Synthesizer Build part-62: 2164 VCF/VCA by Thomas Henry.

This is the Fonitronik Thomas Henry AS2164 state variable filter and VCA in one. THIS FILTER SOUNDS DELICIOUS!!  It's a great sounding combination. I used two stripboards that connect together with pinheaders making this a very eurorack friendly design. However this is not a beginner friendly project. You need good soldering skills for this one. Even for me this was not a 'hole in one' like a lot of the previous projects. I made a few mistakes but I found them in the end so all is well. But this project is certainly in my top 5 of best projects on this website.

This filter uses the AS2164 or V2164 chip which, in its original form, was a chip from SSM (Solid State Music). These chips were used in many late 70's polyphonic analog synthesizers like the Prophet 5 for instance. The V2164 is made by CoolAudio which is a company owned by Behringer which they use to create all the old obsolete chips for Behringer's line of vintage synths that they are reproducing.

The 2164 is not actually a filter chip. It has four independent VCA's on board and in this design two of those are used to make a great sounding 2 pole filter (12dB/Oct.). This filter has that vintage liquidy feel to it when you add a lot of resonance. It sounds amazing.

The left over two VCA blocks in the chip are used to make a single VCA. Of course you don't have to build the VCA if you don't think you need it. You can build just the filter board if you want. However you can not only build the VCA board (using my layouts) for the obvious reason that the 2164 chip is housed on the filter board.

The V2164 chip is very sensitive to missing negative voltage. If negative voltage falls away the chip will be distroyed. There is a diode in the layout that protects the chip however.

If you buy the AS2164 instead of the V2164 then you don't have to worry about this because the AS2164 has internal protection against negative voltage cut out built in. BTW, you can buy the chip(s) from Electric Druid amongst others.

Here's the finished module (on the right) fitted into a Nifty Case. Dispite the depth it still easily fits.

Here is the schematic:

This project will run on both a dual 15V or a dual 12V powersupply. It's designed for 15V as you can see on the schematic but I built it for Eurorack dual 12V and it works fine.

As you can see it's quite a simple design and in my experience those produce the best sounds. The top part of the schematic shows the filter and the bottom part the VCA. I decided to make the two parts that make up this module on two separate pieces of stripboard so that I could make them small enough to fit flat behind a 14hp faceplate, with one board on top of the other. They connect together using pinheaders. The VCA board is connected to the 2164 chip via those pinheaders. The depth of the finished module will be around the 4 CM mark.

LAYOUTS:

Below are the layouts I made for this module. As always they are verified, I used them for my build.

Here's an overview of both boards. In the layouts you can see a Coarse and Fine control for the filter cut-off. The fine control is there in case you want to use the filter as an oscillator in full resonance, so you can tune it, but I never use a filter like that so in my own project I switched the 3M3 resistor for a 100K one and put in an extra socket so I can use that as an extra CV input with level control, So the potmeter labelled as Coarse is in my case labelled as 'Cutoff' and the 'Fine' control is now my CV2 Level potmeter. I placed it all the way down on the faceplate. 

The PTC is an other component you don't need if you don't want to use this filter as a sinewave oscillator. Just put in a 2K resistor instead. That's what I did too. There's also a 7K5 resistor which I coloured purple, in the layout. Leave that out too. If you include it you change the VCA amplifier type from a class AB to a class A type. Totally unnecessary.

I used miniature potmeters in this project to save space and I made my own custom potmeter symbols in the layout software, the little green ones.

Here's the wiring diagram for the filter part. All potmeters are viewed from the back. It may look to you that the Resonance potmeter is wired the wrong way around with ground at the clockwise position but I found out that this is the right way to do it. Resonance is usually wired the other way around in most filters. This filter, I have to say it again, sounds sooo good. I love 2164 based filters and I think this is now my favourite filter on the website. It has a liquidy watery feel to the resonance which is just amazing. Anyway..... here's the layout for the filter board.

VCF stripboard only view:

Below is the wiring diagram for the Linear VCA part. The VCA has two audio inputs. One direct input without level control which is intended more for LFO signals. On the Fonitronik panel it is labelled as DC IN. In my design I did give it a level control but that potmeter is not on the layout. The top audio input has a level control and an AC/DC switch. AC is usually used for audio signals, filtering out any DC components like offset voltages that might be present. DC is used for very low frequency signals like from an LFO or envelope generator; signals that would be distorted if they went through a capacitor first. The VCA is very snappy, it can switch on and off very fast so you can use audio rate signals to open and shut the VCA and get a sort of ringmodulator effect.

