When i tone check the left surround channel over -18db, the amplifier shutts off immediately and the phase control light indicator pulses, even with no speaker connected. The same happens when sound plays through this channel (e. g. music), with or without connected speaker. The problem occurs at a cold start. It seems to be a problem inside the amp, maybe an overvoltaged capacitor, not the speaker; nor the wires. What would it be?