Monday 19 July 2021

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 print 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. ^____^

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 you 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.

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.

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 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 print 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.

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