Simplest way to combine a mono and a stereo source to play through a set of stereo headphones?

Ok, what I am specifically wanting to doing is make the Xbox voice chat (mono) and the game audio (stereo) come out through the same set of headphones. I quickly found out that I couldn't just wire them directly without turning the game audio to mono (not good for online multiplayer). I found a discussion about using an "opamp" and taking the mono signal and feeding it into the right channel then take the opamp output and feeding that back into the left channel to get a "stereo" output. Unfortunetly, I got as many details from that as I just gave here. Can anyone explain how and what parts it would take to do this? I also got the impression it might be passive, and no batteries would be a plus. Preferably, I would like to be able to get the stuff at Radioshack, and ideally, I would want to try and stuff the parts inside the over-the-ear cup style headphones directly.

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lemonie7 years ago
As you know, it isn't simple. If you're connecting L & R to the same mono source, you lose the stereo.
See the schematics on this page:
"stereo active mixer with two types of input", then perhaps the text.

vphreeze (author)  lemonie7 years ago
OK, I read that link, and it sounds like #4 is what I am after. I think I understand the concepts, but looking at the schematics makes my head spin. Is it showing two separate stereo in puts and two separate mono inputs? If so, that would allow me to cut out half the parts right there, right? Also, are the balance and pan parts needed for operation or are they disposable for my purposes?

There's your principle of operation, you could take out unnecessary inputs, and not use the balances.
Not being too familiar with this stuff, I'm not able to advise more though.

I realise u had this issue some time ago but for others that are interested in a simple solution.

attached is a simple opamp circuit (4 op amps on a single ic is common) the circuit is unity gain and i have ommitted capaciters on inputs.

This circuit allows the combination without bleed through of left and right channels.

I used non inverting amplifiers to combine the signals and ommited resisters from the output amps as these become standared to any circuitry u may wish to add.
Yeah that was a while ago, but the diagram is worth attaching for anyone browsing the page.


Multiple +'s to lemonie and Re-design!

First, here's a link to another question about a "simple" mixer. It uses the same page lemonie referenced, which, by the way, was given by Re-design in that question. (Excellent retention guys!) This should help you understand the electronics a bit better.

Now, from your description above, it looks like a slightly modified version of #4 is exactly what you want. Yes, get rid of 1 each of the stereo and mono inputs, leaving you with 1 of each to use. You should keep the input level controls (10k variable resistor - VR for short - after the inputs, before the 1uF capacitor) as you will definitely use them. The Balance and Pan Pots (VR's) are optional. On the OutPut side, get rid of the resistor, capacitor and jack labeled #2 on both the left and right - you wont need them.

For the OpAmp(s), you can use (4) LM741's, (2) LM1458's or (1) LM324. (note - the " 's " after the above numbers indicate more than 1 / multiple, or in english, plural and is not to be considered a part of the OpAmp's number) The 741 is a single OpAmp, the 1458 is a dual OpAmp, and the 324 is a quad OpAmp. So you could use any of these for construction. The easiest and least costly would be the LM324. Here is the datasheet. Look at the pinout on the first page and you will see all the necessary connections in a graphic format that you can correlate to the schematic. I'd use one side (2 OpAmps) for the left channel and the other side for the right channel.

The only thing not covered in the schematic is the connection for power, and it's understandable as they don't specify what OpAmp they are using. For the LM324, you would connect the positive to pin 4 and he negative or ground to pin 11. As standard practice, you should put an electrolytic capacitor form positive to ground (wired + to + and - to -) of around 100 uF or more to keep the power smooth.

You may find that you need some amplification. If this is the case, build a stereo amp from (2) LM386 OpAmps. Look at the many instructables here or see the datasheet for schematics. Just connect the left output from the mixer to the input of one LM386 amp and the right output of the mixer to the other LM386 amp.

That's about it. I hope you can follow these words of advice and be proud of your accomplishment. Good luck!


Re-design7 years ago
Here's an instructable for a passive mixer.  It's setup for stereo and on the mono if you want it in both sides you can connect it to both inputs.  Since is passive there will be some attenuation and maybe you can overcome that by just turning up the volume. 
vphreeze (author)  Re-design7 years ago
I think that's what I tried before, splitting the signal wire of the mono chat source to the signal terminal of the right and left speaker at the same time resulted in the stereo game audio becoming mono (combined where right and left can not be differentiated) too.
Well, if that's what happens, then you'll have to use and active mixer. It'll use and opamp or something to isolate the different signals from each other.

Now looking again at the schematic I can see that that's what would happen.