I’m building the MFOS VCO — well, breadboarding and going to build — and I thought I’d post some notes here in case they’re of interest to someone else.
I breadboarded the VCO core, as designed except:
- I didn’t put CV1 through CV4 in, am just using coarse and fine tune to test CV
- U1 is TL072; I do have an OP275 I intend to use in the build, and will probably stick into the breadboard soon, but haven’t done so yet.
- U3 is SOIC SSM2212
- Q1 is 2N5457 (see below)
It works. With no attempt made to calibrate: Frequency range with coarse tune (fine tune near center) is about 2 Hz to about 13 kHz. It’s roughly 1 V/oct. With coarse tune roughly centered frequency is somewhere in the vicinity of middle C. RAW amplitude is more like 2.1 V instead of 2.2 V but that might be my not quite 12 V bench power supply.
Sync works with a low frequency (~ 1 Hz) square wave. Sync frequency really should be comparable to VCO frequency but I don’t have an audio oscillator on my bench right now.
Lin FM works, though frequency change with ~4 V is small, might be some error somewhere but as long as the frequency changes at all I figure it’s “working” and won’t panic until I see how it does on the PCB.
I tried the following JFETs, no detailed study of performance but just checked if the oscillator worked over the full coarse tune range:
They all worked.