The 'initial' potmeter regulates the output volume of the VCA by adding an offset voltage to the envelope input. You can use it to open the VCA without pressing any keys so you hear continuous sound. If you close it, the sound will only come through when you press a key and an ADSR signal comes in on the CV input. 

(Last revised: 16-12-2024: The two potmeters with ground connections were wired the wrong way around. That is now corrected)

I know that in the schematic the 'Initial' potmeter goes through a 300K resistor and not a 100K like on the layout but I lowered it to 100K because that worked better for me. Should you feel that the Initial potmeter is too overpowering or doesn't work right, then put in a 300K for the bottom 100K resistor.

VCA stripboard only view:

Here's an overview of the pinheaders, wirebridges, and cuts to be made for both boards, seen from the component side:

The VCA board has male pinheaders soldered directly to the copperside so the board connects to the filter board with the copperside facing the component side of the filter board.

This is a bit fiddly to solder, especially because I used a double row of pinheaders to make sure the connections are solid. I used the same method I used with the wavetable oscillator. I put some solder down between the holes where the pins sit and I put some flux on the solder part of the pins and pre-soldered them too. Then I put them in place and I only needed to heat the solder already there to make them connect to the stripboard. Do this before you solder in any components so you have enough room to work and fit the two boards together regularly to make sure it all aligns like it should. 

Be careful not to overheat the pinheaders because the plastic that holds them together can melt. When I solder male pinheaders I always connect female connectors to them so the heat can dissipate

Finally here's the Bill of Materials:

You can use other types of Schottky diodes if you want, like the BAT41 or 42, 43 etc. It doesn't matter as long as they are Schottky diodes.

PICTURES:

Here are some pictures from the build proces:

The stripboards with wirebridges installed:

Here's how I soldered on the male pinheaders:

Both boards finished but without their chips. I only put those in at the last moment to prevent damaging them.

The faceplate with the holes drilled in and de-burred, with the waterslide design applied to it. You can see there are still some bits that are not completely flat but when it is dry it will all be tight.

As you can see in the picture below, it dried up beautifully. Now to cut out all the holes with a very sharp hobby knife and then give it a few more layers of clear lacquer.

Here's the end result, not yet wired up. I put in two 3 CM M3 bolts with counter-sunk heads and screwed them tight with nylon ringed locknuts. Then I put some white paint over the heads and applied the waterslide paper overtop of that. It doesn't make the screw heads totally invisible but it works. I forgot to put in a hole for the 3mm LED. I later drilled one in just underneath the top text.

While I was waiting for some components to come in the mail I wired up the backside of the panel as far as I could. I connected all grounds together with one copper wire and I also soldered all the potmeter pins that needed to be grounded to that same wire. This way I will only need one ground wire going to the stripboard to ground everything. This is how I usually wire up ground connections. Do not rely solely on the metal of the faceplate to be the ground. Remember Aluminium oxidizes and oxides are not good at conducting current.

When all components were in, I wired it all up which took me almost a whole day and then I plugged it in and.... it didn't work at all. I tried to troubleshoot it, I posted in the Facebook group about it but it wouldn't work. Then I left it for two days and came back at it with fresh eyes on a sunday morning and I found the mistakes within half an hour. I made two little errors in the layout and I soldered one wire to the wrong place and I forgot to connect the ground copperwire, which has all the socket grounds and potmeter grounds connected to it, to the stripboard. After I corrected that, it all worked fine. Strangely enough the missing ground wire connection was something I noticed later on, but even without a ground connection everything worked! I was really surprised by that.  

Btw, I normalled the lowpass output of the filter to the input of the VCA so the filter output is automatically enterred into the VCA. To replicate this all you have to do is solder a wire to the audio output of the VCF and then solder the other end to the switch connection of the audio input socket of the VCA. When no patch-cable is connected to the VCA audio input, it gets its audio from the VCF output. If you connect a cable to the VCA input that VCF connection is broken.

Here's a look at the finished module:

As you can see, the boards bend back a little because they are only connected to the panel at one end and there are a lot of wires pushing it up. To pull the other side down, I soldered a wire from the socket ground to the ground of the eurorack powerheader. These points are directly above eachother and the copper wire now pulls the boards down which looks better and helps keep the depth to a minimum. It also takes care of grounding everything on the panel.

