Monday, February 18, 2019

Korg MS20 Filter--the Why in AudioDIWHY.....

OK this one drove me crazy. 

This should have been easy, we know that Rene Schmitz stuff sounds good (I have already built his "FEITW" ADSR) and I thought, let's try his MS20 filter.

So: Did the Eagle design based on what's on Rene's site, put together a board layout, and sent off for fab. 

This is a pretty simple circuit and from a good looking schematic, from a smart synth DIY maven, why wouldn't it sound good?

You know how I sometimes post: worked first time?  This one most definitely didn't work first time. Neither filter worked for beans for at least 3 evenings--and the second one took me more like 5 evenings to get working. This got me to wonder, why not just buy someone else's filter design? Does a kit qualify as DIY (of course it does....but it's not enough torture for me?)

I used a 2x dual concentric pot PCB for filter #2 which I cut down from this PCB.

OK I built two and at first both builds passed audio but both filters sounded--wimpy. Almost like a cheap stereo tone control, not that nice fat MS20 sound with dirt when you crank up the "Peak". I know Rene's designs are high quality, and a skinny sounding filter must be on me.

Building Filter #2....

After probing around a lot with my scope I ended up tracing this to a stupid mistake: I copied the schematic wrong into Eagle!  I drew up a critical voltage divider incorrectly, which was causing a journey to 6db cutoff, wimp ass sounding low pass land. But the 8-28-18 PCB is still useful, although you have to do a trace cut and some kludge wiring to get it sounding MS20-like.

But then again: what would DIY be without trace kludges?

With all that crappy advertising, you can get my PCB design (the one needing a trace cut, 8-28-18, and fixed yet untested version, 1-9-19) here.  My website includes BOM, wiring suggestions, and specifics about the trace fix.

MS20 filter #1 finished, no decals yet, (I think) it sounds Rene-errific

Great! we get that crunchy peak sound, fat lowpass and nice Korg growl!

Next I turned my attention to the second unit (two VCFs so I can HP + LP = band pass? Two on the original MS20 right?) But even with the trace fix, the second MS20 filter had a strange problem--when I cranked the CV up the audio output went completely dead.

Damn! It took me 2 more evenings of comparing the working VCF to the broken one to figure out problem! I suspected a parts placement error, but after checking twice, no, everything is OK.  Bad IC?  No; swapped everything and no change. Could I live with a VCF just not eating any CV above about 4V DC?  No, not really.  And besides, #1 worked, so #2 needs to work the same way.

Arrg!  Too much shotgunning! 

Ended up tracing this to a wiring problem with one of the grounds:

Filter #2, turns out, the normal is the fault--thanks to E-M GRUMBLE for the suggestion....

Goes like this: for an MS20 OTA filter: if you want high pass, stick audio into one end of a critical cap. If you want low pass, ground the same end of the same cap and put your signal into the circuit's main input:

Not having room for a switch in a 1U Frac, I used a normal to ground the cap, and the normal had a cold solder joint.  Yow! So when I put audio into the "LP" input, it would cut out at high CV.  If I put audio into the HP input, it wouldn't work at all. Not at all the way I would think this issue would manifest but there you go.

OK with all that sorted, I did a recorded the quickee demo, here.

Second MS20 filter works! Filter 2 has 2 mod inputs, I was going to use this as the primary filter of the two.

Five evening later, I have to wonder, was all of this worth it?  There are a ton of PCB's for MS20 filters already out there, did I really have to do my own PCB? I doubt it, really. Hell, I could have just bought one of those MS20 repops, and if it went south, bug Guitar Center for help right? Maybe not?

I think I just like to torture myself, building machines that I go crazy trying to fix over the course of 5-6 evenings. Welcome to the world of DIY.

Hey!  They work!

I realized something important: I hate, hate, hate haatteee it when things I build don't work. More than I should. So why DIY at all?  This is like being into sailing but hating water, right?

In my rack--panels redone using FPE

I'll have to ask my girlfriend, who is a mental health professional (really!) why this is, because really I have no idea. Was I dropped on my head at a young age?

Granted--too many fumes. I need a break.


  1. Hi could you add the whiring for the filter#2 in your website ?

  2. i don't buy tools so i can complete projects, i make up projects as an excuse to play with my tools. diy is it's own reward. i too am angry at,...everything...i think it's angry old man syndrome


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