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So I have here sub woofer Simbadda CST 5100N and I want to convert it to bluetooth, so bought an MP3 bluetooth module to convert it. The module needed 5 to 12 volt DC to operate. Inside the speaker, there is step down transformer 220 to 9 volt (that is what is written on the transformer), but I connected the power supply for MP3 module from a capacitor after full bridge rectifier. Before I connected it though, I measured the voltage running through the capacitor (2200 uf), and it says 12 volt. So I can directly connect the module and it would operate. On the module though, there is LM 7805, so I know it runs with 5 volts. The problem is the humming sound is louder. Is there anyway I can reduce the humming sound ? Or it would be best to eliminate it.?

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    \$\begingroup\$ dows the hum go away if you power the mp3 from battery? \$\endgroup\$
    – Jasen Слава Україні
    Commented Oct 3, 2019 at 7:56
  • \$\begingroup\$ TY for answering, Jasen... if I power it with battery, it would create another problem for me like the battery would be exhausted pretty fast, wouldn't it? cz I connect my pc with that speaker.....is there another way? I wanted to install phone charger inside the speaker as the power source for the module, but man... honestly I'm just an amateur in this kind of things... I don't know what would be the result of it... oh yeah this is the link of the module s0.bukalapak.com/img/54359177341/w-1000/data.png.webp \$\endgroup\$
    – Nelson
    Commented Oct 3, 2019 at 8:08
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    \$\begingroup\$ I am trying to diagnose the problem, I suspect a ground loop and the easiest test is to use an isolated supply like batteries. if that works an audio isolating transformer will stop the hum. \$\endgroup\$
    – Jasen Слава Україні
    Commented Oct 3, 2019 at 8:31
  • \$\begingroup\$ Ok, TY... I will try using the battery then.. TY jasen,,, \$\endgroup\$
    – Nelson
    Commented Oct 3, 2019 at 8:42
  • \$\begingroup\$ yes Jansen, the hum disappear when I connected it with battery.. TY man.... \$\endgroup\$
    – Nelson
    Commented Oct 3, 2019 at 18:57

1 Answer 1

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The problem is a ground loop, or possibvly a short-circuit caused by the common connection of the audio signal

To fix it use an audio isolating transformer to carry the music signal between the MP3 player and the amplifier board

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    \$\begingroup\$ I have fixed the problem... I simply install 12 volts adaptor inside of the speaker to power the module... yes I guess it's the ground loop that caused this humming sound.... Thank you Jasen.... man.. it's 4.40 am here in Indonesia, and I haven't sleep because of this... Thank you again jasen for helping me... \$\endgroup\$
    – Nelson
    Commented Oct 3, 2019 at 21:42
  • \$\begingroup\$ that's the other way to do it isolate the supply. \$\endgroup\$
    – Jasen Слава Україні
    Commented Oct 4, 2019 at 5:42

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