The ASM-1 VCO is cool because it's simple, has a low-parts-count, and is fun to build. At the onset of this project I couldn't find a single-ASM-1 VCO, using a single sided PCB anywhere, so I decided to fab one myself.

You can download my ready-to-transfer PCB and the component placement document . I have built two working ASM-1 VCOs from this exact PCB; in other words, to the best of my knowledge, this layout is 100% ready to go and works!

As far as building it: You will need matched NPN for Q1. I used a MAT02. If you don't use a matched set you will have pretty bad tuning stability issues. As far as which resistors need to be 1%: I found that all the antilog convertor stuff pretty much had to be: that's the 150K R15, R13 56K, R3 and R7 100K, R1 or whatever you use for keyboard 1V/oct input, R16 10K, and the 390K. BTW, for all the caps in the document here, EXCEPT the 2n2, 22pF (I have used 18pF or 22pF in my tests of the design; both worked fine), and 100pF, are power bypass caps. Use .1uF, say, 25V or so. I used mylar for the 2n2 and .1, and ceramic for the 100pF and 22pF. Worked for me. You can see a parts list here.

I found that the power to this circuit has to be stable/well regulated for the VCO to operate correctly. Oh yeh, use 10 turn trimmers for all the trimmers in this. The "long skinny one" shown on my parts placement is 100K 10 turn.

Note that the LM311 needs to have PIN 1 see -15V and 5,6 tied together. This isn't on all the ASM-1 schematics you can find on the web! Also, Magnus Danielson's suggestion of adding a 10K trimmer "on top" of the 150K resistor in the exp conv section is critical to making this thing easy to calibrate following the steps below.

See the ASM-1 pages for more details about parts substitutions if you are interested. Actually I'd read the whole section on VCO before trying to build this.

Another thing I learned is that you need a big, fat trace between the MAT-02 output and the 2n2 cap for the circuit to work. If you're breadboarding or doing another PCB, wire the MAT-02 directly to the cap and then to the FET. This was one of those "PCB layout requirements" that took me an evening to figure out.

To calibrate, follow this procedure (many thanks to Magnus Danielson for his help!):

• You will need a voltmeter, a frequency counter or working/in tune synthesizer, an extra -0 to -10 or so adjustable power supply, the VCO under test, and a working MIDI to CV coverter.
• As an idiot check, use your voltmeter to make sure your MIDI to CV converter is really putting out 1V/Octave. If it isn't you may have serious problems getting your VCO to scale.
• unplug any sources of CV to the ASM-1 VCO you are setting up.
• Using a voltmeter, check the voltage on the "op amp" side of the 56K resistor. Not the side that hooks to the MAT-02, but the other side. Tweak P1 trimmer (on my board, it's the long, skinny trimmer by where + and - 15V come into the board) until the voltmeter reads 0V relative to ground.
• Assuming you have a frequency meter, hook up the ASM-1 VCO to your frequency meter's input. Go the R5, the trimmer on top of the 150K resistor (this "R5 trimmer" isn't in all the schematics out there either, but you need it). Adjust it until the output frequency of the ASM-1 VCO you're calibrating reads C#8 or 8869hz. Here is good chart for pitch to frequency.
• (If you don't have a frequency meter, plug in your working synthesizer to an audio amp and call up a non-filtered, always-on saw or sine wave. Making sure you're set to A440, dial in a "high octave" pitch setting for your VCO, then play C#8 and let it drone. Then adjust the ASM to "beat" on this pitch.)
• Measure the output of your extra power supply. Adjust the output until you see exactly -6V between the two terminals.
• Hook the -6V output to your Keyboard 1V/octave CV in (which should be 1% 100K as written above) and the other terminal to your ASM-1 ground plane. Don't hook this up backwards or you may get Kentucky Fried VCO, original recipe.
• Tweak the P2 trimmer (the one by the 1V octave inputs) until the ASM-1 output pitch is C#2, or 138Hz. Again, if you don't have a frequency meter, adjust your already working synthesizer to play C#2 and let it drone....then calibrate the ASM-1 VCO so its frequency "beats" on the frequency of the already tuned up one.
• Adjust P1 down a bunch to the range you use in your studio. For me, I had to crank it WAY down so that C0 was the lowest note on my controller and came up at about 32hz....farting range.
• If you did all this, your ASM-1 will scale accurately across about 7-8 ocatves or more. It worked for me. If you get stuck (I did for awhile) email me. This isn't the easiest thing to build, but it CAN be done!

Now, the final question is adjusting things so this VCO works with your setup--your ADSR's, your LFO's whatever. There is no one solution for this. I would say to mess around with the resistors directly hooked up to the CV inputs. Larger resistors here will scale down the amount of Mod coming in from the outside world. You have to experiment.