Synthesizer Build part-67: KASSUTRONICS PRECISION ADSR.

The best version of the 7555 based ADSR's on this website. This one uses precision rectifiers to eliminate the problems the previous versions have. This project is small enough for Eurorack and runs fine on dual 12V or 15V and is easy to build even for beginners.

There are PCBs available for this ADSR (8V output version). See 'PCB Service'

This is another version of the two 7555 ADSR's you've already seen on this website. The previous ones by Yusynth and Rene Schmitz had the problem that, because of diode voltage drop, the envelope wouldn't get down to 0 Volt after each cycle. The diode in series with the release potmeter would stop conducting when the voltage dropped to the threshold of 700mV in case of a 1N4148 and around 300mV for Schottky diodes.

This ADSR eliminates that problem.

By using precision rectifiers made up of a diode inside the feedback loop of an opamp, you solve the problem of the 700mVolt remaining after the release cycle and so the ADSR not returning to 0 Volt after each cycle. The opamp now has that voltage drop in the feedback loop and compensates for it, effectively creating a perfect diode.

I tried to address the voltage drop problem in the Rene Schmitz version by using Schottky diodes that have a very low voltage drop of about 0.3 V (300mV) and that already helped a lot. This version lowers that even further although on my oscilloscope I could still measure a tiny bit of voltage left over but the majority of that was due to the capacitor I was using. It was about 90mV. I used a normal electrolytic capacitor for testing. I then tried a Tantalum capacitor and that lowered the offset to around 10 to 20mV. That's almost the noise floor so really no problem what so ever. It's 35 times better than using a 1N4148 in the Yusynth ADSR. The reason for this is transistor resistance. The Gate voltage, if switched by a transistor, never reaches zero because of transistor resistance. But this is such a low voltage that you can totally ignore it. So please don't go fretting about 20 thousandths of a Volt. 20mV is equal to 0V!!

Use a 1µF Tantalum capacitor like the schematic says. 

The slowest risetime you can create with 1µF is 1.2 seconds. If you want longer risetimes you need to connect two caps to a switch so you switch between low and high speeds. That's up to you. I didn't include that option in this project but it's very easy to implement. 

If you want to read more about this ADSR then here's the link to the Kassutronics webpage.

SCHEMATIC

I made some changes to the design of this ADSR. For one, I don't like the high value resistors on the gate input. I always get problems with the gate pulse not getting through. So because the Rene Schmitz version works so well I copied the Gate/Trigger section from that ADSR and put it in this one too. It's practically the same circuit but with different resistor values.

I also changed the inverted output to an attenuverted output. I find that much more useful because you can play with the attenuverter while you're feeding the ADSR signal into the CV input of a filter and get all sorts of cool sounds from it. You can turn it into a normal output if you need an extra output. Much more versatile I think. The schematic below has all the changes I made included.

Eventhough I used BAT43 Schottky diodes for D1 and D2 in the layouts below, you can just put in 1N4148 diodes. The voltagedrop isn't important here and both diodes have the same switching speed of 5nSec. They are just used here as reverse voltage protection.

Here's the KiCad version of the schematic. I'm teaching myself to work with KiCad and it's going very well. I taught myself in 3 days -___^

LAYOUTS:

Below are the layouts I made for this project. As always they are verified. I used them to build my ADSR and it worked rightaway. An other hole in one. 

I alterred the layouts a little one day after posting this article in so far that I added a transistor to drive the LED to avoid pulling down the envelope voltage. 

Wiring: (All potmeters viewed from the backside!) As you can see the potmeter wiring is a bit complicated looking so be accurate when wiring up the pots! 

Stripboard only:

If with testing you notice that the envelope doesn't come up when the Attack potmeter is fully closed then use a 330 Ω resistor in series with the Attack potmeter (R8) instead of the 100 Ω in the schematic. This is something I had to do with my build. 

Cuts and wirebridges seen from the component side.

You know the drill, mark the cuts on the component side, stick a pin through the marked holes and mark them again on the copper side and then cut on the marks.

Here's the Bill of Materials.

