbering on the schematic will be different from the layouts.
LAYOUTS:
Below are the layouts I made for this project. As ever they are verified. I used them for my build and it again worked straight away, just like the VCO. No mistakes and everything worked right from the get go. As I mentioned before the LED was an extra I put in myself and it's not on the schematic. It's fed by an opamp buffer connected to the ADSR input with a Bi-Coloured LED on the output and a 2K4 resistor as current limiter.
Wiring diagram:
Stripboard only view:
This layout was small enough to have a few strips left unused which gives up plenty of room to connect some L brackets to mount the stripboard to the faceplate.
Cuts and wirebridges. I did not bother to make a layout with just the cuts on it because they are easy enough to see on this combined view:
Testing was pretty straight forward. I connected my scope to the Max. output and put a VCO signal on the input (yellow line) and an LFO signal on the ADSR input (blue line) to serve as envelope signal. The scope showed a beautifully responsive VCA reaction as you can see on the screenshots below.
When you turn the INITIAL potmeter past the 1 o'clock position the audio signal will have a tendency to clip as you can see in the flat top and bottom parts of the highest amplitude of the yellow signal in the screenshot below. Maybe clipping isn't the right term, it reaches maximum amplitude. In that case the VCA will also not shut completely but the audio will not distort though.
Here you can see the negative voltage rejection on the ADSR input. The LED will shine Blue when there's a negative voltage presented on the ADSR input and the VCA will just stay closed.
CALIBRATING THE VCA:
This is simple. There's only one trimmer and you use it to trim away any DC offset on the audio output. What you do is you connect an oscilloscope to the Max output. Then you put an audio signal on the input and an LFO signal on the ADSR input. Then you turn the trimmer until the signal is centered around the zero Volt line. In other words, the zero Volt line goes straight through the middle.
That's all.
PICTURES:
Here are some pictures of the finished module:
So that's it for now. Three in a row this month and three very good Thomas Henry projects for you to sink your teeth in.
If you have any questions or remarks please comment below or in the special Facebook Group for this website.
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.
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 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.
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.
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 comparitors. The IC has been used in some very cool Eurorack modules too, among them the Wiard Wogglebug.
The opamp used in the exponential converter, with the inputs of the VCO, is also 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 ^___^
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)
And as the article in 'Birth of a Synth' states; one of the finest hard sync effects ever heard from a VCO.
THIS VCO WILL RUN ON BOTH +/-12V OR +/-15V. I 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 there's no problem building this for Eurorack. There's a video on YouTube showing a stripboard eurorack version being built. He uses lots of small pieces of stripboard with the potmeters soldered straight to them.
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.
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 needs to be 500K or higher. (I also used a 1M for that one).
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.
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:
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. You can also just put in a 2K resistor instead of the PTC.:
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 as I always do just by measuring the Hfe on my multimeter and choosing two that have the same value. If the Hfe is the same you can be pretty sure the Vbe will be pretty similar too.
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 it.
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.
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 :)
TUNING THE VCO:
Tuning 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. The sinewave took a bit of time to get right but it's just a matter of trial and error. You need to set the offset voltage for the Triangle and Sawtooth waves so that the zero Volt line goes nicely through the middle.
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.
Tuning the VCO to track with the octaves is also just a matter of using the V/Oct trimmer in combination with the Frequency controls on the front panel. The HF tracking has very little influence. I found it quite difficult to get this VCO in tune but that's a known characteristic of this VCO.
I turned the Frequency fine control to get note C3 in tune and then I checked C2 and C5 and I turned the V/Oct. trimmer to get them closer to the true note. Then I checked all the octaves again.
I again set C3 in tune with the Freq control and turned the V/Oct. trimmer to get the others (C2 - C5) closer to where they need to be. I used the HF tracking to get better results on the higher octave but it has little influence.
I repeated this proces until I got reasonable results. I managed to get C3 to C5 in tune but C2 was about 20 cents too low. I just left it at that because it sounds just fine for my needs.
These are the test results Thomas Henry himself got when tuning his VCO to track over the octaves:
Here are the standard waveforms. The spikes you see on the triangle wave are a characteristic of the VCO. They are very fast and way beyond the human hearing range so no problem at all.
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.
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!
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 little test video in which I try the famous Hard Sync function of the VCO. I must say this VCO paired with the State Variable filter is a killer combination. I'm using a sawtooth wave from the Thomas Henry 555 VCO into the Hard Sync input.
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.
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A very easy to build and awesome sounding state variable filter by Thomas Henry. For Eurorack or Kosmo systems.
