First of all I'd like to thank anyone who's going to spend time reading (and answering) this. I'm a confirmed software developer and a fellow user of stackoverflow, who knows how boring it can be to come across a "stupid" question. I know how it feels to disagree with the accepted answer yet not taking the time to explain why it's wrong (nothing to be proud of).

My issue

I have a mono speakers setup at home, and bought a Bluetooh stereo amplifier. I couldn't find a mono Bluetooth amplifier. I read here and there that merging left and right was wrong then bought a stereo to mono converter, I put it between my amplifier and my speakers (small 8 Ohm passive ceiling speakers) but that didn't work. As a complete newbie in electrical engineering and hifi systems, I googled a lot and understood more or less why my setup wasn't working. To put it simply, the stereo merging implies much more impedance than my speakers, in order to make the left-to-right/right-to-left draw insignificant. If I got it right, merging HAS to be done before amplifying.


1. Formula anyone ???

I've seen various wiring diagrams to merge stereo to mono. I TOTALLY GET why resistors are needed. However, from a diagram to another you'll se different resistor values... HOW do I find out what the value of my resistors has to be ?

2. Shut up and take my money

If I'd buy an additional (mono) amplifier and put it so that my system looks like : BT amplifier -> stereo-to-mono mixer -> mono amplifier -> speakers. Would it just... work ?

3. DIY

Here's two photos of my BT stereo amplifier PCB. Where should (could) I solder the resistors and merge left and right, if possible ? enter image description hereenter image description here

  • 2
    \$\begingroup\$ Stereo channels are mixed together at the input of an amplifier, not at the output. But since the input of your stereo amplifier is Bluetooth then it does not have two audio input signals unless you get its schematic and a technician to mix the audio from its Bluetooth receiver for you. For mono on both output channels their inputs must be connected to the stereo to mono mixing resistors. Then each channel probably will be overloaded if it has more than 2 eight ohm speakers connected on each channel output. \$\endgroup\$
    – Audioguru
    Dec 18, 2021 at 0:18
  • \$\begingroup\$ Just use one of the two channels and ignore the other. \$\endgroup\$
    – user16324
    Dec 18, 2021 at 13:30

1 Answer 1


Can you wield a soldering iron? Do you have any test gear, e.g. a multimeter?

Anyhoo, you need to merge the signal where it gets out of the bluetooth module, which fortunately is a standard type- here it is on Banggood on a little motherboard


We can see from the nice big pictures that the audio outs come out, if we arrange it so that the square IC is at the top and the rectangular one at the bottom, on the topmost four connections on the right hand set of outputs. I've ringed them in red. enter image description here

It then appears that they go to what looks like a standard stereo op amp (5532) where the two channel outputs should be the two pins I've indicated with white arrows.

With this info the next step would be to break the tracks leading from those outputs and inserting a resistor network to merge them, but we really should confirm that there's audio coming out of those two pins before proceeding, because we're getting into the difficult to reverse changes state at that point. Or, thinking about it, the two inputs to the power amp section ought to have resistors setting an input impedance and we could take another pair of resistors of the same value, cross connected.

Anyway, first step would be to confirm that there's audio at those white arrows.

  • \$\begingroup\$ Thank you for helping. What's a safe way to confirm that there's audio coming out of those two pins ? \$\endgroup\$ Dec 20, 2021 at 11:39
  • \$\begingroup\$ If you don't have any other test gear, a rough and ready method I've used is to simply take an audio cable, plug one end into some kind of amplifier (or even computer audio input, etc), cut the connector off the other end, ground the shield and touch the signal wire to the pin in question. Make sure you don't short it to anything else. From the picture I can't tell which way up the chip is, so possibly the correct pins are a 180 rotation position wise. If you have or can get a multimeter, the arrows are for an orientation in which the top right pin is +volts and the bottom left is -volts. \$\endgroup\$
    – Ian Bland
    Dec 20, 2021 at 11:50
  • \$\begingroup\$ Hello, and happy new year I wish you the best. I got a hand on a multimeter and measured 23V between the +V and -V pins. Is that enough ? How comes the channel output pins A and B don't have a + and a - as in the cables that link devices ? Just curious. \$\endgroup\$ Jan 5, 2022 at 10:19
  • \$\begingroup\$ It's not about it being "enough", it's just confirming that the top right pin is the positive supply and bottom left is negative, so you should read +23V with the red probe top right and the black probe bottom left. The outputs use the common ground of the circuit as their "negative wire". \$\endgroup\$
    – Ian Bland
    Jan 5, 2022 at 13:12
  • \$\begingroup\$ Ok ! I figured out, sorry I wasn't totally following... So now that we're sure of the orientation, what's the next step ? \$\endgroup\$ Jan 5, 2022 at 18:27

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