MOSFET Power amplifier

Posted on Jan 12, 2013

The Zen (single-ended MOSFET) amplifier was published by Nelson Pass in The Audio Amateur 1. In a subsequent issue of this magazine, Nelson slightly improved the original design 2. I decided to build the `Revisited` version with the following circuit modifications: 1. More extensive power supply filtering to eliminate hum (C-L-C filter , L1 in combination with C1,C2,C7,C8). 2. Even higher quiescent current (3 Ampere / channel, by lowering R1 from 0.33 to 0.22 Ohms). 3. Slight amount of negative feedback (via resistor R11) to improve damping factor and loudspeaker control.

MOSFET Power amplifier
Click here to download the full size of the above Circuit.

My copy of the amp worked immediately, but not "without any flaw". Although I had used the largest heatsinks which I could obtain (0.3 K/W, one per channel) and had mounted these vertically on an open chassis to ensure proper ventilation, the amp ran EXTREMELY hot. After ten minutes of operation, one could not touch the heatsinks. Even the PCB became very hot due to thermal conduction. I began to feel the burden of TRUE class A operation.... Something had to be done about this, or the lifespan of the MOSfets and electrolytic capacitors would be very short. Since I did not want the amp to become even larger and heavier than it already was, I decided to use forced cooling and mounted a powerful 12 V fan on each heatsink. The fans are series-fed from a separate 24V DC supply based on a LM317 i.c. With the fans in action, the amp does not get more than lukewarm even after several hours of operation. But it should stand in the open to ensure the circulation of air. The movement of air by the fans generates a lot of noise. Thus, the amp must be far from the listening site (preferably in another room). But how does a Zen sound? Or, as Nelson Pass said: What is the sound of one transistor clapping”? When you have appropriate speakers: very good indeed. By "appropriate speakers" I mean: speakers with high efficiency and simple crossovers (or no crossover at all). Low-efficiency speakers with complex crossovers will not mate...

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