Fusion Hybrid Amplifier

Design Objective : 300B Single Ended sound with POWER. The 300B drives the output transformers which drives the speakers at the usual power output of 8-9w. BUT, flick the Fusion switch, and that power is now 50 watts!

Circuit walk through : input gain stage is handled by an EF86 – the classic high gain/low microphonic pentode. Coupling is by one small high quality 0.015uf cap to paralleled sections of a dual triode 5687. The 5687 is direct coupled from the cathode to the 300B, providing a low impedance source to manage the 300B while serving as a 'pass-through' (no coupling cap) for the speed and finesse of the pentode music amplifier.

The chassis is pre-assembled with aluminum front panel, 2 PCBs with pre-mounted components; power transformer M6 laminations in the output transformers. The OPT are the soul of any tube amp and the latest generation is CNC wound for perfect lay, low capacitance and very wide power bandwidth. The large chassis and generous size dissipates heat evenly; it is normal for the chassis and power transformer cover to feel hot due to the efficiency of heat transfer from critical parts. The unit looks good as well. The combination of pentode and no NFB adds up to truly high end sound. Dynamics are impressive, as is openness and sound staging. Its performance belies its price many times over.

But what about 'mosfet mist'? Not here. Mosfets become starchy sounding when driven by high impedances. In this case we are driving the mosfet with the output transformer 8 ohm taps. Zout is 3 or 4 ohms and is exactly what is needed to bring the mosfets into the 'tube sound' producing state they are famous for. So. Just need 8 watts? Run the amp with Fusion switched OUT. Want an instant 8db gain in power (50watts)? Fusion IN. The switch is on a time delay to prevent transient thumps.

Construction Details and Features:

  • Power output: 9.5W x 2 RMS @1khz with 300B; 50 Watts x 2, RMS @1Khz with 300B + mosfet
  • Frequency response: 6Hz to 60KHz (-3dB@10W)
  • Total harmonic distortion: < 2% (36W@1Khz)
  • Input sensitivity and impedance: 500mV, 100k ohm
  • Input Impedance: 100k ohms
  • Output Impedance Speaker Taps 300B driven: 4 , 8 and 16 ohm
  • Output noise: <1.5mv
  • Power Consumption: 110/230V AC, +- 10%, 90W
  • Control Functions: Front panel: Power On/Off, Top deck: Fusion in/out
  • Input Interfaces: 1 groups (RCA)
  • Output Interfaces: 4 Groups 4-way binding Post 0/4/8/16
  • Vacuum tubes: EF86 x 2 or equivalent, 5687 x 2 or equivalent, 300B x 2
  • Dimension (mm) and weight: 340(L) x 410(W) x 230(H), 28kg net
  • Only available assembled