F4 Channel Problem - Running Cold

This circuit is highly sensitive to Idss of Jfets used.

If you have 8mA running through the jfets you will be starving the TL431A shunt

As the amp heats up the Jfets will draw even more current then TL431 turn off
 
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Most TL431 devices require 1mA to operate but you also have around 0.3mA going through the program resistors so you need at least 1.3mA


I was checking. There is another device ATL431. It can run on 100uA so this would be the better device to use for this circuit.

Or drop the resistor values feeding the shunt