25+25 Watt integrated Class-A hybrid Amplifier

Features:

  • 2 x 25 W @ 8 Ohms in Class-A (1,25A bias current)
  • 4 transformer-coupled inputs (2 balanced, 2 single-ended), relay-switched
  • 2-stage tube voltage amplification using 2 half ECC82 per channel in a Loftin-White configuration
  • ECC82s are under-heated to enhance long-term stability of characteristics
  • Lundahl input and interstage transformers
  • Absolutely no capacitor in the signal path
  • Zero dB feedback
  • 2-stage current amplification using Hitachi® Mosfets as driver and Sanken® bipolar power transistors
  • High-voltage valve rectifier EZ80 and extensive PI-filtering
  • High-capacity power supply with PI-filtering

This amplifier is used best with speakers like Roiene, Triangle, Lowther etc., if you stay with this type of speaker you will be rewarded with the combination of the best characteristics of a single ended tube amplifier and a Class-A transistor amplifier. I must admit that the results with my JBL 4430 monitors are not equally satisfying as the output impedance has the same magnitude as a single ended no-feedback 300B amplifier. A few dBs of feedback could be the cure here ...

BTW, Guido Tent published an article in the June 1999 issue of “Audio en Tecnhiek” which features similar design objectives and solutions.

Download schematics as PDF file