From Garageband to Amp/Speaker

[h.finn]

I'm trying to use my Macbook Pro's Garageband with an acoustic electric (either through built-in input or external dac input) and listen to it either on a little portable amp/speaker like this SRS X33 ( https://static.qobuz.com/info/IMG/png/jpg_650x336_SRS-X33_L_Blue_cw.png ) or a proper "class D" amp connected to a large JVC "passive speaker".

Problem with the portable Sony: I only hear screeching sounds, no guitar notes...
Problem with the amp+JVC speaker: sound comes out all right but with a very annoying low buzz (or humm) that makes it basically....un-usable.

Is there a way to fix this?

Thanks!

 

[cbautis2]

^ The hum is from your amp not sharing a common ground with your MBP. Make sure all your gears are connected to the same ground so you won't get the ground loop hum.

 

[h.finn]

^ The hum is from your amp not sharing a common ground with your MBP. Make sure all your gears are connected to the same ground so you won't get the ground loop hum.

Hi,
Thanks for getting back to me!
....how do I do that?
I looked on the net and, for similar issues, it's recommended to either use a 3.5mm to rca cable (which I tried) or a toslink cable (my amp does not have an optical input but...). I used the Mac on it's battery. This is for the Hum when using an amp.

BUT: why the screeching noise when using garageband on mac with a battery powered "boombox" like the Sony?