Disaster Eurorack: D-80 Sallen-Key Filter 'Tentacle Summoner'

'Tentacle Summoner' is a 13HP Eurorack module. I kitbashed the circuit together

from bits and pieces of other modules and made the front panel art (no, I did NOT use fucking AI. Eat my ass.)

I. Pretext.

One of my favorite filters in synthesizers is the filter in the Korg MS-10 and the original, mark 1 Korg MS-20. I love everything about it - the unpolished, dirty 'fuck you' sound, of course, but also the bonkers way that it was implemented electronically through the creative use of transistors and a JFET in the Korg 35 IC.

There is a lot of documentation online regarding DIY implementations of the mark 2 filter - with the LM13700s - but hardly anything that I could find centering on the Korg 35 implementation. The only DIY kits or modules I could find that specifically try to recreate this are the excellent Befaco BF-22 which is sold as a kit only, and the now seemingly out-of-print Miss 10 VCF/VCA. I did buy the Miss 10 VCF PCBs from synthCube last year but shortly after I purchased them, they went out of stock. As of this writing there are full kits available for it at synthCube but the documentation for the module is confusing and spread across multiple threads on Modwiggler, which is not ideal. Furthermore, there were some modifications I wanted to make to enhance the 'fuck you' factor.

While I really like what the BF-22 does, I wanted to take a crack at making my own version of the filter and I wanted to offer my schematics and Gerbers up as open source hardware. I may look at selling the PCBs in the future but I fucking hate dealing with money, so don't hold your breath.

I decided to make a Eurorack module because that's what most people use these days, but I am also working on a Kosmo format (the superior format, if you ask me) for the PCBs and front panel.

Finally, I want to re-iterate something here - I am -not- an engineer. In fact, I only have an American high school degree. I have never gone to college. I was trained as an aircraft electrician in the USAF and I have been working as an industrial mechanic for the last 12 years. When it comes to electronics and the sort of thing I blab about on this blog, I am almost completely self-taught. Go easy on me! I'm delicate.

II. A Brief Schematic Review.

Unlike the page about The Tower I do not plan on going very deep into this. There is an extraordinary source of information that is written by Tim Stinchcombe (someone with far more intellect and who is far more qualified than I) that I will encourage you to peruse here. 

Other sources for my design come from the aforementioned Miss 10 VCA/VCF and the Befaco BF-22, both linked in the second paragraph of this post. The schematic for the BF-22 is available at that link.

I had a few goals with this. The first was to make a version of the Korg 35 filter that didn't hew so closely to the original. Instead of trying to make something that was a clone, I wanted to add some spice to it. 

III. Trim Pots, and why sometimes they should just be regular pots

I noticed when I built the Miss Ten according to BOM on Modwiggler that the damn thing wouldn't resonate. It wouldn't scream. If your Korg35 filter doesn't scream, you just have an electronic muffler.

I wasn't the only one. There were several posts from others who had the same issue and who had to change the value of the feedback resistor in the loop of the first op-amp after the transistor network (A3 in my diagram). This lead me to experimenting with different values using a resistor decade box and I realized that the damn thing really needed a trimmer. So my first revision of Tentacle Summoner had a trim pot for the resonance feedback, but when I got my boards and tested it, I decided... fuck trimmer pots. This thing is way too wild to just leave it behind the panel. This became the 'Sothoth' control, which is just an adjustment on the level of feedback in the resonance/regeneration circuit. 

IV. Nail clippers

After that I decided that it needed some extra middle finger in the form of blue LEDs for clipping diodes instead of boring 4148s.

I decided to use one of my favorite transistors - the MPSA18 - as the VCA transistor. I also hard-wired a feature that was a kludge on the Miss 10 VCA/VCF, whereby one would need to wire the output of C3 to the switch of a switched jack in series with a set of resistors, in order to have an output before the color of the VCA.

One of the other issues I had with the Miss 10 was the CV circuit. It didn't seem to work super well at +/-12V, so I blatantly copied and pasted the CV circuit of the BF-22, which works far better:

The cutoff CV circuit from the BF-22.

V. Unnecessarily high component count

I also decided against all logic and reason that each op-amp should be an individual op-amp, instead of part of a dual package. I know, I know - op-amps don't make a lot of difference to the sound - sure. Whatever. I love the LM308/OP07 and its Soviet partner, the UD1408. I like them for the same reason I like the 6CB6 for distortion - they're fucking terrible. You can't have the Proco Rat sound without the LM308 or the OP07. And I love the Rat distortion sound for synths, so... you know, vibes or whatever. Also the idea of repairability and ease of troubleshooting is front of mind for me and the idea of having all op-amps individually replaceable and/or testable was appealing to me. I also have a shit-ton of UD1408s and a handful of OP07s that I salvaged from discarded industrial temperature controllers. You know, the usual.

The last bit I did was change the input section of the circuit. On the Miss 10, the circuit is basically picked off at the point in the MS-10 where the signal from the VCO enters the filter. Which makes sense, I guess, but I wanted to run all kinds of signal through it - VCO or otherwise. So what I did was shit-can that idea and I used the circuit for the external signal input instead (highlighted in pink):

VI. Results.

The result of all of this is a module that puts out a SUPER HOT FUCKING SIGNAL and is capable of... well, just listen to it:

VII. Front Panel.

I also used this opportunity to make a front panel in KiCad. It came out perfect and everything lined up exactly correct on the first try (!).

There are holes for the jacks under 'PRE-VCA' and 'POST-VCA.' They are hidden by the silkscreen.

And here is what the complete PCB set looks like, showing the back of the front panel:

The Gerbers, BOMs and everything needed to get a fab house to make the boards for you are located at my Github. There are three boards in total - the front panel, the main board, and the control board. They lay up flat, which makes the module skiff-friendly. 

And finally, a demo: