hifidoctor

Hi All, New to the group. I purchase a few Carver amps(M-4.0t and M-1.5T). Both of them have the same audio hum at idle with no input connected from the pre-amp. Each one is from different sides left and right. But once I hook up the patch cords from the pre-amp to the Carvers is goes away???????Any help would really be appreciated. Thanks Don