Power supply with Borbely style 12AU7/MOSFET hybrid

Description: A tube/MOSFET hybrid power amplifier is being constructed based on a circuit published in AudioXpress by Cozza (May 2001), which is derived from a design by Borbely.

The tube/MOSFET hybrid power amplifier integrates the warm, rich sound characteristics of vacuum tubes with the efficiency and power of MOSFET transistors. This design typically employs a vacuum tube in the input stage to provide high gain and low noise, while the output stage utilizes MOSFETs to deliver high power with low distortion.

The circuit generally consists of several key components: the input stage with a vacuum tube, often configured as a common cathode amplifier, which serves to amplify the audio signal. Coupling capacitors are used to block DC while allowing AC signals to pass through. The output stage features a complementary push-pull configuration of MOSFETs, which ensures that the amplifier can drive a load efficiently while maintaining linearity across a wide frequency range.

Feedback loops are often implemented to improve the stability and linearity of the amplifier. This feedback can be either global, affecting the entire amplifier, or local, focusing on specific stages. Additionally, power supply decoupling capacitors are critical in reducing noise and enhancing performance.

Thermal management is also a vital consideration, as both tubes and MOSFETs generate heat during operation. Adequate heat sinking and ventilation must be incorporated into the design to prevent overheating and ensure reliability.

Finally, the circuit layout should be carefully designed to minimize parasitic capacitance and inductance, which can adversely affect performance. Proper grounding techniques and component placement are essential to achieve optimal sound quality and amplifier performance.I`m building a tube/MOSFET hybrid power amplifier based on a circuit in AudioXpress by Cozza (5/01) which in turn is based on a design by Borbely..