• Item: Class A Power Amplifier  
  • Type: Nelson Pass A40  
  • Posted: 21 July 2011  
  • Builder: Mitchco  
  • Country: Canada  
  • Comments: 1  


20 years ago I purchased circuit boards and partial parts from Old Colony Sound. I managed to stuff the boards and then life happened.

Recently I had the opportunity to finish the amp. I decided to go with separate chassis for each amp channel.

Nelson Pass published the design of this deceptively simple 40 watt RMS per channel Class A amplifier in 1978. Documentation: http://passdiy.com/pdf/a40.pdf Part substitution: http://passdiy.com/pdf/part-sub.pdf

Thanks to the folks at diyAudio Pass Labs for helping me out: http://www.diyaudio.com/forums/pass-labs/ Especially Eric for keeping and finding a 15 year old doc where the Old Colony pcb's had a trace error.

Tip of the hat to Nelson Pass for designing such a sweet sounding amplifier!


  • davelang
  • 11 November 2014 at 08:40PM
  • Nice job. I too have stuff boards for 4 channels of pass A40 nirvana. I did this 30 years ago. Built one and have 4 channels left. I have all the original semiconductors including two sets of lambda output devices.

    I think I'm going to build two amps in diyaudio 5U deluxe cases. No excuses after seeing yours.

    Very cool.

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