Synthesizer Extra's No.5: PROFESSIONAL LOOKING PANELS using WATERSLIDE PAPER.

 An easy way to make your panels look really professional.

I've been doing this hobby for over 4 years now and all that time I always used black powdercoated aluminium to make my panels from and I always use a white acryllic pen to write all the labeling etc. 

It works fine but it doesn't look very professional.

For a long time I've seen people using waterslide decals to apply to their panels and it looks very slick. I wanted to try that myself so I ordered some "Water-based ink jet water transfer paper". That's what is says on the label. You can order this from AliExpress or your preferred webshop. If you Google it you'll find lots of sellers.

Make sure, if you use an inkjet printer like I do, to order transfer paper suitable for inkjet printers. It must say inkjet on the package and it must be transparent. If you use a laser printer then there's special transfer paper for that too.

There is also white transfer paper available and i just took delivery of that too. I want to experiment with black backgrounds.

IF THIS IS YOUR FIRST TIME WORKING WITH WATERSLIDE PAPER, MAKE SOME TEST PIECES AND TRY TO APPLY THAT TO A SURFACE FIRST!

First step: Design your panel layout.

I do all my designs in Photoshop. I like working with Photoshop because you can work with layers and the centimeter markings on the side are very accurate to how it will roll out of the printer.

There are also special layout design programs on the internet, like 'Schaeffer AG Front Panel Designer' which you can download for free.  There is also a free program called Inkscape which should work very well too. You can download that by clicking here. 

I got the following link from a comment on the Facebook group. Also a faceplate designer: --- CLICK HERE ---

I have not worked with these yet although I intend to try them out soon.

You can find decal designs on the internet or make your own. Make sure you draw in dots for the places where you need to drill holes in the panel so you know where to drill them.

One tip I can give you is to make your panel design just a little bit smaller than the actual panel. Just a millimeter on the sides and have it start and end inbetween the holes in the panel where you screw it into the synthesizer case. That way the lacquer you apply later will seal those edges and prevent them from coming loose, which can happen if they are exactly on the edge of the panel. 

Here's a picture to illustrate that point. You'll see in the light reflection on the top how the lacquer forms a protective layer over the edge which runs just under the mounting hole:

Btw, that panel had a typo in it, the plus signs should have been multiply signs but I still used it because it came out so well. This was the second panel I ever made using this method and it went perfectly.

Here is my first design for the Digisound-80 VCO, made in Photoshop and printed on A4 paper. (feel free to use it if you want):

I drew in the Octaves and Freq Fine control markings after measuring on the VCO where they should go so they actually indicate where the different octaves are and on the fine control where the notes are within the octave. However I used a 2M7 resistor for the fine control, not a 3M3 as recommended so my fine control covers a little more than the normal half octave. Note the dots that indicate where to drill the holes for the potmeters and sockets.

Step two: Prepare your waterslide paper.

Now print your design onto the waterslide paper. Make sure you print out as many of your designs as you can fit on one sheet of paper. Copy and paste them next to eachother. You can only use an A4 size paper once so if you make a mistake you will have a few others as back-ups. It's a shame to have to throw away 2/3rds of your waterslide paper unused.

Here's how I printed the design of the envelope follower panel. Four on one A-4 piece of waterslide paper:

Now cut the design out with scissors. I use one of those paper cutters with a rotating knife on a rails that you can put the paper under. It guarantees a straight cut. Round off the edges a little with scissors. Make sure you cut it a bit smaller than the panel size so that the waterslide paper doesn't sit right at the edge of your panel. It will come loose over time if that happens.

Now apply a few thin coats of clear acryllic lacquer onto the paper. You can of course also do this first before you cut it out. Make sure there's no dust or hairs on the paper and let it dry a few hours and apply a second coat. These coats not only protect the ink they also make the paper stronger when applying it to the panel. 

If you use a laserprinter that uses toner then you don't have to apply the lacquer. However inkjet prints are not waterproof so they must be protected with lacquer. Make sure it is well dried before moving on.

Step three: Prepare the panel.

Take the design you made and print it out again but this time on a normal piece of paper. We are going to use this as a drill guide, to drill the holes in the right place. 

