Stereo to mono summing circuit

I am building a portable speaker and need to take a stereo signal and convert it to mono for the amplifier as simply as possible. I found some really simple circuits online (1K resistors on the two signal lines before joining them), but wanted to see if anyone here has done something like this and can confirm or suggest something else. Thanks!

(I should probably have just bought a Riva Turbo X, but I have all the parts for the project already).

 

I built a feedback subwoofer and summed the two channels at the input. It may be overkill for your project, but I used a nice op amp to buffer the inputs. The op amp is set at a gain of 1, that is, feedback from the output to the negative input. Two channels have resistors in line (I chose 4.7k, but 1k would fine) and connect together at the '+' input of the op amp. The biggest challenge for you might be powering the op amp.

Good luck in your project.

 

Two resistors is really truly the simplest way to do it. Look at DJ headphones with mono switches and that's all they are.

Or... just buy the Riva+bag combo at the ridiculous outlet price of $129: https://rivaaudio.com/riva-turbo-x-outlet/

 

Well, I did the two resistor thing and it ostensibly worked. The problem is that now I am getting some pretty bad buzzing. Nothing when no input plugged in, but fairly loud when something is plugged in. Lower in volume when plugged into my phone vs when plugged into my Amazon Dot.

Is there a more complex circuit with wires and resistors that might address this? Would one of these might work? https://www.amazon.com/Mpow-Ground-Isolator-Stereo-System/dp/B019393MV2/

 

I'm really not sure... does the buzz sound like a ground loop? What value of resistors are you using?

 

5.6k resistors (only ones I had on hand). Got a ground loop breaker (tiny transformers I think) off Amazon that didn't help. When I touch the barrels of my cable connectors with my fingers it got worse. I haven't used this amp board in a couple years. Maybe it is dying. Going to try a different one that I know works this weekend.

 

You need bigger resistors, remind to grab some Saturday.. dB

 

I agree with dBel, try some higher R values first. My first guess is that each channel is trying to drive the output impedance of the opposite channel.

 

@fraggler did you connect the two input grounds together to the output ground? Just trying to think of something simple that could be overlooked.

 

Thanks guys. I tried adding another set of resistors in series, but that didn't do anything. I finally swapped out the amp board and all my noise issues went away. So my little boom box is up and running! Might need some work with filters or maybe a port to get it sounding good, but for now it is electrically sound. Thanks again for the help.