NOTICE: An error has been discovered in the 535SE schematic in Post #30 (PDF and graphic are now updated). From Steve: The value of R105/205 is incorrect. The 4.3 ohms that was shown was a holdover from a previous biasing scheme (resistor/red LED). When I decided to switch to the current CRD/Zener scheme, I updated the schematic to the CRD/Zener, but left the 4.3 ohm resistor designation until I could establish the new resistor value. This was nearly two years ago when I decided to put the amplifier on the back burner and didn't breadboard it. I subsequently forgot that 4.3 ohms was not the correct value. My apologies. The schematic has been updated and R105/205 given a "TBD" until I can breadboard it and dial in the final value. It will likely be in the range of 10-15 ohms. When that has been done (we're still waiting on the heatsink samples) I will update the schematic with the proper value and power rating. Hat tip to @dsavitsk for catching this. Thanks, Doug!