This is an old thread, an industrial solution is found. Please visit this thread.
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Original passage:
Greetings! About 7 months ago, I posted a thread about the noise from a bluetooth audio receiver.
Link to the thread:
https://www.diyaudio.com/community/threads/bluetooth-audio-receiver-high-frequency-noise-hum.375580/
Unfortunately, none of the solutions in the thread can fully eliminate the noise, and I also didn't find any solutions from the forum. Recently, out of some random chances, a professor from my university mentioned sth about common mode choke, said that it is a component to build a EMI filter, and that's literally how I find the solution to my problem.
I know that there are a lot of people working with bluetooth audio receiver had faced the same problem, then ended up using a more expensive bluetooth module. Therefore, 7 months later, I am here to share my solutions that might eliminate the noise from your Bluetooth audio receiver.
It will be brilliant that we can read till the end of this post as I would love to share my personal best solution for building a Portable Bluetooth Speaker for discussion!
Identifying the problem
First of all, we need to identify which types of noise occurs on your system.
Generally speaking, there are two types of noise can occur with the Bluetooth audio receiver:
I have made audio demonstrations for your reference.
As the name suggests, the noise is in a low and constant frequency.
A humming noise in high frequency. Sometimes the noise is dynamic, the frequency might be phrasing/varying, the noise might be softer after some time.
This two types of noise actually corresponds to different problem of your system and it is possible for both noises to occur. If the noise from your bluetooth receiver is very different than what I have suggested, it could be either your bluetooth receiver is defective, or it's simply too cheap... Yet if that wasn't the case, you are very welcome to discuss with everyone in this thread.
Possible Solution
Low Frequency Buzzing
If this is the type of noise that you are hearing, there is a good chance that you connected the bluetooth module and the amp from the same power source. And the noise will be gone if you supply the bluetooth module with another battery or any other power source.
The buzzing that you are hearing is caused by the current draw of the bluetooth receiver, and since amp is connected with the same power source, the noise from the receiver can feedback to the amp from its power input (from its GND specifically). We call this problem a Ground Loop Noise, or Non-Isolated Ground Feedback Noise.
To solve this problem, you will need a DC-DC Isolator connecting between the power supply and the bluetooth receiver to isolate the bluetooth receiver and the amp.
Sth looks like this:
View attachment 1033527
There are isolators with different rated values, make sure that you use the right one for your receiver.
There are also isolators that can isolate and step up of down the voltage from the power source, you can use one when needed, but I personally would not suggest that. I will explain why later.
For more information about Ground Loop Noise, I have came across a video from GreatScott!
High Frequency Humming
This is the noise that occurs from my last project, took me long enough to find the solution.
sigh
This noise is a bit complicated. It is due to the interference from the Bluetooth signal of the Bluetooth receiver. Although the frequency of the bluetooth signal is way too high for the amp to pick up (2.4GHz), the amp can pick up and playback the lower harmonics of the interference, yet we are still talking about frequencies around MHz, that's why we can hear the humming keeps phrasing with the amp's frequency response. The problem that you are facing is Electromagnetic Interference (EMI). This usually occurs when you are using a cheap Bluetooth module (even on some CSR/QCC module), that's because on a more expensive Bluetooth receiver, there can be special algorithms to suppress the noise. Yet, using the component below is going to give you the best result.
To solve this problem, you will need a EMI/EMC Filter. You can either connect it between the power source and the Bluetooth receiver, or connect it between the amp power supply. The EMI Filter looks like this:
View attachment 1033531View attachment 1033532
Notice that there you can find coils from the module. And that is a Common Mode Choke that I mentioned earlier.
Again, there are different rated values for EMI filters. Some of them can handle very large current, some of them are designed for low load modules.
You may use different EMI Filters at different rated values depends on your usage. If you have tons of modules connected to the same power source with the amp, just get one large power EMI Filter for your amp. If only have one module, get a smol EMI Filter just for that module as they are usually cheaper than the large power ones.
If you want to learn more about EMI filters, here is another video again from GreatScott!:
My Personal Best Solution for DIY Bluetooth Portable Speakers
Thanks for staying with me till here! Now I would like to share my experience on DIY Bluetooth Speakers.
I personally like to build medium sized DIY Bluetooth Speakers, that the power of my speaker is around 20W-50W. Therefore I usually use 12V battery for the Amp's power supply. As most Bluetooth Audio Receiver are rated at 5V, I simply use 12V-5V step down DC Isolator for the job, AND THIS starts my journey to find the solution for High Frequency Humming Noise. Yup, with some reasons, the High Frequency Noise can still feedback to the amp even when there is an Isolator (probably its because the frequency is too high).
Some people didn't have this problem is because they use a DC-DC Converter to step down the voltage first, then supply the Bluetooth Module with the stepped down voltage through a isolator. And I figured out that some DC-DC Converter actually comes with designs to suppress high frequency interference or even have a built in EMI Filter. Im pretty sure that some of these people eliminate the High Frequency Humming without knowing lol.
Anyway, because of this, I suggest that using a DC-DC Converter, and a DC Isolator for the Bluetooth Receiver can be the best solution, as this will be the cheapest to DIY a Bluetooth audio to your speaker.
However, not all DC-DC Converter has great build quality or an EMI Filter, some DC-DC Converter might even generate noise to the circuit, you will need to experiment with different brands of the component to get the best solution.
Also, DC Converter is still a DC Converter, which is meant for stepping up or down the voltage source. If you would like to fully eliminate the EMI, use a EMI filter at the power input of the amp, and DC Isolator for the Bluetooth Receiver.
Why using a Big EMI Filter for the amp instead of using a smol one for the Bluetooth Receiver you ask?
Welp that's simply because I didn't find much EMI Filters for low load components from my supplier. But who knows if you can find one from your place?
And finally, that's all I want to share, this took me long enough! I have struggled for too long since I got this problem, therefore I wrote this brief instructions for everyone who would like to build portable DIY Bluetooth Speakers just like I do.
I hope this post can help you on your project, good luck on your build!
😀
