i'm building a portable stereo system and am having some unwanted noise from the bluetooth receiver. the noise is typically a high pitched tone. 

please see the attachment for my chicken scratch schematic. 

the general setup is an amp connected to a 12V battery. there is also a step-down converter, connected parallel to the same battery, that goes to USB, which then connects to a bluetooth receiver that sends audio to the amp via an 1/8" cable. 

i've tried adding a capacitor at 200µF to the battery terminals with no luck of eliminating the noise. 

the high pitched tone goes away only when i power the bluetooth receiver from a separate power source. 

I've ordered a ground-loop isolator, but haven't received it yet, so i figured if anybody can suggest something, that would be most excellent. 

Also, I've tried subbing in alternate bluetooth converters, batteries, amps, and step-down converters, with no luck.

![enter image description here][2]


  [1]: http://forum.allaboutcircuits.com/attachments/img_2184-jpg.76776/
  [2]: https://i.sstatic.net/BZENQ.jpg