Take your panel material, in my case white powdercoated aluminium, and apply the printed design to the protection layer of the panel with doublesided tape. You can use other methodes but I find this the easiest to do. Usually the panel material will have a protection layer against scratching, if it doesn't have that just put some paper tape on it first. The kind you use to tape off edges when you paint a door for instance. Stick your design to the panel and make sure it can't move.

Now use a very thin drill to make the first pilot holes. I used a 3,3mm drill the first time but that was too big and I couldn't place the drill accurately enough which caused me some problems when I put in the potmeters. 1,5mm will be better. So make sure you drill very accurately. Then drill the holes to the right size and take off the paper/protection layer. Make sure you de-burr the holes you just drilled. There must be no upstanding edges around the holes because that will cause air pockets under the waterslide paper or it will tear the design when you apply it to the panel.

Step four: Apply the waterslide paper to the panel.

Now take a nice big bowl and fill it 2/3rds with slightly luke warm water. Put one drop of washing up liquid in in the bowl and then add the water. This decreases the water surface tension. If the water has gone all bubbly, just scoop out most of the bubbles with your hand so you can see what you're doing. 

Put the waterslide paper into the bowl. It will immediately curl up. Carefully roll it out with your fingers under water and make sure it is completely in contact with the water. The paper will straighten out. Keep it under water for about 30 to 35 seconds and then take it out.

MAKE SURE the image doesn't come off while it is still in the water!! Or all the glue will wash off. That's why I say 30 to 35 seconds in the water is about right.

Put a little water on the panel with your fingers to wetten it a little. Make sure the panel has been de-greased and free of fingerprints. (A little alcohol does wonders in cleaning the surface beforehand).

Now take out the waterslide paper and place it over the panel. Grip it carefully with your fingers on the top side of the panel and then rub the bottom side between your fingers in such a way that the bottom layer slides out from under the image layer. Your thumb stays in place while your fingers slide the backpaper downwards. If this doesn't go smoothly, the paper needs a little more time to absorb the water and liquify the glue. Be patient and careful.

Slide the back layer down from under the image layer. It is important that you don't move the waterslide paper too much once it is applied to the panel because this will impact on the stickiness of the paper. The slippery stuff underneath the image layer is actually the glue so you need to make sure this is not washed away by too much movement.

There are special 'decal setting solutions' available for purchase, that will cure the layer and make for a good bonding between panel and image layer. I don't own any of that myself but I might try this later, however you don't really need it. 

Now once the waterslide paper is in the right place, carefully squeeze out the water underneath the waterslide image by using the backpaper as a squeegee. Carefully move it over the image layer and squeeze out any water underneath. (Use the slippery side.) Be careful not to crease or tear it! This takes a bit of practise but it doesn't have to be perfect. The image layer may still look a bit rippled but that will disappear when it dries up. Make sure there are no folds though. When the water has been squeezed from under the paper, you can use a fine cloth to dry the surface of most of the water. Be very careful though. If you don't trust yourself, leave the water on top. It will evaporate anyway. I put it on the central heating in my room and it was dry within an hour. 

Here's how it looks freshly applied after I went over it with the backpaper to squeeze out most of the water. It's still very wet and a bit wrinkly but that doesn't matter:

After an hour on the central heating the layer has pulled itself tight on the surface and it's completely dry:

(This is from project 62 and it's my most perfect panel to date. No crease in sight.)

Step five: The final touches.

When the panel is dry, cut out the parts that are covering the holes you drilled with a very sharp knife. Take your time with this, don't rush it. You can ruin it otherwise. Use a very sharp knife to clear the holes and make a scissor action (cut) against the metal of the panel holes, so you don't pull on the image layer. Pull out the bits you cut out, from the underside of the panel with tweasers so you can't rip off the decals by accident. When that's done I always take a cigarette lighter and move the flame quickly under the holes, from the underside, to burn off any small bits that didn't come off. Don't overdo it though. Be careful.

After that you must coat it again with a few thick layers of clear lacquer and leave it to dry for a day. When that is dry you can put in all the knobs, switches and sockets. Be liberal with the application of clear lacquer. For my last two panels I put on one really thick layer. Because it stays flat all the time you don't have to worry about the lacquer running. The thicker the layer the better when putting in the components because it will prevent the waterslide decals from tearing when you screw things down.

When you're using matte lacquer instead of glossy then do NOT apply thick layers. Matte lacquer has a tendency to bubble and make a rough surface when you apply it too thickly.

