AD9833 Audio VCO--Works But not Finished

 I finally have the AD9833-VCO working! Eurorack format!

The fun starts with the AD9833 post here. I wrote a C library for it....then I did this thing with the AD9833, then this thing, then that thing, then something else--oh I have to link all those posts?

Nah, if you want to know the history of the function generator IC used in this VCO, please search this blog for "AD9833". 

This blog is chock a block full of posts about this handy little IC--I have written more about this function generator IC than I should have? Why? 

To cut my teeth on embedded C, SPI, UARTs, and everything else we know and sing. 

WORKS, BUT NOT FINISHED!

This has been a laborious project--as always, still, more work needed....

The 12-bit Seeed ADC buffer board--good for a lot of projects--may not be the right solution here. Best I can tell, 12 bit data gets lost in the A-D noise floor (I think?) and thus the audio output of the VCO is a bit jittery. I want to fix that. I could use a 14 bit ADC and just toss out the 2 LSB's. That might help?

Also the triangle to ramp PCB's ramp output (previous post here) a bit too buzzy sounding. There are ideas on Muff Wiggler about ameliorating that puppy.

For now it works, the VCO doesn't smoke, looks OK, tracks V/octave fine, and damn, I need to get this Rube Goldberg thing off my bench for a bit!

THE SIGNAL FLOW:

I mostly designed and built this VCO to see if I could figure out how to design and build this VCO. Makes sense right?  There are lots of designs out there for audio synthesizer VCOs that are a lot simpler. This isn't one of those.

How it works:

Input is a volt/octave CV, modulation CV, and a frequency and fine control.  

These are mixed, buffered and clamped by the buffer board that incorporates a SEEED XAIO RP2040 dev board (previous post here, the buffer PCB is a PCBWAY community project; PCBWAY sponsors this blog--thanks!--if you want to build one, you can get the gerber here).  

The RP2040 on the SEEED dev board is running embedded C (current code revision of the code is on Github, here).  

It uses a python frequency lookup table array creator--which I turned turned into a .h file for this project (github repo for the Python array frequency creator is here). 

I also used the AD9833 C library I wrote for AD9833 (github here) and debugged using UART (post here). SPI is used to drive an AD9833 (again, lots of posts about this IC, for the VCO I used this cheap breakout board).  

The AD9833 has a 600mV unipolar triangle output--so yet another PCB is used to convert the triangle to ramp and pulse/PWM and buffer the 3 signal outputs....post for this subcircuit is here, modwiggler forum thread about the concept and design of the converter begins here. 

Yes, this is a complex build!  To make it even harder, the build uses no hookup wire and is configured in Eurorack skiff format. That makes it harder to modify and troubleshoot, but, well, who likes things to be easy?

Build Photos:

Board fab was from this blog's sponsor, PCBWAY--please help out this blog and check 'em out.

testing the triangle to ramp converter....it works....

 

OUTTRO

I blew up a SEEED dev board getting this going, one AD9833 BoB was defective, and one SEEED board was DOA. A lot of swearing and head scratching has gone into this project so far.  

This experience has been pure DiWHY--such as, why am I building such a complex VCO when a 3340 based analog equivalent will probably sound better and be a hell of a lot easier to lay out and build?

Because it's there?

DiWHY masochists: show you can take the pain and get yer zips for BOMs, gerbers, PDFs for the schematics and boards, and Eagle CAD files, at Github here. 

If anyone wants to join in with further madness, message me through the MW forum and you can play along. or just grab the files and start modifying.  

In the meantime I am going to take some time off DiWhy--my pychiatrist fiance (amazing she accepted my marriage proposal right?) tells me I need a break. The Good Dr. is always right.

work on other projects for a few weeks or more to regain my DiWHY sanity. 

Then go back to this project. 

For instance, I already have a 14 bit ADC board to replace the 12 bit version used for this project. To skiff-ize the PCB, it uses a lot of tiny tiny SMD parts...should be a real pain in the butt to fabricate. Blah blah ginger blah blah?

....this project tied a lot of previous learning into one module, and getting it to perform better (better sounding ramp, better ADC performance, other improvements) will be good learning as well. I will say it yet again: in the words of immortal Emil Faber: "Knowledge is good". 

 

Update 3-5-23: newer version of this VCO is here--greatly improving the design--in spite of all the whining.

Back to work.