Mr Nelson Pass, the designer: As many of you may know, First Watt is dedicated to exploring the performance quality of small simple power amplifiers. Here is the F5. We want to further push the performance boundaries of simple little amplifiers with a FET two-stage class-A push-pull design....

This is 2x25W JFET-MOSFET class-A push-pull custom amplifier by LJ Audio. Custom made only, approx lead time: 2 weeks.

• Model: F5 Ver #1, June 2013 (protoyype).
• Design:
  
  • Circuit design:
    
    • The  topology is familiar; a very simple two-stage conjugate-complementary circuit using two JFET transistors for the input amplification and two power MOSFET devices for the output.
    • Input: Single-ended, JFET complementary common-source stage, class-A self-biased.
    • Output: MOSFET complementary common-source stage, class-A fix-biased.
  • It has very wide bandwidth, DC (0 Hz) to > 1 MHz.
  • No capacitors anywhere in the circuit (except in the power supply, of course!).
  • It has a high input impedance – 100 Kohms, and a high damping factor (~40)
  • The distortion is very low, between .001% and .005% at 1 watt.
  • It will drive low impedances.
  • It’s very quiet, about 60 microvolts or so.
  • Circuittry and technical stuff at www.firstwatt.com
• Features:
  
  • Loudspeaker protector module with triple protection.
  • Soft start module.
  • Quad fan (inflatable fan, and suction fan) controlled by thermal sensor.
• Key Parts/Components:
  
  • Very low noise J-FET: 2SK170.
  • Power MOSFET: complementary pair IRFP240/IRFP9240 (HEXFET, vertical).
  • Rectifier: Motorola 35A bridge diode (upgradeable option).
  • Resistor: Riken (Japan), Kiwame (Japan).
  • Capacitors: Panasonic (upgradeable option).
  • Transformer: LJ custom trafo.
  • Heatsink: The bigger "Block Y".
• Chassis: metal chassis, audio pro style.
• Construction: PCB.
• The parts and components are subject to change.