When you put in anything that needs screwing down, and that's everything, make sure there are rings underneath so the screwing action doesn't cut into the waterslide material. This is a bit problematic with sockets because they don't come with rings, so be careful when you tighten them. (The thick layers of lacquer really helps prevent this) You can hold the screw and turn the socket or potmeter from behind the panel, do what you think works the best for you. Most of the time you will damage the layer slightly by putting in the potmeters, that can't always be helped but if you put enough lacquer over it, the damage will be minimal and covered by the knobs you put on the potmeters.

And that's all there is to it. Anyone with a computer and a printer can do this. I do advise to make a practise piece first and use it to hone in your skills. Here's a look at the Thomas Henry 2164 VCF/VCA in use in my Nifty Case. It sounds amazing and looks amazing too.

VIDEO TUTORIALS:

I found a few video tutorials on YouTube that show the proces from beginning to end. These are all about guitar pedals but it works the same as with synthesizer panels. 

I urge you to watch these before you start, so you have a good idea of what's involved.

--- CLICK HERE FOR TUTORIAL ---

--- CLICK HERE FOR ANOTHER TUTORIAL ---

Okay that's it for this one. I hope this is of use to you. If you have any questions or remarks please put them in the comments below. They won't appear directly, comments are moderated, but everyone of them is answered by me. You can also post your questions in the Facebook Group for this website.

Synthesizer Build part-36: DUAL VOLTAGE PROCESSOR.

This is the Fonik Buchla Style Dual Voltage Processor. A very useful module for altering Control Voltages with five different functions!  Offset, Attenuation, Inverter, Glide or Lag control and, if you follow the tip below the schematic image, it can also be a CV splitter. 

Now also with Eurorack compatible layout. 

I wanted a Voltage Processor module in my synth for a long time and I was thinking of copying the ARP2600 VP, but that one is fairly limited in its options and more specialized specifically for the ARP2600 so when I saw this design I thought it would fit much better in my system. This module lets you alter the offset of a control voltage by 0 to +5V or -5 to +5V. It lets you attenuate and invert a control voltage by means of an Attenuverter and it has a Lag control that is a direct copy of the Lag control from the ARP2600, with a 1 MOhm potmeter and a 470nF capacitor (The ARP used a 100nF cap). This alters the slew rate of, for instance, a Squarewave and rounds off the corners turning it into a Sharkfin Wave. In fact it adds a 90° phase shift to the signal. 

If you want that control to behave more like a Glide control to smoothly go between different notes with a 1V/Octave signal then use a 1µF electrolithic capacitor. Try it and experiment. Maybe use a 100K potmeter instead of a 1M one.

Besides control voltages this module can also handle audio signals.

This module will work fine on either a dual 12V or a dual 15V powersupply so no problem for you Eurorack fanatics =).  In fact, there's a Eurorack friendly layout further down the article. One thing though, with a dual 12V supply normally the maximum offset would be 4 Volt instead of 5 Volt but I addressed that issue and fixed it by changing some resistor values.

I guess you could say that the Eurorack equivalent of this module would be something like the TipTop Audio MISO (Mix, Invert, Scale, Offset) which costs a 110 Euro's. 

The circuit is primarily meant for control voltages but it can handle audio signals just as well. Even at very high frequencies it won't distort the signal. With audio you can use the Lag control to turn a Triangle wave into a Sinewave although with less amplitude. It won't be perfect but it's possible. It can also turn a 0V to +10Vpp signal into a +5/-5V signal by adding a -5V DC Offset voltage to it. The other way around works too of course, turning +5/-5V into 0V to +10Vpp signal. Very useful.

The circuit consists mostly of 47K resistors but you can actually alter the value of those and use for instance all 91K resistors. I actually did this as a test with the second part of this dual module and it didn't change the working of the circuit in any way. Just make sure you use the same value for all 7 resistors. Don't make them lower than 47K though. You can also use other quad-opamp chips instead of the TL084. You can use TL074, LM324 or any other, as long as it's a low noise opamp (good for use with audio, which most of them are these days) and they have the same pin-out as the TL084 (and most quad opamps also have that these days).