Not every component is numbered exactly as in the schematic but most of the resistors are. Order a Tantalum capacitor for C3 1µF/35V. You can use any type of 7555 timer chip. I used the ICM7555. Don't use a normal NE555 though. It might work but they're not ideal. It needs to be a CMOS type.

As usual I didn't put any decoupling caps in but if you want to include it, there's room enough on the stripboard. You can put two 100nF caps; one from plus to ground and one from ground to minus. If you feel you need extra voltage stabilization, put some 10µF caps over the power rails too. That's up to you, the ADSR works fine without them. Do not use linear potmeters. I know the 1M log pots can be difficult to find but you can get them from AliExpress (and they're good enough quality) using this link: https://tinyurl.com/3z7t3ueh

PICTURES and test results:

This ADSR has a very fast risetime. I measured risetimes of 550µSeconds! The output amplitude of the envelope has a maximum voltage of 8.4 Volt when you run this on a +/-12 Volt power supply. Maximum Sustain level is 8 Volts. This is determined by pin 6 of the 7555 (Threshold) which stops charging the capacitor at 2/3rds of VDD. (+8V). The timer stops and the capacitor is discharged through the Decay potmeter and U2-D and D4 to the Sustain level. The output will stay at the Sustain level until the Gate input stops. Then the capacitor will discharge through the Release potmeter, U2-A and D3 to 0V. As I mentioned before, the maximum risetime of the Attack phase is 1.2 seconds with a 1µF cap. If you need longer times you can put a 1µF and a 10µF on a switch and connect that to the stripboard, so you have a choise. The fast times sound amazing though when used on filters (especially the 303 filter).

Here are some pictures of the finished product. They were taken in the test phase so some components that are on the layout are not in these pictures (like the LED driver transistor for instance).

If you have problems with the ADSR not triggering correctly with certain equipment, try putting a pulldown resistor on the trigger/gate input. A 10K from the input to ground should work. This ADSR has a high input impedance so a pulldown resistor can be a solution for problems like that.

I used the same faceplate as I used for my previous 7555 ADSR's. I just exchanged the stripboard for this one and wired everything up again.

Here are some screenshots from the oscilloscope. The first one shows the extremely fast risetime of this ADSR/ With Attack set to zero you can get risetimes of 550µSeconds. This with a Tantalum cap and a 330 Ω resistor in series with the Attack potmeter, instead of 100 Ω in the schematic.

Below is a screenshot of the quickest pulse I could get with all potmeters closed. You can see the risetime is the same as above, about 550µSec and the releasetime is faster because it only has 100 Ω in series. It's about 400µSec.Total time is 992µSec. So you could create a waveform with a top frequency of 1kHz with this ADSR.

Below here is the normal and inverted output. The voltages indicated by the scope are a bit lower because I had the LED connected straight to the output. I now have the LED driven by a transistor which means no voltage pull-down so the real maximum voltage is about 8.4 Volt. Max sustain voltage is 8 Volt as is the case with all ADSR's that use a 7555 and are run on +12V because that voltage is 2/3rds the voltage of the positive powerrails. If you run it on +/-15V it would be +10V.

The sustain is actually very stable because of the use of precision rectifiers. There is no leakage of voltage from the sustain stage.

ABOUT THE +10V OUTPUT OPTION:

I had a section here about making the output voltage +10V instead of +8V but I have a tendency to over think things and I had a rethink about this. This really makes no sense and I took down the schematic and layouts for this because I think it will only confuse people. +8V output is more than enough to open a VCA and to use on a filter. Should you want to do this, you need to put a voltage divider in the feedback line from the ADSR output to pin 6 of the 7555. Put a 1K8 in series in this line and a 8K2 from pin 6 to ground. That will give you +10V output if you want it but my advise is: don't bother.

TIP: using your ADSR as a VCO. Send the squarewave output of a VCO to the Gate input of the ADSR. Now your ADSR acts as a VCO and with the Attack and Decay you can shape your own wave. It's a trick used in Psy-Trance Techno music. This ADSR is fast enough to do this. I tried it and it sounds pretty cool when you then input it into a filter.

There is one more ADSR design that tries to really come down to zero volts after each cycle and that is the ADSR PRO by Davor Slamnig. You can visit his website by clicking here.