I have covered many different types of filters on my website but I had not yet built a State Variable filter. What does that even mean I hear you ask. Well state variable means that it can simultaneously provide two or more types of filtering. In this case the filter has Lowpass, Highpass and Bandpass outputs and rather than having to switch the filter into these different modes, you have them available together, so each filtertype has its own output.
I found this particular schematic on "Birthofasynth.com", a website that has all of Thomas Henry's projects on it.
In the article about this filter he calls it a 'barebones' filter. Very much taken straight from the CA3080 datasheet. That may be so but it's still a great sounding filter. The way it sounds reminded me of the Steiner-Parker filter but I think it may even be better. It has a bit more edge to it I think.
It's a really old school filter with a 12dB/Oct. cutoff slope and it sounds like a 70's synthesizer filter should sound. It's pure sounding, phat in the low end (especially using squarewaves in Lowpass mode) with a beautiful but little bit agressive resonance. It's a beauty!
Anyway I urge you to read the article I linked to above if you want to know more about what TH said about this filter and its development.
THIS FILTER WILL RUN FINE ON BOTH A DUAL 12V OR A DUAL 15V POWERSUPPLY!
SCHEMATIC:
Below is the schematic I used for this filter. It says the filter is to be used with a dual 15V powersupply but I did all my testing running it on a dual 12V powersupply because many of you will be building this for Eurorack and it works just fine. I also used a Eurorack friendly size of stripboard which is 24 by 41 holes. That will fit behind a Eurorack panel.
As you can see the filter uses two OTA chips, the AS3080. These are the modern version of the original CA3080 chips and I believe these are less noisy than the originals too. You can order them from Electric Druid, which is where I got mine.
The LM13700 OTA chip also has two CA3080 chips inside and can be used in this circuit but then you'd have to design your own layout because that is a DIP16 IC. For this particular project you'll need the AS3080 chips. They are also available from Thonk --- click here ---
The numbering of the opamp pins has been changed to fit the layouts below, because I used the opamps in a different order to the original schematic.
The schematic shows a two transistor exponential converter with a PTC as temperature compensation. which we already know from the Thomas Henry 555-VCO. The temperature compensation is only useful if you intend to use the self oscillation of the filter as an extra oscillator. I never used a filter in this way and I can't imagine any of you will ever use the filter for that purpose so you can leave out the PTC and just use a 2K resistor. That's what I did eventhough I have these PTC's in my stock. In fact Thomas Henry himself used a 2K resistor as he mentions in the article linked above.
You do have to match the two PNP transistors though. I matched them as I always do just by measuring the Hfe on my multimeter and picking two transistor that measure the same value.
You might use the filter in full self resonance mode if you're looking for a special sound effect. I tested it and it will track with the keyboard because it has a Volt per Octave input. I have not tested how accurate the tracking is but I do think you can make it track over a few octaves if you want. Beware that the self oscillation is about twice as loud as the normal audio you get from this filter!!
You can leave out the Frequency Fine Tune potmeter too because that's only there to tune the self resonance for tracking. As a Cut-Off Frequency potmeter it is pretty much useless. One thing you can do is change the 3M3 resistor to a 100K and add a socket to that potmeter so it turns into an extra CV input with level control. Wire it up like the Envelope input. That's what I did myself. I find two CV inputs a necessity for a filter.
LAYOUTS:
Below are the layouts I made for this build. As always they are verified. I used them to build my filter. I was very thorough with checking this stripboard layout for faults before I printed it out and used it to start building my filter. I'm glad I checked it over a few times because I did manage to catch some mistakes in the design phase which saved me some hours troubleshooting I think.
Anyway the build went fine and apart from one transistor being faulty which needed changing out the filter worked straightaway.
Here's the wiring diagram:
All potmeters are seen from the BACK SIDE!
The Cutoff Frequency potmeter is wired up in such a way that the filter opens up when you turn it clockwise. The Resonance potmeter is wired up so that it gives more resonance when you turn it clockwise going into self oscillation when turned fully clockwise.
A little remark about the Frequency Fine Control. Only include this potmeter if you intend using the filter as an oscillator with the resonance at self-oscillation. As I mentioned earlier, the Frequency Fine Control is meant to tune the self oscillation of the filter so that it tracks with the keyboard. In itself it has very little influence on the CutOff Frequency so if you intend to use this filter just as a VCF and self oscillation isn't important to you than leave out the fine control potmeter. It will save some space on the faceplate too. I myself left out the Fine control potmeter too. I changed the 3M3 resistor for a 100K one and connected a second CV input to that point, complete with level control potmeter like the envelope input.