This circuit was designed by Chris MacDonald and modified by Peter Grenader and then further improved by Matthias Herrmann who added the Lag (Glide) control function. The only thing I did was adding the Offset switch, changing the potmeter values from 50K to 100K, changing the value of the Lag Capacitor from 1µF to 470nF and adding the 470 Ohm resistor before the Lag potmeter to eliminate noise issues, based on practical testing.

The original schematic and a PCB design can be found in this original PDF and I made a new drawing from that schematic which is posted below. Like I just mentioned, they use 50K panel potmeters in the schematic but I didn't have those so I used 100K potmeters. Again, this made no difference what so ever. You must however use a 1 MegaOhm potmeter for the Lag control because this, together with the capacitor, forms a simple lowpass filter and these values are important to get the correct frequency response. The original schematic uses a 1µF capacitor for the Lag control but with testing I found out that this is way too much. So I changed it for a 150nF in the layout but that turned out to be not quite enough. (The original ARP2600 Lag control uses a 100nF capacitor.) In my own build I experimented with different values and I ended up using a 270nF and a 180nF in parallel to make a total of 450nF and that works fine. So I set the capacitor value on the layout to 470nF. I found that this gives the best Lag control response in my case. Of course, if you don't have a cap of that value available, you can use an other one with a value close by. Anything between 300nF and 700nF will work fine and you can put two (or more) in parallel to create the value you want but test it and check to see it works like you want it to. Use an oscilloscope set to DC mode for measuring the output.

If you don't want a Lag control but a 'Glide' control you can use a 1µF electrolithic cap. My advise again is to experiment and use whatever suits your needs.

Setting the trimmer (T1):

The trim potmeters are for setting the attenuverter mid point, but they don't have too much of an impact so you don't have to use multiturn potmeters for those. The normal ones will do fine.  I added a switch to the offset control so you now have a choise to offset a control voltage from 0 to +5V or from -5 to +5V. 

Here's the procedure for setting the T1 trimmer:

- Turn all potmeters to the fully counter-clockwise position.

- Turn the attenuverter potmeter to the 12 o'clock position (mid point).

- set the offset switch to 0 to +5V position.

Connect an oscilloscope or multimeter to the output and turn the trimmer until the output reads exactly zero volts when the attenuverter is at the 12 o'clock position.

A little quirck I found, at least in my build, is that there can be a lot of noise on the output if the Lag potmeters are set fully closed (counter clockwise). Because this was the case with both sides of the Dual Processor I figured this was a fault in the circuit design so I added a 470 Ohm resistor between the Lag potmeter and R6. The value is low enough not to influence the Lag filter and it gets rid of all noise issues that I had.

The schematic drawing doesn't include any de-coupling capacitors but they are included in the layout. Just four 100nF ceramic caps on the power rails as close to the chips as possible. If you experience hum on the audio output you could even put some 10µF to 47µF electrolytic capacitors on the power rails. There's room enough left for that. Make sure they are rated 25V or higher and put one on the +15V to ground (negative pole to ground) and one on the ground to -15V (negative pole to -15V) rails. I leave that up to you but for my module it wasn't necessary to include them. (The electrolytic capacitors are not included in the layout, only the de-coupling caps.)

Here's the schematic drawing which I re-made from the original, from the above linked PDF file. The Dual Voltage Processor consists of two of these circuits side by side with only the Ground as a common link:

I made a Falstad simulation of this circuit. Some of the component values have been alterred a little to make the simulation act more like the real thing. The Lag potmeter value changed from 1M to 100K, the cap from 470nF to 100nF and the 470 Ohm resistor has been taken out.

--- CLICK HERE ---

LAYOUT

Here is the verified stripboard layout I made for it. It's the same layout once repeated and mirrored to make it a dual module. This layout was made for the Kosmo format modules but now there's also a more compact layout below for Eurorack size modules.

TIP: Solder a wire from the input-socket of stage one to the socket switch of the input socket of stage two. That way the signal on input one will be present on both inputs and can be processed by both stages and split in two. If you connect a patch cable to the input of stage two, that first connection will be broken by the socket switch and it's back to normal. (Normally we call this normalling a connection ;) Very useful me thinks! 

Of course you need input sockets with built in switches for this but most types have that as standard.

Stripboard only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide. 

