Power source interference on Bluetooth audio module?

Question

I am building a Bluetooth Speaker and I been getting static noise or high pitch tone with certain configuration. I am new to electronics, hopefully the seasoned hobbyists here can help me explain what I am observing. I initially build the speaker with the following configuration:

I am experiencing static noise and high pitch tone with this configuration. At first I thought it's due to the 12V->5V step down converter I built, so I replaced that with a off the shelf USB Auto Charger (the one that plugs into the 12V outlet in car and powers a USB port) but still experience the same issue. Later, during some trial and error I came across the following configuration:

With this configuration, where 5V is coming directly from a cellphone charger, the static noise and the high pitch tone are gone. Everything is working as I expected it would except the fact that I have two power adapters powering one project.

I did some research and came across this post by Jake, I believe we ran into the exact problem. It seems to be resolved through the use of ground loop isolator. I read up on the ground loop and how an isolator can address the issue but couldn't get a good understanding of it. So here are my questions:

1. What is ground loop?
2. How can I solve the problem I am having without a ground loop isolator?

Thanks in advance for the advise.

Answer 1

Grounding can be tricky for noisy circuits. Switching power supplies and digital stuff both create noise. Put those two concepts together, and you have the problem you described.

Basically, you're seeing Ohm's law on the ground traces/wires making it not ground anymore at some distance away. Because that trace/wire is being used as a reference for your signal, any noise on the reference is interpreted as signal also. If you had a dedicated reference instead of sharing the power return for that purpose, then you could subtract out that noise using a balanced receiver. This is how balanced cables work: they're all about equal pickup of noise and nothing about opposite signals. (https://sound-au.com/articles/balanced-2.htm)

Ground loop isolators typically use transformers to fix the problem a bit differently, by converting the signal from electric to magnetic and back again, which allows the signal to pass with no electrical connection. The cheap ones, however, may not be able to handle low frequencies at high volumes without distorting because the iron core that makes them more efficient can also saturate. This is the magnetic equivalent of clipping. (The really cheap ones may actually be power transformers in a different package. Same concept, and sorta works, but different design criteria.)

Or, as your experiment showed, you could use two isolated supplies for the parts of your circuit that don't like each other, and connect their grounds at exactly one point, that point being the reference for your signal. This prevents any current from flowing through that reference so that it's only a reference and not part of a power supply.

So, I see three ways to fix your problem, which may be combined if necessary:

1. Use a balanced connection from the receiver to the amp.
2. Use a (good quality) ground loop isolator or audio transformer.
3. Isolate the power supplies from each other and reconnect them only through the audio cable. If you don't want separate 110v adapters, you can use an isolated DC-DC converter instead.