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I'm currently building a portable stereo speaker and have run into some issues. The speaker is based on a set of broken Jabra Move headphones (used as bluetooth receiver) and a XH-A232 amplifier board.

The way I'm planning to make this work is to use the speaker outputs from the headphones (left/right positive and negative) and try to feed these into the amplifier board. The problem is that the amplifier uses positive/negative/common as input (3.5mm/TRS style), which does not match the output from the headphones.

So far I've tried two methods, of which none have given a satisfactory result:

  1. I soldered the two negatives together and fed them to the common/ground of the amplifier. This worked, but the the two speakers played in mono. Somehow it seems like the left and right channels is mixed together by doing this.

  2. I soldered only the right negative to the ground terminal of the amplifier board. This also worked, but a similar behavior was obtained. The sound signal coming from the right channel is played equally loud on both speakers. The left sound signal plays only on the left speaker, but VERY silently.

Now I'm out of ideas, and I need some advice from all of you. What can I do to make the speaker play stereo sound?

Thanks in advance!

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  • \$\begingroup\$ Can you tolerate some loss of volume? If so, you can use a resistor network to combine the signals. If you cannot tolerate loss of volume, you will need an amplifier to combine the signals. \$\endgroup\$ Jun 14, 2021 at 14:37
  • \$\begingroup\$ How would such a resistor network be constructed? Also, how big would the volume loss be? @MathKeepsMeBusy \$\endgroup\$ Jun 15, 2021 at 7:38
  • \$\begingroup\$ Could you explain why a coupling capacitor must be used? If I were to use such components the wiring would be the same as I did in "method 1", but with the capacitors added? Also, is a coupling capacitor any different than a regular capacitor? What is a reasonable capacitance to make it work? @Audioguru \$\endgroup\$ Jun 15, 2021 at 7:52
  • \$\begingroup\$ The XH-A232 amplifier does not have "positive/negative/common as input (3.5mm/TRS style)" as input. Instead it has Left, Right and pb which is ground (0V). \$\endgroup\$
    – Audioguru
    Jun 16, 2021 at 15:00
  • \$\begingroup\$ Your description of the headphones receiver has its negative outputs each powered separately by its own amplifier (bridged) and you shorted them together and grounded them that might have destroyed the outputs. Since they are bridged then each positive output has a DC voltage on it. The power amplifier has no details so it might not want DC on its inputs then DC blocking capacitors must couple the signal. If the amplifier input impedance is known then the capacitor value is simply calculated. \$\endgroup\$
    – Audioguru
    Jun 16, 2021 at 17:18

2 Answers 2

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A guess:

Low voltage amps can well have bridged output stages i.e. both + and - output wires come from amp IC, none of them is GND. If you connect +output or -output to GND the amp of the headphones is shorted. There can be some series resistors or other protection mechanism which luckily have held the smoke inside the ICs.

Try to connect left +output to the tip and right +output to the ring. Make the connections via capacitors to stop DC. Measure its polarity to keep electrolyte capacitors alive or use plastic capacitors. Connect the ground of the headphones circuit board to the sleeve.

BTW. There's still traps left:

  1. The headphones output amp can be a kind of pulse modulation based one and needs a filter which becomes ineffective if the signal is taken between the +output and GND. In theory the speaker elements in the headphones can be alone the necessary lowpass filter.

  2. Connecting the GND of the amp in the phones and the GND of your new speaker amp can cause something unwanted if they both use the same power supply. The system can become oscillator or a voltage which is generated by some DC-DC conversion can be shorted. The oscillatory behaviour fortunately generally needs very high gain.

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  • \$\begingroup\$ Thank you for this answer. Maybe I was lucky, but so far no components have broke. Is your recommendation similar to @Audioguru 's tip above? I'm new to capacitors, is there any difference between electrolyte capacitors and plastic ones? Also, what capacitance should the capacitors have? If I understand it correctly I should wire the components as I did in "method 1" and add capacitors to both +wires? \$\endgroup\$ Jun 15, 2021 at 8:18
  • \$\begingroup\$ The negative output wires should be left unconnected. Get 1uF polycarbonate or ceramic capacitors (=no DC polarity problems) and connect them from +outputs to the tip and the ring. The sleeve should be connected to the ground of the heaphone circuit board. I guess (=GUESS, not know) one of the battery terminals is connected to the same place. GND is often the largest continuous metallized area in the circuit board. Seemingly @Audioguru has written the same words but I'm not at all sure if he writes of headphone circuit board. See in the edit history did I write it first or did I copy it. \$\endgroup\$
    – user136077
    Jun 15, 2021 at 8:31
  • \$\begingroup\$ Okay! I will try this in a couple of days, hopefully it solves my issues! Thanks a lot for the detailed answer! \$\endgroup\$ Jun 15, 2021 at 8:51
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Balanced (differential) vs unbalanced (single-ended) issue.

Your headphone bluetooth circuit is putting out a (pair of) unbalanced, aka single -ended signals.

Meanwhile your amplifier needs as input two balanced (differential) signals.

If this is new to you, do some quick research on single ended vs differential signals. In audio these are referred to as unbalanced vs balanced, but it means the same thing. This is fundamental to working with analog signals.

You will need to convert the unbalanced output to a balanced output. You can find circuits that do that. Look for 'unbalanced to balanced converter', etc.

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  • \$\begingroup\$ If you are talking about an amplifier output you can blow the amp if it is a bridged output or they do not share a common output connection. You can determine this by looking at the schematic. If one half of each output is ground this will work, if not probably smoke! \$\endgroup\$
    – Gil
    Jun 15, 2021 at 1:03
  • \$\begingroup\$ @DanRoss I did some quick research and to my understanding the stereo TRS/aux/3.5mm cable is actually NOT balanced? If this is correct then I would think that a unbal to bal converter is the wrong approach, do you agree? \$\endgroup\$ Jun 15, 2021 at 8:05
  • \$\begingroup\$ You can't say that a TRS cable is balanced or unbalanced. The cable can be used for either type of signal. It is just three conductors. In audio circuits a cable with a TRS connector can be used for a pair of unbalanced signals (Left, Right, Common), or it can be used for a single balanced signal (+,-, ground). You can't tell if a circuit uses balanced or unbalanced signals by looking at the connector. Your Bluetooth Rx is putting out unbalanced pair of audio signals. \$\endgroup\$
    – Dan Ross
    Jun 15, 2021 at 13:13
  • \$\begingroup\$ The question is in regards to the INPUT of the amplifier, not the output. The data sheet ti.com/lit/ds/symlink/… says that the inputs to the amplifier are differential (aka balanced), not single ended. See section 9.3.3. \$\endgroup\$
    – Dan Ross
    Jun 16, 2021 at 19:02
  • \$\begingroup\$ The datasheet shows that if the - inputs of the TPA3110 IC are capacitor coupled to ground then an unbalanced L, R and ground input can be used. Yes, I know this question is 5 years old. \$\endgroup\$
    – Audioguru
    Dec 23, 2021 at 1:13

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