Bill of Materials:

EURORACK LAYOUTS:

As of December 2021 there is now also a new layout for the Eurorack format. The stripboard is 24 by 41 holes. Just like in the Kosmo format layout above, the right part of the dual voltage processor is a mirror image of the left part so I could place all the connections to the potmeters on the edge of the stripboard on both sides. The TL084 has 4 identical opamps in it so it doesn't matter which opamp is used for which part of the circuit. As I mentioned earlier, you can use other quad opamps like the TL074 or LM324 for this without problem.

Make sure you connect the three ground strips at the top together by putting some extra solder on the eurorack power connector. Otherwise put in some wirebridges to connect the three ground pins together.

Here is the wiring diagram:

Stripboard only view:

I built this version on Dec 10th 2021 and everything worked fine except that I had to use a lower value capacitor for the Lag control. The layout has a 470nF cap in it and that works fine in my Kosmo format panel but for this one I had to use a 10nF cap. Not sure why this one is different, maybe it's the fact that this runs on +/-12V instead of +/-15V or it's the potmeters I used I don't know, it's a bit of a mystery. Anyway, it's not important because if you find, when testing, that the Lag potmeter doesn't work over the full throw then you need to lower the capacitance. Just a matter of experimenting. The cap can easily be de-soldered and changed for another one.

The trimpotmeters are for setting the Attenuverter midpoint and as with the previous layout they don't have much influence but you need to set the Attenuverter so that at the midpoint, when the waveform is a flat line, that line is at the zero volt mark. Measure this with a scope and make sure all the other potmeters are turned fully counter clockwise and the offset switch is set to 0/+5V.

Btw, because this module is running on +/-12V the actual offset voltage is plus or minus 4 Volts, not 5 Volts but I only discovered that after I made the panel so I kept the labeling as is.

EDIT: To get a higher offset voltage in this Eurorack version you must change the resistor R6 from a 47K to a 68K. This will actually give you +/-6V offset voltage. R6 on the lefthand side of the stripboard is the 47K between pins 13 and 14 of IC-1 (positions I and J-16). On the righthand side it is the 47K between pins 1 and 2 of IC-2 (positions I and J-27). Change those for 68K's and your offset voltage will be upto +/-6V. If you need +/-5V then use a 56K with a 1K2 in series. You may have to do some tweaking if you want the voltage to be just right for you. You could also use a 100K trimpot to dial it in accurately. 

The 470 Ohm resistor(s) are not in the original circuit schematic. I put those in myself when I built the first version because the Lag control produced some noise when the potmeter was fully counter-clockwise. This resistor sort of prevents that the Lag pot can be fully closed. It has no negative influence on the amplitude or sharpness of the signal so it works fine.

Here's a picture of the finished Eurorack module:

VIDEO DEMO:

Here's a video with a quick overview of the different functions. 

I watched a demonstration video about the ARP Odyssey and in it they showed the effect that the Odyssey's Lag control had on the filter cut-off control voltage. It made the filter make these 'Wah' sounds. And I'm very chuffed to see that the Lag control in this module has the precise same effect on a filter.

Here are some pictures from the build process:

In the picture of the panel (below) the 'Lag' control is still called 'Glide'. That's what it's called on the schematic but I chose to use the same term that ARP uses in the 2600.  I think it's a more accurate description because it actually creates a phase shift of about 90 degrees (see also the article about the ARP Envelope Follower). So that makes the signal lag behind the original in a small way. 

The picture below shows one side of the dual module wired up and the other side has not yet been wired up. The LEDs of that side are still mounted on the print (which was necessary for testing) instead of in the panel.

When the module is in 'rest' position so to speak, all potmeters should be set fully counter clockwise and the switch set to 0/+5V. That way, any signal you put in will come out unchanged. You can then alter it by turning the controls.

Okay that's an other one done. If you have any questions please put them in the comments below or on the EddyBergman Facebook group. Please read the whole article before asking questions.

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.

Synthesizer Build part-35: RESONANT LOPASS GATE (Buchla 292).

 An awesome sounding combination of a Voltage Controlled Amplifier and a LowPass Filter using Vactrols. It has three modes: VCA, VCF or Both. Prepare to fall in love with this one!! 

This is one of my favourite modules on this website because A, it sounds so good and B, it's versatility.

