Sony 2SK82 / 2SJ28 V-FET (SIT) Class-A Amplifier – Hiraga 30W DIY (L’Audiophile 1979)

This amplifier was built by me based on Jean Hiraga’s original 1979 publication in L’Audiophile, using genuine Sony V-FET output devices (2SK82 / 2SJ28). ⚙️ Technical highlights: ±35V supply ~1.7A bias current (pure Class-A operation) 600VA toroidal transformer Dual mono rectification 214,400µF total reservoir capacitance (± rails) Discrete protection board with DC detection Motorized ALPS potentiometer Soft-start Custom PCB design 📷 Detailed build photos (internal layout, PSU, PCB design): [Google Drive link] https://drive.google.com/folderview?i... The speakers in the video are self-built 2-way units, crossed at 140 Hz. (Subwoofers visible in the room are not connected in this recording.) 🔬 Technical note V-FET devices behave closer to triodes than to typical MOSFETs or BJTs. Their relatively low μ and depletion-mode operation make bias stability and power supply quality especially important. In this build, the very large reservoir capacitance and dual rectification were used to minimize supply ripple and dynamic sag, reducing ripple-related intermodulation effects under load. The amplifier operates in true Class-A with constant current draw, which further contributes to supply stability and predictable thermal behavior. SANSUI SP-2500    • V-FET bootstrap power IC amp Sansui SP-2500