Ground Loop Struggles with MAX9744

[sethsch]

Hi there, I'm working on a bluetooth speaker project. I am using this basic bluetooth audio receiver board. When connected directly into the MAX9744's audio inputs there is a high pitched whine which I assume is ground loop (please let me know if it's something else!!) Because this is all one project I need it powered off the same power supply. Currently I have one of these bad boys plugged into the breadboard which the VIN and GND pins of the amplifier are connected to. The bluetooth board and an arduino is powered off a 5v regulator circuit.

Following in the footsteps of this thread I picked up some DA102C transformers (the 1's were out of stock) from digi-key and I have them set up like so:


Using these transformers, the sound is extremely quiet, with everything turned all the way up. Note I have the transformers connected to AGND on the amp side because it's quieter there. Connecting to the ground rail or directly to the GND pin on the amp adds noise and doesn't fix the problem.

What else can I try to filter noise?

[adafruit_support_mike]

Hmm.. that sounds like the audio board isn't sending much current through the transformer's primary coil. Is your board designed to drive speakers directly, or does it create a line signal (low current) that is assumed to go to a power amp?

You might also try the return-path filtering option mentioned earlier in the thread you linked: connect your board's output directly to the MAX9744's RIN and LIN pins, then connect the grounds to AGND through one of the transformer's coils. You'll have DC connections on both sides, but the transformer coils to AGND will act as low-pass filters to block noise coming in through the GND line.

[sethsch]

Hmm.. that sounds like the audio board isn't sending much current through the transformer's primary coil. Is your board designed to drive speakers directly, or does it create a line signal (low current) that is assumed to go to a power amp?

Line signal.

You might also try the return-path filtering option mentioned earlier in the thread you linked: connect your board's output directly to the MAX9744's RIN and LIN pins, then connect the grounds to AGND through one of the transformer's coils. You'll have DC connections on both sides, but the transformer coils to AGND will act as low-pass filters to block noise coming in through the GND line.

How is this wired? Do you mean to attach the bluetooth board's ground to the end of one coil, and AGND to the other end?

[adafruit_support_mike]

How is this wired? Do you mean to attach the bluetooth board's ground to the end of one coil, and AGND to the other end?

Yes. The inductance between the Bluetooth module's GND and the amplifier's AGND will block the high-frequency signals injected through the ground loop.