The Hifisonix nx2-Amplifier

A new high performance 100W CFA amplifier. The nx2 produces >100W into 8 ohms and >200W into 4 Ohms both channels driven. At 100W into 8 ohms, the distortion is below 30ppm while at 200W, it is below 40ppm. The amplifier features 700ns rise/fall times and an EF2 output stage using three pairs of output devices, allowing it to drive difficult loads.

Click here to download the full build document with circuit description and measurements

Click here for the detailed BOM

PCB’s for the amplifier modules and the PSU will be available over in the Hifisonix shop in about 2 weeks.

I will post some decent pictures up in the next few days along with my listening impressions now that I’ve had the amp running for a few weeks playing music. In the meantime, if you have any questions, let me know, and thanks for visiting hifisonix.com

nx2-Amplifier: Some Background

In 2012, shortly after I published the sx-Amplifier, a very simple pared-down class A CFA, I tweaked the design to come up with the nx-Amplifier that delivered 100 Watts RMS into an 8 Ohm load and simulated at 0.007% distortion at 80 Watts RMS. Like the sx-Amp it was based on, the idea was to keep the amplifier simple, and easy to build but still have good overall results and sound. The transimpedance stage (aka VAS) was a simple single transistor implementation preceded by a standard CFA diamond buffer input stage, while the OPS was an EF2. Two iterations of the nx-Amp board were made – the initial release in 2012 and then a further small update in 2016. About 150 sets of each release were sold through Jim’s Audio* and from what I can tell, most of those were built.

Accompanying the amplifier boards was a rather complex and unwieldy power supply and protection board that I called the ‘PSU +Prot’ that offered +-50V DC rails, DC offset and overload current protection, and power ON/OFF muting. The Zobel network for the amplifier modules was, unusually and not very cleverly, incorporated onto the PSU +Prot Board. I received lots of excellent feedback about the sound which builders loved. However, in retrospect, I think some aspects of the design were idiosyncratic as described in the presentation.

Fast forward 13 years to 2025, I’ve learned a lot, had much feedback, and so it is time for an upgrade . . . this can be approached in two ways: start with a blank sheet of paper and a brand-new design or start with what is a good basic amplifier and improve it, ending up with something altogether better than the original. I decided on the latter course of action, adhering to the  ‘if it ain’t broke, don’t fix it, just upgrade it’ dictum.