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Power Amplifier project by Nelson Pass


The F4 is a Push-Pull, Class-A, MOSFET power amplifier. The F4 is unique in that it has no voltage gain, and it's topology adds very little sonic coloration to the signal.



From $ 25.00 USD
3 options

Key Information

  • Power Amplifier Project
  • Designed by Nelson Pass
  • Intermediate Difficulty
  • Only Essentials Kit Available
  • Basic Build Guide


  • No voltage gain
  • Excellent for pairing with a "flea watt" tube amp
  • Build two as monoblocks for bridged or parallel operation


The F4 is a Push-Pull, Class-A, Mosfet power amplifier. It is unique in that all of the active devices (transistors) are connected in a 'source-follower' configuration, which adds less coloration than many other arrangements.

It sounds great, and it is extremely versatile.  Each channel can be used separately as a stereo amplifier, or they can be bridged or run in parallel for higher power into various loads.

The F4 can be paired with almost any normal speaker.  However, for most people, the F4 needs a preamp that has voltage gain.  In order to drive the F4 to it's maximum power output, a pre-amp with gain is needed.  For reasonable listening levels and/or with sensitive speakers, many builders enjoy connecting their F4 directly to their source. 

It can also be used on the output of a "flea-powered" tube power amp, keeping the tone of the tubes, but adding additional power for less-sensitive speakers and/or to add a little oomph to the lower registers where some tube designs might like a little help.  

The sonic signature of the F4 is intended to be close to transparent - the tone of whatever you use to drive the F4 will pass though without any alteration.

Please read the Firstwatt F4 manual for more explanation on various configurations.

    Power supply requirements

    A Bipolar power supply of (+/- 24V) is required. The PSUs for almost all First Watt amplifiers including the F4, F5, F6, M2x, Aleph J, and Aleph Jzm are all essentially identical.

    These Power Supply PCBs are suitable for all First Watt amplifiers.

    Chassis suggestions

    We have a full line of Deluxe Chassis perfect for this project. The two most common choices are the Deluxe 4U and the larger Deluxe 5U, however we heartily encourage you to consider the PASSdiy chassis currently available for pre-order until May 31st 2024. We also have a back panel parts kit to make life easier. Don’t forget the Keratherm.