This module is not like your conventional Lowpass Filter. It's a combination of a VCA and a VCF. It helps if you're trained a bit in your modular synthesizer knowledge to get the best out of this module. As a beginner you might be better of building some normal filters first and leave this one for later. But then again, if you're feeling adventurous, then hop to it. You will certainly learn a thing or two as I did. Plus it's quite easy to build. I believe this design is equal to the Fonitronic version but Doepfer also has a Eurorack LowPass Gate for sale for around €100. It's the A-101-2. That's also the same one as in this project. I also have a Eurorack sized layout further down this article.

There are PCBs available for the Resonant LPG. See 'PCB Service'

THE RESONANT LOPASS GATE WILL RUN EQUALLY WELL ON +/-15V AS ON +/-12V. No extra changes are necessary.

A little bit of history:

When modular synthesizers were first being developed there were two people who became prominent in this world in the United States. Don Buchla on the West Coast and Bob Moog on the East Coast of the States. While Bob Moog preferred a more conventional way of playing the synthesizer by using a black and white piano style keyboard, Don Buchla chose to go an other route and developed a touch sensitive device that would react to the pressure human fingers would impose on it. Buchla didn't even like to call his instruments synthesizers since that name connotes imitating existing sounds and/or instruments. His intentions were to make instruments for creating new sounds. He wanted unrestrained artistic expression un-bound by the conventional chromatic scale used in western music. A completely different approach to modular synthesis but one that sounds out of this world if you get it right. However, piano style keyboards are instantly recognized by musicians as something they can work with, and therefore the Moog system became the most widely adopted system in the world. This module is one from Don Buchla's stables, in fact the first one from his design philosophy on my website. (Hopefully not the last one because I really like the West Coast approach.) The addition of the resonant feedback loop and the refinement of the original Buchla design goes to the credit of Thomas White. The module I built is the Thomas White version as presented on the website modularsynthesis.com. Click here to visit that webpage. 

Here's the link to the NatualRythmMusic website which features the same project.

(I'm not associated with any of those websites.)

Resonant Lopass Gate:

To be honest with you, I had never heard of Resonant Lopass Gates before I held a poll on Facebook to see what people would like me to build for future projects. This was one of the options that was mentioned. It instantly intrigued me  because I didn't know what it was. So I asked for schematics, did some research and started building one. 

This module consists of three parts and there's a mode switch to switch between them. There's a voltage controlled amplifier or VCA and a lowpass filter (12dB) and the option to have both on at the same time. The VCA is nothing more than a voltage controlled attenuator and with the switch in VCA mode that is what you get. Now if you set the switch to 'Both' mode, you get that same VCA function but unlike a pure VCA not all frequencies are attenuated equally. The amplitude will change in accordance with the frequency response. Depending on the height of the Control Voltage, the filter cuts off parts of the high frequency content of the input signal. If we now switch to VCF mode we have the full function of the lowpass filter including resonance (and it can self-oscillate) and the CV voltage determins the cut-off frequency of the filter. The VCA part is no longer working in this mode but we still get a mixture of changing cutoff frequencies and changes in amplitude driven by control voltage and the CV input also affects the amount of resonance that is put on the audio signal. It's very complicated and I can't explain it very well but it makes for a very special sounding module. Because it works best with a constantly changing CV inputs, the lopass gate really shines when used in more percussive typ patches (See demo video lower down the article for sound samples) but that doesn't mean you can't use it for other purposes. It'll work equally well as a VCF module. It just begs to be experimented with.

The CV inputs can be anything from Gate signals to Envelope signals or LFO's or any combination of those. You can experiment with what sounds best. I think it's better to have signals going into both CV inputs at the same time. The CV 2 input has an inverter connected to it in the form of opamp U2-A to form an attenuverter, The more you turn it clockwise the more the CV signal gets inverted. This is one of the changes that has been made (by Thomas White) from the original design as described in the 'modularsysthesis' article in the link below here, which I incorporated into the redrawn schematic. It works very well. The CV-2 control contributes a lot to the funky sound of this module. CV-1 is the more dominant input and if it is fully opened up it will somewhat suppress the working of CV-2 so you need to find the right balance between the two CV's.

Here's the schematic drawing that I re-made from the schematic on 'modularsynthesis.' It has all the changes that are suggested in the linked article implemented. (Click on the image to enlarge it and then right-click and 'Save as' to save it to your computer. Then you can zoom in on it.).

