Now that Nelson Pass published the details
of the JFET upgrade to First Watt F2 and that Newark sells
power JFETs from Semisouth, there was no excuse not to upgrade my Zen V4 with the new active device.
I replaced Q1 (see the original schematic
) with SJEP120R100A; increased R5 to 130 kohm to bias the JFET correctly with Vgs of approx. 1.5V; and reduced R8 to 22 ohm.
The measured result is a nice reduction in THD+N; attached are the graphs for THD+N @ 1W into 8 ohm vs frequency, before and after the upgrade. With IRFP044 as Q1, the distortion was mostly 2nd harmonic (at -65dB), plus some traces of the 3rd harmonic. With the JFET, it is still mostly 2nd harmonic, which is now at -80..85dB, with the 3rd below my measurement floor.
Subjectively, the improvement is remarkable! There is additional detail and depth of the scene.