It is still a pretty deep module. It's 47mm deep. But it will fit most eurorack cases like the Nifty Case just fine. 

VIDEO:

Here's a cool demo video I found on YouTube by Fonitronik:

A few final notes:

I based my panel design on the original Fonitroniks panel and the labeling on that is somewhat different than on the schematic. This caused me some confusion as I only really noticed it after I had finished the panel. For instance the DC IN on the VCA is actually an extra audio input that can also take CV signals. The Linear AM on the schematic is labelled Lin. FM on the original panel. So I would advise you to keep to the labeling of the schematic and the layouts and not use the Fonitroniks panel as inspiration, like I did.

Here is the original panel of the Eurorack module:

Note how it says Linear FM at the bottom right input but with a VCA the control voltage influences the amplitude or volume of the output, not the frequency. So it should be AM.

Here's an explanation of the input options of the panel design above. They can be very confusing.

I took my panel design and added more understandable labels. It used to look like the design above here but I think this will make more sense. There's four designs for an A-4 sized waterslide paper so you have some spares should you mess up. I also added a place for the LED which is connected to the filter CV-1 input (that's the envelope input for the filter). This image is to scale for a 14hp Eurorack panel. You can save it and print it out onto waterslide paper and use it to make you panel.

Okay, that's it for this one.

If you have any questions or remarks about this project then please put them in the comments below. Comments are moderated and don't appear until I approved them which can take a while if you're in a different timezone than me. You can also post questions or show your work in the special Facebook group for this website.

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's a favorite among Psy-Trance and Techno producers! I'm reliably informed the hard sync in this VCO is regarded as being better than that of the Nord Lead. As is the FM function. That's saying something!

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.

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. Some waveforms can be a bit lower in amplitude though. 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:

The KiCad version of the 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 but you can save yourself the trouble.

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

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

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

And here's the Bill of Materials. For the PTC I used the same one as I used on the 555-VCO. See project 37. That article has links to the webshop where I got them from They are 3300ppm instead of 3500ppm but that 200ppm difference you can ignore. It works just fine. 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.

THE BUILD PROCESS:

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

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

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

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

 

Here are some more pictures from the build process:

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

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

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

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

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

TUNING THE VCO:

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:

Source: Birth of a Synth website.

OSCILLOSCOPE SCREENSHOTS:

Here are the standard waveforms. The spikes you see on the triangle wave are a characteristic of the VCO. 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.

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

Okay that's if for this project.

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

Synthesizer Extra's No. 01: SIMPLE AD/AR using the 7555

A fantastic little AD/AR envelope generator that is super easy to build and works very well.  Perfect little adition to the DIY synthesizer.

I was looking for a better AD/AR design than the one I had built and used upto now and I came across the Thomas Henry design. Here is the schematic.
This design uses the CMOS version of the NE555, the 7555 and it can be built on a piece of stripboard that is the same size as the LMNC version that I first used.

If you're new to all this; AD/AR stands for Attack Decay/Attack Release. It's a little Envelope Generator creating a control voltage that can open and close a Voltage Controlled Amplifier (or you can drive a filter with it. There are lots of options.)
In the picture below is the stripboard layout I made for it. I added all the features that Sam Battle has in his design, like the Arcade push-button with internal light and I added a little thing of my own, an option to double the output voltage of the Envelope from 0 to +5V to 0 to +10Vpp. I always find it handy to have a bit of extra charge on the envelope if I want to use it to control a filter for instance. You can of course connect a potmeter to the +10V output and so turn the output amplitude up or down from 0 to 10V. That way you can do away with the +5V output altogether. Since I already made and wired up the panel for this, I couldn't use the potmeter option. I simply exchanged the old stripboard for this new one and soldered all the wires back in place. You can also wire up the opamp in such a way that it inverts the envelope. That would be easy enough to do. As a final extra I put in buffer stages for the envelope output, for both the +5V and the +10V output. You could also wire one of those up to be an inverter. Lots of options here. One thing that is different from the LMNC version is that the Arcade Push Button won't work as long as a key is pressed down. The Gate signal has priority in this design.

The layout below is an early version and although it works fine, it is a bit messy. So I made a new layout which you can find a bit further down the article. I'm leaving the old one up in case someone who built it needs to reference it for troubleshooting or something.

SKIP THESE 2 LAYOUTS AND GO FURTHER DOWN TO THE UPDATED LAYOUTS TO BUILD THIS PROJECT.

