• Item: 300W Class AB AMP  
  • Type: retro-thermionic “Precision”  
  • Posted: 8 February 2020  
  • Builder: mwhouston  
  • Country: Australia  
  • Comments: 1  


Over ten years ago I made two stereo power amps for a friend based on the Holton Precision 200W amp modules. I liked the amp so much I bought two of the 300W (500W into 4ohm) amp modules - nxv500. The modules lay dormant until recently where I built them into an amp. The amp was exceptionally good sounding but the PS I had made (as in the image above) was too big to fit into a 3U rack mount enclosure with two heatsinks etc.

To finish the first build I built a narrower HS with 40,000uf of caps. I only enjoyed this excellent sounding amp for a week then it was sold. Happy customer and a repeat. Now I had this high power PS with 625VAC tranni and 80,000uf of 100V capacitors, 10A bridge and fully snubbed. What to do? Build another amp of course and use this sub-chassis mounted PS.

Anthony Holton's 300W modules have come a long, long way over the last ten years now in release R5. They now support very high performance dual die lateral FETs which can with stand 200V @ 16A EACH! There is eight per channel. The modules are; HPA-NXL500 R5. The power FETs are specifically designed for high-end audio not just power amplications.

The amp is now complete with heavy duty 3-way speaker binding post and gold plated RCAs. The amp has a built in power filter, a choked power lead, fused and earthed. The 10A rec. bridge is soldered directly to the first cap bank to ensure he 100A pulses are not supressed before reaching that first cap. This technique I have used many times before with only positive benefits.

Due to the fact I could not get the HS I wanted this amp runs hot without the lid off and a silent fan underneath. To rectify this this amp will receive a new lower voltage PS (but still 80,000d of snubbed caps) and be converted to a Holton 150W amp with the latest HPA-NXL200 R4.1 modules. These also support the high performance power FETs.

The 300W boards and high power PS will go into 300W monoblocks with appropriate HS to stop any other heat issues. In the meantime I am enjoying this high-tech power amp emencely; Bass is deep and very powerful with NO bloat or boom. Precisely articulated. My 12" ploy cone woofers in a 60ltr sealed box are 100% controlled. Even my 180W Class D amp do not give me the bass depth and control this amp does. Mids and treble are dead linear, smooth and extended from my Altec 511B horns with FailtalPro poly compression drivers. For me its good by Class D hello AB!


  • steven
  • 14 February 2020 at 09:28AM
  • Nicely done. I’m currently building one myself, but using dual HPA-NXL-800. Can’t wait to see the results. One questions though, the larger capacitors that you are using...where did you source them from?

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