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Let me preface this with saying that I'm a complete amateur with electronics and know very little of theory behind it. I think my question is easily answered but I just can't find the right info.

I have a portable speaker that I'm trying to turn into a bluetooth speaker. The speaker originally ran on 4 AA batteries or could be plugged into the the wall. I've removed all the power inputs and soldered an old phone battery to the power in. I just plugged in an 3.5mm cable from my phone to the aux in and it has worked really well. I then bought a usb bluetooth transmitter. I connected it to the same batter that the speaker system is using. I plugged the 3.5mm into the bluetooth and cut the cable. I connected the wires from the 3.5mm to the aux input on the speaker "board". The audio sends just fine and plays through the speakers.

However, I'm getting some sort of interference - some fast, high pitched pulsing coming out of the speaker. Other than this the music sounds great. From what little I know about this kind of thing it seems that it is causing a problem because I'm drawing both of these off the same power source. I've tried to use a capacitor to smooth out the power but it doesn't cut the interference at all.

What can I do to cut this high-frequency noise out? I appreciate any and all help.

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Presumably you bought a bluetooth receiver/transmitter (not simple transmitter) to attach to the amplified speaker?

Audio noise from bluetooth activity could be entering the audio system via the shared power, as you suspected. A mix of high and low value caps on the battery connections to the audio and the bluetooth device would be needed to filter that out. But before going to much effort, try powering the bluetooth off separate power or battery, and see if that solves the problem.

If it doesn't, the amplifier in the battery-powered speakers may be picking up the radio signals from the bluetooth, and, acting like a radio, turning the signal into sound you can hear. It's not supposed to do that, of course, but many amps are susceptible to that problem if there's a transmitter close by. Solving it may be difficult or easy... rearranging wires may do the trick. Or you may have to shield the amplifier circuitry inside a metal box, grounded to the battery/audio ground.

All of this will be easier to experiment with if you can power and position the bluetooth and audio modules separately.

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I had the exact same problem and it was solved by putting a ground loop isolator between the Bluetooth receiver and the amp.

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