El jueves, 26 de noviembre de 2015, 0:23:16 (UTC-3), Phil Allison escribió:
Leonardo Capossio wrote:




In this case, the only thermally dependent element I see is the PTC100.
The circuit surrounding the PT100 seems to be compensating the bias
current of the power amp, though I do not understand it fully, if
anyone can point to a source that explains how this circuit work it
would be great.


** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification.



.... Phil

That claryfies things a little bit.

The mute coming from the input jacks seems to pull the "mute" point to ground, hence setting the base voltage much lower than 10v+0.7v, and thus shutting down Q7.

Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms. I don't know how to calculate Q6's Q point (Vec or Ie), since Veb/PTC100 seems to control Ie directly (unless the only way out is using Shockley's equation). Also the shut down seems way to abrupt, like Q6 is open, and then suddenly it is a closed switch.