But really, the Kassutronics design is good enough and you're not going to notice any difference in practise so don't bother.

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 straightaway!

You can also post your questions on the special facebook group for this website.

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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 REALLY AMAZING!!  2164 based filters are amongst the best sounding you can build. 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. Use a normal paper printout as your drill guide. I put in dots where the holes need to be drilled.

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.

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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!

Synthesizer Build part-43: VACTROL VCF-1 by Skull & Circuits.

This is a bit of a weird one among filters and because it uses Vactrols it doesn't sound like any other filter. It has some percussive qualities too if you feed the CV IN with an LFO.

After not having built anything for two months I decided to get going again and this filter seemed like a nice one to try. It's a very simple design and thus easy to build. Because it's such a small stripboard and because it works on a dual 12V powersupply I made the layout more Eurorack friendly.  I have not tested this circuit on a dual 15V powersupply but I think it will work just as well on 15V.  

This project is more of an experimental one than one where the end product behaves in a way we expect from VCFs. What I mean is that the audio from this filter sounds different to all other filters and the CV IN behaves differently too. When I put an AD (envelope generator) signal on the CV IN, the resonance only comes up as the AD signal fades out, the opposite of how other filters behave and when I try to use a negative AD signal by putting it through the Dual Voltage Processor's Attenuverter, it doesn't give the expected result. Not even when I apply an offset voltage to the AD signal. So this is an experimenters dream. It works, but not like you expect. Just know that before you proceed to build it. ^____^

There are PCBs available for this VCF. See 'PCB Service'

VACTROLS:

This is a Skull and Circuits design and you can find the original information by clicking here.

This is a 12dB LowPass filter, so a two pole instead of the usual four pole filters  This means the frequency rolloff won't be as steep as other (4 pole) filters. 

The Vactrols are something you'll need to make yourself. Well, you can buy them ready made but I advise to make them yourself because new ones can be hard to find and expensive and the ones from China will be fakes, I guarantee it. In the information link above is a video that shows you how to make your Vactrols step by step. I used two 5mm green LEDs (you need two Vactrols) and two LDRs no. 3P (or maybe 39) from a set with 10 pieces of 5 different types of LDR from eBay. They come in 5 little plastic bags and they are all numbered. I used the ones from the bag numbered 3P. I had used those before on the Lopass Gate and they react reasonable fast to light changes and go way down in resistance. The way your filter will turn out will be different from my filter. There are so many variables here that filters may turn out to be alike but never quite the same. This is mostly due of course to the Vactrols and the way you set the three trimmers. I tried two different types of LEDs in my Vactrols. First I tried bright white LEDs but with those I needed to turn most of the trimmers completely clockwise or counterclockwise to get the filter to behave correctly. The second time I used normal red LEDs (5mm) and they worked better. That's what I used in this build. 

Later on I bought the kit from Skull & Circuits and in that kit I used Vactrols made from green LEDs which I found work even better than red ones. (The kit built filter sounds the same as the stripboard one.)

Now I could use the trimmers more accurately to get the right sound. So I advise to use either red or green 5mm LEDs for your Vactrols. The LEDs in the filter itself, mounted on the front panel, do not shine very bright in my module. I used two 3mm yellow LEDs and they only come on when the trim-potmeters are set to extreme positions. I can get them to shine brightly when I turn the trimmers a certain way but then the filter doesn't work properly. Again this is one of those things that will be different with your filter because of component tolerances and differences in the Vactrols etc.

Just out of interest, here are some measurements I took of my DIY Vactrol (out of circuit) with red LED:

OFF resistance = >200MOhm.  ON resistance with a +/-5Vpp signal = about 10K, with a 0/10Vpp signal = about 5K. This is with a 1K resistor in series with the 5mm red LED of the Vactrol.

Of course these values differ when the Vactrol is in circuit because there you can set offset voltage etc. which alters the resistance. With the green LEDs, in the kit I built later on, I could get even lower resistance down to 200 Ohm!

