Main SA-100 MOSFET Output SA-100 BJT Output SA-12

Counterpoint SA-100 MOSFET Output Stage

Counterpoint used Harris and Fairchild vertical MOSFETS in their amplifiers. These MOSFETs were tested at operating temperature at the factory and were so tightly matched that ballast resistors were not needed (their sorting and grading procedure was over-the-top). Unfortunately, the MOSFETs and their surrounding circuitry were very delicate and easily destroyed. These devices have not been available in over 20 years, so if one device gets damaged, then all the devices have to be replaced with something else. One would need to purchase and grade an awful lot of vertical MOSFETs find four matched pairs for a 100W amplifier. Lateral MOSFETs, on the other hand, have a negative temperature coefficient, don't require matching or ballast resistors, and can be biased with a single resistor.

Here is the mod I did over 10 years ago using Exicon lateral MOSFETs. This was my first real attempt at design. These lateral TO-3 MOSFETs are made specifically for audio. I used the George Kay's Moscode 300 and Erno Borberly's Hafler DH-200 schematics as guides, as well as output stage topologies from a long dead Geocities site. I also used the schematic drawn by Federico Paoletti as a reference for my drawing (which is actually part of an LTSpice simulation). This amplifier was used daily for 8 years until I sold it in 2018.

SA-100 schematic.

Modified SA-100 schematic.

Modified SA-100 pictures