Here's the stripboard only view. A little tip, when soldering in the trimmer potmeters put the wipers in the middle position. That way you won't have to do much tuning when you're testing the filter.
Here's an overview of the cuts and wirebridges. Start soldering these in first before you solder in any components. There are 35 wirebridges to solder in. Make sure you're very very accurate here. It's easy to make a mistake and one wirebridge in the wrong position and the filter won't work.
And finally the cuts only as seen from the component side. As always, mark the cuts on the component side first with a waterproof Sharpie or Edding 3000 and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut the strips at the marked places with a sharp hand held 6 or 7mm drill bit. This way you have the least chance of making mistakes.
Once again you need to be very accurate here because the component placement leaves no room for errors.
And here's the Bill Of Materials:
If you're going to build this module for Eurorack then I urge you to order miniature potmeters. You're going to need all the space you can get. There are 5 potmeters to accomodate and 7 sockets.
Adding extra's:
As you can see there's only one audio input but it is easy enough to add more inputs. You can simply connect them through a 100K resistor to pin 9 of the TL074 and they will be summed together. You can also put in more CV inputs if you wish by connecting them through a 100K resistor to pin 2 of the TL074, which is what I did.
The trimpots for the Offset control could actually be left out. They are not really necessary. They are part of the design because the early versions of the CA3080 chip (which was originally used in this filter) had quite a bit of variation in their offset voltages. But the latest generations don't have that problem anymore. I left the trimmers in because I stayed true to the schematic but it's up to you. The filter should work fine without them. Should you take them out then you can remove the trimmers and the 100K resistors in series with the wipers. Leave the 22 Ohm resistors to ground in place.
Finally we have an opamp left unused! We have to do something with that right? :-)
I always like to include a LED if I can, so I altered the layout a little and made a jumpwire from the envelope input to pin 3 of the TL072. The green wirebridge from pin 10 of the TL074 needs to be lengthened and soldered straight to the bottom strip (X). Now we have configured the opamp as a voltage follower or buffer so we can connect a Bi-coloured LED to it without drawing any current from the envelope input. I used a big 4K7 resistor as current limiter so the LED will only be the brightest with the highest voltage. Below is the layout to show this alteration.
The components for this change are not listed in the Bill of Materials because this was done as an after thought, but it's only a LED and a resistor. I used a red/blue bi-coloured LED.
ABOUT THE AC/DC SWITCH:
You may have noticed the audio input of the filter has an AC/DC switch in it. This is provided for instances where the full audio bandwidth of the signal is desired. DC coupling allows very low notes to pass through uninhibited by the input capacitor. This is actually the only filter on this website (apart from the ARP2600 LPF) that doesn't have a capacitor in the audio path, when switched to DC that is.
A DC signal can pass straight through the filter without ever encountering a capacitor so the filter can actually process a control signal! This opens up an entirely different can of worms.
For example you could patch the CV output of a sequencer through the Lowpass mode of this filter before it goes into a VCO. If you then modulate the cutoff frequency with an LFO you can create some really spacey effects.
CALIBRATION:
This is the procedure for the V/Oct. tracking of the filter in full self oscillation:
Connect the lowpass output to your VCA so you can hear the signal. Be careful, the self oscillation signal is quite loud! (Very loud in fact)
Connect a keyboard or other V/Oct. source to the 1V/Oct. input of the filter.
Put the filter in self oscillation by turning the resonance full clockwise and adjust the V/Oct trimmer for as close to a one Volt per Octave interval as possible. Go between C2 and C3 on the keyboard for instance and turn the trimmer to get the best result. Use the Frequency Fine control to help you tune the oscillator to the right notes, if you kept this potmeter.
About the trimmers.... what I actually did was put them in the middle position and just leave it at that. There really is nothing to trim because the modern AS3080 chips don't have erratic offset voltages in their output like the old CA3080's used to have. So I didn't do any trimming and the filter works fine but I'll give you the procedure anyway, but as far as I'm concerned, you can ignore it.
Here is the procedure as mentioned in the official article:
Put the trimmers in the middle position. With multiturn trimmers you start turning them untill they start clicking. Then turn back and count the number of turns until the wiper is at the other side and you hear it clicking again. Now turn the wiper half the number of turns you counted. Then it's in the middle position. You could also just measure the voltage coming from the wiper and turn until it's at zero Volts. Then it's in the middle too.