NOTE: This module pairs extremely well with the Voltage Processor and the Lopass Gate module. If you create a beat with an LFO into the CV IN of the filter but first put the CV through the voltage processor you can accurately control the sound and if you then send the audio through the Lopass Gate you can turn your beat into a sort of galloping beat. Connect a signal from the same LFO used for the VCF to CV1 of the Lopass Gate and use a faster LFO signal on CV 2 of the Lopass Gate. Used in this way the Vactrol VCF can be used to make some cool Techno kickdrum sounds.

LAYOUTS:

Here is the layout I made for this filter. As always the layout is verified. You can see it's small enough for a Eurorack module and I incorporated a Eurorack power connector to make life easier for those of you using that format. I myself didn't use the connector in my build but just soldered the power cable straight to the print like I always do. All potmeters viewed from the front.

IMPORTANT:

Stripboard only view below. Note that the copper strip between pins 3 and 12 of the IC is NOT CUT! Both pins are connected to eachother underneath the chip. Also do not forget the cut at hole B-29 otherwise, when you close the Cutoff potmeter, you'll create a short circuit between +12V and ground through the wiper of the potmeter which will produce smoke, I guarantee it ^^ :

Strangely enough I found that I needed to reverse the wiring of the Cutoff potmeter for it to work the right way around. I left it the way it is presented on the schematic, in the layout so you can decide for yourself whether you need to do that or not. 

Bill of materials:

TRIMMING THE CIRCUIT:

Trimming this circuit is a matter of trial and error. You need to set the trimmers in such a way that you get a good deal of self oscillation when the Resonance potmeter is set almost completely open and you need to get the right range for the Cutoff potmeter. I can not give you a procedure to help you do this. You will have to figure it out yourself but it's pretty straight forward. Just put a squarewave or rampwave signal on the audio input and connect an LFO signal (+/-5Vpp) to the CV input and then connect the output to your VCA. Changing R7 from a 47K resistor to a 100K trimmer helped a lot in getting this filter trimmed. It was suggested in the text of the original Skull and Circuits article about this VCF and I also saw they implemented it on the PCB so that's why I put in a trimmer for R7 in the layout. It might be a good thing to use multiturn trimmers instead of the single turn ones I use in the layout. I used normal trimmers on my board but it can be fiddly to set these correctly. I might replace mine with multiturn ones too. You might think it would be a good idea to put an attenuation potmeter on the audio input but I tried that using the passive attenuator on my mixer but it didn't help at all. The filter only worked if I had the potmeter fully open so there's no need for an audio input level potmeter. As you can see in the schematic, the audio is attenuated by a factor of almost 18 times by the 1K and 56 Ohm resistor voltage divider right at the audio input. Then it is boosted up again in the output opamp.

The whole upper portion of the schematic dealing with the Cutoff and CV input goes straight to the LEDs of the Vactrols so you can see that he whole filter hangs on these Vactrols. The Offset and Range trimmers control the Vactrol LEDs and determin when the LEDs turn on and how sensitive they are.

The LEDs over the opamp go on when the signal is clipping. You should try and trim the potmeters so these LEDs turn just about on when the filter is in full resonance and cut off is 3 quarters closed. So when the filter is really working its hardest these LEDs flicker in time with the CV input voltage. These LEDs should not be burning at full brightness in normal use!!

Schematic drawing:

Here are some pictures from the build proces. In these pictures the third trimmer is not yet put in. I tested it first with R7 as a normal 47K resistor. I later changed it for a 100K trimmer.

The stripboard:

Stripboard mounted behind the panel not yet wired up:

Video of the first test. You can see here that connecting an AD (or Envelope) signal to CV IN results in the opposite reaction to what normally happens. Using a negative or inverted AD signal doesn't fix this issue. Not even with a positive offset voltage applied. That just proves that this is a very quircky filter and all the more interesting for it ^__^  I've heard better results than mine from other people who built this VCF, so it depends a lot on how your vactrols turn out.

If you have any questions or remarks you can put them in the comments below or go to the Facebook Page for this website where we have an awesome little community of cool DIY synthesizer enthusiasts that are more than willing to help you out with any problems you might encounter.

Synthesizer Build part-29: VCA (Yusynth design).

A good working and easy to build VCA for use as extra VCA or as the endstage and line out for your modular set-up. The VCA as presented in this article is specifically meant for audio, not CV however if you leave out the input electrolytic caps you can use it for Control Voltages.

