Bluetooth Interference with High Pitched Tone

Question

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.

Answer 1

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

This more likely means it is power supply noise upsetting your audio amplifier. Audio amps can be very temperamental particularly on the input wires. Ensure that your bluetooth module shares the minimum wire length from battery to audio amp. If this doesn't fix it then ensure that you are somehow not allowing power currents thru the input stage section of your audio amplifer. If this doesn't cure it (or is hard to implement) try an audio transformer coupling just to see if the isolation it provides avoids the noise issue. Other ideas might be to use a high speed (hence high frequency common-mode rejection) differential op-amp between bluetooth module and power amp input.