Tuesday 11 June 2024


Due to family circumstances I won't be available to answer any questions in the comments or on Facebook as often as I have done, at least for the coming weeks. I will look in every now and then but after the loss of my 85 year old mother (whom I cared for for the last 9 years) I have to adapt to a new phase in my life. A lot of teadious paperwork and things to change. It's like a complete re-boot of my life. When it is all done I hope to take a little holiday, preferably to Berlin (hopefully I can meet some of you there when the time comes).

If you have issues with one of the projects that need to be solved then please post on the Facebook group for this website where there are some awesome people around willing to help you out. 

Wednesday 1 May 2024

Synthesizer Build part-59: BASS++ DRUMVOICE by THOMAS HENRY.

Finally a new drum related project and this is a really good one. This circuit creates all sorts of percussive sounds from Bass drum and Tom-Toms to Woodblock and a lot in between. 

Once again a Thomas Henry project. They're just so good and I wanted to build a drum related module for a long time to expand on the 808 Kickdrum from Juanito Moore. This is a relatively simple circuit to produce such a wide variety of drum sounds. 
Here's the schematic I used to make the stripboard layout:

Just two IC's and 4 transistors. The core of the circuit consists of a VCO built up from the first of the two OTA opamps in the LM13700. This is called the Shell VCO. The VCO's pitch is influenced by the little Envelope Generator consisting of the 2,2µF capacitor and the Decay potmeter fed from IC-2a. This opamp takes the Trigger pulse and with the Sensitivity control, which is an offset control really, turns it into a useable Envelope signal. The VCO also has it's own manual pitch control and there's an external CV input with a level control (Range) and this is where the real magic lays because if you input a VCO signal into this CV input all sorts of weird sounds result. The VCO output goes through the second OTA which functions as a VCA with a buffered output. There are DC blocking capacitors (2,2µF) to get rid of any offset voltages.
Then there's an Impact section which provides the beat, so to speak. It creates the initial hit of the stick on the drum which you can control in tone and intensity. This is fed from the same trigger pulse as the Envelope Generator.
I did not have any 2,2µF electrolytic caps in stock so I used 3,3µF or 4,7µF caps in those places where they are used as blocking capacitors. The value of those isn't that critical. However C11 is the capacitor that determins the Decay length and for that I used two 1µF caps in parallel. With the wide tolerances they have that came to 2,3µF when I measured them together so that was ideal. 
C2 is an other critical capacitor. It determins the frequency range of the VCO so keep to the indicated value for that one. You can make that cap switchable with other values to change the base frequency of the VCO. I leave that up to you.

If you want to know more or you want a more detailed explanation of the circuit then I urge you to read the original article on the Birth of a Synth website.

There are some modifications to this circuit out there some of which you can check out via this link to the electro music forum:
One thing you can do is add an extra socket so you can input external envelope signals. Have it normalled to the built in E.G. and then break that connection when a patch cable is inserted.  I think a good place to insert that signal would be between the output of the opamp (pin 1 of the TL072) and the junction that connects to all the diodes. 
An other thing is to add a built-in noise generator. This would be handy for snare like sounds.
You can also add a switch to choose between different values of timing cap (C2) to be able to change the frequency range of the VCO.
With the circuit being this simple it will be easy to add to it and make it your own. I will leave that up to the individual builder. This article just deals with the original version.

Below is the series of layouts I used. They are verified as always and this project was yet an other hole in one. It worked straight away, no trouble shooting required. I kept the layout small enough to fit behind a Eurorack panel. Just 24 strips with a width of 41 holes.
Wiring diagram:

Here's the stripboard only view:

The cuts and wirebridges:

And finally the cuts only view. Seen from the component side. 

Mark the cuts on the component side with a waterproof Sharpie or Edding pen and then stick a pin through the marked holes and mark them again on the copper side. Then cut the copper at the marked positions. Be very accurate here. There's not much room for errors.

