Sunday, May 17, 2020

Noise! Lunetta Circuit Part IV: adding dual VCAs for Audio Gran-Turing-smos

Back again. I'm stuck at home with damn covid Shelter in Place and it's raining. Fortunately the PCB fab shop I use in China is still cranking and I got another set of boards for my Noise! Lunetta stack last week.

Built 'em up rawhide. Worked first time.  Here's all 6 PCBs to date.  The little switch on the left chooses the overall frequency range.

The project so far.....

About Noise!: It's an ongoing project I've been building for many months. I've tried to keep things organic: I think some damn thing, crank it out in Eagle, send it off, get it back, solder it up and see if and how it works. Like the Winchester Mystery House this project may never be completely finished.

It started when Martijn Verhallen from reverselandfill.org and I did a board swap. I really got into his Noise! tone generator and started adding to it, my goal being a full Lunetta type synth for oddball sound generation (I refuse to call this "Sound Design").

Part I--where I build up the base VCO and logic board is here.  Part II: resized and revised, here.  Part III: Adding a quad mux, here.  I created a quad digital pot board and added it to the stack, here.

AD5254 based quad digital pot board, Arduino controlled.  Post is here.

As I was messing with the interconnected PCBs: patching the stack to a bench VCA frequently came up as strange repeating patterns of audio bleeps and blurps. In the mod synth world this is not quite referred to as a "Turing machine"; as Turing modules I've messed with (a popular one has a demo vid here) produce CV, not audio, and on my bench it's the audio patterns that tend to repeat over and over. But whatever. Is a frequency to CV board coming next? Perhaps.

So why not add a couple of VCAs to the stack?  OK, easy, I already have built up a 2164 based dual VCA board (here), I just needed to lay it out again to fit the 3750x2000 mil footprint I am using for the project.

Got em back, built 'em up. No dumb mistakes--yeh baby. You can get eagle files, PDFs of schematics, Eagle files etc. here.

OK now the bench photos--sorry--some folks take photos of their dogs and cats, but I don't have dogs nor cats, so you get circuit boards.

The 3750x2000 dual VCA board.  

Ready to test....

The front facing board is a jacks PCB for the 3750x2000 dual VCA. While I was at it I added buffered CV inputs for the MUX board using the usual opamp/zener thing.




I used the usual super-low-parts-count zener clamp/buffer to keep the Arduino CV inputs from frying:



However I am curious about this clever fragment I lifted from a Mutable Instruments design--looks to me like it allows more stages with only one zener reference; didn't use it this time, will use it down the road.


After that, it became a matter of wiring it all up and testing.

Ready to test!




OK so how does it sound?  I still need to record the damn stack for real but you can see a vid of some of the repeating patterns at work here.

Is it for everyone?  Nope. Time to keep moving. Might do a powersupply and LFO and call it done.  I guess we'll see.  Until next time, don't breathe the fumes.

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