I gave up on the idea of a single transistor phase shift oscillator, since I need to cover a range of frequencies of approximately 1 to 12 Hz. At least with my feeble attempts I couldn't make it work. I have routinely done it with one tube, however.
So what I came up with last night was an opamp design. In the attached LTspice circuit, I am using an LT JFET opamp for everyone's convenience (I had to add the TI opamps to my own LTspice lib). I'll be using the TL072 in the final design, since I have a half opamp available for use at the moment.
See attached *.asc file.
What I've put together seems to work at slightly less than 1Hz to just over 10Hz, which I could probably live with. Increasing it to 12Hz, would give me some extra margin, if the changes were not too radical. I doubt 12Hz would actually get used in practice but I like to leave the option open.
I used two ganged 5k pots to adjust two resistors for that frequency range. I'll use 3k pots if I can get them locally (the extra 2k is unnecessary- diminishing returns).
To cover that freq range, I have to keep the gain of the opamp a little higher than I would otherwise use. This has the bad side effect of more distortion in the output sine wave.
As a compromise, I used a low pass filter after the emitter follower circuit. That helps a little but the output could be better.
Have a look and see what you think. What can be changed to improve:
a) frequency range (ideally 1Hz to 12Hz) b) purity of the sine wave
Thanks, Warren