Trying to add a bluetooth audio receiver chip/module with no background noise - Page 1
Forum section: electronics / projects

author: microvia
author_rank: Regular Contributor
timestamp: July 03, 2025, 08:11:43 am
content: Hi,I designed a small FM receiver module which also has an aux input (single ended).Audio outputs are also single ended.Audio sources are switched using a 3:1 click-less MAX audio switch.I'm trying to add a bluetooth audio receiver. At first I thought about the cheap MH-M18 module but I can't really avoid the background noise it generates since the grounds can't be separated (the module uses the ground of the target device, which contains an audio amplifier).Unless there is some smart filter that does the trick, what other "tiny" bluetooth module could solve the problem ? Are there micro-modules available for this purpose and which suppress backgound noise ? I don't need to use Play/Stop/Vol keys as the functions are used on the bluetooth source (hence why I initially thought about something as small as the MH-M18).Thanks.

author: FartingNoveltyToyEngineer
author_rank: Newbie
timestamp: July 05, 2025, 03:44:46 pm
content: have you considered bm64 or bm83 from microchip? These are easy to configure and they offer an i2s output for separate dac.

author: Buriedcode
author_rank: Super Contributor
timestamp: July 06, 2025, 01:34:21 am
content: Many bluetooth modules have differential drivers so that they can:1) directly drive earphones without an external power amplifier meaning...1a) no bulky output caps and1b) large output swing when running from 3VAnother benefit is that all the noise from the onboard switching regulators, CPU cores and RF amplifier stages is common to both outputs - effectively cancelling out when driving something differentially.You could just use a differential amplifier to make a single ended output, and as long as the resistors are fairly well matched the noise should be attenuated enough to be inaudible.

author: microvia
author_rank: Regular Contributor
timestamp: August 01, 2025, 04:04:44 pm
content: Quote from: FartingNoveltyToyEngineer on July 05, 2025, 03:44:46 pmI have considered using the BM83. I even purchased the BM83EVB evaluation board which is rather complex but complete.The only drawbacks that I saw:- Price range of BM83 compared to Chinese chips a la Jieli (JL).- Very low resolution audio effects (connecting, disconnecting, etc.).I then considered using Feasycom modules. I bought an evaluation board and Feasycom has been kind enough to provide a firmware with the minimal custom configs I needed. But I gave up for several reasons not related to the quality of their modules. Basically I prefered using the KRC-86B which includes an AUX input with automatic switching and which produces negligible background noise.What I would really like to know is whether the MH-M18 background noise can be removed without using a ground isolator.I'm considering the following:- Double pi filter with two inductors (6.8uH and 100uH as often seen in designs around the MH-M18) and two 10uF capacitors.- One 10 to 20 ohm resistor between the MH-M18 / filter grounds and the common ground of my module.Any advise welcome

author: FartingNoveltyToyEngineer
author_rank: Newbie
timestamp: October 14, 2025, 11:25:58 am
content: I remember having some issues with BM83 as well, and eventually managed to solve them. Different firmware versions had different bugs.When using 24-bit output pull down resistor was required on i2s data line due to some bug where zero padding is done with high-z state. I can try to find how i configuredthe module in more detail if you are still interested. Also can you share schematics and layout of your current setup?

author: FartingNoveltyToyEngineer
author_rank: Newbie
timestamp: October 14, 2025, 11:50:08 am
content: I used firmware version 2.1.3

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 14, 2025, 12:10:59 pm
content: Are you saying the "noise" generated from the bluetooth module is picked up by the FM receiver, and shows up on the FM output? Is the noise a problem when using the aux input?Quote from: microvia on August 01, 2025, 04:04:44 pmAre you sure you know the path by which the noise propagates? If it's being radiated to the FM receiver's antenna, then separating the grounds likely won't help at all.

author: microvia
author_rank: Regular Contributor
timestamp: October 14, 2025, 07:24:26 pm
content: Quote from: mtwieg on October 14, 2025, 12:10:59 pmI tried using the KRC-86B and the MH-M18 on an old car radio. Both generated audible background noise which I couldn't eliminate. A buck DC/DC converter couldn't help there as the module had to share the amplifier's ground and the radio had not enough room for a 1:1 isolation transformer between the audio output of the BT module and the LF input of the radio's amplifier.Quote from: mtwieg on October 14, 2025, 12:10:59 pmSo far, I have been using the KRC-86B on old radios along with my FM-DW module, and had no trouble. This is mainly because their amplifiers are powered by the B+ whereas the modules are powered by the tube heater circuit. Although there is a common ground, there is barely any background noise (except the one generated by the charge pump in the KRC-86, which can be bypassed if you don't need the BT/AUX switch circuit).In any case, you can't just separate the grounds between the BT module and the LF amplifier unless you can fit a 1:1 isolation transformer in the audio path.

author: Sacodepatatas
author_rank: Regular Contributor
timestamp: October 15, 2025, 07:46:01 pm
content: Quote from: microvia on October 14, 2025, 07:24:26 pmI used a VHM-314 (HW770C with IC AC6979A2), It shares the ground with the car radio, connected to its audio-in input. For powering the module, i modified a MT3608 boost module replacing the coil with a 1:1 flyback transformer (about 25 turns per wound) hand wound using a T44-52 core (very small). I replaced the potentiometer with fixed resistors for having Vcc=5V.My experience: No noticeable noise at normal volume, eventhough with the engine started. If the module is idle, and i turn up the car radio volume to max, then i notice a very faint whistle that doesn't bother at all.

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2025, 11:31:10 pm
content: If the noise is from the ground loop which includes the audio signal ground, then there are two ways to fix it:The output signal can be converted to a current, by say a Howland current pump, and then the current drives the audio input in parallel with a resistor to the audio input signal ground. The resistor converts the current back into a voltage at the audio input.Alternatively, a difference amplifier at the audio signal input can be used to remove the difference between the audio output ground and the audio input ground, removing the ground loop noise.