Beware if you are using standard 24 x 56 holes stripboard, that the layout only goes from A to U not to X. So only 21 strips!

(Last revised: 1-March-2020: Changed attack and release pots from linear to logarithmic. 4-Oct.-2021: Cosmetic changes to layout.)

Here's a close-up of just the stripboard:

Bill of Materials:

Here's a look at how fast this little AD/AR is and it is super fast! It reacts to the Gate signal with practically no delay what so ever as you can see from these scope images. The risetime is about 12 µSec. The same as the risetime of the Gate signal (Gate = yellow, AD Out = blue). The gate signal has a bit of a skew in it half way up. That's due to some circuit specific stuff elsewhere in the synth but not really relevant because we are zoomed in so much it's practically instant. I mean, it's 12 millionth of a second in total:

UPDATED LAYOUT:

I made an updated version of the layouts above. I built it and changed the old one for this new one and everything works as it should so it's verified.  In the previous layout the output stages and 0 to 10V is a bit clumsy, although I guarantee that it works fine! The layout below is just a bit neater because I gained some knowledge over the past year and applied it here:

Beware if you are using standard 24 x 56 holes stripboard, that the layout only goes from A to U not to X. So only 21 strips instead of the usual 24!

SCHEMATIC:

Here is the schematic for this version. As you can see I added two buffers (which is a bit overkill but I wanted to use all the opamps available) and one opamp with a gain of x2 to get a 0 to +10V output. The buffers help to prevent this AD/AR from 'hanging' if you use it with inputs that have a bit of a low impedance (see text below). Both outputs are connected to a switch so you can choose between them. You could of course connect sockets to both outputs, instead of the switch and have two outputs permanently available, a 0 to +5V and a 0 to +10V. That's up to you.

The AD/AR works as follows: In AR or Gate mode, the Attack remains high for as long as you keep the key on the keyboard pressed down. After you let go the Release kicks in and the signal will fade out in the time you have set with the Release potmeter.
In AD or Trigger mode the Attack/Decay cycle still needs to have been completed before you can trigger it again but as soon as the Attack cycle has been completed the Decay kicks in, regardless of whether the key is still pressed down or not. For fast trigger sequences the Attack and Decay need to be set to short times because it won't trigger again until the cycle is completed, and that's perfectly normal.
So with the Attack a tiny bit open and Decay/Release fully closed you get a powerful envelope pulse of either 5V or 10V depending on how the switch is set.
In Gate mode you can have both Attack and Release fully closed to get fast short envelope pulses as the video below will demonstrate.
If you build this circuit with separate inputs for Trigger and Gate, and you feed it both at once, the Gate signal will take priority.

Here are some pictures of the stripboard using the new layout. I had made a mistake at first because I forgot this layout only had 21 strips instead of 24 so I made some cuts in the wrong place. That's why I placed the warnings with the layouts. And that's why there are some horizontal wirebridges in the lower ground strip (bottom black line).

12V vs 15V:

A little word on operating this from a dual 12V power supply. It will work but you'll need to change one resistor at the output. (R7 on the schematic). The 2K2 (R7) becomes a 3K3. This is necessary to give pin 6 on the 7555 the correct threshold voltage. I myself put in a 5K trimpot for R7 so I could experiment with the threshold voltage. It turned out that changing the resistance value of R7 mainly influenced the amplitude of the Envelope. In other words, you can set the initial envelope voltage with it. So after I learned this I took the trimmer potmeter back out and put in a 3K3 resistor.

The 'hanging' issue:
Because the resistor voltage divider at the original output influences how this AD/AR works I decided to add some extra buffer stages at the end, to stabilize the working of the circuit. I noticed that impedance differences, when connecting it to certain filters in my synth, can make the AD/AR hang sometimes. The release won't activate like it should, probably because the threshold voltage on pin 6 is disturbed somehow. I didn't want to rebuild the whole stripboard so I used a little piece of stripboard with just a single TL072 on it and buffered the +5V aswell as the +10V outputs. I stuck it onto the main board with hot-glue. It now works perfectly. No hanging or anything. I incorporated these buffers on the stripboard layout so they are now part of this design.

This design works a lot better for me than the LMNC one. This AD/AR reacts to trigger signals with an amplitude of +4 V and upwards and gate signals from +1.8 V and upwards with a maximum frequency of at least 60Hz. For triggering to work well, you need to open up the Attack a tiny little bit. The circuit is so fast that the envelope pulse shuts off before it has time to reach full potential. I tried different things to fix this little issue but I wasn't successful upto now. Anyway, it's nothing serious having to turn up the Attack a tiny little bit when using Trigger pulses. When you use Gate signals there's no problem.

