The Nemesis ideology
Nemesis is not only the Greek god of vengeance but, first of all, of compensation and balance.
The basic idea is that every discrete component is imperfect, however well it can be done or its tolerances are low. If the discrete components were perfect, only one transistor would be enough to produce a totally precise power amplifier.
Hence the Nemesis is born, a medium power high speed and high efficiency class AB analog audio amplifier.
Its structure, perfectly symmetrical, is made up of 36 transistors:
- A double differential with constant GM degeneration and advanced current mirrors, much more precise than the classic Wilson 4 transistor.
- High impedance constant current generators and cascoding to guarantee high PSRR, placed on both the differentials and the VAS.
- The low compensation, the right current and the unit gain of the differential allows a signal speed so high that it can deliver a swing from positive voltage to negative voltage rail in only 1uS.
- The output stage is of the CFP type, which, unlike the classic Darlington, requires one tenth of the bias current and is far more linear and thermally more stable.
- The compensation capacitors are NP0 (C0G) ceramic type, i.e. perfectly immune to voltage or heat variations. The electrolytic capacitors have a very low ESR and long life, this, added to the minimum currents and voltages passing through, allows them to last almost forever.
- The local feedback of the various stages and the global one guarantees a minimum output impedance, and therefore a high damping factor with only a pair of output power transistor, necessary for the total control of the speakers.
- The perfectly symmetrical structure not only guarantees an optimal working point regardless of the differences in supply voltage between the positive and negative supply branches, but also a very low output DC offset.
- Unlike many other amplifiers, the input impedance and the feedback impedance are the same. Thanks to this, the noise is very low, the DC offset is just as low, and there is no on or off "bumping".
- Despite the high speed, the amplifier is totally stable even without output networks, inductors or Zobel networks.
- Thanks to the constant current generators and cascoding in the VAS stage, the thermal variation is very low, consequently, the memory distortion is also low.
All this thanks to its strictly corrective structure: without selecting any component, resulting in very low distortions, total immunity to thermal variations, total stability without output networks and same identical performance module by module.
The final concept is easy: if you have to select components to reach a result, then with the slightest variation in temperature, voltage or other variables, the result itself will vary. In audio it is not so much achieving a low amount of distortion that is the problem, but also keeping it while running music, with variations of all genres. And this, an amplifier with few transistors simply cannot guarantee it.