Sphinx Project-18 Upgrade from PWR transistors to PWR MOSFET's

Jan de Groot, former designer of Sphinx High End audio for more than six years, have a passion for creating the best audio sound reproduction in all its faculties, keeping in mind to use the original sound with the utmost integrity. In the period of Sphinx manufacturing (1994-2000), I've designed a prototype of the MOSFET Projcet-18. This model had regarding to the new model some imperfections, like MOSFET protection, biasing, etc. For the Sphinx enthusiast I've finished this study model and write a designmanual for it. Suggestions for further improvement are welcome to me and will published on this site.


Project-18 Standard:
The Project-18 once designed by STC in the nineties, was designed with heavy bipolar semiconductors in the Power Stage. We talk about the 2SA1302 of Toshiba with his complementary opposite one, the 2SC3281. The Stage is fully built around a common emitter circuit (collector output stage). For more details, see the book “Self on Audio from Douglas Self, chapter 23”. In short; the Biasing for the FET is cross-refer to the power supply.

Project-18 redesigned:
The Power Amp peculiarity sounds a little bit too hard, especially when the Amp is connected to a preset-day loudspeaker. Because of that and the possibility to use High End qualified power MOSFET’s, I’ve got the idea to redesign the Project-18 from a common emitter (Collector output stage) to a common source (drain output stage). This document describes the redesign of the Project-18.

Most elementarily design changes are the Thosiba MOSFET's, the accompanied protection circuit (Z1,2 & D1,2) for these MOSFET's and the modified biasing circuit. For more details see PJ-18 Designmanual. This manual explains the modification for the do-it-yourself analogue designers in all his facilities. On request I will deliver more details about this modification. Here below a picture token when the Power Amplifier delivers to both channels maximum current on 4W loads by a frequency of 10kHz, THD is <0.1%.

Current and maximum Power through the 4W load is:

The Designmaunal and technical papers are saved in pdf format and can be downloaded from here:

  PJ-18 Designmanual   2sa1302.pdf   2sj162.pdf
  1n5711.pdf   2sa1943.pdf   2sj200.pdf
  zener.pdf   2sc3281.pdf   2sk1058.pdf
  meggitt.pdf   2sc5200.pdf   2sk1529.pdf