Summing two channels to one (line level signals)

[jarros]

Hello! I'm currently working on some mods for my T-amp to use it in a 2.1 system for my computer. The amp itself will only be used to power the two mains, but I would like to use the same box for the amplifier to control the volume for my sub as well as the mains. I have attached a picture of the schematics I drew for the amp.

The pic might need a bit of explaining as to what I'm all trying to do with this box. I have both a 3.5mm input and two RCA inputs, but I will only be using one or the other. Next, I have decided to add a headphone jack: this is basically just the source signal going through the 100K pot (main volume). The "Speaker Select" is a toggle switch which lets me decide where to send the input signal: to the mains or to the headphones. The other change I have made is to add a "Sub Toggle" toggle switch, which switches between using the signal from the main input or to use the signal from a seperate sub input (I'm using this in my room where I also have a TV and a basic surround sound setup, and I want the option to add the sub from the 2.1 system to my surround setup).

OK, time to the question at hand. I would like to be able to control the volume of the sub "on the fly" like you have in most computer speaker packages. From the 100K pot I will split the signal, one path going to the amp (after going through a few small resistors), the other path going to the sub volume control. The question is, how can I sum the stereo signal into a mono signal for the sub? This way I can use a single gang pot instead of a dual gang (of which I can only get 100K from radioshack). It would also make it simpler once I get to the sub input toggle, because right now I think I have the left channel from the main input going to the left output for the sub regardless of what position the sub toggle switch is in.

Wow, that was a lot longer than I had hoped. If you read through it all, thanks for your time, and please feel free to let me know what you think of my design.

Thanks in advance for any comments!

Jared

[sx881663]

In my amp3 setup I have about the same situation. I took the VC output from each channel and connected to a single sub out RCA with 10k resistors. The sub amp I have has enough gain that the losses don't matter much. Works fine and sounds great.
Roger

[jarros]

Did you wire the resistors in series with each channel?

I know I can't just connect the two channels to make them mono because that would make the source going to the amp for my mains mono as well. Is there a simple, passive way that I can join these two channels to get one mono signal?

Thanks,
Jared

[BWRX]

There is. Just use a Y resistive network and take the output from the center of the Y. It will sum the signals but it will also attenuate them (by a factor of 3 if all resistors are of equal value).

[sx881663]

Sorry I wasn't clearer in my answer; a pair of 10k res connected one end of each to L/R volume control outputs. Open ends tied together form a summing point and are the mono output. This way the common mono output tracks the main output so you only have to set the sub level once.
Roger

[jarros]

Ah, makes sense. Thanks for the input guys, I'm glad the solution was as simple as I hoped!

Jared