This should have been easy, we know that Rene Schmitz stuff sounds good (I have already built his "FEITW" ADSRs and his MS20 Ring Modulator) and I thought, let's try his MS20 filter.
|Original MS20 filter|
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. It didn't work for beans for at least 3 evenings--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 an evening with my scope I ended up tracing this to a stupid mistake: I copied the schematic wrong into Eagle! I had drawn 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 then do with some kludge wiring to get it sounding up to snuff.
|But then again: what would DIY be without trace kludges?|
If you're as big a masochist as I am, you can get my PCB design (the one needing a trace cut, 12-28-18, and fixed yet untested version, 1-9-19) here. My website includes BOM, wiring suggestions, and info about the trace fix--what I think you'd need to know to make a working MS20 filter.
|MS20 filter #1 finished, no decals yet, but it works and (I think) 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 = bandpass? 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|
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!!! 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 then again I am not an electron?
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.|
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 the PCB vendor 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 3 evenings. Welcome to the world of DIY.
I realized something important: I hate, hate, hate haatteee it when things I build don't work. More than I should. 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.