Blog

yoo..

it took a little while but i got a new filter built out a couple weeks ago. i'd have blogged about it sooner but i havent been in as much of a writing mood recently and i'm not sure why. maybe weltschmerz? in any case tho since my plans for the weekend got fucked up¹ im forcing myself to do the blog post now. and it looks like this btw:

the new module is a voltage controlled filter based on the filter section of the edp wasp, a weird little synth from the late 70s that as far as i can tell was produced by a british company that only seems to have existed for like 5 years. originally i was gonna go with an ms-20 type filter like basically every other diy synth head seems to, but i didn't really like the idea of my first vcf having a different roll-off depending on whether it was in low pass (12db/oct) or high pass (6db/oct) mode, and tbh i just didn't really understand how it worked at all. so i did some digging around for a filter design that had a high pass with 12db roll-off and found jurgen haible's wasp clone, which seemed to do exactly what i wanted and also included band pass, notch pass and distortion, which seemed like a nice set of bonus extras.

now, something i always like to do before trying a circuit out on breadboard is to to try and figure out what the thing's outputs should look like using a simulator. this especially is true for cases like this where weird and kinda rare parts² are involved, but unfortunately cos the wasp filter relies on (mis?)using inverters as comparators my usual circuit simulator of choice, circuit.js, just could not handle it at all. so i had to give in and finally learn how to use kicad. finding models for this thing is a pain and i still don't really 100% get how the simulation stuff works but after a couple weeks i finally got my head around it and, now knowing what i should be seeing on the oscilloscope, i went on to breadboarding it....

it looks kinda clean but dont let it fool u this thing was a nightmare to prototype. i think something's up with the translucent breadboard cos it would just cut out at random and screech at me if i touched the pots wrong. this and having to learn how to use kicad were the main reasons this thing took so long to build, but i kept struggling thru it anyway cos there were features to be added.....

first was exponential cv input. for this i took a look at the cgs twin cmos filter and borrowed part of the cv input design, which involved switching the entire filter core to run between 0v and -5v instead of 5v and 0v³. once i'd done that though i found that i was getting no output at all, which had me stumped as to whether it was just my breadboards finally giving up. but nah turns out i needed to add an extra trim pot for centering the frequency control, which is missing from the cgs schematic and only mentioned in passing halfway down the page. my bad for not reading i guess. whatever.

once i'd done that i got to looking at ways of moving the distortion section to be either before or after the filter. i had a few ideas but i was totally pissed off at the breadboard situation by this point and really didn't feel like doing much more prototyping, so instead i decided to just drop the distortion and notch pass sections entirely (i wasn't super feeling the sounds from them anyway) and use the 3 inverter gates that were freed up by this to make it into a dual vcf. i then also decided that it made no sense to drop £5 on a rotary switch for changing filter mode like the original wasp has, so i changed the design to just have separate outputs for lp/bp/hp. i haven't done a huge amount of testing of this, but as long as they're feeding into high impedance inputs i think they'll all be usable simultaneously?

finally with the distortion section gone the remaining parts of the schematic was all stuff i (mostly) understand, and it seemed like there was no real need to be running the core(s) at -5v specifically, so after seeing a forum post where someone had adapted the wasp filter to work as a guitar pedal running between 9v and 0v i decided to just try running the core on -12v instead, which seems to work fine and also means the input and output don't need to be scaled up/down. i did leave the attenuation pot on the input though so that the volume can be tuned to give the resonance room to go wild/crazy without clipping 😈

overall i'm real happy with how this one came out⁴. now that it's fully built and i'm not having to wrestle with the janky prototype setup i can really appreciate just how good it sounds, and i totally get why people seem so enamored with this goofy old synth's filter section. the resonance sounds sick too, and the cv input works great, although i haven't bothered tuning it to v/oct yet⁵. now i just need to build an lfo to plug into it....

as always i've updated the synth page so u can see it in the rack and hover over it and see it light up etc if u want, and u can also check my eurorack module schematics repo if u wanna see the stripboard layout. i realised kicad schematics look kinda nice and they'd probably be useful to have so i'm planning to go back thru and add real schematics for all my previous designs at some point too. idk when that'll happen tho cos tbh i've fallen down a hole playing tony hawk games and will be in that hole for some time i think

see u in the next,

– freya