Oscillators, VCOs


Hard to believe I first fabbed the ASM-VCO over 15 years ago. I still have 3 of my from my original PCB fab (cost for 2 custom PCB's fabbed in Canada in 2002: $85 in 2002 dough; cost now for 5: $5 plus $25 shipping from China) in one of my studios and they work, sound, and track great. I had to re-calibrate one recently and couldn't find my 2003 documentation, arrg! but I did find it eventually and now I post it here. useful to me anyway?

ABOUT THE VCO The basic idea (chopper FET/2n2 cap/comparator) has been around since the late 80's Electronotes days. I see this design crop up over and over; on Ken Stones CGS site; EFM had a version; Elby Designs still sells an updated version. My build was from the original ASM-1 Magnus design before 2002. If it ain't broke don't fix it?

My old schematic and board are way out of date and current Eagle can't open them without a ton of errors. I will re layout the board and put a few updates in there. Almost done! Stay tuned for that.

OK here's my docs in PDF format. With this I successfully recalibrated one VCO in a few minutes and it's tracking over 8 octaves again.

  • Calibration notes . PDF is Here! My design adds the Magnus mod (?) of a 10K multi turn trim where the 15V supply feeds the log converter. Technically I remember you don't need this but for me it made things much quicker to calibrate.

  • Wiring layout go here. When I lay this out again I will get rid of the resistors on the daughterboard and front panel.
  • parts list for 2002 builds are here.
  • Board layouts are here.
  • schematics are here.

    Another classic from the 70's. In goes a sine or ramp waveform, put some CV in there, and out comes a new waveform (triange/rampish/squarish?) Depends on what goes in). It's an easy build you can find all over; Again back in the day EFM copped it, CGS also has a lot of Serge content that you should check out. Also if you follow the CGS link as you can see, put should put 3x of these is series; then get some hard-synch like sounds out of it, the "west coast synth" approach to timber shift. I have fabbed this board in China, got it, and built it, it works! zip of Eagle SCM, BRD, and gerbers are here. PDF of schem is here; of board is here.

    And! the board uses a quarter of a Norton OpAmp (3900). I am pretty sure those aren't made any more, but they are readily available. I can't make heads or tails out of how to design using the damn things--this isn't a traditional op amp and we're not in Kansas any longer either. maybe someone can explain it to me.