By 2011, Nelson Pass published the details of the JFET upgrade to First Watt F2, Newark was selling power JFETs from Semisouth, and there was no excuse not to upgrade my ZenV4 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. I also rebuilt the PCBs with better parts.
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.
The THD+N @ 1W into 8 ohm vs frequency with a MOSFET (IRFP044N) as Q1:
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. I also rebuilt the PCBs with better parts.
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.
The THD+N @ 1W into 8 ohm vs frequency with a MOSFET (IRFP044N) as Q1:
The THD+N @ 1W into 8 ohm vs frequency with a JFET (SJEP120R100A) as Q1:
ZenV4-J PCB, take 3, now with better parts:
