Hi folks,

I hope if I describe my problem there are enough people here that know about amp setup to help.

I have this normal (slightly old) Yamaha amplifier to which I have connected 2 Sony floor standing speakers and a sub-woofer (Polk 100W). If the sound in the amp comes from my laptop or my iPhone (same cable) the sound is incredibly muffled. If it comes from my SC-50 or an older computer's Sound Blaster it is fine (different cable, different input).

The laptop/iPhone is connected to the amp via a 3.5mm cable at one end and a 2 RCA at the other end.

I can make it un-muffled if I feed it mono signal (by slightly pulling out the 3.5mm jack from my laptop) or if I remove either the red or white RCA cable at the other end (where it enters the amp).

Sometimes the muffling sound is in waves as in the attenuation is not constant. it is possible the sound from other sources (SC-50) is attenuated too, maybe I am not noticing it.

Why is that and what I am doing wrong?