Oscillators, VCOs, LFOs

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ASMVCO from 2002
Serge 3900 based Waveshaper
Dual "mini" LFO


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 2002-2003 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. This is a simple and reliable low parts count design which is always good by me. 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My old schematic and board are way out of date and current Eagle can't open them without a ton of errors.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. Update! done but not tested. It should work right?

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.(see below)
  • parts list for 2002 builds are here.
  • Board layouts are here.
  • schematics are here.
  • 2017 board and schem update are here. This is my redeux of the 2002 design, but doesn't add any new features. Board layout is cleaned up a bit. I will try to test this before too long. If anyone wants a free board to test, and lives in US or canada, email me using the contacts tab above and I will send you one if I have any left.

    Another classic from the 70's. In goes a sine or ramp waveform. You can put some CV in and get subtle sound variations, but I like using a VCA in front of this board to get less subtle changes. Anyway, use this small circuit, 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!

    Some files:

  • Zip of Eagle SCM, BRD, and gerbers are here.
  • PDF of schem is here;

  • PDF of the PCB board layout 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.


    Trying to cram two LFOs into a small board, this is the classic opamp/comparitor design, I have added buffers and a couple of stages you can use for DC offset of the output waveforms. This board tests working. As usual I try to make this layout as modular as I can, for instance, get 2 boards and use the 2nd for 2 more bias offset stages and use the LED stages for audio buffers vs. lighting more LEDs.

  • Wiring layout go here. This of course is just a suggestion, there are a hundred ways to wire this up. For me, I wanted 2 knobs and LEDs only on the front panel, and for 2 of the outputs to be to 5V P/P while the other 2 were standard P/P going postive and negative.
  • PDF of dual LFO board layout w traces shown is here.
  • Board layout pdf is here.
  • PDF file of the dual LFO schematic is here.
  • Eagle files--sch and brd are here. If you modify, improve, have ideas, etc. please let me know.
  • Zip of gerbers, testing working is here. Send em off to your favorite fab shop and have fun!