Connect a square/pulse wave to the filter input and monitor the Lowpass output with an oscilloscope. Now check for DC deflection as you turn the Frequency Cutoff potmeter through its range. Adjust the trimmer to get the least DC deflection at the output.
The trimmers are interactive (they influence eachother) so you may have to go back and forth between them a few times.
There should really be very little to trim. As I mentioned earlier the filter will actually work fine even without these offset trimmers and with the AS3080 there should be any offset voltage to speak of.
PICTURES:
Here are some pictures I took during the build proces:
As long as I was waiting for the new powder coated aluminium to come in the mail, I thought I'd make a template for the panel out of cardboard, so I can mount all the components in it and see if all the wiring is long enough. As you can see I changed my mind about the switch placement and I needed to lengthen several wires but it all fits nicely behind a Kosmo sized panel of 20 x 7.5 CM. I want to install this filter in my DIY synthesizer, not my Eurorack system. I think, if you want to fit it behind a eurorack panel, you have to make it a bit wider still.
Here's the finished module all ready to go mounted in my DIY synthesizer: I did all my testing running this filter on +/-12V and it was absolutely fine. But in my synthesizer I have two powersupplies, one for +/-12V and one for +/-15V, so I decided to connect it to the +/-15V supply for permanent use.
(Boy, does this thing sound good. I love it!)
I wrote the labels with a white acryllic pen. I got new pens and this one is not as scratch resistant as the old pens I used to have so after I finished labeling everything I sprayed the panel with a layer of clear lacquer.
The stripboard is mounted behind the panel with a piece of plexiglass that I bent at both ends so it grips the stripboard like the fingers of a hand. I glued two little pieces of plexiglass at the ends so the stripboard can't slide out and I secured it with glue. Then I drilled a 3mm hole through one end and mounted it to the panel with an M3 bolt. I also used superglue to secure it and keep it from rotating should the bolt get loose.
When I first tested the filter I found that the Frequency control wasn't working. The Resonance was fine and I could see on the oscilloscope that the filter was doing it's thing but no Frequency control. I took my scope probe and tested the legs of the transistor pair and sure enough. Transistor Q2 was not working. So I put in a new matched pair of transistors and now everything was fine. It all worked as it should. Strangely enough the transistor was not faulty. It must have been a bad connection.
Here is a video of me testing the filter and the different outputs. It's a video I also uploaded to my YouTube channel.
I found with testing that the Lowpass sounds best with a squarewave on the input. The High- and Bandpass filters sound the best when you use a Sawtooth wave on the input.
Here's a little test video with a demonstration of the extra CV input I installed (with level potmeter). I have a sinewave connected to the second CV input. The rest is like the previous video:
Okay, that's it for now. Enjoy building this filter. It's a really good one!
If you have any questions about this or other projects then please comment below or post your question in the special Facebook Group for this website.
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I built this project for Eurorack but of course it can just as easily be built for a Kosmo sized synthesizer. In fact with a Kosmo module you have more space to put in more stages so that would be even better.
This project works on both -12/+12V and on -15/+15V without needing any changes.
Here's the schematic I used:
HOW IT WORKS:
It's a very simple circuit. You have a DC voltage that can swing between the voltagerails coming into the inverting input of an opamp by means of the 'Init' potmeter (Initial Voltage) and a second connection in the shape of a Control Voltage input with a level control, summed to the same input on the opamp. The output of this opamp controls the lighting up of the LEDs in the Vactrols. One Vactrol does the negative phase and one does the positive phase, or one does input A and the other input B. Whichever you prefer.
The LDR's (Light Dependent Resistors) of the Vactrols are each connected to one input source and the other sides are connected together and go into the second opamp which is simply an output buffer. That's all there is to it.
The CV input comes in via a lower resistance than the Initial Voltage, 22K instead of 100k. This will, in combination with the 100K feedback resistor R4, give the CV input an extra gain. We can calculate that gain with the formula: Gain = (-Rfeedback / Rin) which is -100/22=-4.45 The minus symbol simply means that the output voltage is inverted.
This is done (I think) to give lower control voltages enough effect to influence the vactrols because a CV voltage can never be as high as the Initial Voltage which covers the full powerrails potential. So if your CV input is not effective enough you can try lowering R2 and so get even more gain, but don't overdo it. Measure first before you start altering things. Make sure this change is actually needed. (I left it as it was myself).
I built one Crossfader stage as seen on the schematic but it would be easy enough to build a few of these so you can mix together more inputs. The circuit, as you can see, only has a few parts so it doesn't take up much room.
