Today I got a problem with my Spider Classic 15. I was playing with my cellphone connected to it via jack on the CD-MP3 input. Then a friend tried connecting it to a laptop via the same connector, and the amplifier started hissing heavily. If I touch with my fingers the end of the jack the noise is lower, but its still there. If I do this and at the same time I touch the strings of the guitar it dissapears. If someone is holding the end of the jack while I'm playing it sounds perfect, but I don't know why, and what to do to repair it. Someone has any idea of what happened and what should I do?