The schematic says to use VTL5C3 vactrols but the slower VTL5C4's will work fine too, maybe even better. It's a matter of taste and experimenting. I used home made ones myself. Somehow, slower working Vactrols make this Lopass Gate sound twice as good as with fast reacting ones. With slower LDR's in your Vactrols this module sounds really amazing. You get that snidy 'ripping the fabric of the universe' synthesizer sound from it.

C7 and C8 should be good quality, none ceramic,  capacitors. The rest can be ceramic although I myself always use film capacitors throughout the LPG. You know those green oblong ones.

I did not use any bypass/de-coupling capacitors on the two IC's but if you want them included, or if you're having trouble with noise from the powersupply, then just put a 100nF ceramic cap between the plus and ground and one from ground to minus 15V and as close to the chips as possible  You can also put some 10µF/25V electrolytic caps on the power rails to suppress any hum. The 'Deep' switch is a normal SPDT toggle switch (ON-ON). If you turn it on, the sound will be deeper with less high tones. It has the effect of turning the 'Offset' knob counterclockwise. You can set the amount with the trimmer Tp2. The MODE switch needs to be a 3 pole ON-OFF-ON switch and I have colour-coded the connections so you can easily see what goes where. The 3 by 3 diagram with red, green and blue represents the bottom pins of the switch and the colours match up with the colours in the schematic drawing. You can see it all connected in the layout below. The switch needs to have a middle position and in that position none of the 3 connections in the schematic are made, so they are all open. This is the 'Both' mode and is how it should be although it may look a bit weird at first. 

You can also use a 3 position rotary switch of course but it will have to be a 3 pole, 3 position rotary switch. I myself used a vintage 6 pole 3 way switch I had in my junkbox. I had four of them and used two of those in earlier projects. One in the Digisound 80 ADSR and one in the Steiner-Parker filter.

About the Vactrols:
The layout I made for this module worked rightaway but I did some experimenting with the Vactrols. 

I made my own Vactrols from 5mm red LEDs and LDR's that had an 'off' resistance of over 200MOhm and with a bright red LED shining on them the resistance was about 200 Ohm. I later soldered a 3mm red LED in parallel over the vactrol LED on the left to dim it a little, because I found out that sounded better. Later I mounted that LED on the front panel to have a visual indication of the working of the Vactrols. I only put a LED over one of the Vactrols, the top one going by the layout below.

I made some Vactrols earlier and used bright white LEDs in them but although they did work, the LEDs hardly came on because the maximum voltage over them was about 2,7 Volt which was too close to the threshold voltage of those LEDs. The red LEDs will shine full on with that voltage which works much better. (NOTE: because the LEDs in the Vactrols are part of the circuit and not connected directly to a powersupply they don't require their own current limiting resistors.) 

If you want to build your own vactrols using LDR's from the GL55** series then I refer you to a comment below posted by Tim who tried several LDR's from that type. He had the best results with GL5528's and GL5537's but read the comment below for his full review.

I now understand the function of the Vactrols a bit better. The characteristic filter sweep sound that we normally get from filters by applying an envelope signal to the filter cutoff is created in the LPG by the slowness of the LDR's inside the Vactrols. The LPG filter sweeps through as the Vactrols lower in resistance. So using super fast LDR's in your Vactrols would be counter productive. It sounds better if they're a bit slow reacting so you get a distinctive filter sweep.

Here's a chart of LEDs you can use in your DIY Vactrols and how much voltage drop each LED normally has. As you can see the blue and white ones have a much too high voltage drop which will seriously reduce the possibility to dim these LEDs with lowering CV voltages.

LAYOUTS:

The picture below is the wiring diagram. The module is meant to work on a dual 15V powersupply but it will work fine on a dual 12V powersupply (Eurorack)  I built this module using two TL074 chips, not the TL084 as mentioned in the layout. It doesn't really matter which quad opamp you use as long as they're low noise types. It's up to you. As always the layout is verified. I used it to build my module and I already had confirmation from others who built this successfully. All potmeters in this layout are viewed from the back side.

Stripboard only. As you can see the components are quite spread out over the stripboard, so I'm sure you could design a smaller stripboard layout but I didn't bother with that because I now also have PCB's I designed myself, which easily fit a Eurorack setup. (see Menu: PCB Service):

Below are the cuts and wirebridges seen from component side. I marked the spot where you need to cut the copper strip between holes J3 and J4 with a vertical line, for the 500K trimpot to work properly.