This module is a great solution for all your VCA needs. I needed a new VCA module for the second stage of my synthesizer and I wanted to upgrade from the first one I built, although it functions fine I hasten to add. This VCA is a more luxurious version of the first one you could say. And this one doesn't invert the signal like the first one did.

The function of a VCA:

Before I get ahead of myself, let me explain what a VCA does for all the people who are new to DIY synth building. A VCA or Voltage Controlled Amplifier, is used to stop your synthesizer from continuously making sound and to only produce sound when you press a key on your keyboard. So it's like a volume knob that is only opened up if it receives a signal from the Envelope Generator. Now the higher the voltage from the envelope generator (or ADSR)  is, the louder the output of the VCA will be. VCA's can of course be used for other things, like in drum machines, as a sort of Gate, to let through short pulses of noise to create percussion sounds but this is not what we're going to be dealing with here. This VCA is primary used as the last stage in your synthesizer from where the audio goes to the HiFi amplifier and the speakers. But the output must be further attenuated to make it suitable for the line in of a HiFi amplifier. In fact the term 'Amplifier' in VCA is a bit misleading. It should be Voltage Controlled Attenuator because when the VCA is fully open the output signal will have the same strength as the input signal. It has not been amplified.

So to sum up: The VCA lets through audio when it receives a signal from the Envelope Generator and it stops audio from passing through when there's no envelope signal present on the ADSR input of the VCA. Opening up the 'Gain' control will enable you to bypass the Envelope Generator Input and output sound/audio without pressing any key on the keyboard and therefor without any Envelope Signal being present but normally Gain is set to zero.

I hope that's clear but you can always ask me things in the comments if you need more info.

Here's a cool video showing the how VCA's and ADSR's work together. ---- CLICK HERE ---

The VCA is capable of handling input signals of + and - 10V and outputs them at the same level if the audio and ADSR potmeters are fully opened up and if you use an Envelope Generator that outputs an envelope of  +10Vmax. This VCA is perfect therefore to pair with the Yusynth 7555 ADSR from article 24.
If you need to output the signal to a HiFi audio amplifier or mixing desk then you must use extra attenuation. You can easily do this with a resistor voltage divider that divides the amplitude of the audio to one tenth of the input amplitude. If you then turn the audio and ADSR potmeter back, the signal will be low enough to go into the Line-In of a HiFi amplifier or mixer. There's an extra additional layout below to show you how you can do that.
As to running this VCA on a dual 12 Volt power supply; I don't think there will be any problems with that. After all the opamps and transistors all work fine on 12V so I think it's just a matter adjusting the trimmers. You could have an issue where one of the trimmers is turned all the way to its limit because of the lower voltage but I have no data on this so I can't be sure. The first VCA I describe in chapter 10 is purpose built for +/-12V so you can always build that one if you're not sure. I've build 3 of those so far and they work very well. As you can see on the layout below, this design has one extra potmeter compared to the earlier VCA I built, and that's the audio level control potmeter on the input, which is always a good thing to have.

The two BC547 transistors Q1 and Q2 must be a matched pair. It's enough to just stick them in the Hfe meter of your multimeter and select a pair that have the same Hfe value. When you measure them let the transistors cool a little after you touched them because their values will change with temperature.

Here's the verified stripboard layout. Wiring diagram:

(Last revised: 23-April-2025: cosmetic changes to the layout.)

Stripboard only. 

Here's the layout with just the cuts and wirebridges. Mark the cuts on the component side with a black Sharpie or Edding marker 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 with a hand held 6- or 7mm dril bit.

This is the schematic from which I made the layout. In my build I have the 10µF electrolytic capacitors on the input as is shown in the schematic below. That means this VCA is an AC version meaning it can not handle signals from a Low Frequency Oscillator (LFO). If you need a VCA capable of handling very low frequency signals (a DC version) then leave out the 10µF caps on the input but for normal audio use LEAVE THEM IN! In my own build I also added a 10µF cap on the output (+ towards the opamp) as an extra failsafe against DC offset voltages on the output but you don't have to replicate that. (It's not in the schematic but it's in the layout as extra option.) Alternatively you can put these capacitors on a switch in series with the input(s) and then you can use them when you need them.

