I picked up a Phase Linear 300 that needed repair. The overall build quality wasn't that great so I decided rather than repair the existing boards I'd design and build new ones. My first breadboard used bipolar outputs and I found that I had to run them with a fairly low bias current to keep the amplifier from overheating. Phase Linear amps always seemed like they used heat sinks that were marginal at best without fan cooling. I decided to go with lateral MOSFETs and run them at 50 mA bias current. With the stock transformer I get 130 watts RMS with both channels driven. The THD is less than 0.005% at 20 KHz, and improves at lower power and lower frequencies. So this circuit meets or exceeds the original. I've been using it for 3 months with no issues and I just laid out and ordered a second board with the provisions for boosted power supplies for the driver. That's the schematic I attached to this post.
