Quote from: J M Fahey on October 02, 2012, 03:48:50 AM
@ loudthud.
Yes, maybe you can not pull that source resistor down enough even by straight grounding it, and it would not be an Op Amp defect, of course.
At least for the experiment, hook the Op Amp to +/-15V rails, so it can sink below ground if needed, and see what happens.
The circuit I copied from used +/- 15V supplies, but I'm committed to a single rail power at the moment. It's easy to tell if the opamp is out of gas, the opamp inputs will be at different DC voltages because the loop isn't closed. The resistor to ground across the source cap works fine. The JFETs I'm working with need at least 3V bias, not a problem for the opamp. At times I see low frequency ringing on the opamp output, but if the R1/R2 voltage divider is set properly, the shift in bias point is minimal. Maybe a small resisitor in series with the source cap would improve phase margin. I'm allowing the preamp rail to sag and that also effects the bias point.