I'm building an FM radio / mp3 player device that is controlled by an Arduino. Each module has its own amp/output to speaker. When I try to connect the output from each device into one speaker by connecting the -'s and +'s together from each module, bad things happen.

If this wasn't a build for a handheld device, I'd probably use a 5 V relay and program the Arduino to control the relay depending on which device was being used.

Here is a video link to the breadboard build, if it helps:


I've tried some opto-isolators which worked with limited success... the sound quality was terrible, but I was able to program and connect everything. I read that the opto-isolators are not good for analog signals, which is obviously what the output from the amps would be.

Are Zener diodes a possibility?

I need some small factor switch that can be controlled with low voltage that only connects the circuit to the speaker for one amp'd device at a time. Thoughts?

Based on a solution provided on mixing the circuits I found this link:


and this image:

enter image description here

Could I possibly do something as simple as this? Or would I need more?

  • \$\begingroup\$ "Thought?": You chose the wrong modules for the job... \$\endgroup\$
    – brhans
    Oct 3, 2022 at 16:21
  • 1
    \$\begingroup\$ If both amplifiers are single-ended (meaning one of their 2 speaker outputs is GND), you might be able to connect the speaker to each "hot" output of each amplifier. It is possible that the 3W amp is NOT single-ended, killing this approach. Videos don't help - module schematics (or even block diagrams) might determine if this would work. I wouldn't "just try it" - its a module-killing idea if those amplifiers are not the right type, with coupling capacitor(s). \$\endgroup\$
    – glen_geek
    Oct 3, 2022 at 16:58
  • \$\begingroup\$ You could TRY a couple series capacitors, one per source. This would remove any DC bias, but still pass the AC signal. It may or may not do what you want but it's conceivably correct. A nonpolarized 220uF cap probably be enough eg. parts-express.com/220uF-100V-Non-Polarized-Capacitor-027-368 \$\endgroup\$
    – Kyle B
    Oct 3, 2022 at 22:11

5 Answers 5


An audio mixer circuit mixes together or selects amplifier INPUTS, not outputs.

  • \$\begingroup\$ So what about setting default volume of the mp3 player to say 1, and maybe even use a resistor to further reduce the volume, run that and the output directly from the fm radio (preamp) into this audio mixer circuit, then output that final mixed circuit to my 3w amp? If so this could be the solution I need and would still be small enough factor to fit in my handheld. \$\endgroup\$ Oct 3, 2022 at 16:44
  • \$\begingroup\$ Yes. A passive mixer circuit is simply a resistor (try 10k ohms) from each signal source and joined together at the amplifier input. \$\endgroup\$
    – Audioguru
    Oct 3, 2022 at 16:52
  • \$\begingroup\$ I added an image and link and was going to ask if this would work, but your telepathy must have known and answered it in advance. Works. I accepted your answer and really, really appreciate the help. This is great! \$\endgroup\$ Oct 3, 2022 at 16:55
  • \$\begingroup\$ @Audioguru but the inputs mixed together are outputs from somewhere, so whether or not mixer mixes outputs of other devices together or inputs of itself together is exactly the same thing. And if the speaker outputs are coming from H bridge then outputs are not that simple to connect together for reamplification with a separate amp. \$\endgroup\$
    – Justme
    Oct 3, 2022 at 17:11
  • \$\begingroup\$ Simply attenuate the "output" signals to the mixer input level. From an H-Bridge, select the Master output and attenuate it, not the slave output. \$\endgroup\$
    – Audioguru
    Oct 4, 2022 at 17:44

If you have two amplifiers with a common ground and with independent signal sources, with a single driven loudspeaker output driven against ground, you can just connect your loudspeaker between the two driven loudspeaker outputs. Formally, that will cause one signal source to have reverse voltage, but if the signal sources are independent (and the output does not couple back into the input like with acoustic feedback), nobody will hear the difference.

There are several caveats: you should preferably use a speaker with double the impedance to limit the maximum current. Since the relations between current and voltage at any terminal become quite unpredictable, you cannot use the amps at full power since their output transistors are designed for the use case where maximum voltage across them corresponds to minimal current, and that is just not the case with independently driven terminals.

There was a suggestion to consider a dual-coil loudspeaker. But those are pretty much only available as loudspeakers intended to serve as subwoofers, and those are generally much larger than you want for your application and unsuitable for reproduction of higher frequencies.


Don't you are asking for problems and potentially destroying one or both of the amplifiers. When you do this unless the amplifiers are exactly in phase you are driving energy into the output of the other. My recommendation is to get another amplifier with two inputs and use that to drive the speaker. The other option is to use two speakers.


If modules are meant to connect directly to speaker so each of them have their own speaker amp. These obviously can't be connected together so they are not meant to be used like that.

Optocouplers for connecting speakers to multiple amplifiers - I haven't heard that before but based on experience I also assume is not a good idea.

Generally you would have a single amplifier, and mix all the audio together before the amplifier. How to do that with your modules is unknown.

You need to reverse engineer your modules enough to get unamplified audio for single speaker amp.


Also consider a speaker with two (independent) voice coils, so-called dual voice coil.


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