Sunday, 8 December 2024

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

This is the Fonitronik Thomas Henry AS2164 state variable filter and VCA in one. I used two stripboards that clip 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.

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 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.

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.


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 but it does not open up the VCA when no key is pressed down. Most of the times that's the function of an Initial or Gain potmeter. So you can hear a signal continuously without having to press a key on the keyboard but with this VCA this doesn't seem to be the case. I can not imagine it's a mistake on my part because I have the potmeter wired up to the power-rails like it should.

(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 the VCA did not open completely without pressing a key on the keyboard. It still doesn't though. 

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.

Finally here's the Bill of Materials:


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 output of the VCA to the input of the filter so you can input audio into the VCA and get it out through the filter. You can of course also choose to normal the filter output to the VCA input to have the two in series. I thought it was more interesting to have the VCA in front of the filter. To replicate this all you have to do is solder a wire to the audio output of the VCA and then solder the other end to the switch connection of the audio input socket of the filter. When no patch-cable is connected to the filter audio input, it gets its audio from the VCA output. If you connect a cable to the filter input that VCA connection is broken.
Here's a look at the finished module:



It is still a pretty deep module. It's 47mm deep. But it will fit most eurorack cases I think. It's the wiring that makes the stripboard bend up a little but anyway, it works and that's what matters.
All layouts have been updated and are free of mistakes now.

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. 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:


Here's the A4 size design I used to make my panel. Thankfully I had two designs on one A4 paper because I ruined the first one so I was glad I had a reserve.


Okay, that's it for this one.
If you like this content and would like to support the website and future projects, you can buy me a coffee. There's a button for that underneath the main menu if you're on a PC or Mac.
Otherwise you can use this PAYPAL.ME link and cut out the middle man. All donations go straight into future projects and the upkeep of the website. Thank you!!

11 comments:

  1. Great One! I would like to know more about the PTC Thermistor. Is it essential for the project or can I leave it out and put a regular resistor?
    Eddy, thank you very much for everything. You are awesome!

    ReplyDelete
    Replies
    1. Thank you!! You can leave out the PTC. I didn't use one either. The PTC is only necessary if you want to use the filter as a sinewave oscillator.

      Delete
    2. Perfect. Personally, I've never seen any practical use for a VCF as an oscillator. So I'll leave it out as well. Thanks a lot.

      Delete
    3. Put in a 2K resistor instead. You can leave out the frequency fine control too. Turn it into an extra CV input (see text).

      Delete
  2. Ok, I'll do it like this. I'll order the 2164 and as soon as I build it I'll come back to comment. Thanks for the tips. And Merry Christmas to you.

    ReplyDelete
    Replies
    1. Merry Christmas to you too and good luck with this project.

      Delete
  3. Replies
    1. I made some alterations to the Bill of Materials and I added a level control potmeter to the ADSR input yesterday. Just so you know. Goog luck!

      Delete
  4. Well done! You rock! This made everything easier. Thank you so much for this help.

    ReplyDelete
  5. Thank you very much for your work! It is very difficult for me to get the Schottky 1N5712. Can I replace it with a similar one?

    ReplyDelete
    Replies
    1. Thank you! Yes you can use any Schottky diode. Try the BAT41,42,43 etc. series. They work very well.

      Delete

Note: comments are moderated and do not appear straightaway. Your first comment is not allowed to contain any links.