Here's the Bill of Materials:

Here are some pictures from the build process:

The finished stripboard:

Because this circuit is a bit difficult to understand and therefore the function of the 9 different knobs is also not easy to understand I decided to make a faceplate with a sort of block diagram on it so I could show which knob controls which part of the circuit. This would make the faceplate a lot bigger but it would also make the user understand which part of the circuit he was influencing.
It was not easy to design a flowchart or block diagram was simple enough but also true to the functioning of the circuit and it took me about 4 drafts before I was happy with the design.

Here's the resulting faceplate:

Everything in place ready to be wired up:

And here's the final result. Each section of the circuit has its own colour knobs. The trigger pulse is visible through the yellow LED inside the triangle:

Here's a little video of me trying the module out for the first time. Don't expect too much from this. I was just fooling around with it. I had a pulse wave on the Trigger input and a sawtooth from my VCO (which was connected to the sequencer) going into the CV input.
I recorded the audio of this video separately with a Zoom H8 to get the best bass tones which my phone would never pick up.

This module can replicate that late 70's 'Peewww peeewww sound made famous by Kelly Marie in her song 'It feels like I'm in love' . Remember that one? (LOL)
It's capable of much more than can be seen in this demo video, if you go to the 'Birth of a Synth' website Bass++ article and listen to the sound sample posted there, you can hear what this module is really capable of.

Okay that's an other one done and again a Thomas Henry design. They are just so good. They always deliver.
If you have any questions or remarks about this project then please put them in the comments below or post on the special Facebook Group for this website.

Tuesday 16 April 2024

Synthesizer Build part-58: VCA-1 by THOMAS HENRY.

 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 about time for a new and updated VCA and one with a little more options too. This project is another one from 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 in my own module. I didn't have the room for 3 because I needed 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 your inputs come in through 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 and how strong that signal is, going by the brightness of the LED. Actually a very useful feature to have.

Here's the schematic I used to make the layout. As mentioned, 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. I used a TL074 for the opamps so the opamp pin numbering on the schematic will be different from the layout.

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. Again the LED was an extra I put in myself and it's not in 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. It's a useful addition.

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 (component side view). 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:

And here's the Bill of Materials. 

You can order AS3080 chips from Electric Druid

You can also order them from Thonk.

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 reach its amplitude limit. In that case the VCA will also not shut completely. (The audio will not distort though) You can look at the Initial potmeter as the 'Gain' control on other VCA's. You use it to open the VCA so you have continuous sound without any keys being pressed on the keyboard. The further you turn it clockwise the louder the audio will be.

Here you can see the negative voltage rejection on the ADSR input. As soon as the blue line goes through the zero Volt line and higher, the audio will come on. The LED will shine blue when there's a negative voltage presented on the ADSR input and the VCA will just stay closed.

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. Set the AC/DC switch to DC. Then you put an audio signal on the input and an LFO signal on the ADSR input, like the screenshot's above. 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.

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.

Saturday 13 April 2024

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

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

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

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.

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.


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 but you can save yourself the trouble.

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

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.:

It is mentioned in the article that certain 4046 chips are better for tuning than others. I used a Texas Instruments CD4046 but that one is impossible to get tuned accurately. It was good enough for me but if you want the best chip for this circuit the ones to get are: National CD4046, Fairchild CD4046 or the Motorola MC14046. This last one is the best one you can get.

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

Here are some more pictures from the build process:

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

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

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

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

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

Calibrating the waveforms 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 less straight forward but 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.  It's not a fault in the schematic or layout.
I turned the Frequency fine control to get note C3 in tune and then I checked notes 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. I'll try and get it tuned better later. You really need to take your time for this.
What is very important here is the make of your 4046 chip. Look at the text under the Bill of Materials for a sum up of the best chips for this circuit. Motorola works best, Texas Instruments is not so good (and that's the one I had in stock and used).
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!

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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Thursday 4 April 2024

Synthesizer Build part-56: VCF-1 STATE VARIABLE FILTER by THOMAS HENRY.

 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.

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.

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.

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.

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.

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!
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