I'm in the middle of a project to adapt a mono aviation headset to a 3.5 mm plug.

The headset is mono, the plug will be stereo. Can I just connect both L & R together and connect to the audio of the headset?

The headset has a fairly high impedance (>400 ohm.)

  • \$\begingroup\$ Is it possible that someone could insert the 3.5mm plug into a stereo amplifier? That's not a good scenario - left amplifier would compete with right amplifier, causing a lot of heat. It would be safer to use two series resistors to L&R plug contacts. Maybe 47 ohms each? \$\endgroup\$
    – glen_geek
    Commented Jun 18, 2022 at 1:08
  • \$\begingroup\$ You can't make a short circuit between headphone output left and right signals. You have to ask yourself if you need to downmix stereo to mono or if you just can live with for example the left signal only, or do you want to modify the headset for stereo operation. You also need some data about the aviation headset, such as what type of microphone it has and is it even directly compatible with PC microphone input. \$\endgroup\$
    – Justme
    Commented Jun 18, 2022 at 11:34

2 Answers 2


No. You cannot connect the outputs of two separate audio power amplifiers directly together. Or at least, not for very long. Inserting capacitors removes a DC offset voltage problem, but each amplifier output stage still is seeing a very low impedance load - the other amplifier's output stage. Better than series capacitors is series resistors. they will prevent overheating in the amplifier stages, but they will generate heat of their own. Fortunately, a 400 ohm load does not draw much current.

I would start with two 200 ohm resistors, one in each audio path. Note the volume control position at high volume, turn things down to a low volume, measure the voltage across them at low volume levels to get the current, and calculate the power dissipation. With this information you can scale up the resistor power rating to handle high volume levels.


NO. If you want to mix the two signals to one, you should couple them with two capacitors. This should work, use normal 100µF electrolytic or ceramic SMD capacitors.

Left output ------||-------/--------Mono-Input
Right output -----||-----/

If the signal on the left side is the opposite on the right side, then the current would flow to the capacitors and you would hear nothing. But this is what we want.

  • 2
    \$\begingroup\$ Use RESISTORS, not capacitors. Caps are short-circuit at high-frequencies, with "high" being a relative term. To avoid sounding weird (audible highpass filter or no sound at all), they need to be short at audio frequencies. Thus, the amp channels are still shorted together, and you've really accomplished nothing. Use resistors instead. \$\endgroup\$
    – AaronD
    Commented Jun 18, 2022 at 4:24
  • 2
    \$\begingroup\$ That's a short circuit between outputs for any frequency other than DC. Extremely bad suggestion. \$\endgroup\$
    – Justme
    Commented Jun 18, 2022 at 12:16
  • \$\begingroup\$ @Justme - If the capacitor-size would be infinite high, then you have right. Otherwise not. \$\endgroup\$
    – MikroPower
    Commented Jun 19, 2022 at 15:32

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