Nemesis rebuild

Nemesis was a design of Jean Hiraga, at the time Editor-in-Chief of the French L'Audiophile. It was published in issues 34 and 35 in spring 1985. L'Audiophile was a very high-end audio diy journal, strongly oriented to the Japanese audio diy scene.

This is the circuit schematic (c) L'Audiophile

The inside picture on the right shows the very minimal PC board, holding the few passive components, with the Exicon lateral MOSFET ECF10N16 on the heatsink. The original MOSFET (a 2SK135) has been obsolete for many years, but Exicon (available through Profusion Plc in the UK) have been making very good replacements with often better specs than the originals.

On the right you can see the 3A linear post-regulator with LT1085, and on the real right under the aluminum bracket hides the Hypex 180W SMPS supplying the 40VDC for both amps. The original Nemesis had a linear supply with an EI mains transformer and over 100.000uF of supply caps per channel. Still, because of the output transformer, it was very difficult to get rid of hum that would be coupled from the mains transformer to the output transformer. A large chassis for physical separation of the transformers as well as painstaking rotation of one with respect to the other would minimize hum, but I did not find it satisfying. The solution with the SMPS followed by a simple linear regulator made the amp very quiet.

Follow-up Sept 2010: I just released my tech audio publication Linear Audio Volume 0! At www.linearaudio.net you can browse the ToC and the article abstracts, and you'll see that none other than Nelson Pass wrote an article for me on his take on the Nemesis, the Arch Nemesis. Worth a read; he describes several versions of the single MOSFET single ended amplifier in the finest tradition of his Zen creations.

The output transformers are very large and heavy for the output power (about 10W), because they carry the supply current for the output stage which is about 1A. To avoid saturation, the output transformer needs an air gap in the core. But that air gap drops the inductance so now you need more windings to compensate for that, and in the end you end up with a large transformer.

Update 2012: Linear Audio continues to be a success, and there are now two follow-up articles to the Nemesis. Nelson Pass has taken possession of his custom manufactured Static Induction Transistors (SIT), christened the PASS SIT-1, and has written an article about these devices. Lennart Jarlevang has transplanted the SIT-1 into his initial Nemesis using the original transformers. Both articles appeared in Linear Audio Volume 4, published September 2012.

Update 2015: You can now get individual articles from Linear Audio as PDF downloads, including the Nemesis article.