Oscillators, VCOs, LFOs

Jump to:
ASMVCO from 2002
Serge 3900 based Waveshaper
Dual "mini" LFO
13700 Based VCO


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My old schematic and board are way out of date and current Eagle can't open them without a ton of errors.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!
  • 13700 based OTA VCO

    You can learn more about this on my blog. I'll just link that here. The schematics are straight out of one of Thomas Henry's books, the only novel (?) thing about these VCOs is that PCBs I fab'd of his designs are mounted on a "universal motherboard" vs. perf or whatever. Here it is:

    Yep, a "motherboard" was used to mount the other boards (CV to log converter, core VCO, waveform generators and a non inverting op amp to boost the Sine wave to 5V peak to peak). That's what you see above. I thought I could use this same board for other projects (maybe i can?) but these were still fairly difficult to deploy and didn't save that much wiring.

    Good news is that the VCOs are done and seem to work.

    Get the build notes (there are a lot) here and the brd files etc. for the motherboard here. It's best I figure to not reproduce TH's schematics or whatever without his permission so for that, please look into getting his books.