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AS3340-VCO

README.md

AS3340 VCO

This design is a VCO based on the AS3340 chip, built up from a combination of designs found on the internet, resulting in the following features:

  • Compact size
  • Octave selector
  • Holds tuning over time and temperature
  • Precise finetune by note
  • The usual linear FM, exponential FM and PWM inputs

Main board

Schematic

schematic

PCB

top

bottom

IO Board

Schematic

schematic

PCB

top

bottom

Face plate

faceplate

Calibration

  • Set to lowest octave
  • Tune knob to lowest (1)
  • Turn off LFO mode
  • Set PWM to middle
  • Connect V meter between GND and 1V
  • Tweak RV_1V so we get 1.000V
  • Connect V meter between GND and 7V
  • Tweak RV_7V so we get about 7V
  • Connect mV meter between TP1 and TP2
  • Tweak RV_TEMPCO until voltage is 0V
  • Put in JP1 to get a stable frequency
  • Observe the triangle or square output
  • Tweak RV_REF_PITCH_C6 until pitch is C6 (1046.5 Hz)
  • Observe the sine output
  • Tweak RV_OFFSET so that the middle value is 0V
  • Tweak RV_SYM and RV_ROUND so the sine looks good
  • Remove JP1
  • Apply +5.000V CV
  • Tweak RV_REFIND_C6 until pitch is again C6 (1046.5 Hz)
  • Applu 0.000V CV
  • Tweak RV_SCALE until pitch is 32.7Hz
  • Connect V meter between GND and 1V
  • Tweak RV_1V so we get 1.000V (TODO: figure out why this drifts)
  • Connect V meter between GND and 7V
  • Set to octave 6
  • Tweak RV_7V until pitch is again C6 (1046.5 Hz)

Old notes on advanced tuning schematic

Tune towards C6

  • 1M8: max ~800Hz

  • 1M6: max ~910Hz

  • 1M3: Spot on.

  • Option: Replace 1M8 with 1M + 1M trimmer. But probably just a 0603 pad is fine too, we'll replace it with a fixed value for the main cap we end up using.

  • Add an LFO switch, which adds a capacitor in parallel to the 1nF main cap.

    • normal labels: C1 through C9
    • 470 nF -> x labels: 13s 6.5s 3.2s 1.5s 0.78s 0.38s 0.19s 0.094s

  • Octave switch goes from 0V to 7V (8 position switch, 7x10k 0.1% between the 8 positions. Make 7V with a TL431, R1 is 10k, R1 is 10k + 10k pot, 1k series resistor, plus 100nF bypass). Test point at +5V.

  • Need a buffer opamp after the octave switch, even with the TL431 in front. TL072 with +12/-12V supply.

  • Fine tune goes from 0V to 1V (take 7V from the octave switch, through 470k + 200k trimmer, to 100k pot).

  • Need a buffer opamp after the fine tune as well.

  • Put the high frequency tracking in the schematic, but don't need to initially populate.

Errors in rev1 boards (obsolete)

  • Mainboard silkscreen has the positions of values for R28 and R31 swapped. R28 should be 30k, and R31 should be 10k.
  • Mainboard R41 should not have been to ground, it should be to -12V instead. Solder it to the left pad only (pointing left), and wire the other terminal to the bottom of R7 (which is -12V).
  • IO board tune pot hole needs to be 7mm, to accommodate pots that are mounted