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I built a suitcase boom box using a cheap BT Rx module and an amplifier module. The BT module is 12 vdc and the amp has an upper voltage input of 24Vdc, so a pair of 5 Ah 12vdc batteries was chosen to power it. My plans are to power the amp from the batteries in series, and use the CT for the - side of the BT rx.

The problem is the audio - output is connected to power - on the amplifier, so when I connected everything, delicious smoke emanated from the BT Rx, ending its short life. What is the simplest way to eliminate the 24v - on the audio connection between the amp and rx?

The modules I used: https://www.amazon.com/UEB-Bluetooth-Decoder-Wireless-Speaker/dp/B06Y2BCCV6/ and https://www.amazon.com/WINGONEER-TPA3116D2-Subwoofer-Amplifier-DC12V-24V/dp/B01M70KGSN/

  • \$\begingroup\$ Please add a hyperlink to the BT Rx module datasheet into your question. "My plans are to power the amp from the batteries in series, and use the CT for the - side of the BT rx", but your schematic shows that the CT (centre-tap?) is positive with respect to the other terminal and would therefore be powered backwards - hence the smoke? \$\endgroup\$ – Transistor Apr 13 '18 at 21:14
  • \$\begingroup\$ Some additional information on the BT and amplifier modules would help. The ground loop guess doesn't hold water for me though, seems more likely that the amplifier has some different care and feeding than anticipated. \$\endgroup\$ – W5VO Apr 13 '18 at 21:17
  • \$\begingroup\$ I corrected the schematic. I was actually getting 12vdc from the battery and had it connected properly to the BT module. \$\endgroup\$ – regretoverflow Apr 13 '18 at 21:37
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    \$\begingroup\$ Just a tip: when you use the CircuitLab editor an editable schematic is saved with your post. This makes it handy for you and us to update, copy'n'paste in to our answers, etc. No need for screengrab, image upload and background grid. Put all the info including links into the question rather than sprinkled through the comments. \$\endgroup\$ – Transistor Apr 13 '18 at 21:42
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    \$\begingroup\$ I had a look at the links. They're both Amazon ads with no datasheets. Our recommendation is, "No datasheets? No sale." It will be difficult to guess what's wrong here. \$\endgroup\$ – Transistor Apr 13 '18 at 21:45

Your problem is that you connected the batteries in series and let them share the \$V+\$ while at the same time GND is shared. That can't work.

The GND of the left circuit is the positive terminal of the left battery.
The GND of the right circuit is the negative terminal of the left battery.

Both GNDs are connected via Out- and Audio-.

I.e. the left battery is shorted via V-(right) --- Audio- --- Out- --- V+(left):

enter image description here

The simplest way to eliminate the problem is to supply the left circuit only by the left battery. I.e. both \$V-\$ go to the negative terminal of the left battery:

enter image description here

| improve this answer | |
  • \$\begingroup\$ Would the downvoter please explain what he considers wrong with this answer? \$\endgroup\$ – Curd Apr 14 '18 at 13:52
  • \$\begingroup\$ Not the downvoter, but those two parts aren’t going to be in agreement about the level of the common audio- line with that arrangement. \$\endgroup\$ – Russell Borogove Apr 14 '18 at 14:17
  • \$\begingroup\$ I just tested this arrangement and yes, I now have 12v between audio - and V+ rail on the BT module. I've connected the modules' V- to B- and connected the BT module V+ to the center tap between the batteries, and the amp V+ to the B+ \$\endgroup\$ – regretoverflow Apr 14 '18 at 15:39
  • \$\begingroup\$ @Russell Borogove: maybe; that's a completely different story not related to the problem addressed in this question \$\endgroup\$ – Curd Apr 14 '18 at 16:22
  • \$\begingroup\$ If the PA input is DC coupled with a reference to 0V=V+ of BT, this may not work but mixing 2 resistors to balance the DC with audio to 0V is better or AC couple. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 14 '18 at 21:01

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