The schematic above represents the latest Analog2.0 LFO circuit. This circuit has a long-existing problem. The oscillation affects the power supply and that causes instability in VCO pitch; You would hear the VCO pitch shifting along with the LFO’s oscillation. I’ve tried eliminating this problem.
I did several tests, mainly by listening, and chose BC547. The reasons are:
- It has good taste as a noise source. Easy to find sweet spots when using it with a filter and VCA.
- Noise level was close to 2SC3311, so my previous design works with simple replacement.
- Availability is good.
- Unlikely to get obsoleted soon.
I started the test with listening “raw” noise without any modification and filtering. However, it didn’t work well. Many of the candidates sounds similarly. However, I found characteristics of transistors are so different when I also used a VCF and a VCA driven by an envelope generator.
Also, it was hard to determine “which sounds better” when I conducted A/B comparison test. So I changed the strategy and just played with synth using each candidates, and chose one I felt the most fun to play with. Thus I gave up “measuring” eventually.
Demo using the chosen transistor.