As always, mark the holes on the component side with a Sharpie or equivalent and then stick a pin through the marked holes and mark them again on the copper side where the pin pokes through. Then cut the copper strips at the marked holes with a sharp, hand held, 6 or 7mm dril bit.

Bill of Materials:

The trimmers are listed as multiturn but you might aswell put in single turn (normal) trimmers because that makes tuning the circuit so much easier. There's no real need for precision here.

The layouts above are quite spread out so here is a more compact layout fit for Eurorack. I did not wire up the 3 pole switch to make it easier to view the layout. All connection points are numbered and colour coded. Refer to the other layout above if you can't work it out. The Eurorack layouts are not verified yet. I have not built a module with them but I'm sure they will work. Compare them closely with the ones above and the schematic if you're not sure. Please post a comment if you used these layouts so I can mark them as verified.

Stripboard only:

Cuts and Wirebridges seen from component side:

How to calibrate this module:

There are two trimmers on the board, the 20K trimmer directly influences the voltage that the Vactrols get so it plays a part in determining the sound. So you need to set it for best resonance, at least that's what I did. The influence it has is not that obvious though. 

The second one is for the 'Deep' switch and determins the 'deepness' or the low frequency emphasis of the circuit. It's a sort of tone control and the effect it gives is like turning the Offset knob down. You can set it to whatever you like best.

Here are some pictures from the build proces. The two black thingies at the bottom left of the stripboard are my home made Vactrols. Everything is in place only nothing has been wired up yet in these first two pictures:

I used a vintage 6-pole 3-way switch but unfortunately I drilled the holes for the screws in the wrong place but since they were 3mm holes I put some 3mm LEDs in them and connected them to a free pole of the switch so that the yellow LED goes on when the switch is set to VCA mode and the red one goes on when switched to VCF mode and both go on when in 'Both' mode. =)

Here's a sketch of how I connected the LEDs to achieve that. In 'Both' mode they are a bit dimmer because of the 0,6V voltage drop of the extra diodes but you hardly notice that. I could have used Schottky diodes to prevent that but anyway. It works perfectly fine:

Remember, the LED circuit above only applies to my own self built module. It's not something common to the Lopass Gate. It's not necessary for you to replicate this. It's just something I did to fill two holes in the faceplate. It turned out to be very useful though.

DEMO VIDEOS:

Here's a video demonstrating the sounds you can get from this module (listen with headphones to get the best effect). When I say "In 'Both-Mode' you don't get Resonance" what I mean is that you don't get self-oscillation in 'Both-Mode'. Resonance still works. When watching this video please keep in mind that I didn't yet know how to properly use this module. I'm simply turning knobs to see what happens, nothing more. Imagine what a skilled synth user could get out of this module when it already sounds so cool in the hands of a noob like me. ^____^

TIP: Try altering the pulse width of the squarewave going into the Lopass Gate. You'll get some really cool sounds that way.

Here's a more recent video of me playing around with the PCB version of the LPG behind a self designed faceplate. The LPG is connected to the Klavis Twinwaves mkII digital oscillator, using 8 sawtooth waves with phase shifting. (The Klavis twinwaves II is my alltime favourite digital oscillator.)

I have the feeling it sounds better than the stripboard version but that could just be me. It sounds amazing though. Listen with headphones if you can, to hear the deep bass it has:

Here's a video (not by me) from 2008 showcasing the Resonant Lopass Gate using the VTL5C4 Vactrols which are slower than the VTL5C3's. This gives a more vintage sound (according to some people). People nickname this version the Slowpass Gate. It sounds very TB-303 Acid House to me. I really love it! Slower reacting LDRs in the Vactrols are definitely the way to go with this module. Decide for yourself. Here's the video:

Here's an other one I found from 2015 demonstrating a dual lopass gate:

It would be very cool to have three or four of these Resonant Lopass Gates in a modular synthesizer set-up and to use them partly as VCA's with a twist. You can do some really cool things with this module, I know that. But I myself haven't figured out yet in how many ways you can use this.

Below is one final video that I posted here for people interested in the inner workings of the Lopass Gate. The video goes into all the electronics and their specific functions in the module. It's very interesting especially for electronics students:

Okay, that's it for now. As always, put any questions you might have in the comments below or on the facebook group.

PLEASE CONSIDER DONATING.

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