Dual VCO
My main audio oscillators use Music From Outer Space PCBs. There are two oscillators per module. I also added some additional circuitry for sawtooth doubling (octave up), dividing the square by two (octave down), and a shared PWM modulation input (CV mixer).
Aug 2024
- Upgraded graphics from hand drawn pen to ink jet printed sticker sheet.
- Left side has an issue with the square wave where it is making a ticking sound. Quick look on the scope shows glitching, narrow pulses mixed in with the expected wave. Will be troubleshooting…
- Right side pitch changes up when left side coarse control turned to maximum, all outputs.
- Left side, both pulse out and sub-oct outputs change pitch when right side coarse adjusted.
- While testing with 16 step sequencer, I noted that the oscillator frequency latches up to 67 kHz when the CV input gets to high (see table below). It has to be dropped well below the latch up point before it lets go of the 67 kHz state. When observing with audio output, it just appears like the oscillator has stopped. Definitely not desirable behavior. Perhaps adding a clamp somewhere to limit the input will help. To be continued…
| CV input [V] | Frequency output [Hz] |
|---|---|
| -3.7 | 20 |
| -2.6 | 40 |
| -1.5 | 80 |
| -0.64 | 160 |
| 0.36 | 320 |
| 1.36 | 643 |
| 2.36 | 1300 |
| 3.36 | 2600 |
| 4.36 | 5930 |
| 5.06 | 12400 |
| 5.26 | 67000 |
Setup notes: coarse and fine controls centered, using exponential input; rough freq measure and this is prior to any calibration and a long time since any previous calibration
March 2025
Troubleshooting the square wave ticking issue continued.
I examined the extra board that includes an extra summing mixer for the PWM modulation. For reasons I’ve long forgotten I had connected the wiper of the manual PWM control pot to the daughter board instead of its normal connection on the PCB. I don’t think it really makes much difference, but I decided to put the control pot back to its usual spot.
Poking around at the wiring I was able to get the ticking and unexpected pitch changing to cut in and out, possibly indicating a bad connection. The trouble ended up going away after some minutes without finding a definitive bad connection or source of the problem.
After putting the module back in the system, turning the left hand side coarse knob fully CW caused the right hand side square output to jump in frequency. At that point, the left hand VCO was in the latch up state (67 kHz), so I increased R65 from 47k to 56k and that limited the left hand side coarse knob from triggering the latch up without help from a CV input (still need to prevent the latch up in general though).
