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.

EDIT: 8-April-2025  I forgot the current limiting resistor in series with the LED on the filter. I have now put in a 1K resistor and updated all the layouts etc.

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 other.

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.

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. 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. At first this didn't seem to work but I made a mistake. I had this VCA connected to the Gate Out of my sequencer and the audio out went to my other VCA. That didn't work well and VCA wouldn't open if I pressed a key. So I had two VCA's in series. My mistake.

(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.

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 be absorbed better.

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 this picture 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 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. You can of course also choose to normal the VCA output to the input of the filter but I've tried that and I didn't like it. It's more conventional to have the filter out go into the VCA in. 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. 

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.

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