Dual MS20 Euro Filter--Finished! Works.

Joy! I took a Rene Schmitz MS20 filter schematic you find everywhere and turned it into a compact MS20 filter board--post on that board is here (PCBWAY community page--same PCB--is here).

This time I turned two of these with 2 additional PCB's into a dual Korg MS20 filter clone--a 15HP Eurorack skiff-style synthesizer module. 

You can download files for this project from the PCBWAY Community: Pots and jacks and front panel and MS20 VCF board.  Again, get the gerber for the VCF board itself here.

So you'll need 4 PCBs to build this module.... 

I spent a lot of time making sure all my measurements were correct, which was a pain, but it's a bigger pain to deal with things that don't fit.....

I have the module in my rack and am happy with how it came out (although not 100% happy, see "improvements" section below).  You can get all the cool distorted Daft Punk distortion stuff, MS20 bandpass madness, fat basses, the whole thing. 

Here are the bench photos. Of questionable use, but, I take them anyway.....

Layouts done, schematics checked; gerbers sent to the blog's trusty sponsor, PCBWAY--they're back!

Project uses 4 PCBs: a front panel, pots n' jacks board, and 2x MS20 filter PCB found here.

From here it was like building a kit....which happily, in this case, worked!

Hardware! Check the BOM here for what's needed.

For the filters, make sure to solder in the SOIC op amps first....

.....then solder the other parts.

If this is your first Euro skiff build: make sure to place the hardware into the pots and jacks board but don't solder anything yet. Place the front panel onto the jacks panel, and once everything is in place and lined up, flip it over and solder:

With all 4 boards assembled I tested them:

Ah! Worked first time! Smells Like MS20 Spirit to me....

The audio inputs are normalled together, as is CV1-2 to CV2-1. Another option would be to normal filter out 1 to filter in 2, but that's easily done with a short patch cord.  It could be on a switch? Maybe in a future revision.

Overall in its current form the module sounds great....once again, if you want to build one of these, you can get the front panel and jacks/pots gerbers from the PCBWAY community page here and the filter boards here.  I suggest asking them to fab up the front panel in black with white silks; make sure to add a comment when you order that you want the order number on the rear solder mask or click the "For an extra $1.50 don't put an order number on the PCB" checkbox when ordering.  I forgot to do that and you can see a small order number to the right of the "Peak" knob.

IMPROVEMENTS

Now that it's in my rack I can see changes I'd like to make:

• The 2nd output of 2nd VCF should be a mix of outputs of both. Easy--single TL072 mixer, add this to pots n' jacks board
• Would make more sense for the normal to be from audio out VCF1 to audio In VCF2 (or maybe switch between normal from out1 and in1)?
• For 5V CV systems, current limiting resistor at CV input of each VCF PCB should be a lower value. I have a 5V system for the most part, so I need to take the filter apart again to change these values, but am too lazy.
• For the frequency control for each VCF, add resistors between the 100K pot and +/- Voltages.  This would get rid of "dead spots" in the filters' overall frequency range. Add this to pots n jacks board.
• From bench experiments, it should be possible to increase or further punish the audio signal in feedback loop of each filter, creating a "super Daft Punk" MS20 filter. In the BOM I mention using an A50K instead of A100K for peak, which leads to truly crazy distortion, but also some odd behavior at certain frequencies. Update: still working on this "distortion plus" design. Seems 50K pot alone won't do it. Stay tuned....

Update: redesign of this filter is finished, with some of these "improvements added...please read about that here.

All of these except the distortion redesign are easy fixes, but I am not sure I have time....Someone? Anyone?

OUTTRO

On to new things....I went to an ARP2600 focused geeky meetup the other day and was impressed with the simplicity and utility of its voltage processors so I may work cloning them next.

So! Until next time--make MiStakes, experiment, have fun, and blow things up--beakers are cheap. Happy subtracting.