nx2-Amplifier Update

June 23rd Update. Not much further progress as I’ve been assembling an X-Altra Mini II ‘BB’ for a friend. Once that is completed, I’ll be back onto the nx2-Amplifier again. Stay tuned!

April 18th Update: The first nx2 proto board has been assembled and basic DC tests completed. This covers output stage quiescent current adjustment, DC offset adjustment and checking that all the DC voltages are correct, along with basic stability testing i.e. no oscillation. The next step will be to take an initial look at the amplifier behaviour under dynamic conditions to confirm operation wrt frequency response, square wave performance and so forth. After that comes stability testing. Once that is complete, the amplifier is characterised wrt distortion performance. So far, so good!

April 3rd Update: In the last update on Feb 1st, I had just received the PCB back from the PCB house. Since this was my first design on KiCAD, I wasn’t expecting much, so it was as much a learning exercise as it was a first PCB spin. Well, half the silk screen was missing, some of the SMD pads looked decidedly small etc so I scrapped the board and went back to work. The new layout you see below is much better, and my progress with KiCAD has been good, thanks to some handholding by the KiCAD crew here

Here is the latest board – still to be ordered. I’ll be away for a week or so, but when I return, assembly of the first proto can hopefully begin along with the final housing design (Mini Dissipante from Modushop).

And here’s a partial 3D rendering

Feb 1st 2024 Update: The PCB has arrived back from the PCB house. Unfortunately the silk screen references have not come out, but since this is my first KiCAD PCB, I’ll cut myself a bit of slack and have to make sure that on the second spin I correct that. Hopefully all the tracking is good though, so this board will be good for electrical, mechanical and performance evaluation. Say tuned for more updates over the coming weeks!

January 24th Update: Good progress on the new nx2 PCB – the board has gone off for fabrication. The learning curve on KiCAD 7 was not as steep as I had anticipated – I have to say it is a superb schematic capture and PCB layout tool. There is a lot to take in, and I have a long way to go to fully exploit all the tools and features on hand, but I am already generating useful output from it. Here is a screenshot of the top overlay. More updates to follow in a few weeks!

December 5th Update: The original nx-Amplifier, first published in 2012 delivered 100W RMS into 8 Ohms. Some PCB tweaks were done in 2016, and all told about 350 boards of both versions have been sold around the world from Ukraine to Brazil, India, UAE, South Africa and of course, many in the USA, Canada, UK, France, Germany and the Netherlands to name but a few. However, some of the TO-92 components are now difficult to source and the specified Fairchild TO-126 drivers have been obsoleted by ON Semiconductor. Further, the protection board, although it worked very well, was overly complex, and the terminal layout less than satisfactory if you wanted to build a nice looking amplifier internally.

So, it’s time for an update. The new nx2 boards will feature :-

  • Lower distortion
  • Improved PCB layout for much lower noise and distortion
  • Incorporate the Zobel network and output coupling inductor on the amplifier module PCB
  • Use the same mounting hole locations as the original nx-Amplifier (except for the 4 off optional PCB support standoff holes in the corners you can see in the image above)
  • For the small signal devices, use of SMD components to future-proof the design and ease parts procurement
  • TMC or Miller compensation
  • For power ON/OFF speaker muting, DC offset, and overcurrent protection, the amplifier can use the existing Hifisonix Speaker Protection Board or the builder can use their own. Builders are no longer tied to a Hifisonix solution
  • Builders can use the Improved Soft-Start + DC blocker and PSU ON/OFF controller board aka iSSDCB
  • A basic PSU board will also be available for those that require it.
  • The nx2 supply rails can go to +-55V, producing 110W RMS output into 8 Ohms
  • For builders wanting a complete solution, the heatsink drilling and rear panel drilling and cutout details will be provided so that a 3U x 250mm deep Mini-Dissipante chassis can be ordered directly from Modushop

The nx2-Amplifier, like its predecessor, is a great introduction to current feedback amplifiers, and is easy to build and set up. As with all Hifisonix designs, the build doc will cover the design in detail and provide all the information you will require to get your amp up and running quickly and without problems. The amplifier module PCB’s will be double sided, through hole plated, with solder mask and silk screen.

The plot below is that of the open loop gain without compensation.

Here is a loop gain plot for the new amplifier after Transitional Miller Compensation aka TMC: –


10 responses to “nx2-Amplifier Update”

  1. Andrej says:

    Great news 🙂 Is there already an idea, how PCB will look like?

    I really suggest that new DC protection should omit famous UPC1237 IC. Original devices are hard to find, Duplicates made in China (Alliexpress) behave differently than original ones.

    br Andrej

    • Bonsai says:

      Andrej, thanks for your feedback. I may consider doing a stand-alone protection board that does not use the UPC1237.

      • David Trussler says:

        That would be great to see – perhaps something that adopted a popular low cost platform as the microcontroller for longevity….

  2. Anand says:


    Just a suggestion. Can the main pcb that holds the new NX2 circuit w/zobel & Thiele network be an individual board to allow for flexibility into existing builds? So a diy builder can use other speaker protection boards as well as other softstart boards that are not Hifisonix brand?

    I will be happy to proofread your build document and BOM if would like.


    • Bonsai says:

      Hello Anand,
      The new nx2 amp includes the output inductor and Zobel (I guess this is what you called a Theil network’?). This will make the nx2 completely independent of any power supply or protection board i.e. builders will be able to use their own PSU and protection setups.

      Thanks for the feedback, and the offer to proofread the build doc and BOM – much appreciated!

  3. Anand says:

    Andrew – Can’t wait for the proofreading session! Please also include in the instructions the option to lower the gain *if possible*. 30db gain is great for 84db loudspeakers and those systems that might be dac direct with insensitive loudspeakers (Magnepan, Apogee, etc…). For the rest of us, about 6dB less gain might be ideal. It might require you to check again on Pspice, with regards to altering the compensation circuits. If you are unable, no worries: I do have differential to SE opamp based circuits that can cut gain down by 6dB on the front end.

  4. David Trussler says:

    I’d be really interested in what (implicit) rules you have been using in the PCB layout for this power amplifier module? Making these explicit would be really interesting and helpful – could be a nice article 🙂

  5. Victor says:

    Excellent news, Andrew!!!

    New design looks impressive…

    Looking forward to built this amp on the new boards (together with Hifisonix Speaker Protection Board), as soon as the boards will be available.

Leave a Reply

Your email address will not be published. Required fields are marked *