something weird is happening, i inject the 40 hz signal and i am able to trace it a the pin 5 of the 4558d with the amp off, but as soon as i turn on the amp i lost that signal, instead i got oscilation between 300 to 1 khz, tihs strange signal is present at the input rcas, it seems that this signal overlap the 40 hz signal, when i turn off the amp the 40 hz is there, the same with the channel Rigth i trace the signal and is present in pin 3 of the same op amp, but i lost it with the amp is on.