I do strongly advise you use a logarithmic potmeter for the 1 M Attack potmeter. I used a normal linear one first but had trouble setting short attack times accurately. I've now put in a logarithmic one and it makes a world of difference. Works so much better. I really need to change the Release potmeter into a Logarithmic one too. That would make it much easier to dial in the Resonance or Cut-Off frequency when I use this to activate a filter. For the 4,7µF capacitor you can use a normal electrolythic capacitor. You don't need to use a Bi-polar capacitor in this circuit, unlike the LMNC one. You can put in extra electrolythic capacitors in parallel with the 4,7µF cap. to stretch the Attack time to the maximum length you want. I put in a 3,3µF and two 1µF caps for a total of 5,3µF which gives me almost 10 seconds maximum attack time. If you need longer Attack times just put in a 10­­µ­F cap.

Here are some technical specifications:
Minimum Attack time: 692 µSec
Minimum Decay time: 248 µSec
Maximum Attack time: 6 seconds with 4,7µF cap.  9 seconds with 5,3µF (which is what I installed)
Maximum Decay/Release time: ±30 sec.
Maximum input pulse frequency: ±60Hz

Here's a link to the Electro-Music Forum page that deals with this design:
http://electro-music.com/forum/topic-61297.html

Here's a little demo video of this AD/AR in action:

This second video shows one way of using the AD setting (trigger mode) of the AD/AR to control the cutt-off frequency of the ARP2600 filter. The Attack is fully closed so the instant a key is pressed the envelope voltage opens up the filter and then the Decay sets in and slowly closes the filter off as the envelope voltage fades down to zero. Watch the big blue light and listen to the effect on the sound.

The LED inside the Arcade push-button is connected to the +10V envelope output with a 4K7 resistor. It shines nice and bright. There's also a yellow LED on the panel between the input and the output. That one is connected to the output jack with a 1K resistor. It shines normally when you use +5 V out and extra bright when you use the +10 V output level. This is just a handy indication of how the output switch is set. It also reacts faster to pulses than the LED inside the push-button so it's a better indicator for that too. The LED was already built in so I thought I might aswel use it like this. :)
The Arcade push-button switch, which is the manual trigger, is fed with half the positive rail voltage (+7,5V) by means of the voltage devider formed by the two 68K resistors. I thought that was better than giving it the full whack of the +15V rail voltage. You can of course use other values for these as long as they are both the same. If you want to feed the switch with a different voltage then you can calculate that voltage as follows: Say R1 is the resistor coming from +15 V and R2 is the resistor going to ground. V = 15/(R1+R2)*R2
The arcade push button will not work as long as a Gate signal is present!!
Gate takes priority over manual trigger, just so you know that.

Okay, conclusion time: This design is a big improvement over the LMNC simple AD/AR and I can highly recommend using it. It works very well with patches where you feed it a fast trigger signal to control drum modules for instance. The switch which lets you choose between +5V or +10V output works perfectly fine but if you want more control just build it with the output controlled by a potmeter like I mentioned before. I do recommend you include the extra buffers at the end. They will insure that this AD/AR works perfectly under any condition. The only tiny little down point is that in Trigger mode the Attack needs to be a tiny bit opened to get a full envelope pulse. With Attack fully closed in Trigger mode, the pulse you get on the envelope output stops so fast that is doesn't have time to reach the full voltage potential. You could say it's too fast for its own good. You can see this happening on the oscilloscope. You get really fast pulses that don't reach the full voltage before they're cut off again. In Gate mode you won't have this issue and it works just perfectly. I really like this design and I highly recommend building it.

Here's a picture of how I added the buffer stages by glueing on a little print with a single TL072. This saved me from having to rebuild the whole thing.

Finally, for my own record keeping purposes, here's two pictures of how the finished synthesizer now looks, with two new VCO's and the Envelope Follower and the little oscilloscope of course:

Okay that's it for this article.
This article isn't really part of the synthesizer build itself so I named it 'Synthesizer Extra's'.  That's the header I will use for articles describing enhancements and changes to the original synthesizer that I build in the past 19 articles.
If you have any questions please leave them in the comments or post them on the special Facebook Group for this website. Okay, see you on the next one.