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Interference noise bluetooth Audio receiver and amplifier.

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asimov_18

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Dear Members,
I had my old computer stereo speakers kicking around(with 3.5 mm Jack). In an attempt to make it useful I bought a cheap bluetooth audio receiver. I managed to interface the bluetooth module to the speaker. For powering up the module I used a 7805 which derives 5V from the speaker powers supply. The module works fine and plays music as expected. But as I am sharing the power supply from the original speaker unit I hear a 800 Hz-1000 Hz hum when the bluetooth module is linked to the mobile phones. Infact what I am observing is not something new but other people using similar approach have faced this problem. I am putting a link when another user has observed similar problem and one of the solution for this is to use an independent power supply for powering up the module. Kindly look at the video from 3 minutes 54 seconds onwards
URL: https://www.youtube.com/watch?v=oGTXczCV30A

I am presuming this problem is to do with following problems;-

1) Ground loops that is formed when the audio amplifier and the module share a common ground once at the power supply end and then at the aux cable end.
2) Common analogue and digital ground for the module.
3) EMI from the module corrupting the power rail and injecting noise in the power supply. The signal from the antenna getting couples thru the
power rail or something similar(though bluetooth signals should be around 2.4 Ghz) there could be some frequency downmixing (guessing)

I tried the following solutions.

1) Inserting a 18 Ohm resistor in the analog ground of the Aux cable hoping that this will make this path more resistive and the module will
not use this aux ground for return supply current and would use lower resistance direct ground path instead.
2) Putting a filter inductor and a filter capacitor at the output stage of the linear regulator for suppressing the noise signal.

3) Putting 1K load impedance at the input stage of the amplifier to bring down the input impedance, hoping the noise signal to be more of a voltage signal with small current delivering capability and with this load resistor the amplifier would only see the higher power audio signal rather than the noise signal.
4) Not using the Aux ground conductor at all and relying on the digital ground from the power supply for audio signal.(Just for testing and seeing the response).

Can anyone one suggest me what can be possible cause for this and a possible solution? Something that I wanted to try was putting an inductor in the ground path and if this noise source is due to the hum getting coupled via the ground rather than thru the power line.
I am sure there would be a solution to this as commercial HIFI with bluetooth inputs would be solving this problem!! Could this be due to inbuilt switching regulator in this module IC for power regulation.

I am attaching an image for this cheap module for reference, I admit the PCB layout for this module is not ideal but nothing much can be done about it!!**broken link removed**

20160305_232458.jpg
20160305_232517.jpg
 

1) Ground loops that is formed when the audio amplifier and the module share a common ground once at the power supply end and then at the aux cable end.

Since you mention ground loops, that turned out to be the problem when I plugged my laptop into my entertainment system. It caused mains hum to come from the speakers.

I looked for solutions. Either: (a) run the laptop only from battery or (b) loosen the incoming cable tv, and pull the plug out of the jack maybe 1/16 inch. This disconnected the ground of the cable tv. Instantly the hum stopped.

Apparently the ground loop was caused by the laptop's AC adapter interfering somehow with the incoming cable tv.

As you know, ground loops pop up occasionally, and are difficult to fix.
 
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