With the Init potmeter (Initial Voltage) you can manually fade between the two inputs. This works as long as there's no CV voltage present. When you input a CV voltage that will then take over control of the crossfading the higher you set the CV Level and you can steer the output signal more to one or the other input with the Initial Voltage control potmeter.
I made the Vactrols myself, in fact I used Vactrols I had already made for the LowPass Gate project, so I don't know exactly which LDR's I used to make them with but I used red 5mm LEDs in them. It is preferable to use red LEDs because they have the lowest voltage drop (about 1,5V) but with the extra gain the CV input gets I don't think this is very important. LDR's do react well to red light. The LDR's I used have a resistance of about 300Ω when the Vactrol is switch fully on. I think it is best to use quick reacting LDRs in the Vactrols so it can handle a wide range of LFO frequencies on the CV input.
The test results I got after finishing building this project showed that, when using the CV input I still got some bleed through of one signal onto another. In other words the separation of inputs A and B was not perfect when the CV voltage was on full positive or negative voltage. With just the manual control (Initial Voltage pot) I did get a good separation between the signals when the potmeter was fully clockwise or counter-clockwise. Of course the total voltage of the CV input could be the problem here. If it is not as high as the voltage produced by the Init. potmeter then it won't have as much influence. That is why the CV stage has extra gain on it to boost the signal.
So please don't expect perfection from this analog Vactrol design. It's old school and that's what I like about it.
LAYOUTS:
Here are the layouts I made for this project. As ever they are verified. I used them for my build.
Stripboard only:
Cuts and wirebridges. As ever, mark the cuts with a Sharpie. Stick a pin through the marked holes and mark them again on the copper side. Then cut the strips where marked with a sharp hand held 6 or 7mm drill bit.
Here's the Bill of Materials. If you make your own Vactrols then refer back to the Lopass Gate project to get more info on the LDRs I used there. There's a complete paragraph discussing the Vactrols in that article. I did not include any bypass caps in the BOM so if you want them, order two 100nF ceramic caps and solder them from positive to ground and from ground to negative power at the top of the stripboard.
Here are some pictures of the finished module. Mine is 6hp wide (3CM) and about 5,5CM deep which is a lot but it will fit in a Nifty Case, no problem.
The two striped components on the top near the power header are just 100nF bypass caps. You can include them of leave them out. They're not really necessary and I left them out of the layouts and bill of materials.
In front of the vactrol you can see a little pin header. There's also one on the other side that you can't see. I soldered those in for test purposes. I could connect my multimeter probes to it to measure the resistance of the LDRs in the vactrols.
SCOPE IMAGES:
Here are some screenshots from the oscilloscope. In the three images below the yellow line is the output signal, the blue is input A and the purple is input B.
The first image represents the crossfader with the Initial Voltage potmeter fully counter-clockwise so only the squarewave is let through to the output. There is no CV input present in any of these screenshots.
In the second image the Init potmeter is in the middle position and we get a mix of both inputs A and B.
As you can see here with the blue and purple traces some of the signal is fed back on to the input signals, changing their character a bit but this is easily explained. When both vactrols are fully active the output and both inputs are separated from eachother by only about 600Ω, the series resistance of the vactrol LDRs, so it's obvious some of the input A signal will find its way to input B and vice versa. This is not a problem however and it can not damage your VCO's. Exactly the same happens when you mix signals with a passive multiple for instance, so nothing to worry about.
And in this third image the Init. potmeter is set fully clockwise so only the Triangle wave is let through.
If you connect a sinewave LFO to the CV input and turn the level up, then the result will be a cycling through these three stages. By turning the Initial Voltage potmeter away from the middle position you can emphasize one of the input signals.
The signal that is connected to the CV input needs to be a bi-polar signal, so a signal that has a negative and a positive voltage phase otherwise it will only fade one of the inputs. If you want to use a uni-polar signal, you could try using a capacitor on the CV input socket to turn a uni-polar signal into a bi-polar signal but I haven't tried that myself.
Here is a little test video I made while I was testing the circuit just after finishing it. You hear me discussing a CV leakage problem, that when the CV Level is fully closed it still had some influence, but that turned out to be a grounding issue, as I thought it must be, and once I put the stripboard behind an aluminium faceplate and wired everything up that issue was no longer there. So problem solved.
Okay, that's it for this project. A simple one just like the previous last few projects because I didn't have time to do really big projects in the last few months. However I am working on some new and bigger projects so watch this space :)
If you have any questions about this project please comment below or post your question in the special Facebook Group for this website.
