Trying to figure out the source of line-out hum

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Not sure if this is the right place to post this but I think this is the most appropriate sub-form.

I have a SNES in which I am using the RCA line level ports for audio into a pyle pro amplifier. When using the SNES, only when powered on, I get a substantial hum in the audio. However, when I hook up my roland SC55 to the amplifier is it crystal clear.

Both of these devices are hooked up to the same plug in the wall. So we can rule out some AC line interference (or the Roland SC55 is handling the interference better?).

What else can cause my SNES's line out to hum badly? Could it be the DC adapter used for the SNES? It's not the official nintendo one. Can a power supply rail effect line out? Could it just be a shitty onboard preamp/mixer? How do I go about debugging this? I want to make this my first hifi project. To fix this hum.
 
Only thought i have is ......

are the outer rings of the RCA plugs making a firm contact with the sockets outers . I have had to sqeeze a few plug outers with a suitable pair of pliers to make them make a good contact . An idication of this is when the plugs slide on easily , ideally they should have a firmish scraping feel.
 
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