They will clamp the minimum voltage that can be applied to the base of TR104 & TR105 avoiding a possible catastrophic failure.
Thank you for your explanation.
"catastrophic failure" is unlikely but bad clipping behavior is. Many op-amps and audio amps suffer from rail sticking or even polarity reversal when over driven. The LED reference voltage is all that biases TR 104,5 so they easily saturate at which point the polarity reverses. This may then throw the amp into oscillations or latch-up to one of the rails, ie +/-55VDC on the speaker. I do not have time to run a simulation on this circuit but that is the best way to determine what does happen when this circuit is over driven. Many amplifier circuits are ~over designed, ie too complicated for the designer to foresee all such problems, and the added complexity rarely improves performance. This can still happen with SPICE simulation because each simulation only tests a very specific situation.
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