just wondering how to sum stereo to mono signal? I am aware that you can use resistors in series to the left and right outputs of say a Bluetooth receiver module but when i do this, the channel does sum but the quality is horrendous on the high frequencies, sounds like a low bit rate mp3? Does anyone have a solution to this? The parts that i am using is a sure 30watt mono amp, and a sure Bluetooth receiver module 4.0 aptx etc... Thanks i would really appreciate this as i have been pulling my hair on this speaker build enter image description hereenter image description here

  • \$\begingroup\$ hm, a schematic might help. Generally guessing: is it possible you have the summing resistors, and then a significant length of cabling? Also, by the way, it's probably sound-quality wise a good idea to not transmit stereo over bluetooth just to mix it down to mono at the receiver, since bluetooth audio is mostly compressed and having one only slightly compressed mono channel will probably sound better than the mix of two channels compressed twice as heavily. And transmitting mono from the start would solve your analog problem alltogether. \$\endgroup\$ Commented Apr 14, 2017 at 10:26
  • \$\begingroup\$ Hey, thanks but I've tried using just an aux cable with this same layout, around 470 ohms on the resistors and it still has this weird sound on the high frequencies more than likely some sort of distortion or crosstalk? , but when i just use one channel (for example using only the left channel) it's completely fine and there is none of this distortion but i lose the signal from the right channel? \$\endgroup\$ Commented Apr 14, 2017 at 10:40
  • \$\begingroup\$ Also, transmitting mono isn't really option as many audio files are stereo so when streaming music, it's kinda hard to force mono through like a phone via SoundCloud or Spotify? \$\endgroup\$ Commented Apr 14, 2017 at 10:42
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    \$\begingroup\$ Mono is easy. Just configure your Operating system to downmix to mono. Has nothing to do with spotify. Your player doesn't know about bluetooth. \$\endgroup\$ Commented Apr 14, 2017 at 11:16
  • \$\begingroup\$ if you add resistance in series with your signal, and then have a long cable, you've built something that resembles an RC low pass filter (because the cable might contribute significant capacity). \$\endgroup\$ Commented Apr 14, 2017 at 11:18

1 Answer 1


Two resistors is the simplest way. These should be significantly higher than the output impedance of whatever is producing the stereo signals. That way, the signal from one channel doesn't mess up the output of the other channel. You may have to buffer the result to get it back to a reasonable impedance.

With a opamp, you can make a classic summing amp. These have the characteristic that each input only sees a resistive load to ground, not the signal from the other channels. Here is the basic topology:

OUT = -(In1 + In2 + in3 + ... + InN)

The output is also nicely low impedance due to the feedback around the opamp.

  • \$\begingroup\$ Thanks, guessed that something like an opamp would do the trick, I'll try this later \$\endgroup\$ Commented Apr 14, 2017 at 12:04
  • \$\begingroup\$ What opamp would you recommend? Currently i have a tl072cp on hand and have no idea how to power this thing, can i use a single rail supply? I this it's like this? I'll add a photo of the schematic above \$\endgroup\$ Commented Apr 14, 2017 at 14:06
  • \$\begingroup\$ @Fab: TL072 is fine as long as you give it wide enough +- supply, like +-12 V. \$\endgroup\$ Commented Apr 14, 2017 at 14:31

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