I am currently working on an embedded audio processing system for electric guitar. I have constructed two JFET preamps (left and right channels) based on the j201 transistor using the common source self biased topology. I hand matched the two transistors and biased them from scratch and achieved a voltage gain of around x3. When pushed to saturation, the preamps also clip symmetrically, which is a good sign that the biasing point is correct. See design below.
My problem is that when the outputs from the preamps are connected to the line inputs of the WM8731 codec, almost all gain is attenuated and clipping occurs on the bottom rail. I don't know why this happens. See codec below
I am testing with a signal generator with 10kHz signal at 1.6Vpp with high impedance output.
Codec disconected => pk-pk around 6V, no clipping, clean inverted sine wave
Codec connected => pk-pk around 2.4V, clipping on bottom end
Again, the only thing I've done is plug in the codec between tests. I'm not sure if this is because of a lack of understanding of JFET characteristics or the codec in question, or even something else entirely. One thought that crossed my mind is that maybe the lower half of the voltage divider created by p2 is being loaded down by something in the codec, but then this would only attenuate the signal and not result in clipping on the bottom rail.