TLC , Thermal Limit Control, is used in some Crown amplifiers to provide the amp thermal protection, when a predetermined temperature threshold is reached, the TLC light(s) will begin to glow. This means the temperature sensing circuitry is starting to engage the input compressor . By compressing the input, the amplifier will not generate as much heat and will have a chance to cool down. The degree of compression is directly proportional to the amount of overheating experienced by the amplifier.