Thanks Sakis for bringing protection circuits up. What is the 220k resistor on the trigger of the 555 with a slash through it and the T beside it? It's not a thermistor attached to the heat sink, is it? They have invented some stuff since I got out of school, and wikipedia didn't show this symbol under thermistor. I was looking at the Newark.com catalog Sunday and found a honeywell 140-103LAC-RB1 thermistor with 25 C resistance of 10Kohm and a beta of 3974. I think this means it goes 1070 ohm at 60 deg C, is that about right? I've been disassembling motor drives trying to find the temperature sensor, but I think they are integrated into the thyristor block where I can't play with them, and honeywell doesn't have and ap note online, so I am trying to get a free education. Thanks in advance.
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Agreed. By the time a positive offset triggers the protection, it will likely have damaged the speakers or simply blown some fuses. I'm not sure you can get a negative offset to even trigger it. I would think a window comparator would do a better job of it but in all my years of working with audio gear, I personally have never run into a amp that put out significant DC without taking out its own fuses and I've never seen speakers damaged by continuous DC.
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R5 looks like it has both pins connected together. See the 2 connect points and net under the resistor symbol. It doesn't really matter if you are only looking at the schematic but it may be a problem for the PCB.
It could be that the resistor has had a pin swap on the PCB but not rotated in the schematic.
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