Bill of Materials. The logarithmic potmeters are noted as linear types in the BOM. It hardly makes any difference it's just that a logarithmic taper sounds more natural to the ear but you can use whichever you prefer. The VCA is a linear type of itself.

LINE OUT:

If you want to use the VCA to feed a HiFi amplifier then you can use a little voltage divider network to further attenuate the audio output level to make it suitable for line out levels which are usually around 100mV to maximum 1V. I made a little extra layout to show you how you can add such a voltage divider to this stripboard. I used a 1M resistor and a 100K trimmer potmeter to divide the output voltage by at least a factor of 10. You can set the initial output level with the trimmer and then further adjust the level with the ADSR and Audio Level potmeters on the panel.

These extra components are not in the Bill of Materials!

Calibrating the VCA:

Before we start, do all the measurements on the 'Normal Audio Output', not the AC one. I added the AC output as an afterthought to block any DC offset voltages from coming through but normally you can use the the Normal Output. So connect your probe there for calibrating the circuit.

With trimmer A2 you set the initial bias voltage on the base of transistor Q3. This influences the working of the Gain potmeter and you should set it in such a way that with the Gain potmeter all the way closed the signal is just muted. If you turn Gain up the last played note will then become audible without pressing any keys on the keyboard. So Gain is normally closed.
I found that the audio signal initially starts at 10V and then drops to about 6V. To counteract this you need to open up the Gain potmeter a little and then trim again so the signal is muted with Gain slightly open, This will stop the voltage drop of the signal and keep it at full power all the time. You will see this soon enough if you start testing it and connect this circuit to a scope. It's easy to counteract and it is in itself not a real problem because you hardly hear the voltage drop but you know, I strive for perfection ^___^
Trimmer A1 is the Balance trimmer. You set it so that the part of the signal that is above the zero Volt line has the same level as the part below the zero Volt line. In other words, you set it so that the signal has the same amplitude in both the positive and the negative part of the wave. This is best done using a triangle or sinewave on the input, together with an oscilloscope.
For trimmer A3 I advise to use a multi turn one. With this trimmer you trim away any DC Offset voltage on the audio output. Again you absolutely need a scope to do this but a cheap 20 dollar one will do nicely here.
The circuit has a LED that indicates the presence of an audio signal on the input. It's a sort of one LED VU meter and the brightness varies with the strength of the audio signal. If you turn the audio input level potmeter up, the LED becomes brighter.

Here are some pictures of the finished product. If you look carefully you can see the electrolytic capacitor on the audio output. This is the AC audio output option to prevent any DC offset voltage on the audio output signal. If you are going to use this VCA with very low frequency signals, like from an LFO, you need to use the DC output option: (If you're a beginner then don't worry about that, use the AC output.)

Normally the AC input is used for all audio signals and the DC input is used for Control Voltages.

You can see that I put a little white stripe alongside the counter clockwise first 20° of the throw of the Gain potmeter, to indicate to where you can turn the potmeter with the audio staying muted. If you turn it past the white, the signal becomes audible and normally the Gain should be slightly open but with in the white area.

The day after finishing this module I built two extra VCA's using the old layout and they work fine too. I wanted a few extra VCA's available but there was no need for the luxurious model. I'm going to use that as the end stage VCA in my second stage, with the output going to a HiFi mixer (at Line Out level) and use the other two VCA's for use in different patches.

Here's some pictures of the double model. I also posted these in the first VCA article:

Finally I want to leave you with this very informative video by Moritz Klein about the ins and outs of differential transistor VCA's. He explains why the transistors need to be matched and goes into the technical details of how a circuit like this functions:

TIP: Instead of using a CV signal to open and close the VCA why not try an other audio signal? That way the VCA will act as a (sort of) ring modulator. It's actually AM or Amplitude Modulation. Change the frequency to the lower/bass areas and experiment with modulating the VCO. You can get some weird sounds going that way.

So that's an other one done. One more and I'll have published 30 synthesizer related projects.

Okay, if you have any comments or questions please put them in the comments below or post them in the Facebook Group for this website.