Hey readers! If you want to build the filter in this post, please go to PCBWAY's Community pages--the front panel and pots and jacks PCB's; with gerbers ready to go, are here; gerbers and info about the VCF subcircuit and PCB are here.
Have fun!
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As soon as I wrap up a project I start thinking of ways to improve it. This time I laid out a few improved PCB's, got them fabricated by the blog's generous sponsor, PCBWAY, and built an improved VCF.
Improvements to a working module? Doesn't happen often. But after having a DiWHY MS20 dual filter in my rack for a bit, the lack of an on-board mixer for both filters' outputs was bothering me. Also, the filter frequencies pots cut out when turned fully clockwise. A slightly more complex voltage divider was needed to fix this annoying problem.
For the back story, please read the earlier post here; the filter circuit itself (the module uses 2 of them) is discussed here. There's more! Post for the pre-Euro versions of the same project is here. Yes, I have laid out a lot of MS20 VCF's. You may want to skim those posts?
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| "Rev 2" of the Dual Euro Mixer. Seems Working!!! |
MIXER
I added a simple mixer to the "pots and jacks" PCB--"1 - 2 Mix".
BETTER CUTOFF FREQUENCY CONTROL
The MS20 filter is a sallen-key design, meaning if you turn up filter feedback too much, instead of oscillation, the filter simply stops passing signal to its output. Good video about that is
here. To fix this I added resistors before and after the pot. After some expermentation, 10K for the V+ side and 1K for the V- side seemed to do the trick. Some HP filter frequency settings can still make the filter cut out, but it's much better than it was before.
IMPROVED NORMALS
Filter 1 "Out" is now normaled to Filter 2 in. I originally set this to Filter 1 IN being normalled to Filter 2 IN, but found myself overriding this with a patch cable frequently. I like the REV2 normals better since one filter set to highpass and the other lowpass now creates a good sounding
bandpass response, without needing patch cables.
All of this led me to a module I am happy with. I can get a lot of cool sounds, from punchy bass to "Daft Punk" type VCF distortion.
TIME TO BUILD:
Here are the build photos:
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| New boards from PCBWAY. |
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| The pots and jacks board now uses a single SOIC op amp for the filter mix. |
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| Already built filter boards.... |
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| I didn't solder the pots you see here until after female edge connectors were soldered into place. |
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| Mounting the VCF boards.... |
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| Chicken Sallen Sandwich? |
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| Testing--worked the first time! |
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| Front panel fit.... |
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| MS20 filters ahoy! |
The module can be further tailored:
- For more control of peak distortion, try using A100K pots for "frequency" pots instead of B100K's.
- R15 on each filter board controls the maximum amount of feedback. Higher values means less feedback. Experiment with values from 10K to 100K--what sounds good is a matter of personal preference.
- At the moment I am using B100K's for both frequency pots; first filter has 15k for R15; 2nd 47K for R15. This gives me a filter that mostly wants to get distorted and gritty sounding....I have a lot of filters that sound pretty, this one isn't one of them; that's OK with me.
- OUT2 is the output of both filters in series, which to my ears sounds more like the filters on the MS10/MS20. "1-2 Mix", not as much, producing odd nasal sounds, Daf Punk distortion, and as Frank Zappa accurately put it, "cross tweeze".
That was fun--the damn thing's done. No more MS20 filter clones for a few years, OK?
Next few posts I hope to finish up a few more ARP2600 work-alike modules, until then, watch those fumes.
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